LASER

Light Amplification by Stimulated

Emission of Radiation

Course BTech

Subject Engineering Physics

Unit I

Chapter 2

Objectiveshellip

Introduction and understand the principle ofLASER

bull Light Amplification by Stimulated Emissionof Radiation

bull Absorption

bull Spontaneous Emission

bull Stimulated Emission

bull Population Inversion

bull Optical Pumping

Objectiveshellip

Characteristics or Properties of Laser Light

bull Coherence

bull High Intensity

bull High directionality

bull High monochromaticity

Laser light is highly powerful and it is

capable of propagating over long distances and

it is not easily absorbed by water

Introductionbull LASER

ldquoLight Amplification by StimulatedEmission of Radiationrdquo

bull MASER (1939 Towner)

ldquoMicrowave Amplification by StimulatedEmission of Radiationrdquo

bull Stimulated Emission - Einstein in 1917

bull Ruby Crystal LASER - Maiman California in1960

bull He-Ne LASER - Ali Javan in 1961

bull Diode LASER- Hall in 1962

Light having following Properties

Wavelength

Frequency

Amplitude

Phase

CoherenceIncoherence

Velocity

Direction

Absorption

bull E1 = Ground state

bull E2 = Excited State

bull E = hν (Photon Energy)

1

bull According to Bohrrsquos law atomic system is

characterized by discrete energy level

bull When atoms absorb or release energy it

transit upward or downward

bull Lower level E1 amp Excited level E2

bull So h ƒ = E2 ndash E1

bull The rate of absorption depends on no of

atoms N1 present in E1 amp spectral energy

density u(ƒ) of radiation

bull So P12 α N1 u(ƒ)

bull P12= B12N1 u(ƒ)

Spontaneous Emission

bull E1 = Ground State

bull E2 = Excited State

bull E = E2 ndash E1

= ΔE

= hν

2

bull System having atoms in excited state

bull Goes to downward transition with emitting

photons hƒ = E1 ndash E2

bull Emission is random so if not in same phase

becomes incoherent

bull The transition depends on atoms in excited state

N2

P12(spont) α N2 = A21 N2

bull Where

A21 = Einstein coefficient for spontaneous

Emission we get Incoherent radiation forms heat

by light amplification of radiation by spontaneous

emission

Stimulated Emission

3

bull System having atoms in excited state

bull Goes to downward transition with emitting photons

bull 2hƒ = E1 ndash E2 After applying photon energy hƒ

bull Emission is depends on energy density u(ƒ) amp No of

atoms in excited state N2

bull P12(stimul) α u(ƒ) N2 = B21 N2 u(ƒ)

bull Where B21 = Einstein coefficient for Stimulated

Emission

bull Thus one photon of energy hƒ stimulates two photons

of energy hƒ in same phase amp directions So we get

coherent light amplification of radiation by

stimulated emission

Population Inversion

bull It is the process of increasing exited electrons in

higher energy levels

bull Due to this process the production of laser is

possible

bull The energy level between the ground state E1 (1st

level) and exited state E3 (3rd level) is known as

metastable state E2 (2nd level)

bull By optical pumping electrons from ground state

jumps to exited state by absorbing photons

bull The electrons remain only for 10-8 sec in exited

state E3 so most of them jumps back to the ground

state E1 by emitting photons But some of them

jumps to the metastable state E2

bull They (electron) stay in metastable state for more

then 10-3 sec

bull So electron density increases in metastable state

bull Thus the transitions are possible it takes more no

of electrons together and ν ndash (knew)12 photon beam is

produced which constitute laser beam

Optical PumpingThere are no of techniques for pumping a

collection of atoms to an inverted state

bull Optical pumping

bull Electrical discharge

bull Direct conversion

When photon of blue green light incident onRuby crystal electrons from ground state absorbs andexited and jumps on higher energy state levels andcomes back to metastable state They increasepopulation of electrons in metastable state

This process is called optical pumping which isdone by flash tube

Relation between Einsteinrsquos lsquoArsquo and lsquoBrsquo

coefficientsbull Einstein obtained a mathematical expression for the

existence of two different kinds of processes

(1) Spontaneous emission

(2) Stimulated emission

bull Consider all atoms r in thermal equilibrium at T

bull Radiation of freq ƒ amp energy density u(ƒ)

bull N1 amp N2 r atoms in E1 amp E2 respectively

bull In equilibrium absorption rates amp emission rates must be same

bull ie B12 N1 u(ƒ) = A21 N2+ B21 N2 u(ƒ)

A21 N2= u(ƒ) [B12N1 ndash B21N2]

So u(f) = [A21 N2 (B12 N1 ndash B21 N2)] ---------(1)

------------(2)

bull Boltzmann distribution law

------------(3)

bull So -----------(4)

bull But E2 ndash E1 = hf -----------(5)

bull So -----------(6)

21

21

12 1

21 2

( )

[ ]

ƒ

1

A

Bu

B N

B N

1

2

1 0

2 0

E kT

E kT

N N e

N N e

2 1( )1

2

E E kTNe

N

h 1

2

ƒ kTNe

N

---------- (7)

bull According to plankrsquos radiation formula

----------- (8)

bull Where B12 = B21 amp A21 B21 = ------------ (9)

bull So Ratio of spontaneous to stimulated emission

--------- (10)

21

21

ƒ12

21

h

ƒ

1

( )

[ ]kTe

A

Bu

B

B

3

3 ƒh

8 1( ) ( )

[ ]

ƒƒ

1kTu

c

h

e

3

3

8 ƒh

c

2 21 21

2 21 21

3

3

8

( ) ( ) ( )

ƒ

ƒ ƒ ƒ

N A A hR

B u B u ucN

bull So

--------- (11)

--------- (12)

bull So R = ---------- (13)

If hƒ ltlt kT in thermal equilibrium

then R = ltlt 1

bull hƒltltkT ndash Stimulated emission

ndashValid in microwave region (MASER)

bull hƒgtgtkT ndash Spontaneous emission

ndashValid in visible region incoherent

3

3

3

3

ƒh

8( )

8

ƒƒ

amp

ƒƒ

1

1( ) ( )

[ ]kT

h

uc

uR

h

e

c

ƒh 1[ ]kTe

ƒh 1[ ]kTe

Types of LASER

There are three types of lasers

1 Solid Laser (Ruby Laser)

2 Liquid Laser

3 Gas Laser ( He ndash Ne Laser CO2 Laser)

Ruby LaserhellipTo produce laser from solid Ruby crystal is

used

Ruby is an aluminum oxide crystal (Al2O3) in

which some of the aluminum atoms have been

replaced with Cr+3 chromium atoms (005 by

weight)

It was the first type of laser invented and was

first operated by Maiman in Research

Laboratories on 1960

Chromium gives ruby its characteristic pink or

red color by absorbing green and blue light

Ruby LaserhellipFor a ruby laser a crystal of ruby is formed into

a cylinder The ruby laser is used as a pulsed

laser producing red light at 6943 Aring

4

Ruby crystal is surrounded by xenon tube Rubycrystal is fully silvered at one side and partiallysilvered at the other end

A strong beam of blue green light is made to fall upon crystal from xenon tube and this light is absorbedby the crystal

Because of this many electrons from ground state ornormal state are raised to the excited state or higherstate and electron falls to metastable state

During this transition photon is not emitted butexcess energy of the electrons absorbed in crystallattice

5

As electron drops to metastable state they remain

there for certain time ~ 10-6 sec

Thus the incident blue green light from tube

increases the number of electron in metastable

state and then the population inversion can be

achieved

If a light of different frequency is allowed to fall

on this material the electrons move back and

forth between silvered ends of the crystal

While moving through they get stimulated and

exiced electrons radiate energy

Thus radiation photon has the same frequency as

that of incident photon and is also in exactly same

phase

When the intensity of light beam is increased the

same process is repeated

Finally extremely intensified beam of light

energies from the semi silvered side of the crystal

This way it is possible to get extremely intensified

and coherent beam of light from the crystal This

beam is nothing but higher energetic beam ndash ie

LASER beam

Applications of Ruby LaserhellipRuby lasers have declined in use with thediscovery of better lasing media They are still usedin a number of applications where short pulses ofred light are required Holographys around theworld produce holographic portraits with rubylasers in sizes up to a meter squared

Many non-destructive testing labs use ruby lasersto create holograms of large objects such asaircraft tires to look for weaknesses in the lining

Ruby lasers were used extensively in tattoo andhair removal

Drawbacks of Ruby Laserhellipbull The laser requires high pumping power because

the laser transition terminates at the ground state

and more than half of ground state atoms must

be pumped to higher state to achieve population

inversion

bull The efficiency of ruby laser is very low because

only green component of the pumping light is

used while the rest of components are left unused

bull The laser output is not continues but occurs in

the form of pulses of microseconds duration

bull The defects due to crystalline imperfections are

also present in this laser

Gaseous Laser (He ndash Ne Laser)A helium - neon laser usually called a He-Ne

laser is a type of small gas laser He-Ne lasers

have many industrial and scientific uses and are

often used in laboratory demonstrations of optics

He-Ne laser is an atomic laser which employs a

four-level pumping scheme

The active medium is a mixture of 10 parts of

helium to 1 part of neon

Neon atoms are centers and have energy levels

suitable for laser transitions while helium atoms

help efficient excitation of neon atoms

The most common wavelength is 6328 Aring These

lasers produced powers in the range 05 to 50

mW in the red portion of the visible spectrum

They have long operating life of the order of

50000 hrs

6

Constructionhellip

It consists of a glass discharge tube of about

typically 30 cm long and 15 cm diameter

The tube is filled with a mixture of helium and

neon gases in the 101

Electrodes are provided in the tube to produce a

discharge in the gas

They are connected to a high voltage power

supply The tube is hermetically sealed with glass

windows oriented at Brewster angle to the tube

The cavity mirrors are arranged externally

WorkinghellipWhen the power is switched on a high voltage of

about 10 kV is applied across the gas

It is sufficient to ionize the gas

The electrons and ions are produced in the process

of discharge are accelerated toward the anode and

cathode respectively

The electron have a smaller mass they acquire a

higher velocity They transfer their kinetic energy to

helium atoms through inelastic collisions

The initial excitation effects only the helium atoms

They are in metastable state and cannot return in

ground state by the spontaneous emission

The excited helium atoms can return to the ground

state by transforming their energy to neon atoms

through collision This transformation take place when

two colliding atoms have initial energy state It is

called resonant transfer of energy

So the pumping mechanism of He-Ne Laser is when

the helium atom in the metastable state collides with

neon atom in the ground state the neon atom is excited

and the helium atom drops back to the ground state

The role of helium atom is thus to excite neon atom

and cause population inversion The probability of

energy transfer from helium atoms to neon atoms is

more as there are 10 atoms of helium per 1 neon atom

in gas mixture

Without the Brewster windows the light output

is unpolarized because of it laser output to be

linearly polarized

7

When the excited Ne atom passes from metastable

state (3s) to lower level (2p) it emits photon of

wavelength 632 nm

This photon travels through the gas mixture

parallel to the axis of tube it is reflected back and

forth by the mirror ends until it stimulates an

excited Ne atom and causes it to emit a photon of

632nm with the stimulating photon

The stimulated transition from (3s) level to (2p)

level is laser transition

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

Objectiveshellip

Introduction and understand the principle ofLASER

bull Light Amplification by Stimulated Emissionof Radiation

bull Absorption

bull Spontaneous Emission

bull Stimulated Emission

bull Population Inversion

bull Optical Pumping

Objectiveshellip

Characteristics or Properties of Laser Light

bull Coherence

bull High Intensity

bull High directionality

bull High monochromaticity

Laser light is highly powerful and it is

capable of propagating over long distances and

it is not easily absorbed by water

Introductionbull LASER

ldquoLight Amplification by StimulatedEmission of Radiationrdquo

bull MASER (1939 Towner)

ldquoMicrowave Amplification by StimulatedEmission of Radiationrdquo

bull Stimulated Emission - Einstein in 1917

bull Ruby Crystal LASER - Maiman California in1960

bull He-Ne LASER - Ali Javan in 1961

bull Diode LASER- Hall in 1962

Light having following Properties

Wavelength

Frequency

Amplitude

Phase

CoherenceIncoherence

Velocity

Direction

Absorption

bull E1 = Ground state

bull E2 = Excited State

bull E = hν (Photon Energy)

1

bull According to Bohrrsquos law atomic system is

characterized by discrete energy level

bull When atoms absorb or release energy it

transit upward or downward

bull Lower level E1 amp Excited level E2

bull So h ƒ = E2 ndash E1

bull The rate of absorption depends on no of

atoms N1 present in E1 amp spectral energy

density u(ƒ) of radiation

bull So P12 α N1 u(ƒ)

bull P12= B12N1 u(ƒ)

Spontaneous Emission

bull E1 = Ground State

bull E2 = Excited State

bull E = E2 ndash E1

= ΔE

= hν

2

bull System having atoms in excited state

bull Goes to downward transition with emitting

photons hƒ = E1 ndash E2

bull Emission is random so if not in same phase

becomes incoherent

bull The transition depends on atoms in excited state

N2

P12(spont) α N2 = A21 N2

bull Where

A21 = Einstein coefficient for spontaneous

Emission we get Incoherent radiation forms heat

by light amplification of radiation by spontaneous

emission

Stimulated Emission

3

bull System having atoms in excited state

bull Goes to downward transition with emitting photons

bull 2hƒ = E1 ndash E2 After applying photon energy hƒ

bull Emission is depends on energy density u(ƒ) amp No of

atoms in excited state N2

bull P12(stimul) α u(ƒ) N2 = B21 N2 u(ƒ)

bull Where B21 = Einstein coefficient for Stimulated

Emission

bull Thus one photon of energy hƒ stimulates two photons

of energy hƒ in same phase amp directions So we get

coherent light amplification of radiation by

stimulated emission

Population Inversion

bull It is the process of increasing exited electrons in

higher energy levels

bull Due to this process the production of laser is

possible

bull The energy level between the ground state E1 (1st

level) and exited state E3 (3rd level) is known as

metastable state E2 (2nd level)

bull By optical pumping electrons from ground state

jumps to exited state by absorbing photons

bull The electrons remain only for 10-8 sec in exited

state E3 so most of them jumps back to the ground

state E1 by emitting photons But some of them

jumps to the metastable state E2

bull They (electron) stay in metastable state for more

then 10-3 sec

bull So electron density increases in metastable state

bull Thus the transitions are possible it takes more no

of electrons together and ν ndash (knew)12 photon beam is

produced which constitute laser beam

Optical PumpingThere are no of techniques for pumping a

collection of atoms to an inverted state

bull Optical pumping

bull Electrical discharge

bull Direct conversion

When photon of blue green light incident onRuby crystal electrons from ground state absorbs andexited and jumps on higher energy state levels andcomes back to metastable state They increasepopulation of electrons in metastable state

This process is called optical pumping which isdone by flash tube

Relation between Einsteinrsquos lsquoArsquo and lsquoBrsquo

coefficientsbull Einstein obtained a mathematical expression for the

existence of two different kinds of processes

(1) Spontaneous emission

(2) Stimulated emission

bull Consider all atoms r in thermal equilibrium at T

bull Radiation of freq ƒ amp energy density u(ƒ)

bull N1 amp N2 r atoms in E1 amp E2 respectively

bull In equilibrium absorption rates amp emission rates must be same

bull ie B12 N1 u(ƒ) = A21 N2+ B21 N2 u(ƒ)

A21 N2= u(ƒ) [B12N1 ndash B21N2]

So u(f) = [A21 N2 (B12 N1 ndash B21 N2)] ---------(1)

------------(2)

bull Boltzmann distribution law

------------(3)

bull So -----------(4)

bull But E2 ndash E1 = hf -----------(5)

bull So -----------(6)

21

21

12 1

21 2

( )

[ ]

ƒ

1

A

Bu

B N

B N

1

2

1 0

2 0

E kT

E kT

N N e

N N e

2 1( )1

2

E E kTNe

N

h 1

2

ƒ kTNe

N

---------- (7)

bull According to plankrsquos radiation formula

----------- (8)

bull Where B12 = B21 amp A21 B21 = ------------ (9)

bull So Ratio of spontaneous to stimulated emission

--------- (10)

21

21

ƒ12

21

h

ƒ

1

( )

[ ]kTe

A

Bu

B

B

3

3 ƒh

8 1( ) ( )

[ ]

ƒƒ

1kTu

c

h

e

3

3

8 ƒh

c

2 21 21

2 21 21

3

3

8

( ) ( ) ( )

ƒ

ƒ ƒ ƒ

N A A hR

B u B u ucN

bull So

--------- (11)

--------- (12)

bull So R = ---------- (13)

If hƒ ltlt kT in thermal equilibrium

then R = ltlt 1

bull hƒltltkT ndash Stimulated emission

ndashValid in microwave region (MASER)

bull hƒgtgtkT ndash Spontaneous emission

ndashValid in visible region incoherent

3

3

3

3

ƒh

8( )

8

ƒƒ

amp

ƒƒ

1

1( ) ( )

[ ]kT

h

uc

uR

h

e

c

ƒh 1[ ]kTe

ƒh 1[ ]kTe

Types of LASER

There are three types of lasers

1 Solid Laser (Ruby Laser)

2 Liquid Laser

3 Gas Laser ( He ndash Ne Laser CO2 Laser)

Ruby LaserhellipTo produce laser from solid Ruby crystal is

used

Ruby is an aluminum oxide crystal (Al2O3) in

which some of the aluminum atoms have been

replaced with Cr+3 chromium atoms (005 by

weight)

It was the first type of laser invented and was

first operated by Maiman in Research

Laboratories on 1960

Chromium gives ruby its characteristic pink or

red color by absorbing green and blue light

Ruby LaserhellipFor a ruby laser a crystal of ruby is formed into

a cylinder The ruby laser is used as a pulsed

laser producing red light at 6943 Aring

4

Ruby crystal is surrounded by xenon tube Rubycrystal is fully silvered at one side and partiallysilvered at the other end

A strong beam of blue green light is made to fall upon crystal from xenon tube and this light is absorbedby the crystal

Because of this many electrons from ground state ornormal state are raised to the excited state or higherstate and electron falls to metastable state

During this transition photon is not emitted butexcess energy of the electrons absorbed in crystallattice

5

As electron drops to metastable state they remain

there for certain time ~ 10-6 sec

Thus the incident blue green light from tube

increases the number of electron in metastable

state and then the population inversion can be

achieved

If a light of different frequency is allowed to fall

on this material the electrons move back and

forth between silvered ends of the crystal

While moving through they get stimulated and

exiced electrons radiate energy

Thus radiation photon has the same frequency as

that of incident photon and is also in exactly same

phase

When the intensity of light beam is increased the

same process is repeated

Finally extremely intensified beam of light

energies from the semi silvered side of the crystal

This way it is possible to get extremely intensified

and coherent beam of light from the crystal This

beam is nothing but higher energetic beam ndash ie

LASER beam

Applications of Ruby LaserhellipRuby lasers have declined in use with thediscovery of better lasing media They are still usedin a number of applications where short pulses ofred light are required Holographys around theworld produce holographic portraits with rubylasers in sizes up to a meter squared

Many non-destructive testing labs use ruby lasersto create holograms of large objects such asaircraft tires to look for weaknesses in the lining

Ruby lasers were used extensively in tattoo andhair removal

Drawbacks of Ruby Laserhellipbull The laser requires high pumping power because

the laser transition terminates at the ground state

and more than half of ground state atoms must

be pumped to higher state to achieve population

inversion

bull The efficiency of ruby laser is very low because

only green component of the pumping light is

used while the rest of components are left unused

bull The laser output is not continues but occurs in

the form of pulses of microseconds duration

bull The defects due to crystalline imperfections are

also present in this laser

Gaseous Laser (He ndash Ne Laser)A helium - neon laser usually called a He-Ne

laser is a type of small gas laser He-Ne lasers

have many industrial and scientific uses and are

often used in laboratory demonstrations of optics

He-Ne laser is an atomic laser which employs a

four-level pumping scheme

The active medium is a mixture of 10 parts of

helium to 1 part of neon

Neon atoms are centers and have energy levels

suitable for laser transitions while helium atoms

help efficient excitation of neon atoms

The most common wavelength is 6328 Aring These

lasers produced powers in the range 05 to 50

mW in the red portion of the visible spectrum

They have long operating life of the order of

50000 hrs

6

Constructionhellip

It consists of a glass discharge tube of about

typically 30 cm long and 15 cm diameter

The tube is filled with a mixture of helium and

neon gases in the 101

Electrodes are provided in the tube to produce a

discharge in the gas

They are connected to a high voltage power

supply The tube is hermetically sealed with glass

windows oriented at Brewster angle to the tube

The cavity mirrors are arranged externally

WorkinghellipWhen the power is switched on a high voltage of

about 10 kV is applied across the gas

It is sufficient to ionize the gas

The electrons and ions are produced in the process

of discharge are accelerated toward the anode and

cathode respectively

The electron have a smaller mass they acquire a

higher velocity They transfer their kinetic energy to

helium atoms through inelastic collisions

The initial excitation effects only the helium atoms

They are in metastable state and cannot return in

ground state by the spontaneous emission

The excited helium atoms can return to the ground

state by transforming their energy to neon atoms

through collision This transformation take place when

two colliding atoms have initial energy state It is

called resonant transfer of energy

So the pumping mechanism of He-Ne Laser is when

the helium atom in the metastable state collides with

neon atom in the ground state the neon atom is excited

and the helium atom drops back to the ground state

The role of helium atom is thus to excite neon atom

and cause population inversion The probability of

energy transfer from helium atoms to neon atoms is

more as there are 10 atoms of helium per 1 neon atom

in gas mixture

Without the Brewster windows the light output

is unpolarized because of it laser output to be

linearly polarized

7

When the excited Ne atom passes from metastable

state (3s) to lower level (2p) it emits photon of

wavelength 632 nm

This photon travels through the gas mixture

parallel to the axis of tube it is reflected back and

forth by the mirror ends until it stimulates an

excited Ne atom and causes it to emit a photon of

632nm with the stimulating photon

The stimulated transition from (3s) level to (2p)

level is laser transition

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

Objectiveshellip

Characteristics or Properties of Laser Light

bull Coherence

bull High Intensity

bull High directionality

bull High monochromaticity

Laser light is highly powerful and it is

capable of propagating over long distances and

it is not easily absorbed by water

Introductionbull LASER

ldquoLight Amplification by StimulatedEmission of Radiationrdquo

bull MASER (1939 Towner)

ldquoMicrowave Amplification by StimulatedEmission of Radiationrdquo

bull Stimulated Emission - Einstein in 1917

bull Ruby Crystal LASER - Maiman California in1960

bull He-Ne LASER - Ali Javan in 1961

bull Diode LASER- Hall in 1962

Light having following Properties

Wavelength

Frequency

Amplitude

Phase

CoherenceIncoherence

Velocity

Direction

Absorption

bull E1 = Ground state

bull E2 = Excited State

bull E = hν (Photon Energy)

1

bull According to Bohrrsquos law atomic system is

characterized by discrete energy level

bull When atoms absorb or release energy it

transit upward or downward

bull Lower level E1 amp Excited level E2

bull So h ƒ = E2 ndash E1

bull The rate of absorption depends on no of

atoms N1 present in E1 amp spectral energy

density u(ƒ) of radiation

bull So P12 α N1 u(ƒ)

bull P12= B12N1 u(ƒ)

Spontaneous Emission

bull E1 = Ground State

bull E2 = Excited State

bull E = E2 ndash E1

= ΔE

= hν

2

bull System having atoms in excited state

bull Goes to downward transition with emitting

photons hƒ = E1 ndash E2

bull Emission is random so if not in same phase

becomes incoherent

bull The transition depends on atoms in excited state

N2

P12(spont) α N2 = A21 N2

bull Where

A21 = Einstein coefficient for spontaneous

Emission we get Incoherent radiation forms heat

by light amplification of radiation by spontaneous

emission

Stimulated Emission

3

bull System having atoms in excited state

bull Goes to downward transition with emitting photons

bull 2hƒ = E1 ndash E2 After applying photon energy hƒ

bull Emission is depends on energy density u(ƒ) amp No of

atoms in excited state N2

bull P12(stimul) α u(ƒ) N2 = B21 N2 u(ƒ)

bull Where B21 = Einstein coefficient for Stimulated

Emission

bull Thus one photon of energy hƒ stimulates two photons

of energy hƒ in same phase amp directions So we get

coherent light amplification of radiation by

stimulated emission

Population Inversion

bull It is the process of increasing exited electrons in

higher energy levels

bull Due to this process the production of laser is

possible

bull The energy level between the ground state E1 (1st

level) and exited state E3 (3rd level) is known as

metastable state E2 (2nd level)

bull By optical pumping electrons from ground state

jumps to exited state by absorbing photons

bull The electrons remain only for 10-8 sec in exited

state E3 so most of them jumps back to the ground

state E1 by emitting photons But some of them

jumps to the metastable state E2

bull They (electron) stay in metastable state for more

then 10-3 sec

bull So electron density increases in metastable state

bull Thus the transitions are possible it takes more no

of electrons together and ν ndash (knew)12 photon beam is

produced which constitute laser beam

Optical PumpingThere are no of techniques for pumping a

collection of atoms to an inverted state

bull Optical pumping

bull Electrical discharge

bull Direct conversion

When photon of blue green light incident onRuby crystal electrons from ground state absorbs andexited and jumps on higher energy state levels andcomes back to metastable state They increasepopulation of electrons in metastable state

This process is called optical pumping which isdone by flash tube

Relation between Einsteinrsquos lsquoArsquo and lsquoBrsquo

coefficientsbull Einstein obtained a mathematical expression for the

existence of two different kinds of processes

(1) Spontaneous emission

(2) Stimulated emission

bull Consider all atoms r in thermal equilibrium at T

bull Radiation of freq ƒ amp energy density u(ƒ)

bull N1 amp N2 r atoms in E1 amp E2 respectively

bull In equilibrium absorption rates amp emission rates must be same

bull ie B12 N1 u(ƒ) = A21 N2+ B21 N2 u(ƒ)

A21 N2= u(ƒ) [B12N1 ndash B21N2]

So u(f) = [A21 N2 (B12 N1 ndash B21 N2)] ---------(1)

------------(2)

bull Boltzmann distribution law

------------(3)

bull So -----------(4)

bull But E2 ndash E1 = hf -----------(5)

bull So -----------(6)

21

21

12 1

21 2

( )

[ ]

ƒ

1

A

Bu

B N

B N

1

2

1 0

2 0

E kT

E kT

N N e

N N e

2 1( )1

2

E E kTNe

N

h 1

2

ƒ kTNe

N

---------- (7)

bull According to plankrsquos radiation formula

----------- (8)

bull Where B12 = B21 amp A21 B21 = ------------ (9)

bull So Ratio of spontaneous to stimulated emission

--------- (10)

21

21

ƒ12

21

h

ƒ

1

( )

[ ]kTe

A

Bu

B

B

3

3 ƒh

8 1( ) ( )

[ ]

ƒƒ

1kTu

c

h

e

3

3

8 ƒh

c

2 21 21

2 21 21

3

3

8

( ) ( ) ( )

ƒ

ƒ ƒ ƒ

N A A hR

B u B u ucN

bull So

--------- (11)

--------- (12)

bull So R = ---------- (13)

If hƒ ltlt kT in thermal equilibrium

then R = ltlt 1

bull hƒltltkT ndash Stimulated emission

ndashValid in microwave region (MASER)

bull hƒgtgtkT ndash Spontaneous emission

ndashValid in visible region incoherent

3

3

3

3

ƒh

8( )

8

ƒƒ

amp

ƒƒ

1

1( ) ( )

[ ]kT

h

uc

uR

h

e

c

ƒh 1[ ]kTe

ƒh 1[ ]kTe

Types of LASER

There are three types of lasers

1 Solid Laser (Ruby Laser)

2 Liquid Laser

3 Gas Laser ( He ndash Ne Laser CO2 Laser)

Ruby LaserhellipTo produce laser from solid Ruby crystal is

used

Ruby is an aluminum oxide crystal (Al2O3) in

which some of the aluminum atoms have been

replaced with Cr+3 chromium atoms (005 by

weight)

It was the first type of laser invented and was

first operated by Maiman in Research

Laboratories on 1960

Chromium gives ruby its characteristic pink or

red color by absorbing green and blue light

Ruby LaserhellipFor a ruby laser a crystal of ruby is formed into

a cylinder The ruby laser is used as a pulsed

laser producing red light at 6943 Aring

4

Ruby crystal is surrounded by xenon tube Rubycrystal is fully silvered at one side and partiallysilvered at the other end

A strong beam of blue green light is made to fall upon crystal from xenon tube and this light is absorbedby the crystal

Because of this many electrons from ground state ornormal state are raised to the excited state or higherstate and electron falls to metastable state

During this transition photon is not emitted butexcess energy of the electrons absorbed in crystallattice

5

As electron drops to metastable state they remain

there for certain time ~ 10-6 sec

Thus the incident blue green light from tube

increases the number of electron in metastable

state and then the population inversion can be

achieved

If a light of different frequency is allowed to fall

on this material the electrons move back and

forth between silvered ends of the crystal

While moving through they get stimulated and

exiced electrons radiate energy

Thus radiation photon has the same frequency as

that of incident photon and is also in exactly same

phase

When the intensity of light beam is increased the

same process is repeated

Finally extremely intensified beam of light

energies from the semi silvered side of the crystal

This way it is possible to get extremely intensified

and coherent beam of light from the crystal This

beam is nothing but higher energetic beam ndash ie

LASER beam

Applications of Ruby LaserhellipRuby lasers have declined in use with thediscovery of better lasing media They are still usedin a number of applications where short pulses ofred light are required Holographys around theworld produce holographic portraits with rubylasers in sizes up to a meter squared

Many non-destructive testing labs use ruby lasersto create holograms of large objects such asaircraft tires to look for weaknesses in the lining

Ruby lasers were used extensively in tattoo andhair removal

Drawbacks of Ruby Laserhellipbull The laser requires high pumping power because

the laser transition terminates at the ground state

and more than half of ground state atoms must

be pumped to higher state to achieve population

inversion

bull The efficiency of ruby laser is very low because

only green component of the pumping light is

used while the rest of components are left unused

bull The laser output is not continues but occurs in

the form of pulses of microseconds duration

bull The defects due to crystalline imperfections are

also present in this laser

Gaseous Laser (He ndash Ne Laser)A helium - neon laser usually called a He-Ne

laser is a type of small gas laser He-Ne lasers

have many industrial and scientific uses and are

often used in laboratory demonstrations of optics

He-Ne laser is an atomic laser which employs a

four-level pumping scheme

The active medium is a mixture of 10 parts of

helium to 1 part of neon

Neon atoms are centers and have energy levels

suitable for laser transitions while helium atoms

help efficient excitation of neon atoms

The most common wavelength is 6328 Aring These

lasers produced powers in the range 05 to 50

mW in the red portion of the visible spectrum

They have long operating life of the order of

50000 hrs

6

Constructionhellip

It consists of a glass discharge tube of about

typically 30 cm long and 15 cm diameter

The tube is filled with a mixture of helium and

neon gases in the 101

Electrodes are provided in the tube to produce a

discharge in the gas

They are connected to a high voltage power

supply The tube is hermetically sealed with glass

windows oriented at Brewster angle to the tube

The cavity mirrors are arranged externally

WorkinghellipWhen the power is switched on a high voltage of

about 10 kV is applied across the gas

It is sufficient to ionize the gas

The electrons and ions are produced in the process

of discharge are accelerated toward the anode and

cathode respectively

The electron have a smaller mass they acquire a

higher velocity They transfer their kinetic energy to

helium atoms through inelastic collisions

The initial excitation effects only the helium atoms

They are in metastable state and cannot return in

ground state by the spontaneous emission

The excited helium atoms can return to the ground

state by transforming their energy to neon atoms

through collision This transformation take place when

two colliding atoms have initial energy state It is

called resonant transfer of energy

So the pumping mechanism of He-Ne Laser is when

the helium atom in the metastable state collides with

neon atom in the ground state the neon atom is excited

and the helium atom drops back to the ground state

The role of helium atom is thus to excite neon atom

and cause population inversion The probability of

energy transfer from helium atoms to neon atoms is

more as there are 10 atoms of helium per 1 neon atom

in gas mixture

Without the Brewster windows the light output

is unpolarized because of it laser output to be

linearly polarized

7

When the excited Ne atom passes from metastable

state (3s) to lower level (2p) it emits photon of

wavelength 632 nm

This photon travels through the gas mixture

parallel to the axis of tube it is reflected back and

forth by the mirror ends until it stimulates an

excited Ne atom and causes it to emit a photon of

632nm with the stimulating photon

The stimulated transition from (3s) level to (2p)

level is laser transition

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

Introductionbull LASER

ldquoLight Amplification by StimulatedEmission of Radiationrdquo

bull MASER (1939 Towner)

ldquoMicrowave Amplification by StimulatedEmission of Radiationrdquo

bull Stimulated Emission - Einstein in 1917

bull Ruby Crystal LASER - Maiman California in1960

bull He-Ne LASER - Ali Javan in 1961

bull Diode LASER- Hall in 1962

Light having following Properties

Wavelength

Frequency

Amplitude

Phase

CoherenceIncoherence

Velocity

Direction

Absorption

bull E1 = Ground state

bull E2 = Excited State

bull E = hν (Photon Energy)

1

bull According to Bohrrsquos law atomic system is

characterized by discrete energy level

bull When atoms absorb or release energy it

transit upward or downward

bull Lower level E1 amp Excited level E2

bull So h ƒ = E2 ndash E1

bull The rate of absorption depends on no of

atoms N1 present in E1 amp spectral energy

density u(ƒ) of radiation

bull So P12 α N1 u(ƒ)

bull P12= B12N1 u(ƒ)

Spontaneous Emission

bull E1 = Ground State

bull E2 = Excited State

bull E = E2 ndash E1

= ΔE

= hν

2

bull System having atoms in excited state

bull Goes to downward transition with emitting

photons hƒ = E1 ndash E2

bull Emission is random so if not in same phase

becomes incoherent

bull The transition depends on atoms in excited state

N2

P12(spont) α N2 = A21 N2

bull Where

A21 = Einstein coefficient for spontaneous

Emission we get Incoherent radiation forms heat

by light amplification of radiation by spontaneous

emission

Stimulated Emission

3

bull System having atoms in excited state

bull Goes to downward transition with emitting photons

bull 2hƒ = E1 ndash E2 After applying photon energy hƒ

bull Emission is depends on energy density u(ƒ) amp No of

atoms in excited state N2

bull P12(stimul) α u(ƒ) N2 = B21 N2 u(ƒ)

bull Where B21 = Einstein coefficient for Stimulated

Emission

bull Thus one photon of energy hƒ stimulates two photons

of energy hƒ in same phase amp directions So we get

coherent light amplification of radiation by

stimulated emission

Population Inversion

bull It is the process of increasing exited electrons in

higher energy levels

bull Due to this process the production of laser is

possible

bull The energy level between the ground state E1 (1st

level) and exited state E3 (3rd level) is known as

metastable state E2 (2nd level)

bull By optical pumping electrons from ground state

jumps to exited state by absorbing photons

bull The electrons remain only for 10-8 sec in exited

state E3 so most of them jumps back to the ground

state E1 by emitting photons But some of them

jumps to the metastable state E2

bull They (electron) stay in metastable state for more

then 10-3 sec

bull So electron density increases in metastable state

bull Thus the transitions are possible it takes more no

of electrons together and ν ndash (knew)12 photon beam is

produced which constitute laser beam

Optical PumpingThere are no of techniques for pumping a

collection of atoms to an inverted state

bull Optical pumping

bull Electrical discharge

bull Direct conversion

When photon of blue green light incident onRuby crystal electrons from ground state absorbs andexited and jumps on higher energy state levels andcomes back to metastable state They increasepopulation of electrons in metastable state

This process is called optical pumping which isdone by flash tube

Relation between Einsteinrsquos lsquoArsquo and lsquoBrsquo

coefficientsbull Einstein obtained a mathematical expression for the

existence of two different kinds of processes

(1) Spontaneous emission

(2) Stimulated emission

bull Consider all atoms r in thermal equilibrium at T

bull Radiation of freq ƒ amp energy density u(ƒ)

bull N1 amp N2 r atoms in E1 amp E2 respectively

bull In equilibrium absorption rates amp emission rates must be same

bull ie B12 N1 u(ƒ) = A21 N2+ B21 N2 u(ƒ)

A21 N2= u(ƒ) [B12N1 ndash B21N2]

So u(f) = [A21 N2 (B12 N1 ndash B21 N2)] ---------(1)

------------(2)

bull Boltzmann distribution law

------------(3)

bull So -----------(4)

bull But E2 ndash E1 = hf -----------(5)

bull So -----------(6)

21

21

12 1

21 2

( )

[ ]

ƒ

1

A

Bu

B N

B N

1

2

1 0

2 0

E kT

E kT

N N e

N N e

2 1( )1

2

E E kTNe

N

h 1

2

ƒ kTNe

N

---------- (7)

bull According to plankrsquos radiation formula

----------- (8)

bull Where B12 = B21 amp A21 B21 = ------------ (9)

bull So Ratio of spontaneous to stimulated emission

--------- (10)

21

21

ƒ12

21

h

ƒ

1

( )

[ ]kTe

A

Bu

B

B

3

3 ƒh

8 1( ) ( )

[ ]

ƒƒ

1kTu

c

h

e

3

3

8 ƒh

c

2 21 21

2 21 21

3

3

8

( ) ( ) ( )

ƒ

ƒ ƒ ƒ

N A A hR

B u B u ucN

bull So

--------- (11)

--------- (12)

bull So R = ---------- (13)

If hƒ ltlt kT in thermal equilibrium

then R = ltlt 1

bull hƒltltkT ndash Stimulated emission

ndashValid in microwave region (MASER)

bull hƒgtgtkT ndash Spontaneous emission

ndashValid in visible region incoherent

3

3

3

3

ƒh

8( )

8

ƒƒ

amp

ƒƒ

1

1( ) ( )

[ ]kT

h

uc

uR

h

e

c

ƒh 1[ ]kTe

ƒh 1[ ]kTe

Types of LASER

There are three types of lasers

1 Solid Laser (Ruby Laser)

2 Liquid Laser

3 Gas Laser ( He ndash Ne Laser CO2 Laser)

Ruby LaserhellipTo produce laser from solid Ruby crystal is

used

Ruby is an aluminum oxide crystal (Al2O3) in

which some of the aluminum atoms have been

replaced with Cr+3 chromium atoms (005 by

weight)

It was the first type of laser invented and was

first operated by Maiman in Research

Laboratories on 1960

Chromium gives ruby its characteristic pink or

red color by absorbing green and blue light

Ruby LaserhellipFor a ruby laser a crystal of ruby is formed into

a cylinder The ruby laser is used as a pulsed

laser producing red light at 6943 Aring

4

Ruby crystal is surrounded by xenon tube Rubycrystal is fully silvered at one side and partiallysilvered at the other end

A strong beam of blue green light is made to fall upon crystal from xenon tube and this light is absorbedby the crystal

Because of this many electrons from ground state ornormal state are raised to the excited state or higherstate and electron falls to metastable state

During this transition photon is not emitted butexcess energy of the electrons absorbed in crystallattice

5

As electron drops to metastable state they remain

there for certain time ~ 10-6 sec

Thus the incident blue green light from tube

increases the number of electron in metastable

state and then the population inversion can be

achieved

If a light of different frequency is allowed to fall

on this material the electrons move back and

forth between silvered ends of the crystal

While moving through they get stimulated and

exiced electrons radiate energy

Thus radiation photon has the same frequency as

that of incident photon and is also in exactly same

phase

When the intensity of light beam is increased the

same process is repeated

Finally extremely intensified beam of light

energies from the semi silvered side of the crystal

This way it is possible to get extremely intensified

and coherent beam of light from the crystal This

beam is nothing but higher energetic beam ndash ie

LASER beam

Applications of Ruby LaserhellipRuby lasers have declined in use with thediscovery of better lasing media They are still usedin a number of applications where short pulses ofred light are required Holographys around theworld produce holographic portraits with rubylasers in sizes up to a meter squared

Many non-destructive testing labs use ruby lasersto create holograms of large objects such asaircraft tires to look for weaknesses in the lining

Ruby lasers were used extensively in tattoo andhair removal

Drawbacks of Ruby Laserhellipbull The laser requires high pumping power because

the laser transition terminates at the ground state

and more than half of ground state atoms must

be pumped to higher state to achieve population

inversion

bull The efficiency of ruby laser is very low because

only green component of the pumping light is

used while the rest of components are left unused

bull The laser output is not continues but occurs in

the form of pulses of microseconds duration

bull The defects due to crystalline imperfections are

also present in this laser

Gaseous Laser (He ndash Ne Laser)A helium - neon laser usually called a He-Ne

laser is a type of small gas laser He-Ne lasers

have many industrial and scientific uses and are

often used in laboratory demonstrations of optics

He-Ne laser is an atomic laser which employs a

four-level pumping scheme

The active medium is a mixture of 10 parts of

helium to 1 part of neon

Neon atoms are centers and have energy levels

suitable for laser transitions while helium atoms

help efficient excitation of neon atoms

The most common wavelength is 6328 Aring These

lasers produced powers in the range 05 to 50

mW in the red portion of the visible spectrum

They have long operating life of the order of

50000 hrs

6

Constructionhellip

It consists of a glass discharge tube of about

typically 30 cm long and 15 cm diameter

The tube is filled with a mixture of helium and

neon gases in the 101

Electrodes are provided in the tube to produce a

discharge in the gas

They are connected to a high voltage power

supply The tube is hermetically sealed with glass

windows oriented at Brewster angle to the tube

The cavity mirrors are arranged externally

WorkinghellipWhen the power is switched on a high voltage of

about 10 kV is applied across the gas

It is sufficient to ionize the gas

The electrons and ions are produced in the process

of discharge are accelerated toward the anode and

cathode respectively

The electron have a smaller mass they acquire a

higher velocity They transfer their kinetic energy to

helium atoms through inelastic collisions

The initial excitation effects only the helium atoms

They are in metastable state and cannot return in

ground state by the spontaneous emission

The excited helium atoms can return to the ground

state by transforming their energy to neon atoms

through collision This transformation take place when

two colliding atoms have initial energy state It is

called resonant transfer of energy

So the pumping mechanism of He-Ne Laser is when

the helium atom in the metastable state collides with

neon atom in the ground state the neon atom is excited

and the helium atom drops back to the ground state

The role of helium atom is thus to excite neon atom

and cause population inversion The probability of

energy transfer from helium atoms to neon atoms is

more as there are 10 atoms of helium per 1 neon atom

in gas mixture

Without the Brewster windows the light output

is unpolarized because of it laser output to be

linearly polarized

7

When the excited Ne atom passes from metastable

state (3s) to lower level (2p) it emits photon of

wavelength 632 nm

This photon travels through the gas mixture

parallel to the axis of tube it is reflected back and

forth by the mirror ends until it stimulates an

excited Ne atom and causes it to emit a photon of

632nm with the stimulating photon

The stimulated transition from (3s) level to (2p)

level is laser transition

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

Light having following Properties

Wavelength

Frequency

Amplitude

Phase

CoherenceIncoherence

Velocity

Direction

Absorption

bull E1 = Ground state

bull E2 = Excited State

bull E = hν (Photon Energy)

1

bull According to Bohrrsquos law atomic system is

characterized by discrete energy level

bull When atoms absorb or release energy it

transit upward or downward

bull Lower level E1 amp Excited level E2

bull So h ƒ = E2 ndash E1

bull The rate of absorption depends on no of

atoms N1 present in E1 amp spectral energy

density u(ƒ) of radiation

bull So P12 α N1 u(ƒ)

bull P12= B12N1 u(ƒ)

Spontaneous Emission

bull E1 = Ground State

bull E2 = Excited State

bull E = E2 ndash E1

= ΔE

= hν

2

bull System having atoms in excited state

bull Goes to downward transition with emitting

photons hƒ = E1 ndash E2

bull Emission is random so if not in same phase

becomes incoherent

bull The transition depends on atoms in excited state

N2

P12(spont) α N2 = A21 N2

bull Where

A21 = Einstein coefficient for spontaneous

Emission we get Incoherent radiation forms heat

by light amplification of radiation by spontaneous

emission

Stimulated Emission

3

bull System having atoms in excited state

bull Goes to downward transition with emitting photons

bull 2hƒ = E1 ndash E2 After applying photon energy hƒ

bull Emission is depends on energy density u(ƒ) amp No of

atoms in excited state N2

bull P12(stimul) α u(ƒ) N2 = B21 N2 u(ƒ)

bull Where B21 = Einstein coefficient for Stimulated

Emission

bull Thus one photon of energy hƒ stimulates two photons

of energy hƒ in same phase amp directions So we get

coherent light amplification of radiation by

stimulated emission

Population Inversion

bull It is the process of increasing exited electrons in

higher energy levels

bull Due to this process the production of laser is

possible

bull The energy level between the ground state E1 (1st

level) and exited state E3 (3rd level) is known as

metastable state E2 (2nd level)

bull By optical pumping electrons from ground state

jumps to exited state by absorbing photons

bull The electrons remain only for 10-8 sec in exited

state E3 so most of them jumps back to the ground

state E1 by emitting photons But some of them

jumps to the metastable state E2

bull They (electron) stay in metastable state for more

then 10-3 sec

bull So electron density increases in metastable state

bull Thus the transitions are possible it takes more no

of electrons together and ν ndash (knew)12 photon beam is

produced which constitute laser beam

Optical PumpingThere are no of techniques for pumping a

collection of atoms to an inverted state

bull Optical pumping

bull Electrical discharge

bull Direct conversion

When photon of blue green light incident onRuby crystal electrons from ground state absorbs andexited and jumps on higher energy state levels andcomes back to metastable state They increasepopulation of electrons in metastable state

This process is called optical pumping which isdone by flash tube

Relation between Einsteinrsquos lsquoArsquo and lsquoBrsquo

coefficientsbull Einstein obtained a mathematical expression for the

existence of two different kinds of processes

(1) Spontaneous emission

(2) Stimulated emission

bull Consider all atoms r in thermal equilibrium at T

bull Radiation of freq ƒ amp energy density u(ƒ)

bull N1 amp N2 r atoms in E1 amp E2 respectively

bull In equilibrium absorption rates amp emission rates must be same

bull ie B12 N1 u(ƒ) = A21 N2+ B21 N2 u(ƒ)

A21 N2= u(ƒ) [B12N1 ndash B21N2]

So u(f) = [A21 N2 (B12 N1 ndash B21 N2)] ---------(1)

------------(2)

bull Boltzmann distribution law

------------(3)

bull So -----------(4)

bull But E2 ndash E1 = hf -----------(5)

bull So -----------(6)

21

21

12 1

21 2

( )

[ ]

ƒ

1

A

Bu

B N

B N

1

2

1 0

2 0

E kT

E kT

N N e

N N e

2 1( )1

2

E E kTNe

N

h 1

2

ƒ kTNe

N

---------- (7)

bull According to plankrsquos radiation formula

----------- (8)

bull Where B12 = B21 amp A21 B21 = ------------ (9)

bull So Ratio of spontaneous to stimulated emission

--------- (10)

21

21

ƒ12

21

h

ƒ

1

( )

[ ]kTe

A

Bu

B

B

3

3 ƒh

8 1( ) ( )

[ ]

ƒƒ

1kTu

c

h

e

3

3

8 ƒh

c

2 21 21

2 21 21

3

3

8

( ) ( ) ( )

ƒ

ƒ ƒ ƒ

N A A hR

B u B u ucN

bull So

--------- (11)

--------- (12)

bull So R = ---------- (13)

If hƒ ltlt kT in thermal equilibrium

then R = ltlt 1

bull hƒltltkT ndash Stimulated emission

ndashValid in microwave region (MASER)

bull hƒgtgtkT ndash Spontaneous emission

ndashValid in visible region incoherent

3

3

3

3

ƒh

8( )

8

ƒƒ

amp

ƒƒ

1

1( ) ( )

[ ]kT

h

uc

uR

h

e

c

ƒh 1[ ]kTe

ƒh 1[ ]kTe

Types of LASER

There are three types of lasers

1 Solid Laser (Ruby Laser)

2 Liquid Laser

3 Gas Laser ( He ndash Ne Laser CO2 Laser)

Ruby LaserhellipTo produce laser from solid Ruby crystal is

used

Ruby is an aluminum oxide crystal (Al2O3) in

which some of the aluminum atoms have been

replaced with Cr+3 chromium atoms (005 by

weight)

It was the first type of laser invented and was

first operated by Maiman in Research

Laboratories on 1960

Chromium gives ruby its characteristic pink or

red color by absorbing green and blue light

Ruby LaserhellipFor a ruby laser a crystal of ruby is formed into

a cylinder The ruby laser is used as a pulsed

laser producing red light at 6943 Aring

4

Ruby crystal is surrounded by xenon tube Rubycrystal is fully silvered at one side and partiallysilvered at the other end

A strong beam of blue green light is made to fall upon crystal from xenon tube and this light is absorbedby the crystal

Because of this many electrons from ground state ornormal state are raised to the excited state or higherstate and electron falls to metastable state

During this transition photon is not emitted butexcess energy of the electrons absorbed in crystallattice

5

As electron drops to metastable state they remain

there for certain time ~ 10-6 sec

Thus the incident blue green light from tube

increases the number of electron in metastable

state and then the population inversion can be

achieved

If a light of different frequency is allowed to fall

on this material the electrons move back and

forth between silvered ends of the crystal

While moving through they get stimulated and

exiced electrons radiate energy

Thus radiation photon has the same frequency as

that of incident photon and is also in exactly same

phase

When the intensity of light beam is increased the

same process is repeated

Finally extremely intensified beam of light

energies from the semi silvered side of the crystal

This way it is possible to get extremely intensified

and coherent beam of light from the crystal This

beam is nothing but higher energetic beam ndash ie

LASER beam

Applications of Ruby LaserhellipRuby lasers have declined in use with thediscovery of better lasing media They are still usedin a number of applications where short pulses ofred light are required Holographys around theworld produce holographic portraits with rubylasers in sizes up to a meter squared

Many non-destructive testing labs use ruby lasersto create holograms of large objects such asaircraft tires to look for weaknesses in the lining

Ruby lasers were used extensively in tattoo andhair removal

Drawbacks of Ruby Laserhellipbull The laser requires high pumping power because

the laser transition terminates at the ground state

and more than half of ground state atoms must

be pumped to higher state to achieve population

inversion

bull The efficiency of ruby laser is very low because

only green component of the pumping light is

used while the rest of components are left unused

bull The laser output is not continues but occurs in

the form of pulses of microseconds duration

bull The defects due to crystalline imperfections are

also present in this laser

Gaseous Laser (He ndash Ne Laser)A helium - neon laser usually called a He-Ne

laser is a type of small gas laser He-Ne lasers

have many industrial and scientific uses and are

often used in laboratory demonstrations of optics

He-Ne laser is an atomic laser which employs a

four-level pumping scheme

The active medium is a mixture of 10 parts of

helium to 1 part of neon

Neon atoms are centers and have energy levels

suitable for laser transitions while helium atoms

help efficient excitation of neon atoms

The most common wavelength is 6328 Aring These

lasers produced powers in the range 05 to 50

mW in the red portion of the visible spectrum

They have long operating life of the order of

50000 hrs

6

Constructionhellip

It consists of a glass discharge tube of about

typically 30 cm long and 15 cm diameter

The tube is filled with a mixture of helium and

neon gases in the 101

Electrodes are provided in the tube to produce a

discharge in the gas

They are connected to a high voltage power

supply The tube is hermetically sealed with glass

windows oriented at Brewster angle to the tube

The cavity mirrors are arranged externally

WorkinghellipWhen the power is switched on a high voltage of

about 10 kV is applied across the gas

It is sufficient to ionize the gas

The electrons and ions are produced in the process

of discharge are accelerated toward the anode and

cathode respectively

The electron have a smaller mass they acquire a

higher velocity They transfer their kinetic energy to

helium atoms through inelastic collisions

The initial excitation effects only the helium atoms

They are in metastable state and cannot return in

ground state by the spontaneous emission

The excited helium atoms can return to the ground

state by transforming their energy to neon atoms

through collision This transformation take place when

two colliding atoms have initial energy state It is

called resonant transfer of energy

So the pumping mechanism of He-Ne Laser is when

the helium atom in the metastable state collides with

neon atom in the ground state the neon atom is excited

and the helium atom drops back to the ground state

The role of helium atom is thus to excite neon atom

and cause population inversion The probability of

energy transfer from helium atoms to neon atoms is

more as there are 10 atoms of helium per 1 neon atom

in gas mixture

Without the Brewster windows the light output

is unpolarized because of it laser output to be

linearly polarized

7

When the excited Ne atom passes from metastable

state (3s) to lower level (2p) it emits photon of

wavelength 632 nm

This photon travels through the gas mixture

parallel to the axis of tube it is reflected back and

forth by the mirror ends until it stimulates an

excited Ne atom and causes it to emit a photon of

632nm with the stimulating photon

The stimulated transition from (3s) level to (2p)

level is laser transition

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

Absorption

bull E1 = Ground state

bull E2 = Excited State

bull E = hν (Photon Energy)

1

bull According to Bohrrsquos law atomic system is

characterized by discrete energy level

bull When atoms absorb or release energy it

transit upward or downward

bull Lower level E1 amp Excited level E2

bull So h ƒ = E2 ndash E1

bull The rate of absorption depends on no of

atoms N1 present in E1 amp spectral energy

density u(ƒ) of radiation

bull So P12 α N1 u(ƒ)

bull P12= B12N1 u(ƒ)

Spontaneous Emission

bull E1 = Ground State

bull E2 = Excited State

bull E = E2 ndash E1

= ΔE

= hν

2

bull System having atoms in excited state

bull Goes to downward transition with emitting

photons hƒ = E1 ndash E2

bull Emission is random so if not in same phase

becomes incoherent

bull The transition depends on atoms in excited state

N2

P12(spont) α N2 = A21 N2

bull Where

A21 = Einstein coefficient for spontaneous

Emission we get Incoherent radiation forms heat

by light amplification of radiation by spontaneous

emission

Stimulated Emission

3

bull System having atoms in excited state

bull Goes to downward transition with emitting photons

bull 2hƒ = E1 ndash E2 After applying photon energy hƒ

bull Emission is depends on energy density u(ƒ) amp No of

atoms in excited state N2

bull P12(stimul) α u(ƒ) N2 = B21 N2 u(ƒ)

bull Where B21 = Einstein coefficient for Stimulated

Emission

bull Thus one photon of energy hƒ stimulates two photons

of energy hƒ in same phase amp directions So we get

coherent light amplification of radiation by

stimulated emission

Population Inversion

bull It is the process of increasing exited electrons in

higher energy levels

bull Due to this process the production of laser is

possible

bull The energy level between the ground state E1 (1st

level) and exited state E3 (3rd level) is known as

metastable state E2 (2nd level)

bull By optical pumping electrons from ground state

jumps to exited state by absorbing photons

bull The electrons remain only for 10-8 sec in exited

state E3 so most of them jumps back to the ground

state E1 by emitting photons But some of them

jumps to the metastable state E2

bull They (electron) stay in metastable state for more

then 10-3 sec

bull So electron density increases in metastable state

bull Thus the transitions are possible it takes more no

of electrons together and ν ndash (knew)12 photon beam is

produced which constitute laser beam

Optical PumpingThere are no of techniques for pumping a

collection of atoms to an inverted state

bull Optical pumping

bull Electrical discharge

bull Direct conversion

When photon of blue green light incident onRuby crystal electrons from ground state absorbs andexited and jumps on higher energy state levels andcomes back to metastable state They increasepopulation of electrons in metastable state

This process is called optical pumping which isdone by flash tube

Relation between Einsteinrsquos lsquoArsquo and lsquoBrsquo

coefficientsbull Einstein obtained a mathematical expression for the

existence of two different kinds of processes

(1) Spontaneous emission

(2) Stimulated emission

bull Consider all atoms r in thermal equilibrium at T

bull Radiation of freq ƒ amp energy density u(ƒ)

bull N1 amp N2 r atoms in E1 amp E2 respectively

bull In equilibrium absorption rates amp emission rates must be same

bull ie B12 N1 u(ƒ) = A21 N2+ B21 N2 u(ƒ)

A21 N2= u(ƒ) [B12N1 ndash B21N2]

So u(f) = [A21 N2 (B12 N1 ndash B21 N2)] ---------(1)

------------(2)

bull Boltzmann distribution law

------------(3)

bull So -----------(4)

bull But E2 ndash E1 = hf -----------(5)

bull So -----------(6)

21

21

12 1

21 2

( )

[ ]

ƒ

1

A

Bu

B N

B N

1

2

1 0

2 0

E kT

E kT

N N e

N N e

2 1( )1

2

E E kTNe

N

h 1

2

ƒ kTNe

N

---------- (7)

bull According to plankrsquos radiation formula

----------- (8)

bull Where B12 = B21 amp A21 B21 = ------------ (9)

bull So Ratio of spontaneous to stimulated emission

--------- (10)

21

21

ƒ12

21

h

ƒ

1

( )

[ ]kTe

A

Bu

B

B

3

3 ƒh

8 1( ) ( )

[ ]

ƒƒ

1kTu

c

h

e

3

3

8 ƒh

c

2 21 21

2 21 21

3

3

8

( ) ( ) ( )

ƒ

ƒ ƒ ƒ

N A A hR

B u B u ucN

bull So

--------- (11)

--------- (12)

bull So R = ---------- (13)

If hƒ ltlt kT in thermal equilibrium

then R = ltlt 1

bull hƒltltkT ndash Stimulated emission

ndashValid in microwave region (MASER)

bull hƒgtgtkT ndash Spontaneous emission

ndashValid in visible region incoherent

3

3

3

3

ƒh

8( )

8

ƒƒ

amp

ƒƒ

1

1( ) ( )

[ ]kT

h

uc

uR

h

e

c

ƒh 1[ ]kTe

ƒh 1[ ]kTe

Types of LASER

There are three types of lasers

1 Solid Laser (Ruby Laser)

2 Liquid Laser

3 Gas Laser ( He ndash Ne Laser CO2 Laser)

Ruby LaserhellipTo produce laser from solid Ruby crystal is

used

Ruby is an aluminum oxide crystal (Al2O3) in

which some of the aluminum atoms have been

replaced with Cr+3 chromium atoms (005 by

weight)

It was the first type of laser invented and was

first operated by Maiman in Research

Laboratories on 1960

Chromium gives ruby its characteristic pink or

red color by absorbing green and blue light

Ruby LaserhellipFor a ruby laser a crystal of ruby is formed into

a cylinder The ruby laser is used as a pulsed

laser producing red light at 6943 Aring

4

Ruby crystal is surrounded by xenon tube Rubycrystal is fully silvered at one side and partiallysilvered at the other end

A strong beam of blue green light is made to fall upon crystal from xenon tube and this light is absorbedby the crystal

Because of this many electrons from ground state ornormal state are raised to the excited state or higherstate and electron falls to metastable state

During this transition photon is not emitted butexcess energy of the electrons absorbed in crystallattice

5

As electron drops to metastable state they remain

there for certain time ~ 10-6 sec

Thus the incident blue green light from tube

increases the number of electron in metastable

state and then the population inversion can be

achieved

If a light of different frequency is allowed to fall

on this material the electrons move back and

forth between silvered ends of the crystal

While moving through they get stimulated and

exiced electrons radiate energy

Thus radiation photon has the same frequency as

that of incident photon and is also in exactly same

phase

When the intensity of light beam is increased the

same process is repeated

Finally extremely intensified beam of light

energies from the semi silvered side of the crystal

This way it is possible to get extremely intensified

and coherent beam of light from the crystal This

beam is nothing but higher energetic beam ndash ie

LASER beam

Applications of Ruby LaserhellipRuby lasers have declined in use with thediscovery of better lasing media They are still usedin a number of applications where short pulses ofred light are required Holographys around theworld produce holographic portraits with rubylasers in sizes up to a meter squared

Many non-destructive testing labs use ruby lasersto create holograms of large objects such asaircraft tires to look for weaknesses in the lining

Ruby lasers were used extensively in tattoo andhair removal

Drawbacks of Ruby Laserhellipbull The laser requires high pumping power because

the laser transition terminates at the ground state

and more than half of ground state atoms must

be pumped to higher state to achieve population

inversion

bull The efficiency of ruby laser is very low because

only green component of the pumping light is

used while the rest of components are left unused

bull The laser output is not continues but occurs in

the form of pulses of microseconds duration

bull The defects due to crystalline imperfections are

also present in this laser

Gaseous Laser (He ndash Ne Laser)A helium - neon laser usually called a He-Ne

laser is a type of small gas laser He-Ne lasers

have many industrial and scientific uses and are

often used in laboratory demonstrations of optics

He-Ne laser is an atomic laser which employs a

four-level pumping scheme

The active medium is a mixture of 10 parts of

helium to 1 part of neon

Neon atoms are centers and have energy levels

suitable for laser transitions while helium atoms

help efficient excitation of neon atoms

The most common wavelength is 6328 Aring These

lasers produced powers in the range 05 to 50

mW in the red portion of the visible spectrum

They have long operating life of the order of

50000 hrs

6

Constructionhellip

It consists of a glass discharge tube of about

typically 30 cm long and 15 cm diameter

The tube is filled with a mixture of helium and

neon gases in the 101

Electrodes are provided in the tube to produce a

discharge in the gas

They are connected to a high voltage power

supply The tube is hermetically sealed with glass

windows oriented at Brewster angle to the tube

The cavity mirrors are arranged externally

WorkinghellipWhen the power is switched on a high voltage of

about 10 kV is applied across the gas

It is sufficient to ionize the gas

The electrons and ions are produced in the process

of discharge are accelerated toward the anode and

cathode respectively

The electron have a smaller mass they acquire a

higher velocity They transfer their kinetic energy to

helium atoms through inelastic collisions

The initial excitation effects only the helium atoms

They are in metastable state and cannot return in

ground state by the spontaneous emission

The excited helium atoms can return to the ground

state by transforming their energy to neon atoms

through collision This transformation take place when

two colliding atoms have initial energy state It is

called resonant transfer of energy

So the pumping mechanism of He-Ne Laser is when

the helium atom in the metastable state collides with

neon atom in the ground state the neon atom is excited

and the helium atom drops back to the ground state

The role of helium atom is thus to excite neon atom

and cause population inversion The probability of

energy transfer from helium atoms to neon atoms is

more as there are 10 atoms of helium per 1 neon atom

in gas mixture

Without the Brewster windows the light output

is unpolarized because of it laser output to be

linearly polarized

7

When the excited Ne atom passes from metastable

state (3s) to lower level (2p) it emits photon of

wavelength 632 nm

This photon travels through the gas mixture

parallel to the axis of tube it is reflected back and

forth by the mirror ends until it stimulates an

excited Ne atom and causes it to emit a photon of

632nm with the stimulating photon

The stimulated transition from (3s) level to (2p)

level is laser transition

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

bull According to Bohrrsquos law atomic system is

characterized by discrete energy level

bull When atoms absorb or release energy it

transit upward or downward

bull Lower level E1 amp Excited level E2

bull So h ƒ = E2 ndash E1

bull The rate of absorption depends on no of

atoms N1 present in E1 amp spectral energy

density u(ƒ) of radiation

bull So P12 α N1 u(ƒ)

bull P12= B12N1 u(ƒ)

Spontaneous Emission

bull E1 = Ground State

bull E2 = Excited State

bull E = E2 ndash E1

= ΔE

= hν

2

bull System having atoms in excited state

bull Goes to downward transition with emitting

photons hƒ = E1 ndash E2

bull Emission is random so if not in same phase

becomes incoherent

bull The transition depends on atoms in excited state

N2

P12(spont) α N2 = A21 N2

bull Where

A21 = Einstein coefficient for spontaneous

Emission we get Incoherent radiation forms heat

by light amplification of radiation by spontaneous

emission

Stimulated Emission

3

bull System having atoms in excited state

bull Goes to downward transition with emitting photons

bull 2hƒ = E1 ndash E2 After applying photon energy hƒ

bull Emission is depends on energy density u(ƒ) amp No of

atoms in excited state N2

bull P12(stimul) α u(ƒ) N2 = B21 N2 u(ƒ)

bull Where B21 = Einstein coefficient for Stimulated

Emission

bull Thus one photon of energy hƒ stimulates two photons

of energy hƒ in same phase amp directions So we get

coherent light amplification of radiation by

stimulated emission

Population Inversion

bull It is the process of increasing exited electrons in

higher energy levels

bull Due to this process the production of laser is

possible

bull The energy level between the ground state E1 (1st

level) and exited state E3 (3rd level) is known as

metastable state E2 (2nd level)

bull By optical pumping electrons from ground state

jumps to exited state by absorbing photons

bull The electrons remain only for 10-8 sec in exited

state E3 so most of them jumps back to the ground

state E1 by emitting photons But some of them

jumps to the metastable state E2

bull They (electron) stay in metastable state for more

then 10-3 sec

bull So electron density increases in metastable state

bull Thus the transitions are possible it takes more no

of electrons together and ν ndash (knew)12 photon beam is

produced which constitute laser beam

Optical PumpingThere are no of techniques for pumping a

collection of atoms to an inverted state

bull Optical pumping

bull Electrical discharge

bull Direct conversion

When photon of blue green light incident onRuby crystal electrons from ground state absorbs andexited and jumps on higher energy state levels andcomes back to metastable state They increasepopulation of electrons in metastable state

This process is called optical pumping which isdone by flash tube

Relation between Einsteinrsquos lsquoArsquo and lsquoBrsquo

coefficientsbull Einstein obtained a mathematical expression for the

existence of two different kinds of processes

(1) Spontaneous emission

(2) Stimulated emission

bull Consider all atoms r in thermal equilibrium at T

bull Radiation of freq ƒ amp energy density u(ƒ)

bull N1 amp N2 r atoms in E1 amp E2 respectively

bull In equilibrium absorption rates amp emission rates must be same

bull ie B12 N1 u(ƒ) = A21 N2+ B21 N2 u(ƒ)

A21 N2= u(ƒ) [B12N1 ndash B21N2]

So u(f) = [A21 N2 (B12 N1 ndash B21 N2)] ---------(1)

------------(2)

bull Boltzmann distribution law

------------(3)

bull So -----------(4)

bull But E2 ndash E1 = hf -----------(5)

bull So -----------(6)

21

21

12 1

21 2

( )

[ ]

ƒ

1

A

Bu

B N

B N

1

2

1 0

2 0

E kT

E kT

N N e

N N e

2 1( )1

2

E E kTNe

N

h 1

2

ƒ kTNe

N

---------- (7)

bull According to plankrsquos radiation formula

----------- (8)

bull Where B12 = B21 amp A21 B21 = ------------ (9)

bull So Ratio of spontaneous to stimulated emission

--------- (10)

21

21

ƒ12

21

h

ƒ

1

( )

[ ]kTe

A

Bu

B

B

3

3 ƒh

8 1( ) ( )

[ ]

ƒƒ

1kTu

c

h

e

3

3

8 ƒh

c

2 21 21

2 21 21

3

3

8

( ) ( ) ( )

ƒ

ƒ ƒ ƒ

N A A hR

B u B u ucN

bull So

--------- (11)

--------- (12)

bull So R = ---------- (13)

If hƒ ltlt kT in thermal equilibrium

then R = ltlt 1

bull hƒltltkT ndash Stimulated emission

ndashValid in microwave region (MASER)

bull hƒgtgtkT ndash Spontaneous emission

ndashValid in visible region incoherent

3

3

3

3

ƒh

8( )

8

ƒƒ

amp

ƒƒ

1

1( ) ( )

[ ]kT

h

uc

uR

h

e

c

ƒh 1[ ]kTe

ƒh 1[ ]kTe

Types of LASER

There are three types of lasers

1 Solid Laser (Ruby Laser)

2 Liquid Laser

3 Gas Laser ( He ndash Ne Laser CO2 Laser)

Ruby LaserhellipTo produce laser from solid Ruby crystal is

used

Ruby is an aluminum oxide crystal (Al2O3) in

which some of the aluminum atoms have been

replaced with Cr+3 chromium atoms (005 by

weight)

It was the first type of laser invented and was

first operated by Maiman in Research

Laboratories on 1960

Chromium gives ruby its characteristic pink or

red color by absorbing green and blue light

Ruby LaserhellipFor a ruby laser a crystal of ruby is formed into

a cylinder The ruby laser is used as a pulsed

laser producing red light at 6943 Aring

4

Ruby crystal is surrounded by xenon tube Rubycrystal is fully silvered at one side and partiallysilvered at the other end

A strong beam of blue green light is made to fall upon crystal from xenon tube and this light is absorbedby the crystal

Because of this many electrons from ground state ornormal state are raised to the excited state or higherstate and electron falls to metastable state

During this transition photon is not emitted butexcess energy of the electrons absorbed in crystallattice

5

As electron drops to metastable state they remain

there for certain time ~ 10-6 sec

Thus the incident blue green light from tube

increases the number of electron in metastable

state and then the population inversion can be

achieved

If a light of different frequency is allowed to fall

on this material the electrons move back and

forth between silvered ends of the crystal

While moving through they get stimulated and

exiced electrons radiate energy

Thus radiation photon has the same frequency as

that of incident photon and is also in exactly same

phase

When the intensity of light beam is increased the

same process is repeated

Finally extremely intensified beam of light

energies from the semi silvered side of the crystal

This way it is possible to get extremely intensified

and coherent beam of light from the crystal This

beam is nothing but higher energetic beam ndash ie

LASER beam

Applications of Ruby LaserhellipRuby lasers have declined in use with thediscovery of better lasing media They are still usedin a number of applications where short pulses ofred light are required Holographys around theworld produce holographic portraits with rubylasers in sizes up to a meter squared

Many non-destructive testing labs use ruby lasersto create holograms of large objects such asaircraft tires to look for weaknesses in the lining

Ruby lasers were used extensively in tattoo andhair removal

Drawbacks of Ruby Laserhellipbull The laser requires high pumping power because

the laser transition terminates at the ground state

and more than half of ground state atoms must

be pumped to higher state to achieve population

inversion

bull The efficiency of ruby laser is very low because

only green component of the pumping light is

used while the rest of components are left unused

bull The laser output is not continues but occurs in

the form of pulses of microseconds duration

bull The defects due to crystalline imperfections are

also present in this laser

Gaseous Laser (He ndash Ne Laser)A helium - neon laser usually called a He-Ne

laser is a type of small gas laser He-Ne lasers

have many industrial and scientific uses and are

often used in laboratory demonstrations of optics

He-Ne laser is an atomic laser which employs a

four-level pumping scheme

The active medium is a mixture of 10 parts of

helium to 1 part of neon

Neon atoms are centers and have energy levels

suitable for laser transitions while helium atoms

help efficient excitation of neon atoms

The most common wavelength is 6328 Aring These

lasers produced powers in the range 05 to 50

mW in the red portion of the visible spectrum

They have long operating life of the order of

50000 hrs

6

Constructionhellip

It consists of a glass discharge tube of about

typically 30 cm long and 15 cm diameter

The tube is filled with a mixture of helium and

neon gases in the 101

Electrodes are provided in the tube to produce a

discharge in the gas

They are connected to a high voltage power

supply The tube is hermetically sealed with glass

windows oriented at Brewster angle to the tube

The cavity mirrors are arranged externally

WorkinghellipWhen the power is switched on a high voltage of

about 10 kV is applied across the gas

It is sufficient to ionize the gas

The electrons and ions are produced in the process

of discharge are accelerated toward the anode and

cathode respectively

The electron have a smaller mass they acquire a

higher velocity They transfer their kinetic energy to

helium atoms through inelastic collisions

The initial excitation effects only the helium atoms

They are in metastable state and cannot return in

ground state by the spontaneous emission

The excited helium atoms can return to the ground

state by transforming their energy to neon atoms

through collision This transformation take place when

two colliding atoms have initial energy state It is

called resonant transfer of energy

So the pumping mechanism of He-Ne Laser is when

the helium atom in the metastable state collides with

neon atom in the ground state the neon atom is excited

and the helium atom drops back to the ground state

The role of helium atom is thus to excite neon atom

and cause population inversion The probability of

energy transfer from helium atoms to neon atoms is

more as there are 10 atoms of helium per 1 neon atom

in gas mixture

Without the Brewster windows the light output

is unpolarized because of it laser output to be

linearly polarized

7

When the excited Ne atom passes from metastable

state (3s) to lower level (2p) it emits photon of

wavelength 632 nm

This photon travels through the gas mixture

parallel to the axis of tube it is reflected back and

forth by the mirror ends until it stimulates an

excited Ne atom and causes it to emit a photon of

632nm with the stimulating photon

The stimulated transition from (3s) level to (2p)

level is laser transition

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

Spontaneous Emission

bull E1 = Ground State

bull E2 = Excited State

bull E = E2 ndash E1

= ΔE

= hν

2

bull System having atoms in excited state

bull Goes to downward transition with emitting

photons hƒ = E1 ndash E2

bull Emission is random so if not in same phase

becomes incoherent

bull The transition depends on atoms in excited state

N2

P12(spont) α N2 = A21 N2

bull Where

A21 = Einstein coefficient for spontaneous

Emission we get Incoherent radiation forms heat

by light amplification of radiation by spontaneous

emission

Stimulated Emission

3

bull System having atoms in excited state

bull Goes to downward transition with emitting photons

bull 2hƒ = E1 ndash E2 After applying photon energy hƒ

bull Emission is depends on energy density u(ƒ) amp No of

atoms in excited state N2

bull P12(stimul) α u(ƒ) N2 = B21 N2 u(ƒ)

bull Where B21 = Einstein coefficient for Stimulated

Emission

bull Thus one photon of energy hƒ stimulates two photons

of energy hƒ in same phase amp directions So we get

coherent light amplification of radiation by

stimulated emission

Population Inversion

bull It is the process of increasing exited electrons in

higher energy levels

bull Due to this process the production of laser is

possible

bull The energy level between the ground state E1 (1st

level) and exited state E3 (3rd level) is known as

metastable state E2 (2nd level)

bull By optical pumping electrons from ground state

jumps to exited state by absorbing photons

bull The electrons remain only for 10-8 sec in exited

state E3 so most of them jumps back to the ground

state E1 by emitting photons But some of them

jumps to the metastable state E2

bull They (electron) stay in metastable state for more

then 10-3 sec

bull So electron density increases in metastable state

bull Thus the transitions are possible it takes more no

of electrons together and ν ndash (knew)12 photon beam is

produced which constitute laser beam

Optical PumpingThere are no of techniques for pumping a

collection of atoms to an inverted state

bull Optical pumping

bull Electrical discharge

bull Direct conversion

When photon of blue green light incident onRuby crystal electrons from ground state absorbs andexited and jumps on higher energy state levels andcomes back to metastable state They increasepopulation of electrons in metastable state

This process is called optical pumping which isdone by flash tube

Relation between Einsteinrsquos lsquoArsquo and lsquoBrsquo

coefficientsbull Einstein obtained a mathematical expression for the

existence of two different kinds of processes

(1) Spontaneous emission

(2) Stimulated emission

bull Consider all atoms r in thermal equilibrium at T

bull Radiation of freq ƒ amp energy density u(ƒ)

bull N1 amp N2 r atoms in E1 amp E2 respectively

bull In equilibrium absorption rates amp emission rates must be same

bull ie B12 N1 u(ƒ) = A21 N2+ B21 N2 u(ƒ)

A21 N2= u(ƒ) [B12N1 ndash B21N2]

So u(f) = [A21 N2 (B12 N1 ndash B21 N2)] ---------(1)

------------(2)

bull Boltzmann distribution law

------------(3)

bull So -----------(4)

bull But E2 ndash E1 = hf -----------(5)

bull So -----------(6)

21

21

12 1

21 2

( )

[ ]

ƒ

1

A

Bu

B N

B N

1

2

1 0

2 0

E kT

E kT

N N e

N N e

2 1( )1

2

E E kTNe

N

h 1

2

ƒ kTNe

N

---------- (7)

bull According to plankrsquos radiation formula

----------- (8)

bull Where B12 = B21 amp A21 B21 = ------------ (9)

bull So Ratio of spontaneous to stimulated emission

--------- (10)

21

21

ƒ12

21

h

ƒ

1

( )

[ ]kTe

A

Bu

B

B

3

3 ƒh

8 1( ) ( )

[ ]

ƒƒ

1kTu

c

h

e

3

3

8 ƒh

c

2 21 21

2 21 21

3

3

8

( ) ( ) ( )

ƒ

ƒ ƒ ƒ

N A A hR

B u B u ucN

bull So

--------- (11)

--------- (12)

bull So R = ---------- (13)

If hƒ ltlt kT in thermal equilibrium

then R = ltlt 1

bull hƒltltkT ndash Stimulated emission

ndashValid in microwave region (MASER)

bull hƒgtgtkT ndash Spontaneous emission

ndashValid in visible region incoherent

3

3

3

3

ƒh

8( )

8

ƒƒ

amp

ƒƒ

1

1( ) ( )

[ ]kT

h

uc

uR

h

e

c

ƒh 1[ ]kTe

ƒh 1[ ]kTe

Types of LASER

There are three types of lasers

1 Solid Laser (Ruby Laser)

2 Liquid Laser

3 Gas Laser ( He ndash Ne Laser CO2 Laser)

Ruby LaserhellipTo produce laser from solid Ruby crystal is

used

Ruby is an aluminum oxide crystal (Al2O3) in

which some of the aluminum atoms have been

replaced with Cr+3 chromium atoms (005 by

weight)

It was the first type of laser invented and was

first operated by Maiman in Research

Laboratories on 1960

Chromium gives ruby its characteristic pink or

red color by absorbing green and blue light

Ruby LaserhellipFor a ruby laser a crystal of ruby is formed into

a cylinder The ruby laser is used as a pulsed

laser producing red light at 6943 Aring

4

Ruby crystal is surrounded by xenon tube Rubycrystal is fully silvered at one side and partiallysilvered at the other end

A strong beam of blue green light is made to fall upon crystal from xenon tube and this light is absorbedby the crystal

Because of this many electrons from ground state ornormal state are raised to the excited state or higherstate and electron falls to metastable state

During this transition photon is not emitted butexcess energy of the electrons absorbed in crystallattice

5

As electron drops to metastable state they remain

there for certain time ~ 10-6 sec

Thus the incident blue green light from tube

increases the number of electron in metastable

state and then the population inversion can be

achieved

If a light of different frequency is allowed to fall

on this material the electrons move back and

forth between silvered ends of the crystal

While moving through they get stimulated and

exiced electrons radiate energy

Thus radiation photon has the same frequency as

that of incident photon and is also in exactly same

phase

When the intensity of light beam is increased the

same process is repeated

Finally extremely intensified beam of light

energies from the semi silvered side of the crystal

This way it is possible to get extremely intensified

and coherent beam of light from the crystal This

beam is nothing but higher energetic beam ndash ie

LASER beam

Applications of Ruby LaserhellipRuby lasers have declined in use with thediscovery of better lasing media They are still usedin a number of applications where short pulses ofred light are required Holographys around theworld produce holographic portraits with rubylasers in sizes up to a meter squared

Many non-destructive testing labs use ruby lasersto create holograms of large objects such asaircraft tires to look for weaknesses in the lining

Ruby lasers were used extensively in tattoo andhair removal

Drawbacks of Ruby Laserhellipbull The laser requires high pumping power because

the laser transition terminates at the ground state

and more than half of ground state atoms must

be pumped to higher state to achieve population

inversion

bull The efficiency of ruby laser is very low because

only green component of the pumping light is

used while the rest of components are left unused

bull The laser output is not continues but occurs in

the form of pulses of microseconds duration

bull The defects due to crystalline imperfections are

also present in this laser

Gaseous Laser (He ndash Ne Laser)A helium - neon laser usually called a He-Ne

laser is a type of small gas laser He-Ne lasers

have many industrial and scientific uses and are

often used in laboratory demonstrations of optics

He-Ne laser is an atomic laser which employs a

four-level pumping scheme

The active medium is a mixture of 10 parts of

helium to 1 part of neon

Neon atoms are centers and have energy levels

suitable for laser transitions while helium atoms

help efficient excitation of neon atoms

The most common wavelength is 6328 Aring These

lasers produced powers in the range 05 to 50

mW in the red portion of the visible spectrum

They have long operating life of the order of

50000 hrs

6

Constructionhellip

It consists of a glass discharge tube of about

typically 30 cm long and 15 cm diameter

The tube is filled with a mixture of helium and

neon gases in the 101

Electrodes are provided in the tube to produce a

discharge in the gas

They are connected to a high voltage power

supply The tube is hermetically sealed with glass

windows oriented at Brewster angle to the tube

The cavity mirrors are arranged externally

WorkinghellipWhen the power is switched on a high voltage of

about 10 kV is applied across the gas

It is sufficient to ionize the gas

The electrons and ions are produced in the process

of discharge are accelerated toward the anode and

cathode respectively

The electron have a smaller mass they acquire a

higher velocity They transfer their kinetic energy to

helium atoms through inelastic collisions

The initial excitation effects only the helium atoms

They are in metastable state and cannot return in

ground state by the spontaneous emission

The excited helium atoms can return to the ground

state by transforming their energy to neon atoms

through collision This transformation take place when

two colliding atoms have initial energy state It is

called resonant transfer of energy

So the pumping mechanism of He-Ne Laser is when

the helium atom in the metastable state collides with

neon atom in the ground state the neon atom is excited

and the helium atom drops back to the ground state

The role of helium atom is thus to excite neon atom

and cause population inversion The probability of

energy transfer from helium atoms to neon atoms is

more as there are 10 atoms of helium per 1 neon atom

in gas mixture

Without the Brewster windows the light output

is unpolarized because of it laser output to be

linearly polarized

7

When the excited Ne atom passes from metastable

state (3s) to lower level (2p) it emits photon of

wavelength 632 nm

This photon travels through the gas mixture

parallel to the axis of tube it is reflected back and

forth by the mirror ends until it stimulates an

excited Ne atom and causes it to emit a photon of

632nm with the stimulating photon

The stimulated transition from (3s) level to (2p)

level is laser transition

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

bull System having atoms in excited state

bull Goes to downward transition with emitting

photons hƒ = E1 ndash E2

bull Emission is random so if not in same phase

becomes incoherent

bull The transition depends on atoms in excited state

N2

P12(spont) α N2 = A21 N2

bull Where

A21 = Einstein coefficient for spontaneous

Emission we get Incoherent radiation forms heat

by light amplification of radiation by spontaneous

emission

Stimulated Emission

3

bull System having atoms in excited state

bull Goes to downward transition with emitting photons

bull 2hƒ = E1 ndash E2 After applying photon energy hƒ

bull Emission is depends on energy density u(ƒ) amp No of

atoms in excited state N2

bull P12(stimul) α u(ƒ) N2 = B21 N2 u(ƒ)

bull Where B21 = Einstein coefficient for Stimulated

Emission

bull Thus one photon of energy hƒ stimulates two photons

of energy hƒ in same phase amp directions So we get

coherent light amplification of radiation by

stimulated emission

Population Inversion

bull It is the process of increasing exited electrons in

higher energy levels

bull Due to this process the production of laser is

possible

bull The energy level between the ground state E1 (1st

level) and exited state E3 (3rd level) is known as

metastable state E2 (2nd level)

bull By optical pumping electrons from ground state

jumps to exited state by absorbing photons

bull The electrons remain only for 10-8 sec in exited

state E3 so most of them jumps back to the ground

state E1 by emitting photons But some of them

jumps to the metastable state E2

bull They (electron) stay in metastable state for more

then 10-3 sec

bull So electron density increases in metastable state

bull Thus the transitions are possible it takes more no

of electrons together and ν ndash (knew)12 photon beam is

produced which constitute laser beam

Optical PumpingThere are no of techniques for pumping a

collection of atoms to an inverted state

bull Optical pumping

bull Electrical discharge

bull Direct conversion

When photon of blue green light incident onRuby crystal electrons from ground state absorbs andexited and jumps on higher energy state levels andcomes back to metastable state They increasepopulation of electrons in metastable state

This process is called optical pumping which isdone by flash tube

Relation between Einsteinrsquos lsquoArsquo and lsquoBrsquo

coefficientsbull Einstein obtained a mathematical expression for the

existence of two different kinds of processes

(1) Spontaneous emission

(2) Stimulated emission

bull Consider all atoms r in thermal equilibrium at T

bull Radiation of freq ƒ amp energy density u(ƒ)

bull N1 amp N2 r atoms in E1 amp E2 respectively

bull In equilibrium absorption rates amp emission rates must be same

bull ie B12 N1 u(ƒ) = A21 N2+ B21 N2 u(ƒ)

A21 N2= u(ƒ) [B12N1 ndash B21N2]

So u(f) = [A21 N2 (B12 N1 ndash B21 N2)] ---------(1)

------------(2)

bull Boltzmann distribution law

------------(3)

bull So -----------(4)

bull But E2 ndash E1 = hf -----------(5)

bull So -----------(6)

21

21

12 1

21 2

( )

[ ]

ƒ

1

A

Bu

B N

B N

1

2

1 0

2 0

E kT

E kT

N N e

N N e

2 1( )1

2

E E kTNe

N

h 1

2

ƒ kTNe

N

---------- (7)

bull According to plankrsquos radiation formula

----------- (8)

bull Where B12 = B21 amp A21 B21 = ------------ (9)

bull So Ratio of spontaneous to stimulated emission

--------- (10)

21

21

ƒ12

21

h

ƒ

1

( )

[ ]kTe

A

Bu

B

B

3

3 ƒh

8 1( ) ( )

[ ]

ƒƒ

1kTu

c

h

e

3

3

8 ƒh

c

2 21 21

2 21 21

3

3

8

( ) ( ) ( )

ƒ

ƒ ƒ ƒ

N A A hR

B u B u ucN

bull So

--------- (11)

--------- (12)

bull So R = ---------- (13)

If hƒ ltlt kT in thermal equilibrium

then R = ltlt 1

bull hƒltltkT ndash Stimulated emission

ndashValid in microwave region (MASER)

bull hƒgtgtkT ndash Spontaneous emission

ndashValid in visible region incoherent

3

3

3

3

ƒh

8( )

8

ƒƒ

amp

ƒƒ

1

1( ) ( )

[ ]kT

h

uc

uR

h

e

c

ƒh 1[ ]kTe

ƒh 1[ ]kTe

Types of LASER

There are three types of lasers

1 Solid Laser (Ruby Laser)

2 Liquid Laser

3 Gas Laser ( He ndash Ne Laser CO2 Laser)

Ruby LaserhellipTo produce laser from solid Ruby crystal is

used

Ruby is an aluminum oxide crystal (Al2O3) in

which some of the aluminum atoms have been

replaced with Cr+3 chromium atoms (005 by

weight)

It was the first type of laser invented and was

first operated by Maiman in Research

Laboratories on 1960

Chromium gives ruby its characteristic pink or

red color by absorbing green and blue light

Ruby LaserhellipFor a ruby laser a crystal of ruby is formed into

a cylinder The ruby laser is used as a pulsed

laser producing red light at 6943 Aring

4

Ruby crystal is surrounded by xenon tube Rubycrystal is fully silvered at one side and partiallysilvered at the other end

A strong beam of blue green light is made to fall upon crystal from xenon tube and this light is absorbedby the crystal

Because of this many electrons from ground state ornormal state are raised to the excited state or higherstate and electron falls to metastable state

During this transition photon is not emitted butexcess energy of the electrons absorbed in crystallattice

5

As electron drops to metastable state they remain

there for certain time ~ 10-6 sec

Thus the incident blue green light from tube

increases the number of electron in metastable

state and then the population inversion can be

achieved

If a light of different frequency is allowed to fall

on this material the electrons move back and

forth between silvered ends of the crystal

While moving through they get stimulated and

exiced electrons radiate energy

Thus radiation photon has the same frequency as

that of incident photon and is also in exactly same

phase

When the intensity of light beam is increased the

same process is repeated

Finally extremely intensified beam of light

energies from the semi silvered side of the crystal

This way it is possible to get extremely intensified

and coherent beam of light from the crystal This

beam is nothing but higher energetic beam ndash ie

LASER beam

Applications of Ruby LaserhellipRuby lasers have declined in use with thediscovery of better lasing media They are still usedin a number of applications where short pulses ofred light are required Holographys around theworld produce holographic portraits with rubylasers in sizes up to a meter squared

Many non-destructive testing labs use ruby lasersto create holograms of large objects such asaircraft tires to look for weaknesses in the lining

Ruby lasers were used extensively in tattoo andhair removal

Drawbacks of Ruby Laserhellipbull The laser requires high pumping power because

the laser transition terminates at the ground state

and more than half of ground state atoms must

be pumped to higher state to achieve population

inversion

bull The efficiency of ruby laser is very low because

only green component of the pumping light is

used while the rest of components are left unused

bull The laser output is not continues but occurs in

the form of pulses of microseconds duration

bull The defects due to crystalline imperfections are

also present in this laser

Gaseous Laser (He ndash Ne Laser)A helium - neon laser usually called a He-Ne

laser is a type of small gas laser He-Ne lasers

have many industrial and scientific uses and are

often used in laboratory demonstrations of optics

He-Ne laser is an atomic laser which employs a

four-level pumping scheme

The active medium is a mixture of 10 parts of

helium to 1 part of neon

Neon atoms are centers and have energy levels

suitable for laser transitions while helium atoms

help efficient excitation of neon atoms

The most common wavelength is 6328 Aring These

lasers produced powers in the range 05 to 50

mW in the red portion of the visible spectrum

They have long operating life of the order of

50000 hrs

6

Constructionhellip

It consists of a glass discharge tube of about

typically 30 cm long and 15 cm diameter

The tube is filled with a mixture of helium and

neon gases in the 101

Electrodes are provided in the tube to produce a

discharge in the gas

They are connected to a high voltage power

supply The tube is hermetically sealed with glass

windows oriented at Brewster angle to the tube

The cavity mirrors are arranged externally

WorkinghellipWhen the power is switched on a high voltage of

about 10 kV is applied across the gas

It is sufficient to ionize the gas

The electrons and ions are produced in the process

of discharge are accelerated toward the anode and

cathode respectively

The electron have a smaller mass they acquire a

higher velocity They transfer their kinetic energy to

helium atoms through inelastic collisions

The initial excitation effects only the helium atoms

They are in metastable state and cannot return in

ground state by the spontaneous emission

The excited helium atoms can return to the ground

state by transforming their energy to neon atoms

through collision This transformation take place when

two colliding atoms have initial energy state It is

called resonant transfer of energy

So the pumping mechanism of He-Ne Laser is when

the helium atom in the metastable state collides with

neon atom in the ground state the neon atom is excited

and the helium atom drops back to the ground state

The role of helium atom is thus to excite neon atom

and cause population inversion The probability of

energy transfer from helium atoms to neon atoms is

more as there are 10 atoms of helium per 1 neon atom

in gas mixture

Without the Brewster windows the light output

is unpolarized because of it laser output to be

linearly polarized

7

When the excited Ne atom passes from metastable

state (3s) to lower level (2p) it emits photon of

wavelength 632 nm

This photon travels through the gas mixture

parallel to the axis of tube it is reflected back and

forth by the mirror ends until it stimulates an

excited Ne atom and causes it to emit a photon of

632nm with the stimulating photon

The stimulated transition from (3s) level to (2p)

level is laser transition

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

Stimulated Emission

3

bull System having atoms in excited state

bull Goes to downward transition with emitting photons

bull 2hƒ = E1 ndash E2 After applying photon energy hƒ

bull Emission is depends on energy density u(ƒ) amp No of

atoms in excited state N2

bull P12(stimul) α u(ƒ) N2 = B21 N2 u(ƒ)

bull Where B21 = Einstein coefficient for Stimulated

Emission

bull Thus one photon of energy hƒ stimulates two photons

of energy hƒ in same phase amp directions So we get

coherent light amplification of radiation by

stimulated emission

Population Inversion

bull It is the process of increasing exited electrons in

higher energy levels

bull Due to this process the production of laser is

possible

bull The energy level between the ground state E1 (1st

level) and exited state E3 (3rd level) is known as

metastable state E2 (2nd level)

bull By optical pumping electrons from ground state

jumps to exited state by absorbing photons

bull The electrons remain only for 10-8 sec in exited

state E3 so most of them jumps back to the ground

state E1 by emitting photons But some of them

jumps to the metastable state E2

bull They (electron) stay in metastable state for more

then 10-3 sec

bull So electron density increases in metastable state

bull Thus the transitions are possible it takes more no

of electrons together and ν ndash (knew)12 photon beam is

produced which constitute laser beam

Optical PumpingThere are no of techniques for pumping a

collection of atoms to an inverted state

bull Optical pumping

bull Electrical discharge

bull Direct conversion

When photon of blue green light incident onRuby crystal electrons from ground state absorbs andexited and jumps on higher energy state levels andcomes back to metastable state They increasepopulation of electrons in metastable state

This process is called optical pumping which isdone by flash tube

Relation between Einsteinrsquos lsquoArsquo and lsquoBrsquo

coefficientsbull Einstein obtained a mathematical expression for the

existence of two different kinds of processes

(1) Spontaneous emission

(2) Stimulated emission

bull Consider all atoms r in thermal equilibrium at T

bull Radiation of freq ƒ amp energy density u(ƒ)

bull N1 amp N2 r atoms in E1 amp E2 respectively

bull In equilibrium absorption rates amp emission rates must be same

bull ie B12 N1 u(ƒ) = A21 N2+ B21 N2 u(ƒ)

A21 N2= u(ƒ) [B12N1 ndash B21N2]

So u(f) = [A21 N2 (B12 N1 ndash B21 N2)] ---------(1)

------------(2)

bull Boltzmann distribution law

------------(3)

bull So -----------(4)

bull But E2 ndash E1 = hf -----------(5)

bull So -----------(6)

21

21

12 1

21 2

( )

[ ]

ƒ

1

A

Bu

B N

B N

1

2

1 0

2 0

E kT

E kT

N N e

N N e

2 1( )1

2

E E kTNe

N

h 1

2

ƒ kTNe

N

---------- (7)

bull According to plankrsquos radiation formula

----------- (8)

bull Where B12 = B21 amp A21 B21 = ------------ (9)

bull So Ratio of spontaneous to stimulated emission

--------- (10)

21

21

ƒ12

21

h

ƒ

1

( )

[ ]kTe

A

Bu

B

B

3

3 ƒh

8 1( ) ( )

[ ]

ƒƒ

1kTu

c

h

e

3

3

8 ƒh

c

2 21 21

2 21 21

3

3

8

( ) ( ) ( )

ƒ

ƒ ƒ ƒ

N A A hR

B u B u ucN

bull So

--------- (11)

--------- (12)

bull So R = ---------- (13)

If hƒ ltlt kT in thermal equilibrium

then R = ltlt 1

bull hƒltltkT ndash Stimulated emission

ndashValid in microwave region (MASER)

bull hƒgtgtkT ndash Spontaneous emission

ndashValid in visible region incoherent

3

3

3

3

ƒh

8( )

8

ƒƒ

amp

ƒƒ

1

1( ) ( )

[ ]kT

h

uc

uR

h

e

c

ƒh 1[ ]kTe

ƒh 1[ ]kTe

Types of LASER

There are three types of lasers

1 Solid Laser (Ruby Laser)

2 Liquid Laser

3 Gas Laser ( He ndash Ne Laser CO2 Laser)

Ruby LaserhellipTo produce laser from solid Ruby crystal is

used

Ruby is an aluminum oxide crystal (Al2O3) in

which some of the aluminum atoms have been

replaced with Cr+3 chromium atoms (005 by

weight)

It was the first type of laser invented and was

first operated by Maiman in Research

Laboratories on 1960

Chromium gives ruby its characteristic pink or

red color by absorbing green and blue light

Ruby LaserhellipFor a ruby laser a crystal of ruby is formed into

a cylinder The ruby laser is used as a pulsed

laser producing red light at 6943 Aring

4

Ruby crystal is surrounded by xenon tube Rubycrystal is fully silvered at one side and partiallysilvered at the other end

A strong beam of blue green light is made to fall upon crystal from xenon tube and this light is absorbedby the crystal

Because of this many electrons from ground state ornormal state are raised to the excited state or higherstate and electron falls to metastable state

During this transition photon is not emitted butexcess energy of the electrons absorbed in crystallattice

5

As electron drops to metastable state they remain

there for certain time ~ 10-6 sec

Thus the incident blue green light from tube

increases the number of electron in metastable

state and then the population inversion can be

achieved

If a light of different frequency is allowed to fall

on this material the electrons move back and

forth between silvered ends of the crystal

While moving through they get stimulated and

exiced electrons radiate energy

Thus radiation photon has the same frequency as

that of incident photon and is also in exactly same

phase

When the intensity of light beam is increased the

same process is repeated

Finally extremely intensified beam of light

energies from the semi silvered side of the crystal

This way it is possible to get extremely intensified

and coherent beam of light from the crystal This

beam is nothing but higher energetic beam ndash ie

LASER beam

Applications of Ruby LaserhellipRuby lasers have declined in use with thediscovery of better lasing media They are still usedin a number of applications where short pulses ofred light are required Holographys around theworld produce holographic portraits with rubylasers in sizes up to a meter squared

Many non-destructive testing labs use ruby lasersto create holograms of large objects such asaircraft tires to look for weaknesses in the lining

Ruby lasers were used extensively in tattoo andhair removal

Drawbacks of Ruby Laserhellipbull The laser requires high pumping power because

the laser transition terminates at the ground state

and more than half of ground state atoms must

be pumped to higher state to achieve population

inversion

bull The efficiency of ruby laser is very low because

only green component of the pumping light is

used while the rest of components are left unused

bull The laser output is not continues but occurs in

the form of pulses of microseconds duration

bull The defects due to crystalline imperfections are

also present in this laser

Gaseous Laser (He ndash Ne Laser)A helium - neon laser usually called a He-Ne

laser is a type of small gas laser He-Ne lasers

have many industrial and scientific uses and are

often used in laboratory demonstrations of optics

He-Ne laser is an atomic laser which employs a

four-level pumping scheme

The active medium is a mixture of 10 parts of

helium to 1 part of neon

Neon atoms are centers and have energy levels

suitable for laser transitions while helium atoms

help efficient excitation of neon atoms

The most common wavelength is 6328 Aring These

lasers produced powers in the range 05 to 50

mW in the red portion of the visible spectrum

They have long operating life of the order of

50000 hrs

6

Constructionhellip

It consists of a glass discharge tube of about

typically 30 cm long and 15 cm diameter

The tube is filled with a mixture of helium and

neon gases in the 101

Electrodes are provided in the tube to produce a

discharge in the gas

They are connected to a high voltage power

supply The tube is hermetically sealed with glass

windows oriented at Brewster angle to the tube

The cavity mirrors are arranged externally

WorkinghellipWhen the power is switched on a high voltage of

about 10 kV is applied across the gas

It is sufficient to ionize the gas

The electrons and ions are produced in the process

of discharge are accelerated toward the anode and

cathode respectively

The electron have a smaller mass they acquire a

higher velocity They transfer their kinetic energy to

helium atoms through inelastic collisions

The initial excitation effects only the helium atoms

They are in metastable state and cannot return in

ground state by the spontaneous emission

The excited helium atoms can return to the ground

state by transforming their energy to neon atoms

through collision This transformation take place when

two colliding atoms have initial energy state It is

called resonant transfer of energy

So the pumping mechanism of He-Ne Laser is when

the helium atom in the metastable state collides with

neon atom in the ground state the neon atom is excited

and the helium atom drops back to the ground state

The role of helium atom is thus to excite neon atom

and cause population inversion The probability of

energy transfer from helium atoms to neon atoms is

more as there are 10 atoms of helium per 1 neon atom

in gas mixture

Without the Brewster windows the light output

is unpolarized because of it laser output to be

linearly polarized

7

When the excited Ne atom passes from metastable

state (3s) to lower level (2p) it emits photon of

wavelength 632 nm

This photon travels through the gas mixture

parallel to the axis of tube it is reflected back and

forth by the mirror ends until it stimulates an

excited Ne atom and causes it to emit a photon of

632nm with the stimulating photon

The stimulated transition from (3s) level to (2p)

level is laser transition

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

bull System having atoms in excited state

bull Goes to downward transition with emitting photons

bull 2hƒ = E1 ndash E2 After applying photon energy hƒ

bull Emission is depends on energy density u(ƒ) amp No of

atoms in excited state N2

bull P12(stimul) α u(ƒ) N2 = B21 N2 u(ƒ)

bull Where B21 = Einstein coefficient for Stimulated

Emission

bull Thus one photon of energy hƒ stimulates two photons

of energy hƒ in same phase amp directions So we get

coherent light amplification of radiation by

stimulated emission

Population Inversion

bull It is the process of increasing exited electrons in

higher energy levels

bull Due to this process the production of laser is

possible

bull The energy level between the ground state E1 (1st

level) and exited state E3 (3rd level) is known as

metastable state E2 (2nd level)

bull By optical pumping electrons from ground state

jumps to exited state by absorbing photons

bull The electrons remain only for 10-8 sec in exited

state E3 so most of them jumps back to the ground

state E1 by emitting photons But some of them

jumps to the metastable state E2

bull They (electron) stay in metastable state for more

then 10-3 sec

bull So electron density increases in metastable state

bull Thus the transitions are possible it takes more no

of electrons together and ν ndash (knew)12 photon beam is

produced which constitute laser beam

Optical PumpingThere are no of techniques for pumping a

collection of atoms to an inverted state

bull Optical pumping

bull Electrical discharge

bull Direct conversion

When photon of blue green light incident onRuby crystal electrons from ground state absorbs andexited and jumps on higher energy state levels andcomes back to metastable state They increasepopulation of electrons in metastable state

This process is called optical pumping which isdone by flash tube

Relation between Einsteinrsquos lsquoArsquo and lsquoBrsquo

coefficientsbull Einstein obtained a mathematical expression for the

existence of two different kinds of processes

(1) Spontaneous emission

(2) Stimulated emission

bull Consider all atoms r in thermal equilibrium at T

bull Radiation of freq ƒ amp energy density u(ƒ)

bull N1 amp N2 r atoms in E1 amp E2 respectively

bull In equilibrium absorption rates amp emission rates must be same

bull ie B12 N1 u(ƒ) = A21 N2+ B21 N2 u(ƒ)

A21 N2= u(ƒ) [B12N1 ndash B21N2]

So u(f) = [A21 N2 (B12 N1 ndash B21 N2)] ---------(1)

------------(2)

bull Boltzmann distribution law

------------(3)

bull So -----------(4)

bull But E2 ndash E1 = hf -----------(5)

bull So -----------(6)

21

21

12 1

21 2

( )

[ ]

ƒ

1

A

Bu

B N

B N

1

2

1 0

2 0

E kT

E kT

N N e

N N e

2 1( )1

2

E E kTNe

N

h 1

2

ƒ kTNe

N

---------- (7)

bull According to plankrsquos radiation formula

----------- (8)

bull Where B12 = B21 amp A21 B21 = ------------ (9)

bull So Ratio of spontaneous to stimulated emission

--------- (10)

21

21

ƒ12

21

h

ƒ

1

( )

[ ]kTe

A

Bu

B

B

3

3 ƒh

8 1( ) ( )

[ ]

ƒƒ

1kTu

c

h

e

3

3

8 ƒh

c

2 21 21

2 21 21

3

3

8

( ) ( ) ( )

ƒ

ƒ ƒ ƒ

N A A hR

B u B u ucN

bull So

--------- (11)

--------- (12)

bull So R = ---------- (13)

If hƒ ltlt kT in thermal equilibrium

then R = ltlt 1

bull hƒltltkT ndash Stimulated emission

ndashValid in microwave region (MASER)

bull hƒgtgtkT ndash Spontaneous emission

ndashValid in visible region incoherent

3

3

3

3

ƒh

8( )

8

ƒƒ

amp

ƒƒ

1

1( ) ( )

[ ]kT

h

uc

uR

h

e

c

ƒh 1[ ]kTe

ƒh 1[ ]kTe

Types of LASER

There are three types of lasers

1 Solid Laser (Ruby Laser)

2 Liquid Laser

3 Gas Laser ( He ndash Ne Laser CO2 Laser)

Ruby LaserhellipTo produce laser from solid Ruby crystal is

used

Ruby is an aluminum oxide crystal (Al2O3) in

which some of the aluminum atoms have been

replaced with Cr+3 chromium atoms (005 by

weight)

It was the first type of laser invented and was

first operated by Maiman in Research

Laboratories on 1960

Chromium gives ruby its characteristic pink or

red color by absorbing green and blue light

Ruby LaserhellipFor a ruby laser a crystal of ruby is formed into

a cylinder The ruby laser is used as a pulsed

laser producing red light at 6943 Aring

4

Ruby crystal is surrounded by xenon tube Rubycrystal is fully silvered at one side and partiallysilvered at the other end

A strong beam of blue green light is made to fall upon crystal from xenon tube and this light is absorbedby the crystal

Because of this many electrons from ground state ornormal state are raised to the excited state or higherstate and electron falls to metastable state

During this transition photon is not emitted butexcess energy of the electrons absorbed in crystallattice

5

As electron drops to metastable state they remain

there for certain time ~ 10-6 sec

Thus the incident blue green light from tube

increases the number of electron in metastable

state and then the population inversion can be

achieved

If a light of different frequency is allowed to fall

on this material the electrons move back and

forth between silvered ends of the crystal

While moving through they get stimulated and

exiced electrons radiate energy

Thus radiation photon has the same frequency as

that of incident photon and is also in exactly same

phase

When the intensity of light beam is increased the

same process is repeated

Finally extremely intensified beam of light

energies from the semi silvered side of the crystal

This way it is possible to get extremely intensified

and coherent beam of light from the crystal This

beam is nothing but higher energetic beam ndash ie

LASER beam

Applications of Ruby LaserhellipRuby lasers have declined in use with thediscovery of better lasing media They are still usedin a number of applications where short pulses ofred light are required Holographys around theworld produce holographic portraits with rubylasers in sizes up to a meter squared

Many non-destructive testing labs use ruby lasersto create holograms of large objects such asaircraft tires to look for weaknesses in the lining

Ruby lasers were used extensively in tattoo andhair removal

Drawbacks of Ruby Laserhellipbull The laser requires high pumping power because

the laser transition terminates at the ground state

and more than half of ground state atoms must

be pumped to higher state to achieve population

inversion

bull The efficiency of ruby laser is very low because

only green component of the pumping light is

used while the rest of components are left unused

bull The laser output is not continues but occurs in

the form of pulses of microseconds duration

bull The defects due to crystalline imperfections are

also present in this laser

Gaseous Laser (He ndash Ne Laser)A helium - neon laser usually called a He-Ne

laser is a type of small gas laser He-Ne lasers

have many industrial and scientific uses and are

often used in laboratory demonstrations of optics

He-Ne laser is an atomic laser which employs a

four-level pumping scheme

The active medium is a mixture of 10 parts of

helium to 1 part of neon

Neon atoms are centers and have energy levels

suitable for laser transitions while helium atoms

help efficient excitation of neon atoms

The most common wavelength is 6328 Aring These

lasers produced powers in the range 05 to 50

mW in the red portion of the visible spectrum

They have long operating life of the order of

50000 hrs

6

Constructionhellip

It consists of a glass discharge tube of about

typically 30 cm long and 15 cm diameter

The tube is filled with a mixture of helium and

neon gases in the 101

Electrodes are provided in the tube to produce a

discharge in the gas

They are connected to a high voltage power

supply The tube is hermetically sealed with glass

windows oriented at Brewster angle to the tube

The cavity mirrors are arranged externally

WorkinghellipWhen the power is switched on a high voltage of

about 10 kV is applied across the gas

It is sufficient to ionize the gas

The electrons and ions are produced in the process

of discharge are accelerated toward the anode and

cathode respectively

The electron have a smaller mass they acquire a

higher velocity They transfer their kinetic energy to

helium atoms through inelastic collisions

The initial excitation effects only the helium atoms

They are in metastable state and cannot return in

ground state by the spontaneous emission

The excited helium atoms can return to the ground

state by transforming their energy to neon atoms

through collision This transformation take place when

two colliding atoms have initial energy state It is

called resonant transfer of energy

So the pumping mechanism of He-Ne Laser is when

the helium atom in the metastable state collides with

neon atom in the ground state the neon atom is excited

and the helium atom drops back to the ground state

The role of helium atom is thus to excite neon atom

and cause population inversion The probability of

energy transfer from helium atoms to neon atoms is

more as there are 10 atoms of helium per 1 neon atom

in gas mixture

Without the Brewster windows the light output

is unpolarized because of it laser output to be

linearly polarized

7

When the excited Ne atom passes from metastable

state (3s) to lower level (2p) it emits photon of

wavelength 632 nm

This photon travels through the gas mixture

parallel to the axis of tube it is reflected back and

forth by the mirror ends until it stimulates an

excited Ne atom and causes it to emit a photon of

632nm with the stimulating photon

The stimulated transition from (3s) level to (2p)

level is laser transition

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

Population Inversion

bull It is the process of increasing exited electrons in

higher energy levels

bull Due to this process the production of laser is

possible

bull The energy level between the ground state E1 (1st

level) and exited state E3 (3rd level) is known as

metastable state E2 (2nd level)

bull By optical pumping electrons from ground state

jumps to exited state by absorbing photons

bull The electrons remain only for 10-8 sec in exited

state E3 so most of them jumps back to the ground

state E1 by emitting photons But some of them

jumps to the metastable state E2

bull They (electron) stay in metastable state for more

then 10-3 sec

bull So electron density increases in metastable state

bull Thus the transitions are possible it takes more no

of electrons together and ν ndash (knew)12 photon beam is

produced which constitute laser beam

Optical PumpingThere are no of techniques for pumping a

collection of atoms to an inverted state

bull Optical pumping

bull Electrical discharge

bull Direct conversion

When photon of blue green light incident onRuby crystal electrons from ground state absorbs andexited and jumps on higher energy state levels andcomes back to metastable state They increasepopulation of electrons in metastable state

This process is called optical pumping which isdone by flash tube

Relation between Einsteinrsquos lsquoArsquo and lsquoBrsquo

coefficientsbull Einstein obtained a mathematical expression for the

existence of two different kinds of processes

(1) Spontaneous emission

(2) Stimulated emission

bull Consider all atoms r in thermal equilibrium at T

bull Radiation of freq ƒ amp energy density u(ƒ)

bull N1 amp N2 r atoms in E1 amp E2 respectively

bull In equilibrium absorption rates amp emission rates must be same

bull ie B12 N1 u(ƒ) = A21 N2+ B21 N2 u(ƒ)

A21 N2= u(ƒ) [B12N1 ndash B21N2]

So u(f) = [A21 N2 (B12 N1 ndash B21 N2)] ---------(1)

------------(2)

bull Boltzmann distribution law

------------(3)

bull So -----------(4)

bull But E2 ndash E1 = hf -----------(5)

bull So -----------(6)

21

21

12 1

21 2

( )

[ ]

ƒ

1

A

Bu

B N

B N

1

2

1 0

2 0

E kT

E kT

N N e

N N e

2 1( )1

2

E E kTNe

N

h 1

2

ƒ kTNe

N

---------- (7)

bull According to plankrsquos radiation formula

----------- (8)

bull Where B12 = B21 amp A21 B21 = ------------ (9)

bull So Ratio of spontaneous to stimulated emission

--------- (10)

21

21

ƒ12

21

h

ƒ

1

( )

[ ]kTe

A

Bu

B

B

3

3 ƒh

8 1( ) ( )

[ ]

ƒƒ

1kTu

c

h

e

3

3

8 ƒh

c

2 21 21

2 21 21

3

3

8

( ) ( ) ( )

ƒ

ƒ ƒ ƒ

N A A hR

B u B u ucN

bull So

--------- (11)

--------- (12)

bull So R = ---------- (13)

If hƒ ltlt kT in thermal equilibrium

then R = ltlt 1

bull hƒltltkT ndash Stimulated emission

ndashValid in microwave region (MASER)

bull hƒgtgtkT ndash Spontaneous emission

ndashValid in visible region incoherent

3

3

3

3

ƒh

8( )

8

ƒƒ

amp

ƒƒ

1

1( ) ( )

[ ]kT

h

uc

uR

h

e

c

ƒh 1[ ]kTe

ƒh 1[ ]kTe

Types of LASER

There are three types of lasers

1 Solid Laser (Ruby Laser)

2 Liquid Laser

3 Gas Laser ( He ndash Ne Laser CO2 Laser)

Ruby LaserhellipTo produce laser from solid Ruby crystal is

used

Ruby is an aluminum oxide crystal (Al2O3) in

which some of the aluminum atoms have been

replaced with Cr+3 chromium atoms (005 by

weight)

It was the first type of laser invented and was

first operated by Maiman in Research

Laboratories on 1960

Chromium gives ruby its characteristic pink or

red color by absorbing green and blue light

Ruby LaserhellipFor a ruby laser a crystal of ruby is formed into

a cylinder The ruby laser is used as a pulsed

laser producing red light at 6943 Aring

4

Ruby crystal is surrounded by xenon tube Rubycrystal is fully silvered at one side and partiallysilvered at the other end

A strong beam of blue green light is made to fall upon crystal from xenon tube and this light is absorbedby the crystal

Because of this many electrons from ground state ornormal state are raised to the excited state or higherstate and electron falls to metastable state

During this transition photon is not emitted butexcess energy of the electrons absorbed in crystallattice

5

As electron drops to metastable state they remain

there for certain time ~ 10-6 sec

Thus the incident blue green light from tube

increases the number of electron in metastable

state and then the population inversion can be

achieved

If a light of different frequency is allowed to fall

on this material the electrons move back and

forth between silvered ends of the crystal

While moving through they get stimulated and

exiced electrons radiate energy

Thus radiation photon has the same frequency as

that of incident photon and is also in exactly same

phase

When the intensity of light beam is increased the

same process is repeated

Finally extremely intensified beam of light

energies from the semi silvered side of the crystal

This way it is possible to get extremely intensified

and coherent beam of light from the crystal This

beam is nothing but higher energetic beam ndash ie

LASER beam

Applications of Ruby LaserhellipRuby lasers have declined in use with thediscovery of better lasing media They are still usedin a number of applications where short pulses ofred light are required Holographys around theworld produce holographic portraits with rubylasers in sizes up to a meter squared

Many non-destructive testing labs use ruby lasersto create holograms of large objects such asaircraft tires to look for weaknesses in the lining

Ruby lasers were used extensively in tattoo andhair removal

Drawbacks of Ruby Laserhellipbull The laser requires high pumping power because

the laser transition terminates at the ground state

and more than half of ground state atoms must

be pumped to higher state to achieve population

inversion

bull The efficiency of ruby laser is very low because

only green component of the pumping light is

used while the rest of components are left unused

bull The laser output is not continues but occurs in

the form of pulses of microseconds duration

bull The defects due to crystalline imperfections are

also present in this laser

Gaseous Laser (He ndash Ne Laser)A helium - neon laser usually called a He-Ne

laser is a type of small gas laser He-Ne lasers

have many industrial and scientific uses and are

often used in laboratory demonstrations of optics

He-Ne laser is an atomic laser which employs a

four-level pumping scheme

The active medium is a mixture of 10 parts of

helium to 1 part of neon

Neon atoms are centers and have energy levels

suitable for laser transitions while helium atoms

help efficient excitation of neon atoms

The most common wavelength is 6328 Aring These

lasers produced powers in the range 05 to 50

mW in the red portion of the visible spectrum

They have long operating life of the order of

50000 hrs

6

Constructionhellip

It consists of a glass discharge tube of about

typically 30 cm long and 15 cm diameter

The tube is filled with a mixture of helium and

neon gases in the 101

Electrodes are provided in the tube to produce a

discharge in the gas

They are connected to a high voltage power

supply The tube is hermetically sealed with glass

windows oriented at Brewster angle to the tube

The cavity mirrors are arranged externally

WorkinghellipWhen the power is switched on a high voltage of

about 10 kV is applied across the gas

It is sufficient to ionize the gas

The electrons and ions are produced in the process

of discharge are accelerated toward the anode and

cathode respectively

The electron have a smaller mass they acquire a

higher velocity They transfer their kinetic energy to

helium atoms through inelastic collisions

The initial excitation effects only the helium atoms

They are in metastable state and cannot return in

ground state by the spontaneous emission

The excited helium atoms can return to the ground

state by transforming their energy to neon atoms

through collision This transformation take place when

two colliding atoms have initial energy state It is

called resonant transfer of energy

So the pumping mechanism of He-Ne Laser is when

the helium atom in the metastable state collides with

neon atom in the ground state the neon atom is excited

and the helium atom drops back to the ground state

The role of helium atom is thus to excite neon atom

and cause population inversion The probability of

energy transfer from helium atoms to neon atoms is

more as there are 10 atoms of helium per 1 neon atom

in gas mixture

Without the Brewster windows the light output

is unpolarized because of it laser output to be

linearly polarized

7

When the excited Ne atom passes from metastable

state (3s) to lower level (2p) it emits photon of

wavelength 632 nm

This photon travels through the gas mixture

parallel to the axis of tube it is reflected back and

forth by the mirror ends until it stimulates an

excited Ne atom and causes it to emit a photon of

632nm with the stimulating photon

The stimulated transition from (3s) level to (2p)

level is laser transition

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

bull The electrons remain only for 10-8 sec in exited

state E3 so most of them jumps back to the ground

state E1 by emitting photons But some of them

jumps to the metastable state E2

bull They (electron) stay in metastable state for more

then 10-3 sec

bull So electron density increases in metastable state

bull Thus the transitions are possible it takes more no

of electrons together and ν ndash (knew)12 photon beam is

produced which constitute laser beam

Optical PumpingThere are no of techniques for pumping a

collection of atoms to an inverted state

bull Optical pumping

bull Electrical discharge

bull Direct conversion

When photon of blue green light incident onRuby crystal electrons from ground state absorbs andexited and jumps on higher energy state levels andcomes back to metastable state They increasepopulation of electrons in metastable state

This process is called optical pumping which isdone by flash tube

Relation between Einsteinrsquos lsquoArsquo and lsquoBrsquo

coefficientsbull Einstein obtained a mathematical expression for the

existence of two different kinds of processes

(1) Spontaneous emission

(2) Stimulated emission

bull Consider all atoms r in thermal equilibrium at T

bull Radiation of freq ƒ amp energy density u(ƒ)

bull N1 amp N2 r atoms in E1 amp E2 respectively

bull In equilibrium absorption rates amp emission rates must be same

bull ie B12 N1 u(ƒ) = A21 N2+ B21 N2 u(ƒ)

A21 N2= u(ƒ) [B12N1 ndash B21N2]

So u(f) = [A21 N2 (B12 N1 ndash B21 N2)] ---------(1)

------------(2)

bull Boltzmann distribution law

------------(3)

bull So -----------(4)

bull But E2 ndash E1 = hf -----------(5)

bull So -----------(6)

21

21

12 1

21 2

( )

[ ]

ƒ

1

A

Bu

B N

B N

1

2

1 0

2 0

E kT

E kT

N N e

N N e

2 1( )1

2

E E kTNe

N

h 1

2

ƒ kTNe

N

---------- (7)

bull According to plankrsquos radiation formula

----------- (8)

bull Where B12 = B21 amp A21 B21 = ------------ (9)

bull So Ratio of spontaneous to stimulated emission

--------- (10)

21

21

ƒ12

21

h

ƒ

1

( )

[ ]kTe

A

Bu

B

B

3

3 ƒh

8 1( ) ( )

[ ]

ƒƒ

1kTu

c

h

e

3

3

8 ƒh

c

2 21 21

2 21 21

3

3

8

( ) ( ) ( )

ƒ

ƒ ƒ ƒ

N A A hR

B u B u ucN

bull So

--------- (11)

--------- (12)

bull So R = ---------- (13)

If hƒ ltlt kT in thermal equilibrium

then R = ltlt 1

bull hƒltltkT ndash Stimulated emission

ndashValid in microwave region (MASER)

bull hƒgtgtkT ndash Spontaneous emission

ndashValid in visible region incoherent

3

3

3

3

ƒh

8( )

8

ƒƒ

amp

ƒƒ

1

1( ) ( )

[ ]kT

h

uc

uR

h

e

c

ƒh 1[ ]kTe

ƒh 1[ ]kTe

Types of LASER

There are three types of lasers

1 Solid Laser (Ruby Laser)

2 Liquid Laser

3 Gas Laser ( He ndash Ne Laser CO2 Laser)

Ruby LaserhellipTo produce laser from solid Ruby crystal is

used

Ruby is an aluminum oxide crystal (Al2O3) in

which some of the aluminum atoms have been

replaced with Cr+3 chromium atoms (005 by

weight)

It was the first type of laser invented and was

first operated by Maiman in Research

Laboratories on 1960

Chromium gives ruby its characteristic pink or

red color by absorbing green and blue light

Ruby LaserhellipFor a ruby laser a crystal of ruby is formed into

a cylinder The ruby laser is used as a pulsed

laser producing red light at 6943 Aring

4

Ruby crystal is surrounded by xenon tube Rubycrystal is fully silvered at one side and partiallysilvered at the other end

A strong beam of blue green light is made to fall upon crystal from xenon tube and this light is absorbedby the crystal

Because of this many electrons from ground state ornormal state are raised to the excited state or higherstate and electron falls to metastable state

During this transition photon is not emitted butexcess energy of the electrons absorbed in crystallattice

5

As electron drops to metastable state they remain

there for certain time ~ 10-6 sec

Thus the incident blue green light from tube

increases the number of electron in metastable

state and then the population inversion can be

achieved

If a light of different frequency is allowed to fall

on this material the electrons move back and

forth between silvered ends of the crystal

While moving through they get stimulated and

exiced electrons radiate energy

Thus radiation photon has the same frequency as

that of incident photon and is also in exactly same

phase

When the intensity of light beam is increased the

same process is repeated

Finally extremely intensified beam of light

energies from the semi silvered side of the crystal

This way it is possible to get extremely intensified

and coherent beam of light from the crystal This

beam is nothing but higher energetic beam ndash ie

LASER beam

Applications of Ruby LaserhellipRuby lasers have declined in use with thediscovery of better lasing media They are still usedin a number of applications where short pulses ofred light are required Holographys around theworld produce holographic portraits with rubylasers in sizes up to a meter squared

Many non-destructive testing labs use ruby lasersto create holograms of large objects such asaircraft tires to look for weaknesses in the lining

Ruby lasers were used extensively in tattoo andhair removal

Drawbacks of Ruby Laserhellipbull The laser requires high pumping power because

the laser transition terminates at the ground state

and more than half of ground state atoms must

be pumped to higher state to achieve population

inversion

bull The efficiency of ruby laser is very low because

only green component of the pumping light is

used while the rest of components are left unused

bull The laser output is not continues but occurs in

the form of pulses of microseconds duration

bull The defects due to crystalline imperfections are

also present in this laser

Gaseous Laser (He ndash Ne Laser)A helium - neon laser usually called a He-Ne

laser is a type of small gas laser He-Ne lasers

have many industrial and scientific uses and are

often used in laboratory demonstrations of optics

He-Ne laser is an atomic laser which employs a

four-level pumping scheme

The active medium is a mixture of 10 parts of

helium to 1 part of neon

Neon atoms are centers and have energy levels

suitable for laser transitions while helium atoms

help efficient excitation of neon atoms

The most common wavelength is 6328 Aring These

lasers produced powers in the range 05 to 50

mW in the red portion of the visible spectrum

They have long operating life of the order of

50000 hrs

6

Constructionhellip

It consists of a glass discharge tube of about

typically 30 cm long and 15 cm diameter

The tube is filled with a mixture of helium and

neon gases in the 101

Electrodes are provided in the tube to produce a

discharge in the gas

They are connected to a high voltage power

supply The tube is hermetically sealed with glass

windows oriented at Brewster angle to the tube

The cavity mirrors are arranged externally

WorkinghellipWhen the power is switched on a high voltage of

about 10 kV is applied across the gas

It is sufficient to ionize the gas

The electrons and ions are produced in the process

of discharge are accelerated toward the anode and

cathode respectively

The electron have a smaller mass they acquire a

higher velocity They transfer their kinetic energy to

helium atoms through inelastic collisions

The initial excitation effects only the helium atoms

They are in metastable state and cannot return in

ground state by the spontaneous emission

The excited helium atoms can return to the ground

state by transforming their energy to neon atoms

through collision This transformation take place when

two colliding atoms have initial energy state It is

called resonant transfer of energy

So the pumping mechanism of He-Ne Laser is when

the helium atom in the metastable state collides with

neon atom in the ground state the neon atom is excited

and the helium atom drops back to the ground state

The role of helium atom is thus to excite neon atom

and cause population inversion The probability of

energy transfer from helium atoms to neon atoms is

more as there are 10 atoms of helium per 1 neon atom

in gas mixture

Without the Brewster windows the light output

is unpolarized because of it laser output to be

linearly polarized

7

When the excited Ne atom passes from metastable

state (3s) to lower level (2p) it emits photon of

wavelength 632 nm

This photon travels through the gas mixture

parallel to the axis of tube it is reflected back and

forth by the mirror ends until it stimulates an

excited Ne atom and causes it to emit a photon of

632nm with the stimulating photon

The stimulated transition from (3s) level to (2p)

level is laser transition

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

Optical PumpingThere are no of techniques for pumping a

collection of atoms to an inverted state

bull Optical pumping

bull Electrical discharge

bull Direct conversion

When photon of blue green light incident onRuby crystal electrons from ground state absorbs andexited and jumps on higher energy state levels andcomes back to metastable state They increasepopulation of electrons in metastable state

This process is called optical pumping which isdone by flash tube

Relation between Einsteinrsquos lsquoArsquo and lsquoBrsquo

coefficientsbull Einstein obtained a mathematical expression for the

existence of two different kinds of processes

(1) Spontaneous emission

(2) Stimulated emission

bull Consider all atoms r in thermal equilibrium at T

bull Radiation of freq ƒ amp energy density u(ƒ)

bull N1 amp N2 r atoms in E1 amp E2 respectively

bull In equilibrium absorption rates amp emission rates must be same

bull ie B12 N1 u(ƒ) = A21 N2+ B21 N2 u(ƒ)

A21 N2= u(ƒ) [B12N1 ndash B21N2]

So u(f) = [A21 N2 (B12 N1 ndash B21 N2)] ---------(1)

------------(2)

bull Boltzmann distribution law

------------(3)

bull So -----------(4)

bull But E2 ndash E1 = hf -----------(5)

bull So -----------(6)

21

21

12 1

21 2

( )

[ ]

ƒ

1

A

Bu

B N

B N

1

2

1 0

2 0

E kT

E kT

N N e

N N e

2 1( )1

2

E E kTNe

N

h 1

2

ƒ kTNe

N

---------- (7)

bull According to plankrsquos radiation formula

----------- (8)

bull Where B12 = B21 amp A21 B21 = ------------ (9)

bull So Ratio of spontaneous to stimulated emission

--------- (10)

21

21

ƒ12

21

h

ƒ

1

( )

[ ]kTe

A

Bu

B

B

3

3 ƒh

8 1( ) ( )

[ ]

ƒƒ

1kTu

c

h

e

3

3

8 ƒh

c

2 21 21

2 21 21

3

3

8

( ) ( ) ( )

ƒ

ƒ ƒ ƒ

N A A hR

B u B u ucN

bull So

--------- (11)

--------- (12)

bull So R = ---------- (13)

If hƒ ltlt kT in thermal equilibrium

then R = ltlt 1

bull hƒltltkT ndash Stimulated emission

ndashValid in microwave region (MASER)

bull hƒgtgtkT ndash Spontaneous emission

ndashValid in visible region incoherent

3

3

3

3

ƒh

8( )

8

ƒƒ

amp

ƒƒ

1

1( ) ( )

[ ]kT

h

uc

uR

h

e

c

ƒh 1[ ]kTe

ƒh 1[ ]kTe

Types of LASER

There are three types of lasers

1 Solid Laser (Ruby Laser)

2 Liquid Laser

3 Gas Laser ( He ndash Ne Laser CO2 Laser)

Ruby LaserhellipTo produce laser from solid Ruby crystal is

used

Ruby is an aluminum oxide crystal (Al2O3) in

which some of the aluminum atoms have been

replaced with Cr+3 chromium atoms (005 by

weight)

It was the first type of laser invented and was

first operated by Maiman in Research

Laboratories on 1960

Chromium gives ruby its characteristic pink or

red color by absorbing green and blue light

Ruby LaserhellipFor a ruby laser a crystal of ruby is formed into

a cylinder The ruby laser is used as a pulsed

laser producing red light at 6943 Aring

4

Ruby crystal is surrounded by xenon tube Rubycrystal is fully silvered at one side and partiallysilvered at the other end

A strong beam of blue green light is made to fall upon crystal from xenon tube and this light is absorbedby the crystal

Because of this many electrons from ground state ornormal state are raised to the excited state or higherstate and electron falls to metastable state

During this transition photon is not emitted butexcess energy of the electrons absorbed in crystallattice

5

As electron drops to metastable state they remain

there for certain time ~ 10-6 sec

Thus the incident blue green light from tube

increases the number of electron in metastable

state and then the population inversion can be

achieved

If a light of different frequency is allowed to fall

on this material the electrons move back and

forth between silvered ends of the crystal

While moving through they get stimulated and

exiced electrons radiate energy

Thus radiation photon has the same frequency as

that of incident photon and is also in exactly same

phase

When the intensity of light beam is increased the

same process is repeated

Finally extremely intensified beam of light

energies from the semi silvered side of the crystal

This way it is possible to get extremely intensified

and coherent beam of light from the crystal This

beam is nothing but higher energetic beam ndash ie

LASER beam

Applications of Ruby LaserhellipRuby lasers have declined in use with thediscovery of better lasing media They are still usedin a number of applications where short pulses ofred light are required Holographys around theworld produce holographic portraits with rubylasers in sizes up to a meter squared

Many non-destructive testing labs use ruby lasersto create holograms of large objects such asaircraft tires to look for weaknesses in the lining

Ruby lasers were used extensively in tattoo andhair removal

Drawbacks of Ruby Laserhellipbull The laser requires high pumping power because

the laser transition terminates at the ground state

and more than half of ground state atoms must

be pumped to higher state to achieve population

inversion

bull The efficiency of ruby laser is very low because

only green component of the pumping light is

used while the rest of components are left unused

bull The laser output is not continues but occurs in

the form of pulses of microseconds duration

bull The defects due to crystalline imperfections are

also present in this laser

Gaseous Laser (He ndash Ne Laser)A helium - neon laser usually called a He-Ne

laser is a type of small gas laser He-Ne lasers

have many industrial and scientific uses and are

often used in laboratory demonstrations of optics

He-Ne laser is an atomic laser which employs a

four-level pumping scheme

The active medium is a mixture of 10 parts of

helium to 1 part of neon

Neon atoms are centers and have energy levels

suitable for laser transitions while helium atoms

help efficient excitation of neon atoms

The most common wavelength is 6328 Aring These

lasers produced powers in the range 05 to 50

mW in the red portion of the visible spectrum

They have long operating life of the order of

50000 hrs

6

Constructionhellip

It consists of a glass discharge tube of about

typically 30 cm long and 15 cm diameter

The tube is filled with a mixture of helium and

neon gases in the 101

Electrodes are provided in the tube to produce a

discharge in the gas

They are connected to a high voltage power

supply The tube is hermetically sealed with glass

windows oriented at Brewster angle to the tube

The cavity mirrors are arranged externally

WorkinghellipWhen the power is switched on a high voltage of

about 10 kV is applied across the gas

It is sufficient to ionize the gas

The electrons and ions are produced in the process

of discharge are accelerated toward the anode and

cathode respectively

The electron have a smaller mass they acquire a

higher velocity They transfer their kinetic energy to

helium atoms through inelastic collisions

The initial excitation effects only the helium atoms

They are in metastable state and cannot return in

ground state by the spontaneous emission

The excited helium atoms can return to the ground

state by transforming their energy to neon atoms

through collision This transformation take place when

two colliding atoms have initial energy state It is

called resonant transfer of energy

So the pumping mechanism of He-Ne Laser is when

the helium atom in the metastable state collides with

neon atom in the ground state the neon atom is excited

and the helium atom drops back to the ground state

The role of helium atom is thus to excite neon atom

and cause population inversion The probability of

energy transfer from helium atoms to neon atoms is

more as there are 10 atoms of helium per 1 neon atom

in gas mixture

Without the Brewster windows the light output

is unpolarized because of it laser output to be

linearly polarized

7

When the excited Ne atom passes from metastable

state (3s) to lower level (2p) it emits photon of

wavelength 632 nm

This photon travels through the gas mixture

parallel to the axis of tube it is reflected back and

forth by the mirror ends until it stimulates an

excited Ne atom and causes it to emit a photon of

632nm with the stimulating photon

The stimulated transition from (3s) level to (2p)

level is laser transition

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

Relation between Einsteinrsquos lsquoArsquo and lsquoBrsquo

coefficientsbull Einstein obtained a mathematical expression for the

existence of two different kinds of processes

(1) Spontaneous emission

(2) Stimulated emission

bull Consider all atoms r in thermal equilibrium at T

bull Radiation of freq ƒ amp energy density u(ƒ)

bull N1 amp N2 r atoms in E1 amp E2 respectively

bull In equilibrium absorption rates amp emission rates must be same

bull ie B12 N1 u(ƒ) = A21 N2+ B21 N2 u(ƒ)

A21 N2= u(ƒ) [B12N1 ndash B21N2]

So u(f) = [A21 N2 (B12 N1 ndash B21 N2)] ---------(1)

------------(2)

bull Boltzmann distribution law

------------(3)

bull So -----------(4)

bull But E2 ndash E1 = hf -----------(5)

bull So -----------(6)

21

21

12 1

21 2

( )

[ ]

ƒ

1

A

Bu

B N

B N

1

2

1 0

2 0

E kT

E kT

N N e

N N e

2 1( )1

2

E E kTNe

N

h 1

2

ƒ kTNe

N

---------- (7)

bull According to plankrsquos radiation formula

----------- (8)

bull Where B12 = B21 amp A21 B21 = ------------ (9)

bull So Ratio of spontaneous to stimulated emission

--------- (10)

21

21

ƒ12

21

h

ƒ

1

( )

[ ]kTe

A

Bu

B

B

3

3 ƒh

8 1( ) ( )

[ ]

ƒƒ

1kTu

c

h

e

3

3

8 ƒh

c

2 21 21

2 21 21

3

3

8

( ) ( ) ( )

ƒ

ƒ ƒ ƒ

N A A hR

B u B u ucN

bull So

--------- (11)

--------- (12)

bull So R = ---------- (13)

If hƒ ltlt kT in thermal equilibrium

then R = ltlt 1

bull hƒltltkT ndash Stimulated emission

ndashValid in microwave region (MASER)

bull hƒgtgtkT ndash Spontaneous emission

ndashValid in visible region incoherent

3

3

3

3

ƒh

8( )

8

ƒƒ

amp

ƒƒ

1

1( ) ( )

[ ]kT

h

uc

uR

h

e

c

ƒh 1[ ]kTe

ƒh 1[ ]kTe

Types of LASER

There are three types of lasers

1 Solid Laser (Ruby Laser)

2 Liquid Laser

3 Gas Laser ( He ndash Ne Laser CO2 Laser)

Ruby LaserhellipTo produce laser from solid Ruby crystal is

used

Ruby is an aluminum oxide crystal (Al2O3) in

which some of the aluminum atoms have been

replaced with Cr+3 chromium atoms (005 by

weight)

It was the first type of laser invented and was

first operated by Maiman in Research

Laboratories on 1960

Chromium gives ruby its characteristic pink or

red color by absorbing green and blue light

Ruby LaserhellipFor a ruby laser a crystal of ruby is formed into

a cylinder The ruby laser is used as a pulsed

laser producing red light at 6943 Aring

4

Ruby crystal is surrounded by xenon tube Rubycrystal is fully silvered at one side and partiallysilvered at the other end

A strong beam of blue green light is made to fall upon crystal from xenon tube and this light is absorbedby the crystal

Because of this many electrons from ground state ornormal state are raised to the excited state or higherstate and electron falls to metastable state

During this transition photon is not emitted butexcess energy of the electrons absorbed in crystallattice

5

As electron drops to metastable state they remain

there for certain time ~ 10-6 sec

Thus the incident blue green light from tube

increases the number of electron in metastable

state and then the population inversion can be

achieved

If a light of different frequency is allowed to fall

on this material the electrons move back and

forth between silvered ends of the crystal

While moving through they get stimulated and

exiced electrons radiate energy

Thus radiation photon has the same frequency as

that of incident photon and is also in exactly same

phase

When the intensity of light beam is increased the

same process is repeated

Finally extremely intensified beam of light

energies from the semi silvered side of the crystal

This way it is possible to get extremely intensified

and coherent beam of light from the crystal This

beam is nothing but higher energetic beam ndash ie

LASER beam

Applications of Ruby LaserhellipRuby lasers have declined in use with thediscovery of better lasing media They are still usedin a number of applications where short pulses ofred light are required Holographys around theworld produce holographic portraits with rubylasers in sizes up to a meter squared

Many non-destructive testing labs use ruby lasersto create holograms of large objects such asaircraft tires to look for weaknesses in the lining

Ruby lasers were used extensively in tattoo andhair removal

Drawbacks of Ruby Laserhellipbull The laser requires high pumping power because

the laser transition terminates at the ground state

and more than half of ground state atoms must

be pumped to higher state to achieve population

inversion

bull The efficiency of ruby laser is very low because

only green component of the pumping light is

used while the rest of components are left unused

bull The laser output is not continues but occurs in

the form of pulses of microseconds duration

bull The defects due to crystalline imperfections are

also present in this laser

Gaseous Laser (He ndash Ne Laser)A helium - neon laser usually called a He-Ne

laser is a type of small gas laser He-Ne lasers

have many industrial and scientific uses and are

often used in laboratory demonstrations of optics

He-Ne laser is an atomic laser which employs a

four-level pumping scheme

The active medium is a mixture of 10 parts of

helium to 1 part of neon

Neon atoms are centers and have energy levels

suitable for laser transitions while helium atoms

help efficient excitation of neon atoms

The most common wavelength is 6328 Aring These

lasers produced powers in the range 05 to 50

mW in the red portion of the visible spectrum

They have long operating life of the order of

50000 hrs

6

Constructionhellip

It consists of a glass discharge tube of about

typically 30 cm long and 15 cm diameter

The tube is filled with a mixture of helium and

neon gases in the 101

Electrodes are provided in the tube to produce a

discharge in the gas

They are connected to a high voltage power

supply The tube is hermetically sealed with glass

windows oriented at Brewster angle to the tube

The cavity mirrors are arranged externally

WorkinghellipWhen the power is switched on a high voltage of

about 10 kV is applied across the gas

It is sufficient to ionize the gas

The electrons and ions are produced in the process

of discharge are accelerated toward the anode and

cathode respectively

The electron have a smaller mass they acquire a

higher velocity They transfer their kinetic energy to

helium atoms through inelastic collisions

The initial excitation effects only the helium atoms

They are in metastable state and cannot return in

ground state by the spontaneous emission

The excited helium atoms can return to the ground

state by transforming their energy to neon atoms

through collision This transformation take place when

two colliding atoms have initial energy state It is

called resonant transfer of energy

So the pumping mechanism of He-Ne Laser is when

the helium atom in the metastable state collides with

neon atom in the ground state the neon atom is excited

and the helium atom drops back to the ground state

The role of helium atom is thus to excite neon atom

and cause population inversion The probability of

energy transfer from helium atoms to neon atoms is

more as there are 10 atoms of helium per 1 neon atom

in gas mixture

Without the Brewster windows the light output

is unpolarized because of it laser output to be

linearly polarized

7

When the excited Ne atom passes from metastable

state (3s) to lower level (2p) it emits photon of

wavelength 632 nm

This photon travels through the gas mixture

parallel to the axis of tube it is reflected back and

forth by the mirror ends until it stimulates an

excited Ne atom and causes it to emit a photon of

632nm with the stimulating photon

The stimulated transition from (3s) level to (2p)

level is laser transition

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

------------(2)

bull Boltzmann distribution law

------------(3)

bull So -----------(4)

bull But E2 ndash E1 = hf -----------(5)

bull So -----------(6)

21

21

12 1

21 2

( )

[ ]

ƒ

1

A

Bu

B N

B N

1

2

1 0

2 0

E kT

E kT

N N e

N N e

2 1( )1

2

E E kTNe

N

h 1

2

ƒ kTNe

N

---------- (7)

bull According to plankrsquos radiation formula

----------- (8)

bull Where B12 = B21 amp A21 B21 = ------------ (9)

bull So Ratio of spontaneous to stimulated emission

--------- (10)

21

21

ƒ12

21

h

ƒ

1

( )

[ ]kTe

A

Bu

B

B

3

3 ƒh

8 1( ) ( )

[ ]

ƒƒ

1kTu

c

h

e

3

3

8 ƒh

c

2 21 21

2 21 21

3

3

8

( ) ( ) ( )

ƒ

ƒ ƒ ƒ

N A A hR

B u B u ucN

bull So

--------- (11)

--------- (12)

bull So R = ---------- (13)

If hƒ ltlt kT in thermal equilibrium

then R = ltlt 1

bull hƒltltkT ndash Stimulated emission

ndashValid in microwave region (MASER)

bull hƒgtgtkT ndash Spontaneous emission

ndashValid in visible region incoherent

3

3

3

3

ƒh

8( )

8

ƒƒ

amp

ƒƒ

1

1( ) ( )

[ ]kT

h

uc

uR

h

e

c

ƒh 1[ ]kTe

ƒh 1[ ]kTe

Types of LASER

There are three types of lasers

1 Solid Laser (Ruby Laser)

2 Liquid Laser

3 Gas Laser ( He ndash Ne Laser CO2 Laser)

Ruby LaserhellipTo produce laser from solid Ruby crystal is

used

Ruby is an aluminum oxide crystal (Al2O3) in

which some of the aluminum atoms have been

replaced with Cr+3 chromium atoms (005 by

weight)

It was the first type of laser invented and was

first operated by Maiman in Research

Laboratories on 1960

Chromium gives ruby its characteristic pink or

red color by absorbing green and blue light

Ruby LaserhellipFor a ruby laser a crystal of ruby is formed into

a cylinder The ruby laser is used as a pulsed

laser producing red light at 6943 Aring

4

Ruby crystal is surrounded by xenon tube Rubycrystal is fully silvered at one side and partiallysilvered at the other end

A strong beam of blue green light is made to fall upon crystal from xenon tube and this light is absorbedby the crystal

Because of this many electrons from ground state ornormal state are raised to the excited state or higherstate and electron falls to metastable state

During this transition photon is not emitted butexcess energy of the electrons absorbed in crystallattice

5

As electron drops to metastable state they remain

there for certain time ~ 10-6 sec

Thus the incident blue green light from tube

increases the number of electron in metastable

state and then the population inversion can be

achieved

If a light of different frequency is allowed to fall

on this material the electrons move back and

forth between silvered ends of the crystal

While moving through they get stimulated and

exiced electrons radiate energy

Thus radiation photon has the same frequency as

that of incident photon and is also in exactly same

phase

When the intensity of light beam is increased the

same process is repeated

Finally extremely intensified beam of light

energies from the semi silvered side of the crystal

This way it is possible to get extremely intensified

and coherent beam of light from the crystal This

beam is nothing but higher energetic beam ndash ie

LASER beam

Applications of Ruby LaserhellipRuby lasers have declined in use with thediscovery of better lasing media They are still usedin a number of applications where short pulses ofred light are required Holographys around theworld produce holographic portraits with rubylasers in sizes up to a meter squared

Many non-destructive testing labs use ruby lasersto create holograms of large objects such asaircraft tires to look for weaknesses in the lining

Ruby lasers were used extensively in tattoo andhair removal

Drawbacks of Ruby Laserhellipbull The laser requires high pumping power because

the laser transition terminates at the ground state

and more than half of ground state atoms must

be pumped to higher state to achieve population

inversion

bull The efficiency of ruby laser is very low because

only green component of the pumping light is

used while the rest of components are left unused

bull The laser output is not continues but occurs in

the form of pulses of microseconds duration

bull The defects due to crystalline imperfections are

also present in this laser

Gaseous Laser (He ndash Ne Laser)A helium - neon laser usually called a He-Ne

laser is a type of small gas laser He-Ne lasers

have many industrial and scientific uses and are

often used in laboratory demonstrations of optics

He-Ne laser is an atomic laser which employs a

four-level pumping scheme

The active medium is a mixture of 10 parts of

helium to 1 part of neon

Neon atoms are centers and have energy levels

suitable for laser transitions while helium atoms

help efficient excitation of neon atoms

The most common wavelength is 6328 Aring These

lasers produced powers in the range 05 to 50

mW in the red portion of the visible spectrum

They have long operating life of the order of

50000 hrs

6

Constructionhellip

It consists of a glass discharge tube of about

typically 30 cm long and 15 cm diameter

The tube is filled with a mixture of helium and

neon gases in the 101

Electrodes are provided in the tube to produce a

discharge in the gas

They are connected to a high voltage power

supply The tube is hermetically sealed with glass

windows oriented at Brewster angle to the tube

The cavity mirrors are arranged externally

WorkinghellipWhen the power is switched on a high voltage of

about 10 kV is applied across the gas

It is sufficient to ionize the gas

The electrons and ions are produced in the process

of discharge are accelerated toward the anode and

cathode respectively

The electron have a smaller mass they acquire a

higher velocity They transfer their kinetic energy to

helium atoms through inelastic collisions

The initial excitation effects only the helium atoms

They are in metastable state and cannot return in

ground state by the spontaneous emission

The excited helium atoms can return to the ground

state by transforming their energy to neon atoms

through collision This transformation take place when

two colliding atoms have initial energy state It is

called resonant transfer of energy

So the pumping mechanism of He-Ne Laser is when

the helium atom in the metastable state collides with

neon atom in the ground state the neon atom is excited

and the helium atom drops back to the ground state

The role of helium atom is thus to excite neon atom

and cause population inversion The probability of

energy transfer from helium atoms to neon atoms is

more as there are 10 atoms of helium per 1 neon atom

in gas mixture

Without the Brewster windows the light output

is unpolarized because of it laser output to be

linearly polarized

7

When the excited Ne atom passes from metastable

state (3s) to lower level (2p) it emits photon of

wavelength 632 nm

This photon travels through the gas mixture

parallel to the axis of tube it is reflected back and

forth by the mirror ends until it stimulates an

excited Ne atom and causes it to emit a photon of

632nm with the stimulating photon

The stimulated transition from (3s) level to (2p)

level is laser transition

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

---------- (7)

bull According to plankrsquos radiation formula

----------- (8)

bull Where B12 = B21 amp A21 B21 = ------------ (9)

bull So Ratio of spontaneous to stimulated emission

--------- (10)

21

21

ƒ12

21

h

ƒ

1

( )

[ ]kTe

A

Bu

B

B

3

3 ƒh

8 1( ) ( )

[ ]

ƒƒ

1kTu

c

h

e

3

3

8 ƒh

c

2 21 21

2 21 21

3

3

8

( ) ( ) ( )

ƒ

ƒ ƒ ƒ

N A A hR

B u B u ucN

bull So

--------- (11)

--------- (12)

bull So R = ---------- (13)

If hƒ ltlt kT in thermal equilibrium

then R = ltlt 1

bull hƒltltkT ndash Stimulated emission

ndashValid in microwave region (MASER)

bull hƒgtgtkT ndash Spontaneous emission

ndashValid in visible region incoherent

3

3

3

3

ƒh

8( )

8

ƒƒ

amp

ƒƒ

1

1( ) ( )

[ ]kT

h

uc

uR

h

e

c

ƒh 1[ ]kTe

ƒh 1[ ]kTe

Types of LASER

There are three types of lasers

1 Solid Laser (Ruby Laser)

2 Liquid Laser

3 Gas Laser ( He ndash Ne Laser CO2 Laser)

Ruby LaserhellipTo produce laser from solid Ruby crystal is

used

Ruby is an aluminum oxide crystal (Al2O3) in

which some of the aluminum atoms have been

replaced with Cr+3 chromium atoms (005 by

weight)

It was the first type of laser invented and was

first operated by Maiman in Research

Laboratories on 1960

Chromium gives ruby its characteristic pink or

red color by absorbing green and blue light

Ruby LaserhellipFor a ruby laser a crystal of ruby is formed into

a cylinder The ruby laser is used as a pulsed

laser producing red light at 6943 Aring

4

Ruby crystal is surrounded by xenon tube Rubycrystal is fully silvered at one side and partiallysilvered at the other end

A strong beam of blue green light is made to fall upon crystal from xenon tube and this light is absorbedby the crystal

Because of this many electrons from ground state ornormal state are raised to the excited state or higherstate and electron falls to metastable state

During this transition photon is not emitted butexcess energy of the electrons absorbed in crystallattice

5

As electron drops to metastable state they remain

there for certain time ~ 10-6 sec

Thus the incident blue green light from tube

increases the number of electron in metastable

state and then the population inversion can be

achieved

If a light of different frequency is allowed to fall

on this material the electrons move back and

forth between silvered ends of the crystal

While moving through they get stimulated and

exiced electrons radiate energy

Thus radiation photon has the same frequency as

that of incident photon and is also in exactly same

phase

When the intensity of light beam is increased the

same process is repeated

Finally extremely intensified beam of light

energies from the semi silvered side of the crystal

This way it is possible to get extremely intensified

and coherent beam of light from the crystal This

beam is nothing but higher energetic beam ndash ie

LASER beam

Applications of Ruby LaserhellipRuby lasers have declined in use with thediscovery of better lasing media They are still usedin a number of applications where short pulses ofred light are required Holographys around theworld produce holographic portraits with rubylasers in sizes up to a meter squared

Many non-destructive testing labs use ruby lasersto create holograms of large objects such asaircraft tires to look for weaknesses in the lining

Ruby lasers were used extensively in tattoo andhair removal

Drawbacks of Ruby Laserhellipbull The laser requires high pumping power because

the laser transition terminates at the ground state

and more than half of ground state atoms must

be pumped to higher state to achieve population

inversion

bull The efficiency of ruby laser is very low because

only green component of the pumping light is

used while the rest of components are left unused

bull The laser output is not continues but occurs in

the form of pulses of microseconds duration

bull The defects due to crystalline imperfections are

also present in this laser

Gaseous Laser (He ndash Ne Laser)A helium - neon laser usually called a He-Ne

laser is a type of small gas laser He-Ne lasers

have many industrial and scientific uses and are

often used in laboratory demonstrations of optics

He-Ne laser is an atomic laser which employs a

four-level pumping scheme

The active medium is a mixture of 10 parts of

helium to 1 part of neon

Neon atoms are centers and have energy levels

suitable for laser transitions while helium atoms

help efficient excitation of neon atoms

The most common wavelength is 6328 Aring These

lasers produced powers in the range 05 to 50

mW in the red portion of the visible spectrum

They have long operating life of the order of

50000 hrs

6

Constructionhellip

It consists of a glass discharge tube of about

typically 30 cm long and 15 cm diameter

The tube is filled with a mixture of helium and

neon gases in the 101

Electrodes are provided in the tube to produce a

discharge in the gas

They are connected to a high voltage power

supply The tube is hermetically sealed with glass

windows oriented at Brewster angle to the tube

The cavity mirrors are arranged externally

WorkinghellipWhen the power is switched on a high voltage of

about 10 kV is applied across the gas

It is sufficient to ionize the gas

The electrons and ions are produced in the process

of discharge are accelerated toward the anode and

cathode respectively

The electron have a smaller mass they acquire a

higher velocity They transfer their kinetic energy to

helium atoms through inelastic collisions

The initial excitation effects only the helium atoms

They are in metastable state and cannot return in

ground state by the spontaneous emission

The excited helium atoms can return to the ground

state by transforming their energy to neon atoms

through collision This transformation take place when

two colliding atoms have initial energy state It is

called resonant transfer of energy

So the pumping mechanism of He-Ne Laser is when

the helium atom in the metastable state collides with

neon atom in the ground state the neon atom is excited

and the helium atom drops back to the ground state

The role of helium atom is thus to excite neon atom

and cause population inversion The probability of

energy transfer from helium atoms to neon atoms is

more as there are 10 atoms of helium per 1 neon atom

in gas mixture

Without the Brewster windows the light output

is unpolarized because of it laser output to be

linearly polarized

7

When the excited Ne atom passes from metastable

state (3s) to lower level (2p) it emits photon of

wavelength 632 nm

This photon travels through the gas mixture

parallel to the axis of tube it is reflected back and

forth by the mirror ends until it stimulates an

excited Ne atom and causes it to emit a photon of

632nm with the stimulating photon

The stimulated transition from (3s) level to (2p)

level is laser transition

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

bull So

--------- (11)

--------- (12)

bull So R = ---------- (13)

If hƒ ltlt kT in thermal equilibrium

then R = ltlt 1

bull hƒltltkT ndash Stimulated emission

ndashValid in microwave region (MASER)

bull hƒgtgtkT ndash Spontaneous emission

ndashValid in visible region incoherent

3

3

3

3

ƒh

8( )

8

ƒƒ

amp

ƒƒ

1

1( ) ( )

[ ]kT

h

uc

uR

h

e

c

ƒh 1[ ]kTe

ƒh 1[ ]kTe

Types of LASER

There are three types of lasers

1 Solid Laser (Ruby Laser)

2 Liquid Laser

3 Gas Laser ( He ndash Ne Laser CO2 Laser)

Ruby LaserhellipTo produce laser from solid Ruby crystal is

used

Ruby is an aluminum oxide crystal (Al2O3) in

which some of the aluminum atoms have been

replaced with Cr+3 chromium atoms (005 by

weight)

It was the first type of laser invented and was

first operated by Maiman in Research

Laboratories on 1960

Chromium gives ruby its characteristic pink or

red color by absorbing green and blue light

Ruby LaserhellipFor a ruby laser a crystal of ruby is formed into

a cylinder The ruby laser is used as a pulsed

laser producing red light at 6943 Aring

4

Ruby crystal is surrounded by xenon tube Rubycrystal is fully silvered at one side and partiallysilvered at the other end

A strong beam of blue green light is made to fall upon crystal from xenon tube and this light is absorbedby the crystal

Because of this many electrons from ground state ornormal state are raised to the excited state or higherstate and electron falls to metastable state

During this transition photon is not emitted butexcess energy of the electrons absorbed in crystallattice

5

As electron drops to metastable state they remain

there for certain time ~ 10-6 sec

Thus the incident blue green light from tube

increases the number of electron in metastable

state and then the population inversion can be

achieved

If a light of different frequency is allowed to fall

on this material the electrons move back and

forth between silvered ends of the crystal

While moving through they get stimulated and

exiced electrons radiate energy

Thus radiation photon has the same frequency as

that of incident photon and is also in exactly same

phase

When the intensity of light beam is increased the

same process is repeated

Finally extremely intensified beam of light

energies from the semi silvered side of the crystal

This way it is possible to get extremely intensified

and coherent beam of light from the crystal This

beam is nothing but higher energetic beam ndash ie

LASER beam

Applications of Ruby LaserhellipRuby lasers have declined in use with thediscovery of better lasing media They are still usedin a number of applications where short pulses ofred light are required Holographys around theworld produce holographic portraits with rubylasers in sizes up to a meter squared

Many non-destructive testing labs use ruby lasersto create holograms of large objects such asaircraft tires to look for weaknesses in the lining

Ruby lasers were used extensively in tattoo andhair removal

Drawbacks of Ruby Laserhellipbull The laser requires high pumping power because

the laser transition terminates at the ground state

and more than half of ground state atoms must

be pumped to higher state to achieve population

inversion

bull The efficiency of ruby laser is very low because

only green component of the pumping light is

used while the rest of components are left unused

bull The laser output is not continues but occurs in

the form of pulses of microseconds duration

bull The defects due to crystalline imperfections are

also present in this laser

Gaseous Laser (He ndash Ne Laser)A helium - neon laser usually called a He-Ne

laser is a type of small gas laser He-Ne lasers

have many industrial and scientific uses and are

often used in laboratory demonstrations of optics

He-Ne laser is an atomic laser which employs a

four-level pumping scheme

The active medium is a mixture of 10 parts of

helium to 1 part of neon

Neon atoms are centers and have energy levels

suitable for laser transitions while helium atoms

help efficient excitation of neon atoms

The most common wavelength is 6328 Aring These

lasers produced powers in the range 05 to 50

mW in the red portion of the visible spectrum

They have long operating life of the order of

50000 hrs

6

Constructionhellip

It consists of a glass discharge tube of about

typically 30 cm long and 15 cm diameter

The tube is filled with a mixture of helium and

neon gases in the 101

Electrodes are provided in the tube to produce a

discharge in the gas

They are connected to a high voltage power

supply The tube is hermetically sealed with glass

windows oriented at Brewster angle to the tube

The cavity mirrors are arranged externally

WorkinghellipWhen the power is switched on a high voltage of

about 10 kV is applied across the gas

It is sufficient to ionize the gas

The electrons and ions are produced in the process

of discharge are accelerated toward the anode and

cathode respectively

The electron have a smaller mass they acquire a

higher velocity They transfer their kinetic energy to

helium atoms through inelastic collisions

The initial excitation effects only the helium atoms

They are in metastable state and cannot return in

ground state by the spontaneous emission

The excited helium atoms can return to the ground

state by transforming their energy to neon atoms

through collision This transformation take place when

two colliding atoms have initial energy state It is

called resonant transfer of energy

So the pumping mechanism of He-Ne Laser is when

the helium atom in the metastable state collides with

neon atom in the ground state the neon atom is excited

and the helium atom drops back to the ground state

The role of helium atom is thus to excite neon atom

and cause population inversion The probability of

energy transfer from helium atoms to neon atoms is

more as there are 10 atoms of helium per 1 neon atom

in gas mixture

Without the Brewster windows the light output

is unpolarized because of it laser output to be

linearly polarized

7

When the excited Ne atom passes from metastable

state (3s) to lower level (2p) it emits photon of

wavelength 632 nm

This photon travels through the gas mixture

parallel to the axis of tube it is reflected back and

forth by the mirror ends until it stimulates an

excited Ne atom and causes it to emit a photon of

632nm with the stimulating photon

The stimulated transition from (3s) level to (2p)

level is laser transition

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

Types of LASER

There are three types of lasers

1 Solid Laser (Ruby Laser)

2 Liquid Laser

3 Gas Laser ( He ndash Ne Laser CO2 Laser)

Ruby LaserhellipTo produce laser from solid Ruby crystal is

used

Ruby is an aluminum oxide crystal (Al2O3) in

which some of the aluminum atoms have been

replaced with Cr+3 chromium atoms (005 by

weight)

It was the first type of laser invented and was

first operated by Maiman in Research

Laboratories on 1960

Chromium gives ruby its characteristic pink or

red color by absorbing green and blue light

Ruby LaserhellipFor a ruby laser a crystal of ruby is formed into

a cylinder The ruby laser is used as a pulsed

laser producing red light at 6943 Aring

4

Ruby crystal is surrounded by xenon tube Rubycrystal is fully silvered at one side and partiallysilvered at the other end

A strong beam of blue green light is made to fall upon crystal from xenon tube and this light is absorbedby the crystal

Because of this many electrons from ground state ornormal state are raised to the excited state or higherstate and electron falls to metastable state

During this transition photon is not emitted butexcess energy of the electrons absorbed in crystallattice

5

As electron drops to metastable state they remain

there for certain time ~ 10-6 sec

Thus the incident blue green light from tube

increases the number of electron in metastable

state and then the population inversion can be

achieved

If a light of different frequency is allowed to fall

on this material the electrons move back and

forth between silvered ends of the crystal

While moving through they get stimulated and

exiced electrons radiate energy

Thus radiation photon has the same frequency as

that of incident photon and is also in exactly same

phase

When the intensity of light beam is increased the

same process is repeated

Finally extremely intensified beam of light

energies from the semi silvered side of the crystal

This way it is possible to get extremely intensified

and coherent beam of light from the crystal This

beam is nothing but higher energetic beam ndash ie

LASER beam

Applications of Ruby LaserhellipRuby lasers have declined in use with thediscovery of better lasing media They are still usedin a number of applications where short pulses ofred light are required Holographys around theworld produce holographic portraits with rubylasers in sizes up to a meter squared

Many non-destructive testing labs use ruby lasersto create holograms of large objects such asaircraft tires to look for weaknesses in the lining

Ruby lasers were used extensively in tattoo andhair removal

Drawbacks of Ruby Laserhellipbull The laser requires high pumping power because

the laser transition terminates at the ground state

and more than half of ground state atoms must

be pumped to higher state to achieve population

inversion

bull The efficiency of ruby laser is very low because

only green component of the pumping light is

used while the rest of components are left unused

bull The laser output is not continues but occurs in

the form of pulses of microseconds duration

bull The defects due to crystalline imperfections are

also present in this laser

Gaseous Laser (He ndash Ne Laser)A helium - neon laser usually called a He-Ne

laser is a type of small gas laser He-Ne lasers

have many industrial and scientific uses and are

often used in laboratory demonstrations of optics

He-Ne laser is an atomic laser which employs a

four-level pumping scheme

The active medium is a mixture of 10 parts of

helium to 1 part of neon

Neon atoms are centers and have energy levels

suitable for laser transitions while helium atoms

help efficient excitation of neon atoms

The most common wavelength is 6328 Aring These

lasers produced powers in the range 05 to 50

mW in the red portion of the visible spectrum

They have long operating life of the order of

50000 hrs

6

Constructionhellip

It consists of a glass discharge tube of about

typically 30 cm long and 15 cm diameter

The tube is filled with a mixture of helium and

neon gases in the 101

Electrodes are provided in the tube to produce a

discharge in the gas

They are connected to a high voltage power

supply The tube is hermetically sealed with glass

windows oriented at Brewster angle to the tube

The cavity mirrors are arranged externally

WorkinghellipWhen the power is switched on a high voltage of

about 10 kV is applied across the gas

It is sufficient to ionize the gas

The electrons and ions are produced in the process

of discharge are accelerated toward the anode and

cathode respectively

The electron have a smaller mass they acquire a

higher velocity They transfer their kinetic energy to

helium atoms through inelastic collisions

The initial excitation effects only the helium atoms

They are in metastable state and cannot return in

ground state by the spontaneous emission

The excited helium atoms can return to the ground

state by transforming their energy to neon atoms

through collision This transformation take place when

two colliding atoms have initial energy state It is

called resonant transfer of energy

So the pumping mechanism of He-Ne Laser is when

the helium atom in the metastable state collides with

neon atom in the ground state the neon atom is excited

and the helium atom drops back to the ground state

The role of helium atom is thus to excite neon atom

and cause population inversion The probability of

energy transfer from helium atoms to neon atoms is

more as there are 10 atoms of helium per 1 neon atom

in gas mixture

Without the Brewster windows the light output

is unpolarized because of it laser output to be

linearly polarized

7

When the excited Ne atom passes from metastable

state (3s) to lower level (2p) it emits photon of

wavelength 632 nm

This photon travels through the gas mixture

parallel to the axis of tube it is reflected back and

forth by the mirror ends until it stimulates an

excited Ne atom and causes it to emit a photon of

632nm with the stimulating photon

The stimulated transition from (3s) level to (2p)

level is laser transition

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

Ruby LaserhellipTo produce laser from solid Ruby crystal is

used

Ruby is an aluminum oxide crystal (Al2O3) in

which some of the aluminum atoms have been

replaced with Cr+3 chromium atoms (005 by

weight)

It was the first type of laser invented and was

first operated by Maiman in Research

Laboratories on 1960

Chromium gives ruby its characteristic pink or

red color by absorbing green and blue light

Ruby LaserhellipFor a ruby laser a crystal of ruby is formed into

a cylinder The ruby laser is used as a pulsed

laser producing red light at 6943 Aring

4

Ruby crystal is surrounded by xenon tube Rubycrystal is fully silvered at one side and partiallysilvered at the other end

A strong beam of blue green light is made to fall upon crystal from xenon tube and this light is absorbedby the crystal

Because of this many electrons from ground state ornormal state are raised to the excited state or higherstate and electron falls to metastable state

During this transition photon is not emitted butexcess energy of the electrons absorbed in crystallattice

5

As electron drops to metastable state they remain

there for certain time ~ 10-6 sec

Thus the incident blue green light from tube

increases the number of electron in metastable

state and then the population inversion can be

achieved

If a light of different frequency is allowed to fall

on this material the electrons move back and

forth between silvered ends of the crystal

While moving through they get stimulated and

exiced electrons radiate energy

Thus radiation photon has the same frequency as

that of incident photon and is also in exactly same

phase

When the intensity of light beam is increased the

same process is repeated

Finally extremely intensified beam of light

energies from the semi silvered side of the crystal

This way it is possible to get extremely intensified

and coherent beam of light from the crystal This

beam is nothing but higher energetic beam ndash ie

LASER beam

Applications of Ruby LaserhellipRuby lasers have declined in use with thediscovery of better lasing media They are still usedin a number of applications where short pulses ofred light are required Holographys around theworld produce holographic portraits with rubylasers in sizes up to a meter squared

Many non-destructive testing labs use ruby lasersto create holograms of large objects such asaircraft tires to look for weaknesses in the lining

Ruby lasers were used extensively in tattoo andhair removal

Drawbacks of Ruby Laserhellipbull The laser requires high pumping power because

the laser transition terminates at the ground state

and more than half of ground state atoms must

be pumped to higher state to achieve population

inversion

bull The efficiency of ruby laser is very low because

only green component of the pumping light is

used while the rest of components are left unused

bull The laser output is not continues but occurs in

the form of pulses of microseconds duration

bull The defects due to crystalline imperfections are

also present in this laser

Gaseous Laser (He ndash Ne Laser)A helium - neon laser usually called a He-Ne

laser is a type of small gas laser He-Ne lasers

have many industrial and scientific uses and are

often used in laboratory demonstrations of optics

He-Ne laser is an atomic laser which employs a

four-level pumping scheme

The active medium is a mixture of 10 parts of

helium to 1 part of neon

Neon atoms are centers and have energy levels

suitable for laser transitions while helium atoms

help efficient excitation of neon atoms

The most common wavelength is 6328 Aring These

lasers produced powers in the range 05 to 50

mW in the red portion of the visible spectrum

They have long operating life of the order of

50000 hrs

6

Constructionhellip

It consists of a glass discharge tube of about

typically 30 cm long and 15 cm diameter

The tube is filled with a mixture of helium and

neon gases in the 101

Electrodes are provided in the tube to produce a

discharge in the gas

They are connected to a high voltage power

supply The tube is hermetically sealed with glass

windows oriented at Brewster angle to the tube

The cavity mirrors are arranged externally

WorkinghellipWhen the power is switched on a high voltage of

about 10 kV is applied across the gas

It is sufficient to ionize the gas

The electrons and ions are produced in the process

of discharge are accelerated toward the anode and

cathode respectively

The electron have a smaller mass they acquire a

higher velocity They transfer their kinetic energy to

helium atoms through inelastic collisions

The initial excitation effects only the helium atoms

They are in metastable state and cannot return in

ground state by the spontaneous emission

The excited helium atoms can return to the ground

state by transforming their energy to neon atoms

through collision This transformation take place when

two colliding atoms have initial energy state It is

called resonant transfer of energy

So the pumping mechanism of He-Ne Laser is when

the helium atom in the metastable state collides with

neon atom in the ground state the neon atom is excited

and the helium atom drops back to the ground state

The role of helium atom is thus to excite neon atom

and cause population inversion The probability of

energy transfer from helium atoms to neon atoms is

more as there are 10 atoms of helium per 1 neon atom

in gas mixture

Without the Brewster windows the light output

is unpolarized because of it laser output to be

linearly polarized

7

When the excited Ne atom passes from metastable

state (3s) to lower level (2p) it emits photon of

wavelength 632 nm

This photon travels through the gas mixture

parallel to the axis of tube it is reflected back and

forth by the mirror ends until it stimulates an

excited Ne atom and causes it to emit a photon of

632nm with the stimulating photon

The stimulated transition from (3s) level to (2p)

level is laser transition

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

Ruby LaserhellipFor a ruby laser a crystal of ruby is formed into

a cylinder The ruby laser is used as a pulsed

laser producing red light at 6943 Aring

4

Ruby crystal is surrounded by xenon tube Rubycrystal is fully silvered at one side and partiallysilvered at the other end

A strong beam of blue green light is made to fall upon crystal from xenon tube and this light is absorbedby the crystal

Because of this many electrons from ground state ornormal state are raised to the excited state or higherstate and electron falls to metastable state

During this transition photon is not emitted butexcess energy of the electrons absorbed in crystallattice

5

As electron drops to metastable state they remain

there for certain time ~ 10-6 sec

Thus the incident blue green light from tube

increases the number of electron in metastable

state and then the population inversion can be

achieved

If a light of different frequency is allowed to fall

on this material the electrons move back and

forth between silvered ends of the crystal

While moving through they get stimulated and

exiced electrons radiate energy

Thus radiation photon has the same frequency as

that of incident photon and is also in exactly same

phase

When the intensity of light beam is increased the

same process is repeated

Finally extremely intensified beam of light

energies from the semi silvered side of the crystal

This way it is possible to get extremely intensified

and coherent beam of light from the crystal This

beam is nothing but higher energetic beam ndash ie

LASER beam

Applications of Ruby LaserhellipRuby lasers have declined in use with thediscovery of better lasing media They are still usedin a number of applications where short pulses ofred light are required Holographys around theworld produce holographic portraits with rubylasers in sizes up to a meter squared

Many non-destructive testing labs use ruby lasersto create holograms of large objects such asaircraft tires to look for weaknesses in the lining

Ruby lasers were used extensively in tattoo andhair removal

Drawbacks of Ruby Laserhellipbull The laser requires high pumping power because

the laser transition terminates at the ground state

and more than half of ground state atoms must

be pumped to higher state to achieve population

inversion

bull The efficiency of ruby laser is very low because

only green component of the pumping light is

used while the rest of components are left unused

bull The laser output is not continues but occurs in

the form of pulses of microseconds duration

bull The defects due to crystalline imperfections are

also present in this laser

Gaseous Laser (He ndash Ne Laser)A helium - neon laser usually called a He-Ne

laser is a type of small gas laser He-Ne lasers

have many industrial and scientific uses and are

often used in laboratory demonstrations of optics

He-Ne laser is an atomic laser which employs a

four-level pumping scheme

The active medium is a mixture of 10 parts of

helium to 1 part of neon

Neon atoms are centers and have energy levels

suitable for laser transitions while helium atoms

help efficient excitation of neon atoms

The most common wavelength is 6328 Aring These

lasers produced powers in the range 05 to 50

mW in the red portion of the visible spectrum

They have long operating life of the order of

50000 hrs

6

Constructionhellip

It consists of a glass discharge tube of about

typically 30 cm long and 15 cm diameter

The tube is filled with a mixture of helium and

neon gases in the 101

Electrodes are provided in the tube to produce a

discharge in the gas

They are connected to a high voltage power

supply The tube is hermetically sealed with glass

windows oriented at Brewster angle to the tube

The cavity mirrors are arranged externally

WorkinghellipWhen the power is switched on a high voltage of

about 10 kV is applied across the gas

It is sufficient to ionize the gas

The electrons and ions are produced in the process

of discharge are accelerated toward the anode and

cathode respectively

The electron have a smaller mass they acquire a

higher velocity They transfer their kinetic energy to

helium atoms through inelastic collisions

The initial excitation effects only the helium atoms

They are in metastable state and cannot return in

ground state by the spontaneous emission

The excited helium atoms can return to the ground

state by transforming their energy to neon atoms

through collision This transformation take place when

two colliding atoms have initial energy state It is

called resonant transfer of energy

So the pumping mechanism of He-Ne Laser is when

the helium atom in the metastable state collides with

neon atom in the ground state the neon atom is excited

and the helium atom drops back to the ground state

The role of helium atom is thus to excite neon atom

and cause population inversion The probability of

energy transfer from helium atoms to neon atoms is

more as there are 10 atoms of helium per 1 neon atom

in gas mixture

Without the Brewster windows the light output

is unpolarized because of it laser output to be

linearly polarized

7

When the excited Ne atom passes from metastable

state (3s) to lower level (2p) it emits photon of

wavelength 632 nm

This photon travels through the gas mixture

parallel to the axis of tube it is reflected back and

forth by the mirror ends until it stimulates an

excited Ne atom and causes it to emit a photon of

632nm with the stimulating photon

The stimulated transition from (3s) level to (2p)

level is laser transition

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

Ruby crystal is surrounded by xenon tube Rubycrystal is fully silvered at one side and partiallysilvered at the other end

A strong beam of blue green light is made to fall upon crystal from xenon tube and this light is absorbedby the crystal

Because of this many electrons from ground state ornormal state are raised to the excited state or higherstate and electron falls to metastable state

During this transition photon is not emitted butexcess energy of the electrons absorbed in crystallattice

5

As electron drops to metastable state they remain

there for certain time ~ 10-6 sec

Thus the incident blue green light from tube

increases the number of electron in metastable

state and then the population inversion can be

achieved

If a light of different frequency is allowed to fall

on this material the electrons move back and

forth between silvered ends of the crystal

While moving through they get stimulated and

exiced electrons radiate energy

Thus radiation photon has the same frequency as

that of incident photon and is also in exactly same

phase

When the intensity of light beam is increased the

same process is repeated

Finally extremely intensified beam of light

energies from the semi silvered side of the crystal

This way it is possible to get extremely intensified

and coherent beam of light from the crystal This

beam is nothing but higher energetic beam ndash ie

LASER beam

Applications of Ruby LaserhellipRuby lasers have declined in use with thediscovery of better lasing media They are still usedin a number of applications where short pulses ofred light are required Holographys around theworld produce holographic portraits with rubylasers in sizes up to a meter squared

Many non-destructive testing labs use ruby lasersto create holograms of large objects such asaircraft tires to look for weaknesses in the lining

Ruby lasers were used extensively in tattoo andhair removal

Drawbacks of Ruby Laserhellipbull The laser requires high pumping power because

the laser transition terminates at the ground state

and more than half of ground state atoms must

be pumped to higher state to achieve population

inversion

bull The efficiency of ruby laser is very low because

only green component of the pumping light is

used while the rest of components are left unused

bull The laser output is not continues but occurs in

the form of pulses of microseconds duration

bull The defects due to crystalline imperfections are

also present in this laser

Gaseous Laser (He ndash Ne Laser)A helium - neon laser usually called a He-Ne

laser is a type of small gas laser He-Ne lasers

have many industrial and scientific uses and are

often used in laboratory demonstrations of optics

He-Ne laser is an atomic laser which employs a

four-level pumping scheme

The active medium is a mixture of 10 parts of

helium to 1 part of neon

Neon atoms are centers and have energy levels

suitable for laser transitions while helium atoms

help efficient excitation of neon atoms

The most common wavelength is 6328 Aring These

lasers produced powers in the range 05 to 50

mW in the red portion of the visible spectrum

They have long operating life of the order of

50000 hrs

6

Constructionhellip

It consists of a glass discharge tube of about

typically 30 cm long and 15 cm diameter

The tube is filled with a mixture of helium and

neon gases in the 101

Electrodes are provided in the tube to produce a

discharge in the gas

They are connected to a high voltage power

supply The tube is hermetically sealed with glass

windows oriented at Brewster angle to the tube

The cavity mirrors are arranged externally

WorkinghellipWhen the power is switched on a high voltage of

about 10 kV is applied across the gas

It is sufficient to ionize the gas

The electrons and ions are produced in the process

of discharge are accelerated toward the anode and

cathode respectively

The electron have a smaller mass they acquire a

higher velocity They transfer their kinetic energy to

helium atoms through inelastic collisions

The initial excitation effects only the helium atoms

They are in metastable state and cannot return in

ground state by the spontaneous emission

The excited helium atoms can return to the ground

state by transforming their energy to neon atoms

through collision This transformation take place when

two colliding atoms have initial energy state It is

called resonant transfer of energy

So the pumping mechanism of He-Ne Laser is when

the helium atom in the metastable state collides with

neon atom in the ground state the neon atom is excited

and the helium atom drops back to the ground state

The role of helium atom is thus to excite neon atom

and cause population inversion The probability of

energy transfer from helium atoms to neon atoms is

more as there are 10 atoms of helium per 1 neon atom

in gas mixture

Without the Brewster windows the light output

is unpolarized because of it laser output to be

linearly polarized

7

When the excited Ne atom passes from metastable

state (3s) to lower level (2p) it emits photon of

wavelength 632 nm

This photon travels through the gas mixture

parallel to the axis of tube it is reflected back and

forth by the mirror ends until it stimulates an

excited Ne atom and causes it to emit a photon of

632nm with the stimulating photon

The stimulated transition from (3s) level to (2p)

level is laser transition

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

5

As electron drops to metastable state they remain

there for certain time ~ 10-6 sec

Thus the incident blue green light from tube

increases the number of electron in metastable

state and then the population inversion can be

achieved

If a light of different frequency is allowed to fall

on this material the electrons move back and

forth between silvered ends of the crystal

While moving through they get stimulated and

exiced electrons radiate energy

Thus radiation photon has the same frequency as

that of incident photon and is also in exactly same

phase

When the intensity of light beam is increased the

same process is repeated

Finally extremely intensified beam of light

energies from the semi silvered side of the crystal

This way it is possible to get extremely intensified

and coherent beam of light from the crystal This

beam is nothing but higher energetic beam ndash ie

LASER beam

Applications of Ruby LaserhellipRuby lasers have declined in use with thediscovery of better lasing media They are still usedin a number of applications where short pulses ofred light are required Holographys around theworld produce holographic portraits with rubylasers in sizes up to a meter squared

Many non-destructive testing labs use ruby lasersto create holograms of large objects such asaircraft tires to look for weaknesses in the lining

Ruby lasers were used extensively in tattoo andhair removal

Drawbacks of Ruby Laserhellipbull The laser requires high pumping power because

the laser transition terminates at the ground state

and more than half of ground state atoms must

be pumped to higher state to achieve population

inversion

bull The efficiency of ruby laser is very low because

only green component of the pumping light is

used while the rest of components are left unused

bull The laser output is not continues but occurs in

the form of pulses of microseconds duration

bull The defects due to crystalline imperfections are

also present in this laser

Gaseous Laser (He ndash Ne Laser)A helium - neon laser usually called a He-Ne

laser is a type of small gas laser He-Ne lasers

have many industrial and scientific uses and are

often used in laboratory demonstrations of optics

He-Ne laser is an atomic laser which employs a

four-level pumping scheme

The active medium is a mixture of 10 parts of

helium to 1 part of neon

Neon atoms are centers and have energy levels

suitable for laser transitions while helium atoms

help efficient excitation of neon atoms

The most common wavelength is 6328 Aring These

lasers produced powers in the range 05 to 50

mW in the red portion of the visible spectrum

They have long operating life of the order of

50000 hrs

6

Constructionhellip

It consists of a glass discharge tube of about

typically 30 cm long and 15 cm diameter

The tube is filled with a mixture of helium and

neon gases in the 101

Electrodes are provided in the tube to produce a

discharge in the gas

They are connected to a high voltage power

supply The tube is hermetically sealed with glass

windows oriented at Brewster angle to the tube

The cavity mirrors are arranged externally

WorkinghellipWhen the power is switched on a high voltage of

about 10 kV is applied across the gas

It is sufficient to ionize the gas

The electrons and ions are produced in the process

of discharge are accelerated toward the anode and

cathode respectively

The electron have a smaller mass they acquire a

higher velocity They transfer their kinetic energy to

helium atoms through inelastic collisions

The initial excitation effects only the helium atoms

They are in metastable state and cannot return in

ground state by the spontaneous emission

The excited helium atoms can return to the ground

state by transforming their energy to neon atoms

through collision This transformation take place when

two colliding atoms have initial energy state It is

called resonant transfer of energy

So the pumping mechanism of He-Ne Laser is when

the helium atom in the metastable state collides with

neon atom in the ground state the neon atom is excited

and the helium atom drops back to the ground state

The role of helium atom is thus to excite neon atom

and cause population inversion The probability of

energy transfer from helium atoms to neon atoms is

more as there are 10 atoms of helium per 1 neon atom

in gas mixture

Without the Brewster windows the light output

is unpolarized because of it laser output to be

linearly polarized

7

When the excited Ne atom passes from metastable

state (3s) to lower level (2p) it emits photon of

wavelength 632 nm

This photon travels through the gas mixture

parallel to the axis of tube it is reflected back and

forth by the mirror ends until it stimulates an

excited Ne atom and causes it to emit a photon of

632nm with the stimulating photon

The stimulated transition from (3s) level to (2p)

level is laser transition

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

As electron drops to metastable state they remain

there for certain time ~ 10-6 sec

Thus the incident blue green light from tube

increases the number of electron in metastable

state and then the population inversion can be

achieved

If a light of different frequency is allowed to fall

on this material the electrons move back and

forth between silvered ends of the crystal

While moving through they get stimulated and

exiced electrons radiate energy

Thus radiation photon has the same frequency as

that of incident photon and is also in exactly same

phase

When the intensity of light beam is increased the

same process is repeated

Finally extremely intensified beam of light

energies from the semi silvered side of the crystal

This way it is possible to get extremely intensified

and coherent beam of light from the crystal This

beam is nothing but higher energetic beam ndash ie

LASER beam

Applications of Ruby LaserhellipRuby lasers have declined in use with thediscovery of better lasing media They are still usedin a number of applications where short pulses ofred light are required Holographys around theworld produce holographic portraits with rubylasers in sizes up to a meter squared

Many non-destructive testing labs use ruby lasersto create holograms of large objects such asaircraft tires to look for weaknesses in the lining

Ruby lasers were used extensively in tattoo andhair removal

Drawbacks of Ruby Laserhellipbull The laser requires high pumping power because

the laser transition terminates at the ground state

and more than half of ground state atoms must

be pumped to higher state to achieve population

inversion

bull The efficiency of ruby laser is very low because

only green component of the pumping light is

used while the rest of components are left unused

bull The laser output is not continues but occurs in

the form of pulses of microseconds duration

bull The defects due to crystalline imperfections are

also present in this laser

Gaseous Laser (He ndash Ne Laser)A helium - neon laser usually called a He-Ne

laser is a type of small gas laser He-Ne lasers

have many industrial and scientific uses and are

often used in laboratory demonstrations of optics

He-Ne laser is an atomic laser which employs a

four-level pumping scheme

The active medium is a mixture of 10 parts of

helium to 1 part of neon

Neon atoms are centers and have energy levels

suitable for laser transitions while helium atoms

help efficient excitation of neon atoms

The most common wavelength is 6328 Aring These

lasers produced powers in the range 05 to 50

mW in the red portion of the visible spectrum

They have long operating life of the order of

50000 hrs

6

Constructionhellip

It consists of a glass discharge tube of about

typically 30 cm long and 15 cm diameter

The tube is filled with a mixture of helium and

neon gases in the 101

Electrodes are provided in the tube to produce a

discharge in the gas

They are connected to a high voltage power

supply The tube is hermetically sealed with glass

windows oriented at Brewster angle to the tube

The cavity mirrors are arranged externally

WorkinghellipWhen the power is switched on a high voltage of

about 10 kV is applied across the gas

It is sufficient to ionize the gas

The electrons and ions are produced in the process

of discharge are accelerated toward the anode and

cathode respectively

The electron have a smaller mass they acquire a

higher velocity They transfer their kinetic energy to

helium atoms through inelastic collisions

The initial excitation effects only the helium atoms

They are in metastable state and cannot return in

ground state by the spontaneous emission

The excited helium atoms can return to the ground

state by transforming their energy to neon atoms

through collision This transformation take place when

two colliding atoms have initial energy state It is

called resonant transfer of energy

So the pumping mechanism of He-Ne Laser is when

the helium atom in the metastable state collides with

neon atom in the ground state the neon atom is excited

and the helium atom drops back to the ground state

The role of helium atom is thus to excite neon atom

and cause population inversion The probability of

energy transfer from helium atoms to neon atoms is

more as there are 10 atoms of helium per 1 neon atom

in gas mixture

Without the Brewster windows the light output

is unpolarized because of it laser output to be

linearly polarized

7

When the excited Ne atom passes from metastable

state (3s) to lower level (2p) it emits photon of

wavelength 632 nm

This photon travels through the gas mixture

parallel to the axis of tube it is reflected back and

forth by the mirror ends until it stimulates an

excited Ne atom and causes it to emit a photon of

632nm with the stimulating photon

The stimulated transition from (3s) level to (2p)

level is laser transition

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

Thus radiation photon has the same frequency as

that of incident photon and is also in exactly same

phase

When the intensity of light beam is increased the

same process is repeated

Finally extremely intensified beam of light

energies from the semi silvered side of the crystal

This way it is possible to get extremely intensified

and coherent beam of light from the crystal This

beam is nothing but higher energetic beam ndash ie

LASER beam

Applications of Ruby LaserhellipRuby lasers have declined in use with thediscovery of better lasing media They are still usedin a number of applications where short pulses ofred light are required Holographys around theworld produce holographic portraits with rubylasers in sizes up to a meter squared

Many non-destructive testing labs use ruby lasersto create holograms of large objects such asaircraft tires to look for weaknesses in the lining

Ruby lasers were used extensively in tattoo andhair removal

Drawbacks of Ruby Laserhellipbull The laser requires high pumping power because

the laser transition terminates at the ground state

and more than half of ground state atoms must

be pumped to higher state to achieve population

inversion

bull The efficiency of ruby laser is very low because

only green component of the pumping light is

used while the rest of components are left unused

bull The laser output is not continues but occurs in

the form of pulses of microseconds duration

bull The defects due to crystalline imperfections are

also present in this laser

Gaseous Laser (He ndash Ne Laser)A helium - neon laser usually called a He-Ne

laser is a type of small gas laser He-Ne lasers

have many industrial and scientific uses and are

often used in laboratory demonstrations of optics

He-Ne laser is an atomic laser which employs a

four-level pumping scheme

The active medium is a mixture of 10 parts of

helium to 1 part of neon

Neon atoms are centers and have energy levels

suitable for laser transitions while helium atoms

help efficient excitation of neon atoms

The most common wavelength is 6328 Aring These

lasers produced powers in the range 05 to 50

mW in the red portion of the visible spectrum

They have long operating life of the order of

50000 hrs

6

Constructionhellip

It consists of a glass discharge tube of about

typically 30 cm long and 15 cm diameter

The tube is filled with a mixture of helium and

neon gases in the 101

Electrodes are provided in the tube to produce a

discharge in the gas

They are connected to a high voltage power

supply The tube is hermetically sealed with glass

windows oriented at Brewster angle to the tube

The cavity mirrors are arranged externally

WorkinghellipWhen the power is switched on a high voltage of

about 10 kV is applied across the gas

It is sufficient to ionize the gas

The electrons and ions are produced in the process

of discharge are accelerated toward the anode and

cathode respectively

The electron have a smaller mass they acquire a

higher velocity They transfer their kinetic energy to

helium atoms through inelastic collisions

The initial excitation effects only the helium atoms

They are in metastable state and cannot return in

ground state by the spontaneous emission

The excited helium atoms can return to the ground

state by transforming their energy to neon atoms

through collision This transformation take place when

two colliding atoms have initial energy state It is

called resonant transfer of energy

So the pumping mechanism of He-Ne Laser is when

the helium atom in the metastable state collides with

neon atom in the ground state the neon atom is excited

and the helium atom drops back to the ground state

The role of helium atom is thus to excite neon atom

and cause population inversion The probability of

energy transfer from helium atoms to neon atoms is

more as there are 10 atoms of helium per 1 neon atom

in gas mixture

Without the Brewster windows the light output

is unpolarized because of it laser output to be

linearly polarized

7

When the excited Ne atom passes from metastable

state (3s) to lower level (2p) it emits photon of

wavelength 632 nm

This photon travels through the gas mixture

parallel to the axis of tube it is reflected back and

forth by the mirror ends until it stimulates an

excited Ne atom and causes it to emit a photon of

632nm with the stimulating photon

The stimulated transition from (3s) level to (2p)

level is laser transition

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

Applications of Ruby LaserhellipRuby lasers have declined in use with thediscovery of better lasing media They are still usedin a number of applications where short pulses ofred light are required Holographys around theworld produce holographic portraits with rubylasers in sizes up to a meter squared

Many non-destructive testing labs use ruby lasersto create holograms of large objects such asaircraft tires to look for weaknesses in the lining

Ruby lasers were used extensively in tattoo andhair removal

Drawbacks of Ruby Laserhellipbull The laser requires high pumping power because

the laser transition terminates at the ground state

and more than half of ground state atoms must

be pumped to higher state to achieve population

inversion

bull The efficiency of ruby laser is very low because

only green component of the pumping light is

used while the rest of components are left unused

bull The laser output is not continues but occurs in

the form of pulses of microseconds duration

bull The defects due to crystalline imperfections are

also present in this laser

Gaseous Laser (He ndash Ne Laser)A helium - neon laser usually called a He-Ne

laser is a type of small gas laser He-Ne lasers

have many industrial and scientific uses and are

often used in laboratory demonstrations of optics

He-Ne laser is an atomic laser which employs a

four-level pumping scheme

The active medium is a mixture of 10 parts of

helium to 1 part of neon

Neon atoms are centers and have energy levels

suitable for laser transitions while helium atoms

help efficient excitation of neon atoms

The most common wavelength is 6328 Aring These

lasers produced powers in the range 05 to 50

mW in the red portion of the visible spectrum

They have long operating life of the order of

50000 hrs

6

Constructionhellip

It consists of a glass discharge tube of about

typically 30 cm long and 15 cm diameter

The tube is filled with a mixture of helium and

neon gases in the 101

Electrodes are provided in the tube to produce a

discharge in the gas

They are connected to a high voltage power

supply The tube is hermetically sealed with glass

windows oriented at Brewster angle to the tube

The cavity mirrors are arranged externally

WorkinghellipWhen the power is switched on a high voltage of

about 10 kV is applied across the gas

It is sufficient to ionize the gas

The electrons and ions are produced in the process

of discharge are accelerated toward the anode and

cathode respectively

The electron have a smaller mass they acquire a

higher velocity They transfer their kinetic energy to

helium atoms through inelastic collisions

The initial excitation effects only the helium atoms

They are in metastable state and cannot return in

ground state by the spontaneous emission

The excited helium atoms can return to the ground

state by transforming their energy to neon atoms

through collision This transformation take place when

two colliding atoms have initial energy state It is

called resonant transfer of energy

So the pumping mechanism of He-Ne Laser is when

the helium atom in the metastable state collides with

neon atom in the ground state the neon atom is excited

and the helium atom drops back to the ground state

The role of helium atom is thus to excite neon atom

and cause population inversion The probability of

energy transfer from helium atoms to neon atoms is

more as there are 10 atoms of helium per 1 neon atom

in gas mixture

Without the Brewster windows the light output

is unpolarized because of it laser output to be

linearly polarized

7

When the excited Ne atom passes from metastable

state (3s) to lower level (2p) it emits photon of

wavelength 632 nm

This photon travels through the gas mixture

parallel to the axis of tube it is reflected back and

forth by the mirror ends until it stimulates an

excited Ne atom and causes it to emit a photon of

632nm with the stimulating photon

The stimulated transition from (3s) level to (2p)

level is laser transition

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

Drawbacks of Ruby Laserhellipbull The laser requires high pumping power because

the laser transition terminates at the ground state

and more than half of ground state atoms must

be pumped to higher state to achieve population

inversion

bull The efficiency of ruby laser is very low because

only green component of the pumping light is

used while the rest of components are left unused

bull The laser output is not continues but occurs in

the form of pulses of microseconds duration

bull The defects due to crystalline imperfections are

also present in this laser

Gaseous Laser (He ndash Ne Laser)A helium - neon laser usually called a He-Ne

laser is a type of small gas laser He-Ne lasers

have many industrial and scientific uses and are

often used in laboratory demonstrations of optics

He-Ne laser is an atomic laser which employs a

four-level pumping scheme

The active medium is a mixture of 10 parts of

helium to 1 part of neon

Neon atoms are centers and have energy levels

suitable for laser transitions while helium atoms

help efficient excitation of neon atoms

The most common wavelength is 6328 Aring These

lasers produced powers in the range 05 to 50

mW in the red portion of the visible spectrum

They have long operating life of the order of

50000 hrs

6

Constructionhellip

It consists of a glass discharge tube of about

typically 30 cm long and 15 cm diameter

The tube is filled with a mixture of helium and

neon gases in the 101

Electrodes are provided in the tube to produce a

discharge in the gas

They are connected to a high voltage power

supply The tube is hermetically sealed with glass

windows oriented at Brewster angle to the tube

The cavity mirrors are arranged externally

WorkinghellipWhen the power is switched on a high voltage of

about 10 kV is applied across the gas

It is sufficient to ionize the gas

The electrons and ions are produced in the process

of discharge are accelerated toward the anode and

cathode respectively

The electron have a smaller mass they acquire a

higher velocity They transfer their kinetic energy to

helium atoms through inelastic collisions

The initial excitation effects only the helium atoms

They are in metastable state and cannot return in

ground state by the spontaneous emission

The excited helium atoms can return to the ground

state by transforming their energy to neon atoms

through collision This transformation take place when

two colliding atoms have initial energy state It is

called resonant transfer of energy

So the pumping mechanism of He-Ne Laser is when

the helium atom in the metastable state collides with

neon atom in the ground state the neon atom is excited

and the helium atom drops back to the ground state

The role of helium atom is thus to excite neon atom

and cause population inversion The probability of

energy transfer from helium atoms to neon atoms is

more as there are 10 atoms of helium per 1 neon atom

in gas mixture

Without the Brewster windows the light output

is unpolarized because of it laser output to be

linearly polarized

7

When the excited Ne atom passes from metastable

state (3s) to lower level (2p) it emits photon of

wavelength 632 nm

This photon travels through the gas mixture

parallel to the axis of tube it is reflected back and

forth by the mirror ends until it stimulates an

excited Ne atom and causes it to emit a photon of

632nm with the stimulating photon

The stimulated transition from (3s) level to (2p)

level is laser transition

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

Gaseous Laser (He ndash Ne Laser)A helium - neon laser usually called a He-Ne

laser is a type of small gas laser He-Ne lasers

have many industrial and scientific uses and are

often used in laboratory demonstrations of optics

He-Ne laser is an atomic laser which employs a

four-level pumping scheme

The active medium is a mixture of 10 parts of

helium to 1 part of neon

Neon atoms are centers and have energy levels

suitable for laser transitions while helium atoms

help efficient excitation of neon atoms

The most common wavelength is 6328 Aring These

lasers produced powers in the range 05 to 50

mW in the red portion of the visible spectrum

They have long operating life of the order of

50000 hrs

6

Constructionhellip

It consists of a glass discharge tube of about

typically 30 cm long and 15 cm diameter

The tube is filled with a mixture of helium and

neon gases in the 101

Electrodes are provided in the tube to produce a

discharge in the gas

They are connected to a high voltage power

supply The tube is hermetically sealed with glass

windows oriented at Brewster angle to the tube

The cavity mirrors are arranged externally

WorkinghellipWhen the power is switched on a high voltage of

about 10 kV is applied across the gas

It is sufficient to ionize the gas

The electrons and ions are produced in the process

of discharge are accelerated toward the anode and

cathode respectively

The electron have a smaller mass they acquire a

higher velocity They transfer their kinetic energy to

helium atoms through inelastic collisions

The initial excitation effects only the helium atoms

They are in metastable state and cannot return in

ground state by the spontaneous emission

The excited helium atoms can return to the ground

state by transforming their energy to neon atoms

through collision This transformation take place when

two colliding atoms have initial energy state It is

called resonant transfer of energy

So the pumping mechanism of He-Ne Laser is when

the helium atom in the metastable state collides with

neon atom in the ground state the neon atom is excited

and the helium atom drops back to the ground state

The role of helium atom is thus to excite neon atom

and cause population inversion The probability of

energy transfer from helium atoms to neon atoms is

more as there are 10 atoms of helium per 1 neon atom

in gas mixture

Without the Brewster windows the light output

is unpolarized because of it laser output to be

linearly polarized

7

When the excited Ne atom passes from metastable

state (3s) to lower level (2p) it emits photon of

wavelength 632 nm

This photon travels through the gas mixture

parallel to the axis of tube it is reflected back and

forth by the mirror ends until it stimulates an

excited Ne atom and causes it to emit a photon of

632nm with the stimulating photon

The stimulated transition from (3s) level to (2p)

level is laser transition

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

The most common wavelength is 6328 Aring These

lasers produced powers in the range 05 to 50

mW in the red portion of the visible spectrum

They have long operating life of the order of

50000 hrs

6

Constructionhellip

It consists of a glass discharge tube of about

typically 30 cm long and 15 cm diameter

The tube is filled with a mixture of helium and

neon gases in the 101

Electrodes are provided in the tube to produce a

discharge in the gas

They are connected to a high voltage power

supply The tube is hermetically sealed with glass

windows oriented at Brewster angle to the tube

The cavity mirrors are arranged externally

WorkinghellipWhen the power is switched on a high voltage of

about 10 kV is applied across the gas

It is sufficient to ionize the gas

The electrons and ions are produced in the process

of discharge are accelerated toward the anode and

cathode respectively

The electron have a smaller mass they acquire a

higher velocity They transfer their kinetic energy to

helium atoms through inelastic collisions

The initial excitation effects only the helium atoms

They are in metastable state and cannot return in

ground state by the spontaneous emission

The excited helium atoms can return to the ground

state by transforming their energy to neon atoms

through collision This transformation take place when

two colliding atoms have initial energy state It is

called resonant transfer of energy

So the pumping mechanism of He-Ne Laser is when

the helium atom in the metastable state collides with

neon atom in the ground state the neon atom is excited

and the helium atom drops back to the ground state

The role of helium atom is thus to excite neon atom

and cause population inversion The probability of

energy transfer from helium atoms to neon atoms is

more as there are 10 atoms of helium per 1 neon atom

in gas mixture

Without the Brewster windows the light output

is unpolarized because of it laser output to be

linearly polarized

7

When the excited Ne atom passes from metastable

state (3s) to lower level (2p) it emits photon of

wavelength 632 nm

This photon travels through the gas mixture

parallel to the axis of tube it is reflected back and

forth by the mirror ends until it stimulates an

excited Ne atom and causes it to emit a photon of

632nm with the stimulating photon

The stimulated transition from (3s) level to (2p)

level is laser transition

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

Constructionhellip

It consists of a glass discharge tube of about

typically 30 cm long and 15 cm diameter

The tube is filled with a mixture of helium and

neon gases in the 101

Electrodes are provided in the tube to produce a

discharge in the gas

They are connected to a high voltage power

supply The tube is hermetically sealed with glass

windows oriented at Brewster angle to the tube

The cavity mirrors are arranged externally

WorkinghellipWhen the power is switched on a high voltage of

about 10 kV is applied across the gas

It is sufficient to ionize the gas

The electrons and ions are produced in the process

of discharge are accelerated toward the anode and

cathode respectively

The electron have a smaller mass they acquire a

higher velocity They transfer their kinetic energy to

helium atoms through inelastic collisions

The initial excitation effects only the helium atoms

They are in metastable state and cannot return in

ground state by the spontaneous emission

The excited helium atoms can return to the ground

state by transforming their energy to neon atoms

through collision This transformation take place when

two colliding atoms have initial energy state It is

called resonant transfer of energy

So the pumping mechanism of He-Ne Laser is when

the helium atom in the metastable state collides with

neon atom in the ground state the neon atom is excited

and the helium atom drops back to the ground state

The role of helium atom is thus to excite neon atom

and cause population inversion The probability of

energy transfer from helium atoms to neon atoms is

more as there are 10 atoms of helium per 1 neon atom

in gas mixture

Without the Brewster windows the light output

is unpolarized because of it laser output to be

linearly polarized

7

When the excited Ne atom passes from metastable

state (3s) to lower level (2p) it emits photon of

wavelength 632 nm

This photon travels through the gas mixture

parallel to the axis of tube it is reflected back and

forth by the mirror ends until it stimulates an

excited Ne atom and causes it to emit a photon of

632nm with the stimulating photon

The stimulated transition from (3s) level to (2p)

level is laser transition

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

WorkinghellipWhen the power is switched on a high voltage of

about 10 kV is applied across the gas

It is sufficient to ionize the gas

The electrons and ions are produced in the process

of discharge are accelerated toward the anode and

cathode respectively

The electron have a smaller mass they acquire a

higher velocity They transfer their kinetic energy to

helium atoms through inelastic collisions

The initial excitation effects only the helium atoms

They are in metastable state and cannot return in

ground state by the spontaneous emission

The excited helium atoms can return to the ground

state by transforming their energy to neon atoms

through collision This transformation take place when

two colliding atoms have initial energy state It is

called resonant transfer of energy

So the pumping mechanism of He-Ne Laser is when

the helium atom in the metastable state collides with

neon atom in the ground state the neon atom is excited

and the helium atom drops back to the ground state

The role of helium atom is thus to excite neon atom

and cause population inversion The probability of

energy transfer from helium atoms to neon atoms is

more as there are 10 atoms of helium per 1 neon atom

in gas mixture

Without the Brewster windows the light output

is unpolarized because of it laser output to be

linearly polarized

7

When the excited Ne atom passes from metastable

state (3s) to lower level (2p) it emits photon of

wavelength 632 nm

This photon travels through the gas mixture

parallel to the axis of tube it is reflected back and

forth by the mirror ends until it stimulates an

excited Ne atom and causes it to emit a photon of

632nm with the stimulating photon

The stimulated transition from (3s) level to (2p)

level is laser transition

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

The excited helium atoms can return to the ground

state by transforming their energy to neon atoms

through collision This transformation take place when

two colliding atoms have initial energy state It is

called resonant transfer of energy

So the pumping mechanism of He-Ne Laser is when

the helium atom in the metastable state collides with

neon atom in the ground state the neon atom is excited

and the helium atom drops back to the ground state

The role of helium atom is thus to excite neon atom

and cause population inversion The probability of

energy transfer from helium atoms to neon atoms is

more as there are 10 atoms of helium per 1 neon atom

in gas mixture

Without the Brewster windows the light output

is unpolarized because of it laser output to be

linearly polarized

7

When the excited Ne atom passes from metastable

state (3s) to lower level (2p) it emits photon of

wavelength 632 nm

This photon travels through the gas mixture

parallel to the axis of tube it is reflected back and

forth by the mirror ends until it stimulates an

excited Ne atom and causes it to emit a photon of

632nm with the stimulating photon

The stimulated transition from (3s) level to (2p)

level is laser transition

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

Without the Brewster windows the light output

is unpolarized because of it laser output to be

linearly polarized

7

When the excited Ne atom passes from metastable

state (3s) to lower level (2p) it emits photon of

wavelength 632 nm

This photon travels through the gas mixture

parallel to the axis of tube it is reflected back and

forth by the mirror ends until it stimulates an

excited Ne atom and causes it to emit a photon of

632nm with the stimulating photon

The stimulated transition from (3s) level to (2p)

level is laser transition

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

When the excited Ne atom passes from metastable

state (3s) to lower level (2p) it emits photon of

wavelength 632 nm

This photon travels through the gas mixture

parallel to the axis of tube it is reflected back and

forth by the mirror ends until it stimulates an

excited Ne atom and causes it to emit a photon of

632nm with the stimulating photon

The stimulated transition from (3s) level to (2p)

level is laser transition

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

Although 6328 Aring is standard wavelength of He-

Ne Laser other visible wavelengths 5430 Aring

(Green) 5940 Aring (yellow-orange) 6120 Aring (red-

orange) can also produced

Overall gain is very low and is typically about

0010 to 01

The laser is simple practical and less expensive

The Laser beam is highly collimated coherent

and monochromatic

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

Applications of He-Ne Laserhellip

The Narrow red beam of He-Ne laser is used in

supermarkets to read bar codes

The He-Ne Laser is used in Holography in

producing the 3D images of objects

He-Ne lasers have many industrial and scientific

uses and are often used in laboratory

demonstrations of optics

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

Semiconductor Laser (Diode Laser)

bull A semiconductor laser is a laser in which asemiconductor serves as a photon source

bull The most common semiconductor material thathas been used in lasers is gallium arsenide

bull Einsteinrsquos Photoelectric theory states that lightshould be understood as discrete lumps of energy(photons) and it takes only a single photon withhigh enough energy to knock an electron loosefrom the atom its bound to

bull Stimulated organized photon emission occurswhen two electrons with the same energy andphase meet The two photons leave with thesame frequency and direction

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

P type Semiconductors

bull In the compound GaAs each Ga atom has threeelectrons in its outermost shell of electrons andeach As atom has five

bull When a trace of an impurity element with twoouter electrons such as Zn (zinc) is added to thecrystal

bull The result is the shortage of one electron fromone of the pairs causing an imbalance in whichthere is a ldquoholerdquo for an electron but there is noelectron available

bull This forms a p-type semiconductor

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

N type Semiconductorsbull When a trace of an impurity element with six

outer electrons such as Se (selenium) is added

to a crystal of GaAs it provides on additional

electron which is not needed for the bonding

bull This electron can be free to move through the

crystal

bull Thus it provides a mechanism for electrical

conductivity

bull This type is called an n-type semiconductor

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

bull Under forward bias (the p-type side is made

positive) the majority carriers electrons in the n-

side holes in the p-side are injected across the

depletion region in both directions to create a

population inversion in a narrow active region

The light produced by radioactive recombination

across the band gap is confined in this active

region

8

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

Application of Lasershellip

Laser beam is used to measure distances of sun

moon stars and satellites very accurately

It can be used for measuring velocity of light to

study spectrum of matters to study Raman

effect

It can be is used for increasing speed and

efficiency of computer

It is used for welding

It is used in biomedical science

It is used in 3D photography

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

Application of Lasershellip It is used for communication T V transmission

to search the objects under sea

It can be used to predict earthquake

Laser tools are used in surgery

It is used for detection and treatment of

cancer

It is used to aline straight line for construction

of dam tunnels etc

It is used in holography

It is used in fiber optic communication

It is also used in military like LIDAR

It is used to accelerate some chemical reactions

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png

Image references links

1 https6postimgorg5mb8uomn1New_Picturepng

2 https6postimgorg3t8c6d1gdNew_Picture_1png

3 https6postimgorg72mrctpjxNew_Picture_2png

4 https6postimgorgj8gk0ytnhNew_Picture_3png

5 https6postimgorgo8e08wza5New_Picture_4png

6 https6postimgorgpby4kvjx9New_Picture_5png

7 https6postimgorgnf1egat99New_Picture_6png

8 https6postimgorgjjy0dqa3hNew_Picture_7png