laser,optical fibres and ultrasonics

1. DEFINITION OF LASER

A laser is a device that generates light by aprocess called STIMULATED EMISSION.

The acronym LASER stands for LightAmplification by Stimulated Emission ofRadiation

Semiconducting lasers are multilayersemiconductor devices that generates acoherent beam of monochromatic light bylaser action. A coherent beam resultedwhich all of the photons are in phase.

3 MECHANISMS OF LIGHT EMISSION

1. Absorption

2. Spontaneous Emission

3. Stimulated Emission

Therefore 3 process of

light emission:

BEFORE AFTER

(i) Stimulated absorption

Spontaneous emission)

Stimulated emission

)II) SPONTANEOUS EMISSION

Consider an atom (or molecule) of the material is existed

initially in an excited state E2 No external radiation is

required to initiate the emission. Since E2>E1, the atom will

tend to spontaneously decay to the ground state E1, a

photon of energy h =E2-E1 is released in a random direction

as shown in (Fig. 1-ii). This process is called “spontaneous

emission ”

Note that; when the release energy difference (E2-E1) is

delivered in the form of an e.m wave, the process called

"radiative emission" which is one of the two possible ways

“non-radiative” decay is occurred when the energy

difference (E2-E1) is delivered in some form other than e.m

radiation (e.g. it may transfer to kinetic energy of the

surrounding)

(III) STIMULATED EMISSION

Quite by contrast “stimulated emission” (Fig. 1-iii)

requires the presence of external radiation when an

incident photon of energy h =E2-E1 passes by an atom in

an excited state E2, it stimulates the atom to drop or

decay to the lower state E1. In this process, the atom

releases a photon of the same energy, direction, phase

and polarization as that of the photon passing by, the net

effect is two identical photons (2h) in the place of one,

or an increase in the intensity of the incident beam. It is

precisely this processes of stimulated emission that

makes possible the amplification of light in lasers.

ND (NEODYMIUM) – YAG (YTTRIUM ALUMINIUM GARNET)

LASERPRINCIPLE CHARACTERISTICS

Doped Insulator laser refers to yttrium

aluminium garnet doped with neodymium. The

Nd ion has many energy levels and due

to optical pumping these ions are raised to excited levels. During the transition from the metastable state to E1,

the laser beam of wavelength 1.064μm is

emitted

Type : Doped Insulator Laser

Active Medium : Yttrium Aluminium Garnet

Active Centre : Neodymium

Pumping

Method

: Optical Pumping

Pumping

Source

: Xenon Flash Pump

Optical

Resonator

: Ends of rods silver coated

Two mirrors partially and

totally reflecting

Power Output : 20 kWatts

Nature of

Output

: Pulsed

Wavelength

Emitted

: 1.064 μm

ND (NEODYMIUM) – YAG (YTTRIUM ALUMINIUM

GARNET) LASER

Power Supply

Capacitor

Resistor

Laser Rod

Flash Tube

M1– 100%

reflector mirrorM2 – partial

reflector mirror

E1, E2, E3 – ENERGY LEVELS OF ND

E4 – META STABLE STATE

E0 – GROUND STATE ENERGY LEVEL

APPLICATIONS

TRANSMISSION OF SIGNALS OVER LARGE DISTANCES

LONG HAUL COMMUNICATION SYSTEM

ENDOSCOPIC APPLICATIONS

REMAOTE SENSING

Energy Level Diagram of Nd– YAG LASER

Non radiative decay

Laser

1.064μm

Non radiative decay

E3

E2

E0

E1

E4

CARBON DI OXIDE LASERPRINCIPLE

THE TRANSITION BETWEEN THE ROTATIONAL AND VIBRATIONAL ENERGY LEVELS LENDS TO

THE CONSTRUCTION OF A MOLECULAR GAS LASER. NITROGEN ATOMS ARE RAISED TO

THE EXCITED STATE WHICH IN TURN DELIVER ENERGY TO THE CO2 ATOMS WHOSE

ENERGY LEVELS ARE CLOSE TO IT. TRANSITION TAKES PLACE BETWEEN THE ENERGY

LEVELS OF CO2 ATOMS AND THE LASER BEAM IS EMITTED.

Type : Molecular gas laser

Active Medium : Mixture of CO2, N2, He or H2O vapour

Active Centre : CO2

Pumping Method : Electric Discharge Method

Optical Resonator : Gold mirror or Si mirror coated with Al

Power Output : 10 kW

Nature of Output : Continuous or pulsed

Wavelength Emitted : 9.6 μm or 10.6 μm

FIBER OPTICS TECHNOLOGY

OPTICAL FIBER: ADVANTAGES

Capacity: much wider bandwidth(10 GHz)

Crosstalk immunity

Immunity to static interference

Lightening

Electric motor

Florescent light

Higher environment immunity

Weather, temperature, etc.

OPTICAL FIBER: ADVANTAGES

Safety: Fiber is non-metalic

No explosion, no chock

Longer lasting

Security: tapping is difficult

Economics: Fewer repeaters

Low transmission loss (dB/km)

Fewer repeaters

Less cable

Remember: Fiber is non-conductive

Hence, change of magnetic field has

No impact!

DISADVANTAGES

Higher initial cost in installation

Interfacing cost

Strength

Lower tensile strength

Remote electric power

More expensive to repair/maintain

Tools: Specialized and sophisticated

OPTICAL FIBER ARCHITECTURE

Transmitter

Input

Signal

Coder or

Converter

Light

Source

Source-to-Fiber

Interface

Fiber-to-light

Interface

Light

DetectorAmplifier/Shaper

Decoder

Output

Fiber-optic Cable

Receiver

TX, RX, and Fiber Link

OPTICAL FIBER ARCHITECTURE –

COMPONENTS

Light source:

Amount of light emitted is proportional to the drive current

Two common types:

LED (Light Emitting Diode)

ILD (Injection Laser Diode)

Source–to-fiber-coupler (similar to a lens):

A mechanical interface to couple the light emitted by the source into the optical fiber

Input

Signal

Coder or

Converter

Light

Source

Source-to-Fiber

Interface

Fiber-to-light

Interface

Light

DetectorAmplifier/Shaper

Decoder

Output

Fiber-optic Cable

Receiver

Light detector:

PIN (p-type-intrinsic-n-type)

APD (avalanche photo diode)

Both convert light energy into current

OPTICAL FIBER CONSTRUCTION

Core – thin glass center of the

fiber where light travels.

Cladding – outer optical

material surrounding the core

Buffer Coating – plastic

coating that protects

the fiber.

FIBER TYPES

Plastic core and cladding

Glass core with plastic cladding PCS (Plastic-Clad Silicon)

Glass core and glass cladding SCS: Silica-clad silica

Under research: non silicate: Zinc-chloride

1000 time as efficient as glass

Core Cladding

PLASTIC FIBER

Used for short distances

Higher attenuation, but easy to install

Better withstand stress

Less expensive

60% less weight

A LITTLE ABOUT LIGHT

When electrons are excited and

moved to a higher energy state they

absorb energy

When electrons are moved to a

lower energy state loose energy

emit light

photon of light is generated

Energy (joule) = h.f

Planck’s constant: h=6.625E-23

Joule.sec

f is the frequency

http://www.student.nada.kth.se/~f93-jhu/phys_sim/compton/Compton.htm

DE=h.f

REFRACTION

Refraction is the change in direction of a

wave due to a change in its speed

Refraction of light is the most commonly seen

example

Any type of wave can refract when it

interacts with a medium

Refraction is described by Snell's law, which

states that the angle of incidence is related to

the angle of refraction by :

The index of refraction is defined as the

speed of light in vacuum divided by the speed

of light in the medium: n=c/v

FIBER TYPES

Modes of operation (the path which the light is

traveling on)

Index profile

Step

Graded

TYPES OF OPTICAL FIBER

Single-mode step-index Fiber

Multimode step-index Fiber

Multimode graded-index Fiber

n1 core

n2 cladding

no air

n2 cladding

n1 core

Variable

n

no air

Light

ray

Index profile

SINGLE-MODE STEP-INDEX FIBER

Advantages: Minimum dispersion: all rays take same path, same time to travel

down the cable. A pulse can be reproduced at the receiver very

accurately.

Less attenuation, can run over longer distance without repeaters.

Larger bandwidth and higher information rate

Disadvantages: Difficult to couple light in and out of the tiny core

Highly directive light source (laser) is required

Interfacing modules are more expensive

MULTI MODE

Multimode step-index Fibers:

inexpensive

easy to couple light into Fiber

result in higher signal distortion

lower TX rate

Multimode graded-index Fiber:

intermediate between the other two types of Fibers

ACCEPTANCE CONE & NUMERICAL APERTURE

n2 cladding

n2 cladding

n1 core

Acceptance

Cone

Acceptance angle, qc, is the maximum angle in which

external light rays may strike the air/Fiber interface

and still propagate down the Fiber with <10 dB loss.

Note: n1 belongs to core and n2 refers to cladding)

2

2

2

1

1sin nnC q

qC

PH0101 UNIT 1 LECTURE 6 26

Introduction to Ultrasonics

The word ultrasonic combines the Latin roots ultra,meaning ‘beyond’ and sonic, or sound.

The sound waves having frequencies above the audiblerange i.e. above 20000Hz are called ultrasonic waves.

Generally these waves are called as high frequencywaves.

The field of ultrasonics have applications for imaging,detection and navigation.

The broad sectors of society that regularly apply ultrasonictechnology are the medical community, industry, themilitary and private citizens.


PROPERTIES OF ULTRASONIC WAVES

(1) They have a high energy content.

(2) Just like ordinary sound waves, ultrasonic waves

get reflected, refracted and absorbed.

(3) They can be transmitted over large distances

with no appreciable loss of energy.

(4) If an arrangement is made to form stationary waves ofultrasonics in a liquid, it serves as a diffraction grating. It is calledan acoustic grating.

(5) They produce intense heating effect when passed through asubstance.


ULTRASONICS PRODUCTIONS

Ultrasonic waves are produced by the

following methods.

(1) Magneto-striction generator or oscillator

(2) Piezo-electric generator or oscillator


MAGNETOAGNETOSTRICTION GENERATOR

Principle: Magnetostriction effectWhen a ferromagnetic rod like iron ornickel is placed in a magnetic fieldparallel to its length, the rodexperiences a small change in itslength.This is called magnetostricioneffect.


The change in length (increase or decrease) produced in the

rod depends upon the strength of the magnetic field, the

nature of the materials and is independent of the direction of

the magnetic field applied.

PH

01

01 U

NIT

1 L

EC

TU

RE

6

31

CONSTRUCTION

The experimental arrangement is shown in Figure

Magnetostriction oscillator


XY is a rod of ferromagnetic materials like iron or nickel.The rod is clamped in the middle.

The alternating magnetic field is generated by electronicoscillator.

The coil L1 wound on the right hand portion of the rodalong with a variable capacitor C.

This forms the resonant circuit of the collector tunedoscillator. The frequency of oscillator is controlled by thevariable capacitor.

The coil L2 wound on the left hand portion of the rod is connected to the base circuit. The coil L2 acts as feed –back loop.


WORKING

When High Tension (H.T) battery is switched on, thecollector circuit oscillates with a frequency,

f =

This alternating current flowing through the coil L1produces an alternating magnetic field along thelength of the rod. The result is that the rod startsvibrating due to magnetostrictive effect.

1

1

2 L C


ADVANTAGES

1. The design of this oscillator is very simple and its

production cost is low

2. At low ultrasonic frequencies, the large power output can

be produced without the risk of damage of the oscillatory

circuit.

Disadvantages

1. It has low upper frequency limit and cannot generate

ultrasonic frequency above 3000 kHz (ie. 3MHz).

2. The frequency of oscillations depends on temperature.

3. There will be losses of energy due to hysteresis and eddy

current.


PIEZO ELECTRIC GENERATOR OR OSCILLATOR

Principle : Inverse piezo electric effect

If mechanical pressure is applied to one pair of oppositefaces of certain crystals like quartz, equal and oppositeelectrical charges appear across its other faces.This iscalled as piezo-electric effect.

The converse of piezo electric effect is also true.

If an electric field is applied to one pair of faces, thecorresponding changes in the dimensions of the otherpair of faces of the crystal are produced.This is known asinverse piezo electric effect or electrostriction.

PH

01

01 U

NIT

1 L

EC

TU

RE

6

36

CONSTRUCTION

The circuit diagram is shown in Figure

Piezo electric oscillator


The quartz crystal is placed between two metal plates Aand B.

The plates are connected to the primary (L3) of atransformer which is inductively coupled to the electronicsoscillator.

The electronic oscillator circuit is a base tuned oscillatorcircuit.

The coils L1 and L2 of oscillator circuit are taken fromthe secondary of a transformer T.

The collector coil L2 is inductively coupled to base coilL1.

The coil L1 and variable capacitor C1 form the tank circuit of the oscillator.


Advantages

Ultrasonic frequencies as high as 5 x 108Hz or 500 MHz can be obtained with this arrangement.

The output of this oscillator is very high.

It is not affected by temperature and humidity.

Disadvantages

The cost of piezo electric quartz is very high

The cutting and shaping of quartz crystal are very complex.


(1)DETECTION OF FLAWS IN METALS (NON

DESTRUCTIVE TESTING –NDT)Principle

Ultrasonic waves are used to detect the presenceof flaws or defects in the form of cracks, blowholesporosity etc., in the internal structure of a material

By sending out ultrasonic beam and by measuringthe time interval of the reflected beam, flaws in themetal block can be determined.

Applications of Ultrasonic Waves in Engineering

PH

01

01 U

NIT

1 L

EC

TU

RE

6

40

EXPERIMENTAL SETUPIt consists of an ultrasonic frequency generator and a cathode

ray oscilloscope (CRO),transmitting transducer(A), receiving

transducer(B) and an amplifier.


WORKING

In flaws, there is a change of medium and this

produces reflection of ultrasonic at the cavities or

cracks.

The reflected beam (echoes) is recorded by using

cathode ray oscilloscope.

The time interval between initial and flaw echoes

depends on the range of flaw.

By examining echoes on CRO, flaws can be detected

and their sizes can be estimated.


THANK YOU

THANK YOU

laser,optical fibres and ultrasonics

Engineering

radiative emission

process of light emission

contrast stimulated

kinetic energy

excited state e2

acronym laser

laser action

spontaneous emission