optical instrumentation u1
Post on 04-Apr-2018
221 Views
Preview:
TRANSCRIPT
-
7/31/2019 Optical Instrumentation u1
1/94
OPTICAL
INSTRUMENTATION
ANUJ BHARDWAJ (UNIT-1) 1
-
7/31/2019 Optical Instrumentation u1
2/94
-
7/31/2019 Optical Instrumentation u1
3/94
INTRODUCTION
Optical instrumentation has become even more firmly established as a
subject in its own right & an increasing number of courses in physics &
electronic engineering include aspects of optoelectronics.
An optical instrument either processes light waves to enhance an image forviewing, or analyzes light waves (or photons) to determine one of a
number of characteristic properties
ANUJ BHARDWAJ (UNIT-1) 3
http://en.wikipedia.org/wiki/Light_wavehttp://en.wikipedia.org/wiki/Photonhttp://en.wikipedia.org/wiki/Photonhttp://en.wikipedia.org/wiki/Light_wavehttp://en.wikipedia.org/wiki/Light_wavehttp://en.wikipedia.org/wiki/Light_wave -
7/31/2019 Optical Instrumentation u1
4/94
SYLLABUS
Unit:1 ) light sourcing, transmitting and receiving
Concept of light , Classification of different phenomenon based on theories of
light, Basic light sources and its characterization , Polarisation ,Coherent and
Incoherent sources , Grating theory, Application of diffraction grating, Electro-optic
effect, Acousto-optic effect, Magneto- optic effect Unit:2 ) Opto- Electronic devices and Optical components
Photo diode, PIN, Photo- Conductors, solar cells, Photo transistors, Materials
used to fabricate LEDs and Lasers Design of LED for optical communication,
Response times of LEDs , LED drive circuitry, Lasers classification: Ruby lasers,
Neodymium Lasers, He-Ne lasers, Dye lasers, Semiconductors lasers, Lasersapplications
ANUJ BHARDWAJ (UNIT-1) 4
-
7/31/2019 Optical Instrumentation u1
5/94
Unit-3)InterferometryInterference effect, Radio-metery, Types of interference phenomenon and its
application,Michelsons Interferometer and its application , Fabry-Perotinterferometer , Refractometer,Rayleighs interferometers, Spectrographs andMonochromators,Spectrophotometers,Calorimeters,Medical Optical Instruments
unit-4) HolographyPrinciple of Holography, On-axis and Off axis Holography , Application ofHolography , Optical data storage
optical fiber sensorsActive and passive optical fiber sensor, Intensity modulated displacement type
sensors, Multimode active optical fiber sensor (Microbend sensor ) Single Modefiber sensor-Phase Modulates and polarization sensors
ANUJ BHARDWAJ (UNIT-1) 5
-
7/31/2019 Optical Instrumentation u1
6/94
-
7/31/2019 Optical Instrumentation u1
7/94
Unit-1( Light sourcing, Transmitting and
Receiving )Concept of light , Classification of different phenomenon based ontheories of light, Basic light sources and its characterization ,
Polarisation ,Coherent and Incoherent sources , Grating theory,
Application of diffraction grating, Electro-optic effect, Acousto-optic
effect, Magneto- optic effect
ANUJ BHARDWAJ (UNIT-1) 7
-
7/31/2019 Optical Instrumentation u1
8/94
UNIT-1 ( Light sourcing, Transmitting and
Receiving )
Concept of light : Light is electromagnetic radiation of wavelength that is visible
to the human eye ( about 400-700 nm ).Light is composed of elementary
particles called photons.
Three properties of light :
a)Intensity b) frequency c) polarization
Light can exhibit properties of both waves and particles.
ANUJ BHARDWAJ (UNIT-1) 8
-
7/31/2019 Optical Instrumentation u1
9/94
Nature of light
What am I?
Waves? Particles?ANUJ BHARDWAJ (UNIT-1) 9
-
7/31/2019 Optical Instrumentation u1
10/94
The Nature of Light- described by different
theories
Quantum TheoryLight consists of small
particles (photons)
Wave TheoryLight travels as a transverse
electromagnetic wave
Ray TheoryLight travels along a straight
line and obeys laws ofgeometrical optics.
Ray theory is valid when the objects aremuch larger than the wavelength
(multimode fibers)
ANUJ BHARDWAJ (UNIT-1) 10
-
7/31/2019 Optical Instrumentation u1
11/94
Classification of different phenomenon based on theories
of light
Image formation - ray theory
Wavelength color,
polarization anddiffraction -
wave theory (electricityand magneticsm)
Interaction of light withatoms - quantum theory of
photons
Constant speed of light
no matter how fast the
source or observer is
moving - special theory of relativity
ANUJ BHARDWAJ (UNIT-1) 11
-
7/31/2019 Optical Instrumentation u1
12/94
Different theories of light
given by different scientist
ANUJ BHARDWAJ (UNIT-1) 12
-
7/31/2019 Optical Instrumentation u1
13/94
A long time ago
Aristotle (384 - 322 B.C.), an ancient Greek thinker, thought that
we saw the world by sending something out of our eye and that
reflected from the object.
In Platos time (427 347 B.C.), the reflection of light from
smooth surfaces was known. He was also a Greek.
The ancient Greeks (about 200 A.D.) also first observed therefraction of light which occurs at the boundary of two
transparent media of different refractive indices.
ANUJ BHARDWAJ (UNIT-1) 13
-
7/31/2019 Optical Instrumentation u1
14/94
Isaac Newton1643 - 1727
Christian Huygens1629 - 1695
In the17th century, two scientists had different views
about the nature of light
Light is
particles
No! Light is
waves
ANUJ BHARDWAJ (UNIT-1) 14
-
7/31/2019 Optical Instrumentation u1
15/94
-
7/31/2019 Optical Instrumentation u1
16/94
Laws of Reflection
According to the Laws of Reflection,
angle of incidence = angle of reflection (i= r)
Incident light ray Reflected light ray
Normal
i r
ANUJ BHARDWAJ (UNIT-1) 16
-
7/31/2019 Optical Instrumentation u1
17/94
Laws of Refraction
Willebrord Snell discovered in 1621 that when a wave travelsfrom a medium of refractive index, n1 , to one of differentrefractive index, n2 ,
n1sin(1) = n2sin(2)
This relationship is calledSnells Law
Incident light rayNormal
Refracted light ray
1
2
n1
n2
Interface
Light bends towards the normal when ittravels from an optically less dense medium
to an optically more dense medium.ANUJ BHARDWAJ (UNIT-1) 17
-
7/31/2019 Optical Instrumentation u1
18/94
Newton proposed his particle theory of light (orcorpuscular theory of light) to explain the characteristics of
light.(source: Opticks, published by Isaac Newton in 1704)
I think light is a stream of tiny
particles, called Corpuscles
ANUJ BHARDWAJ (UNIT-1) 18
-
7/31/2019 Optical Instrumentation u1
19/94
Why does light have different colours?
The particles of different colours have different
properties, such as mass, size and speed.
Why can light travel through a vacuum?
Light, being particles, can naturally pass through vacuum.(At Newtons time, no known wave could travel through a
vacuum.)
Particle Theory
ANUJ BHARDWAJ (UNIT-1) 19
-
7/31/2019 Optical Instrumentation u1
20/94
Why does light travel in straight lines?
A ball thrown into space follows a curved path because of
gravity.
Yet if the ball is thrown with greater and greater speed, its
path curves less and less.
Thus, billions of tiny light particles of extremely low mass
travelling at enormous speeds will have paths which are
essentially straight lines.
ANUJ BHARDWAJ (UNIT-1) 20
-
7/31/2019 Optical Instrumentation u1
21/94
How does the particle theory explain the Laws of
Reflection?
The rebounding of a steel ball froma smooth plate is similar to the
reflection of light from the
surface of a mirror.
Steel Ball Rebound
Light Reflection
Mirror
Many light
particles in a light rayANUJ BHARDWAJ (UNIT-1) 21
-
7/31/2019 Optical Instrumentation u1
22/94
A cannon ball hits the surface of water, it is acted upon by arefracting force which is perpendicular to the water surface. Ittherefore slows down and bends away from the normal. Light doesthe opposite. Newton explained this observation by assuming thatlight travels faster in water, so it bends towards the normal.
(What was the problem in this explanation?)
The problem:
Does light really travel
faster in water?
In fact nobody could measure
the speed of light at the time of
Newton and Huygens
How does Newton's particle theory explain the Laws of Refraction?
Cannon ball Light
Water
Air
ANUJ BHARDWAJ (UNIT-1) 22
-
7/31/2019 Optical Instrumentation u1
23/94
-
7/31/2019 Optical Instrumentation u1
24/94
Lets see how Huygens used his wave theory toexplain the characteristics of light
I think light is emitted as a series of
waves in a medium he called aether
(source: Treatise on light, published by Huygens in 1690)
(aether commonly also called ether)
ANUJ BHARDWAJ (UNIT-1) 24
-
7/31/2019 Optical Instrumentation u1
25/94
A wave starts at P and a wavefront W
moves outwards in all directions.
After a time, t, it has a radius r, so that
r= ct if c is the speed of the wave.
Each point on the wavefront starts
a secondary wavelet. These secondarywavelets interfere to form a new wavefront
W at time t.
How do waves propagate?
P
ANUJ BHARDWAJ (UNIT-1) 25
-
7/31/2019 Optical Instrumentation u1
26/94
-
7/31/2019 Optical Instrumentation u1
27/94
How can wave theory explain the Laws of Refraction?
Opticallydensermedium
n1sin1 = n2sin2
can be proved by geometry.
Refer to the appendix of the
worksheet or your textbookfor the proof.
Click here for animation
Wavefront W1 reaches the boundary between media 1 & 2, point A of
wavefront W1 starts to spread out. When the incoming wavefrontreaches B, the secondary wave from A has travelled a shorter
distance to reach D. It starts a new wavefront W2. As a result the
wave path bends towards the normal.
Air A
C
B
D
W1
W2
1
2
ANUJ BHARDWAJ (UNIT-1) 27
http://d/Eudora/Attach/propagation.htmlhttp://d/Eudora/Attach/propagation.htmlhttp://d/Eudora/Attach/propagation.htmlhttp://d/Eudora/Attach/propagation.html -
7/31/2019 Optical Instrumentation u1
28/94
If light behaves as waves, diffraction and interference should be seen. Theseare two important features of waves. This was known in the 17th century.
(You can see this easily with water waves in a ripple tank)
The wave theory of light predicts interference anddiffraction. However, Huygens could not provide any
strong evidence to show that diffraction and interference
of light occurred.
Diffraction and interferenceof water waves
ANUJ BHARDWAJ (UNIT-1) 28
-
7/31/2019 Optical Instrumentation u1
29/94
Newton was the winner.. (at that time!)
Newtons particle theory of light dominated optics during
the 18th century.
Most scientists believed Newtons particle theory of light
because it had greater explanatory power.
Lets consider the reasons
ANUJ BHARDWAJ (UNIT-1) 29
-
7/31/2019 Optical Instrumentation u1
30/94
Sounds can easily be heard around an obstacle butlight cannot be seen around an obstacle. Light, unlikesound, does not demonstrates the property ofdiffraction and it is unlikely to be a type of wave.
(1)Waves do not travel only in straight lines,
so light cannot be waves.
ANUJ BHARDWAJ (UNIT-1) 30
-
7/31/2019 Optical Instrumentation u1
31/94
-
7/31/2019 Optical Instrumentation u1
32/94
(3) Particle theory of light can explain why there are
different colours of light.
Huygens could not explain why light has different colours
at all. He did not know that different colours of light havedifferent wavelengths.
Though Newtons explanation was not correct (particles
of different colours of light have different mass and size),his particles theory could explain this phenomenonlogically in the 17th century.
?ANUJ BHARDWAJ (UNIT-1) 32
-
7/31/2019 Optical Instrumentation u1
33/94
(4) Reputation of Newton
People tend to accept authority when there is notenough evidence to make judgement. Newtonsparticle theory could only explain refraction by
incorrectly assuming that light travels faster in a densermedium. No one could prove he was wrong at thattime.
The uncertainty about the speed of light in water remained
unresolved for over one hundred years after Newton's death.
ANUJ BHARDWAJ (UNIT-1) 33
-
7/31/2019 Optical Instrumentation u1
34/94
However, the wave theory of light wasre-examined 100 years after Newtons particle theory of light had been
accepted
Thomas Young
1773 - 1829
Light is not
particles!
ANUJ BHARDWAJ (UNIT-1) 34
-
7/31/2019 Optical Instrumentation u1
35/94
Thomas Young successfully
demonstrated theinterference of light (which
Huygens failed to show),
by his famous double-slit
experiments.
Since then the wave
theory of light has been
firmly established.
ANUJ BHARDWAJ (UNIT-1) 35
-
7/31/2019 Optical Instrumentation u1
36/94
The wave theory of light was widely accepted until1905
Albert Einstein
1879 - 1955
Wave theory oflight? No way!
ANUJ BHARDWAJ (UNIT-1) 36
-
7/31/2019 Optical Instrumentation u1
37/94
The photoelectric effect is observed when light strikes a metal,
and emits electrons.
Einstein used the idea of photons (light consists of tiny particles)
to explain results which demonstrate the photoelectric effect.
ANUJ BHARDWAJ (UNIT-1) 37
-
7/31/2019 Optical Instrumentation u1
38/94
Light
What evidence did Einstein find in his
photoelectric effect experiments that
helps to support the particle theory of
light?
In the setup investigating the photoelectric effect (as shown
below), the intensity of the light, its frequency, the voltageand the size of the current generated are measured.
e-
ANUJ BHARDWAJ (UNIT-1) 38
-
7/31/2019 Optical Instrumentation u1
39/94
fC
For certain metals, dim blue light can generate a currentwhile intense red light causes no current at all.
Below a certain cut off frequency
of light ( ), no voltage is
measurable.
Why does the wave theory of light
not explain the result?
Results from photoelectric effect experiments
Voltage
Frequencyof light
n0
fC fANUJ BHARDWAJ (UNIT-1) 39
-
7/31/2019 Optical Instrumentation u1
40/94
-
7/31/2019 Optical Instrumentation u1
41/94
Albert Einstein provided a piece of convincing evidence for the particle
nature of light
Has the story ended yet?
Is light particles or waves?
Louis de Broglie
1892 - 1987
Light is not particles, not
waves, but BOTH!
ANUJ BHARDWAJ (UNIT-1) 41
-
7/31/2019 Optical Instrumentation u1
42/94
Louis de Broglie in 1924 proposed that particles also have
wave-like properties, this was confirmed experimentallythree years later.
Most scientists did not understand de Broglies Ph.D.dissertation at that time. One scientist passed it on to
Einstein for his interpretation. Einstein replied that deBroglie did not just deserve a doctorate but a Nobel Prize!
De Broglie was awarded the
Nobel Prize in 1929.
ANUJ BHARDWAJ (UNIT-1) 42
-
7/31/2019 Optical Instrumentation u1
43/94
Aristotle ( Light was emitted from our eyes )
Christian Huygens ( Wave theory of light )
Isaac Newton ( Particle theory of light )
Thomas Young ( Wave theory of light )
Albert Einstein ( Particle theory of light )
de Broglie ( Wave-particle duality of all matter)
Summary
ANUJ BHARDWAJ (UNIT-1) 43
-
7/31/2019 Optical Instrumentation u1
44/94
Summary: What can we learn from the historical development about the
understanding the nature of light?
Evidence(e.g. Youngs double slit experiment, photoelectricexperimental resultsetc) can establish or refute a theory.
Gathering scientific knowledge (the nature of light) is hardwork, building upon the hard work of other scientists in thepast or present. (collaboration across time)
Scientific knowledge is ever changing, sometimes even
revolutionary (Einstein discovered the particle nature of light,de Broglie discovered the
wave-particle duality of all matter)
ANUJ BHARDWAJ (UNIT-1) 44
-
7/31/2019 Optical Instrumentation u1
45/94
Some properties of light...
light travels more slowly in an optically dense
medium than it does in a less dense medium
A measure of this effect is the refractive index (indexof refraction)
Gives us refraction and reflection
materialinlightofSpeed
invacuumlightofSpeed=h h > 1
ANUJ BHARDWAJ (UNIT-1) 45
-
7/31/2019 Optical Instrumentation u1
46/94
Reflected Rays
h1
h2
a
a= acceptance angle
c= critical angle
Incident Ray Reflected
Ray
c
Partial Reflection
Exit Ray
ANUJ BHARDWAJ (UNIT-1) 46
T l i l fl i i f b di f h
-
7/31/2019 Optical Instrumentation u1
47/94
Total internal reflection is an extreme case of a ray bending awayfrom the
normal as it goes from a higherto alowerindex of refraction medium (from a
slowerto afastermedium)
Glass orwater
(slow)
Normal
Air (fast medium)
Just below thecritical angle for totalinternal reflection there is a reflectedand a transmitted (refracted) ray
Glass or
water
(slow)
Normal
Just abovethe critical angle for total
internal reflection there is a reflected raybut no transmitted (refracted) ray
Critical
angle
For the glass-air interface
ANUJ BHARDWAJ (UNIT-1) 47
-
7/31/2019 Optical Instrumentation u1
48/94
Total internal reflection
ANUJ BHARDWAJ (UNIT-1) 48
-
7/31/2019 Optical Instrumentation u1
49/94
Basic light sources and its characterisation:There are many sources of light. The most common light sources are
black body radiation
Incandescent light bulbs
Flames
Certain other mechanisms can also produce light: scintillation
Electroluminescence
Sonoluminescence
Cherenkov radiation
For characterisation of these sources , we have to discuss their properties,that how they emit light, what is the spectrum region..etc etc.
ANUJ BHARDWAJ (UNIT-1) 49
-
7/31/2019 Optical Instrumentation u1
50/94
Polarisation :Thisphenomenon tells the
transverse nature of light.
We can understand this by
doing simple experiment.
ANUJ BHARDWAJ (UNIT-1) 50
-
7/31/2019 Optical Instrumentation u1
51/94
Experiment:
light from a source S fall on a tourmaline crystal A cut parallel to itscrystallographic axis shown dotted in fig on next slide.
Second crystal B is placed in the path of beam.
Now if B is rotated, the intensity of the emergent light decreases and no light
comes out of the crystal B when B is right angle to A .
Thus it can be easily inferred that light is transverse motion.
ANUJ BHARDWAJ (UNIT-1) 51
-
7/31/2019 Optical Instrumentation u1
52/94
experiment
ANUJ BHARDWAJ (UNIT-1) 52
-
7/31/2019 Optical Instrumentation u1
53/94
Plane polarised light may bedefined as the light in which
the light vector vibrates
along a fixed straight line in
a plane perpendicular tothe direction of
propagation.
ANUJ BHARDWAJ (UNIT-1) 53
-
7/31/2019 Optical Instrumentation u1
54/94
Pictorial representation of light vibrations
ANUJ BHARDWAJ (UNIT-1) 54
-
7/31/2019 Optical Instrumentation u1
55/94
Brewsters law
According to this law,when light is incident at the
polarizing angle the
reflected and refracted rays
are perpendicular to eachother.
ANUJ BHARDWAJ (UNIT-1) 55
-
7/31/2019 Optical Instrumentation u1
56/94
Coherence
Purpose Neglect temporal dependence Coherence light -> stable interference Degree of coherence interference fringes visibility
What light is coherent Monochromatic temporal coherence Coherence length
Spherical waves spatial coherence Coherence area
Formal description Binary relation Cross correlation between two signals
ANUJ BHARDWAJ (UNIT-1) 56
-
7/31/2019 Optical Instrumentation u1
57/94
I t f i i h t f h ith d
-
7/31/2019 Optical Instrumentation u1
58/94
Interfering many waves: in phase, out of phase, or with random
phase
Waves adding exactly
in phase(coherent
constructive addition)
Waves adding withrandom phase,partially canceling
(incoherentaddition)
If we plot the
complex
amplitudes:
Re
Im
Waves adding exactly
out of phase, adding tozero (coherentdestructive addition)
ANUJ BHARDWAJ (UNIT-1) 58
-
7/31/2019 Optical Instrumentation u1
59/94
h f l h b lb h
-
7/31/2019 Optical Instrumentation u1
60/94
Light from a light bulb is incoherent
Itotal = I1 + I2+ + In
When many light waves add withrandom phases, we say the light is
incoherent, and the light wave totalirradiance is just the sum of the
individual irradiances.
Other characteristics of incoherent light:
1. Its relatively weak.
2. Its omni-directional.
3. Its irradiance is proportional to the number of emitters.ANUJ BHARDWAJ (UNIT-1) 60
C h t I h t Li ht
-
7/31/2019 Optical Instrumentation u1
61/94
Coherent vs. Incoherent Light
Itotal = I1 + I2+ + InEtotal = E1 + E2
+ + En
Coherent light:1. Its strong.
2. Its uni-directional.
3. Total irradiance N2 or0.
4. Total irradiance is the mag-squareof the sum of individual fields.
Incoherent light:1. Its relatively weak.
2. Its omni-directional.
3. Total irradiance N.
4. Total irradiance is the sum ofindividual irradiances.
Laser
ANUJ BHARDWAJ (UNIT-1) 61
-
7/31/2019 Optical Instrumentation u1
62/94
Diffraction
What exactly is diffraction
Everything not being reflection or refraction
Interference of many sources
Scalar Diffraction
Easier in certain environment
Huygens-Fresnel principle More precise formulations
Kirchhoff
Rayleigh-Sommerfeld
t
t
t-Dt
t-Dt
t+Dt
t+Dt
Direction of
propagation
Direction of
propagation
) ) sdsoiyxu srsrik
S
cos
~
,~
|)(|
|))(|exp(
=1
ANUJ BHARDWAJ (UNIT-1) 62
-
7/31/2019 Optical Instrumentation u1
63/94
Diffraction is a phenomenon when a wave
that passes through an aperture or around an
obstacle forms a pattern on a screen.
What causes diffraction is interference of an
infinite number of waves that are emitted by
the points of the aperture
ANUJ BHARDWAJ (UNIT-1) 63
-
7/31/2019 Optical Instrumentation u1
64/94
-
7/31/2019 Optical Instrumentation u1
65/94
There are two different limiting types ofdiffraction observations
- Fresnel diffraction patterns
- Fraunhofer diffraction patterns
ANUJ BHARDWAJ (UNIT-1) 65
-
7/31/2019 Optical Instrumentation u1
66/94
For Fraunhofer diffraction pattern there is alarge distance between aperture and the
screen.
For Fresnel diffraction the distance between
the aperture and the screen is generally
small.
ANUJ BHARDWAJ (UNIT-1) 66
l f l diff i
-
7/31/2019 Optical Instrumentation u1
67/94
Example of Fresnel diffraction
ANUJ BHARDWAJ (UNIT-1) 67
-
7/31/2019 Optical Instrumentation u1
68/94
Airy disk: This is a Fraunhofer diffraction.
ANUJ BHARDWAJ (UNIT-1) 68
-
7/31/2019 Optical Instrumentation u1
69/94
Grating theory: diffraction
grating is an arrangementequivalent to a large number of
parallel slits of equal widths and
separated from each other byequal opaque spaces.
ANUJ BHARDWAJ (UNIT-1) 69
A single beam of light will scatter off of the grating and emerge as many beams with indices m
0 1 2 3 The outgoing beam with m 0 is called the "zeroth order" beam and it goes
-
7/31/2019 Optical Instrumentation u1
70/94
= 0,1,2,3,,,,. The outgoing beam with m = 0 is called the zeroth order beam and it goes
straight through. The beam with m=1 is called the "first-order" beam and the beam with
m=2 is called the "second order" beam.
All of these higher orders beams have the same color (wavelength) as the incident beam.
Each order m refers to a pair of diffracted beams that emerge from the grating at
angles from the initial beam's direction.
The directions of the beams diffracted from the initial beam of wavelength is given by the
relation,
ANUJ BHARDWAJ (UNIT-1) 70
-
7/31/2019 Optical Instrumentation u1
71/94
A diffraction grating is the tool of choicefor separating the colors in incident light.
The condition for maximum intensity is
the same as that for a double slit.
However, angular separation of the
maxima is generally much greater because
the slit spacing is so small for a diffraction
grating.
ANUJ BHARDWAJ (UNIT-1) 71
http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/slits.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/slits.html -
7/31/2019 Optical Instrumentation u1
72/94
Diff ti
-
7/31/2019 Optical Instrumentation u1
73/94
Diffraction
ordersBecause the diffraction angle depends
on , different wavelengths are
separated in the nonzero orders.
No
wavelength
dependence
occurs in
zero order.
The longer the wavelength, the larger its deflection in each
nonzero order.
Diffraction angle, m()
Zeroth order
First order
Minus
first order
Incidence
angle, i
ANUJ BHARDWAJ (UNIT-1) 73
Diffraction grating dispersion
-
7/31/2019 Optical Instrumentation u1
74/94
Because diffraction gratings are used to separate colors, its helpful to
know the variation of the diffracted angle vs. wavelength.
Differentiating the grating equation,
with respect to wavelength:
Diffraction-grating dispersion
cos( ) mm
da md
=
sin( ) sin( )m ia m - =
cos( )
m
m
d m
d a
=
Rearranging:
[i is constant]
Thus, to separate different colors maximally, make a small, work in high
order (make m large), and use a diffraction angle near 90 degrees.
Gratings typically have an
order of magnitude moredispersion than prisms.
ANUJ BHARDWAJ (UNIT-1) 74
Any surface or medium with
-
7/31/2019 Optical Instrumentation u1
75/94
periodically varying or n is a
diffraction grating.
Transmission gratings can be amplitude () or phase (n) gratings.
Gratings can work in reflection (r) or transmission (t).
ANUJ BHARDWAJ (UNIT-1) 75
Real diffraction
-
7/31/2019 Optical Instrumentation u1
76/94
gratings
Diffracted white light
White light diffracted by a real grating.
m = 0 m =1m = 2
m = -1
The dots on a CD areequally spaced (although
some are missing, of
course), so it acts like a
diffraction grating.
Diffraction
gratings
ANUJ BHARDWAJ (UNIT-1) 76
-
7/31/2019 Optical Instrumentation u1
77/94
The tracks of a compact
disc act as a diffraction
grating, producing a separation ofthe colors of white light. The nominal
track separation on a CD is 1.6micrometers, corresponding to about
625 tracks per millimeter. This is in
the range of ordinary laboratory
diffraction gratings. For red light of
wavelength 600 nm, this would give
a first order diffraction maximum atabout 22 .
ANUJ BHARDWAJ (UNIT-1) 77
Application of diffraction grating
http://hyperphysics.phy-astr.gsu.edu/hbase/audio/cdplay.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/audio/cdplay.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/audio/cdplay.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/audio/cdplay.html -
7/31/2019 Optical Instrumentation u1
78/94
Application of diffraction grating
GRATINGS FOR INSTRUMENTAL ANALYSISAtomic and molecular spectroscopy
Fluorescence spectroscopy
Colorimetry
Raman spectroscopy
GRATINGS IN LASER SYSTEMS
Laser tuning
Pulse stretching and compression
GRATINGS IN ASTRONOMICAL APPLICATIONS
Ground-based astronomy
Space-borne astronomy
GRATINGS IN SYNCHROTRON RADIATION BEAMLINES
SPECIAL USES FOR GRATINGS
Gratings as filters Gratings in fiber-optic telecommunications
Gratings as beam splitters
Gratings as optical couplers
.Gratings in metrological applications
ANUJ BHARDWAJ (UNIT-1) 78
http://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asphttp://gratings.newport.com/information/handbook/chapter13.asp -
7/31/2019 Optical Instrumentation u1
79/94
Colorimetryis the measurementand specification of color, used inanalytical chemistry, color matching,color reproduction and appearancestudies. Because color as perceivedcannot be associated with a singlewavelength it is a more complicated
function of how the three differentlight receptors in the human eyerespond to the entire visiblespectrum when looking at an objectit is common to use amultiwavelength instrument such asa grating spectrometer.
ANUJ BHARDWAJ (UNIT-1) 79
-
7/31/2019 Optical Instrumentation u1
80/94
-
7/31/2019 Optical Instrumentation u1
81/94
Real life application of diffraction grating
The diffraction grating is animmensely useful tool for theseparation of the spectral linesassociated with atomictransitions. It acts as a "superprism", separating the different
colors of light much more thanthe dispersion effect in a prism.The illustration shows thehydrogen spectrum. Thehydrogen gas in a thin glass tubeis excited by an electrical
discharge and the spectrum canbe viewed through the grating.
ANUJ BHARDWAJ (UNIT-1) 81
http://hyperphysics.phy-astr.gsu.edu/hbase/hyde.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/hyde.html -
7/31/2019 Optical Instrumentation u1
82/94
Electro-optic effect:when an electric field Is applied across an optical medium the distribution of
electrons within it is distorted so that the polarizability and hence the refractive
index of the medium changes anisotropically. The result of this electro-optic
effect may be to introduce new optic axes into naturally doubly refracting
crystalse.g. KDP,GaS
ANUJ BHARDWAJ (UNIT-1) 82
-
7/31/2019 Optical Instrumentation u1
83/94
-
7/31/2019 Optical Instrumentation u1
84/94
ANUJ BHARDWAJ (UNIT-1) 84
-
7/31/2019 Optical Instrumentation u1
85/94
ANUJ BHARDWAJ (UNIT-1) 85
-
7/31/2019 Optical Instrumentation u1
86/94
-
7/31/2019 Optical Instrumentation u1
87/94
ANUJ BHARDWAJ (UNIT-1) 87
-
7/31/2019 Optical Instrumentation u1
88/94
ANUJ BHARDWAJ (UNIT-1) 88
-
7/31/2019 Optical Instrumentation u1
89/94
Acousto-optic effect: theacousto-optic effect is the change in
the refractive index of a medium
caused by the mechanical strains
accompanying the passage of an
acousic(strain) wave through themedium.
ANUJ BHARDWAJ (UNIT-1) 89
Acousto-Optics:
h f i i d f i l di i l d b
-
7/31/2019 Optical Instrumentation u1
90/94
The refractive index of an optical medium is altered by
the presence of sound; sound modifies the effect of
medium on light; Many useful devices make use of this acousto optic
effect: optical modulator,switches, deflectors, filters,
isolators, frequency shifters, and spectrum analyzers.
Sound is a dynamic strain involving molecularvibrations that take the form of waves which travel at a
velocity characteristic of the medium; vibration of
molecular => polarizability => refractive indexx
z
=2/s in
Bragg diffraction: an
acoustic plane
wave acts as a partial
reflector of light
beams litter .ANUJ BHARDWAJ (UNIT-1) 90
Interaction of light and sound:
B Diff i
-
7/31/2019 Optical Instrumentation u1
91/94
Bragg Diffraction:
Consider an acoustic plane wave traveling in thex
direction in a medium with velocity vs, frequencyf,and wavelength = vs/f. The strain (relative
displacement) at positionx and time tis
)c o s (),(0 q xtstxs -=
s0: amplitude; = 2f; q = 2/ (wavenumber).
The acoustic intensity (W/m2) is 2/2
0
3svI ss =
: mass density of medium.
The medium is assumed to be optical transparent and
the refractive index in the absence of sound is n. The
strain s => perturbation of the refractive index
(analogous to the Pockels effect)
),(),(3
21
txsntxn p-=Dp: photoelastic constant
(strain-optic coefficient)ANUJ BHARDWAJ (UNIT-1) 91
-
7/31/2019 Optical Instrumentation u1
92/94
ANUJ BHARDWAJ (UNIT-1) 92
-
7/31/2019 Optical Instrumentation u1
93/94
Magneto-optic devices:the presence of magnetic fields may
also affect the optical properties of
some substances thereby giving rise
to a number of useful devices.This is
based on the faraday effect.
ANUJ BHARDWAJ (UNIT-1) 93
Assignment
-
7/31/2019 Optical Instrumentation u1
94/94
Assignment
Discuss basic light sources and its characterisation.
What is Bire frengence? How it is useful to generate electro-
optic effect.
What is Acousto optic effect? Explain Raman Nath effect.
How magneto optic effect is used to design current
sensors.Explain with relevant diagram.
What is Diffraction grating? Write down its application.
top related