Engineering Physics-II, As per RTU Syllabus

1. Laser & Holography
Unit-IV (B.Tech-II sem)
Dr. Vishal Jain
Assistant Professor
Department of Physics

2. Introduction
LASER Light Amplification by Stimulated Emission of Radiation
A laser is a device that produce an intense, concentrated and highly
parallel beam of coherent light. The first successful built by Townes and
his associates at Colombia University between 1951 to 1954.
Or
A laser is a device that emits coherent light beam through a process
of optical amplification based on the stimulated
emission of electromagnetic radiation.
Red (660 & 635 nm), green (532 & 520 nm)
and blue-violet (445 & 405 nm) lasers
He-Ne
Laser

3. Properties of Laser
• Monochromatic:- Concentrate in a narrow range of wavelengths
(one specific colour).
• Coherent: - All the emitted photons bear a constant phase
relationship with each other in both time and phase
• Directional:- A very tight beam which is very strong and
concentrated.
Early History of Laser
• 1917 Einstein’s paper on Quantum Theory of Radiation
• 1954-58 basic idea’s about laser by Townes and associates.
• 1960 laser coined by Gould
• 1960 first ruby laser by Maiman
Basic concepts for a laser
(1) Absorption (2) Spontaneous Emission (3) Stimulated Emission (4)
Population inversion

4. Atom is initially in state 1, it will be raised to state 2 by absorbing a photon of light whose
frequency is hν =E2-E1 . This process is called Absorption. (as shown in first figure)
The probability of such a transition depends on the energy density of incident photo
P12 = B12 u(ν) --------------------Eq 1
where P12 is the probability of absorption , u(ν) is the spectral energy density and B12 is the
Einstein Constant.
The rate of absorption will be given by
R12 = N1 B12 u(ν) ----------------Eq 2
where N1 is the number of atoms in lower energy level

5. Atom present in Higher Energy level E2 has a small life time (10-8 sec) so it can be
spontaneously jump from state E2 to state E1 by emitting a photon as shown in figure two.
This process is called Spontaneous Emission.
It is independent of the presence of any photon present in the system. Therefore any of
electron can jump to lower state randomly and the photon emitted are never be in phase. So
spontaneous emission provides incoherent photons. Thus the probabilty here (P21)s is always
constant
(P21 )s = A21 --------------------Eq 3
Where (P21 )s is the probability of Spontaneous emission , u(ν) is the spectral energy density
and A21 is the Einstein Constant.
The rate of Spontaneous emission will be given by
(R21 )s = N2 A21 ------------------Eq 4
where N2 is the number of atoms in excited state.

6. Einstein proposed that of a photon of appropriate frequency is present, it is not essential that
it may always be absorbed. It can also interact with Atom present in Higher Energy level E2
and induce that atom to emit a new photon. Such a process is known as Induced or
Stimulated Emission.
This type of emission is in phase, directional and energy of the emitted photo is same as that
of incident photon. It means both the photons are coherent with each other. This type of
emission is preferred to get a highly coherent laser light. The probability of induced
emission will be given as
(P21 )i = B21 u(ν)--------------------Eq 5
Where (P21 )i is the probability of Stimulated emission , u(ν) is the spectral energy density
and B21 is the Einstein Constant.
The rate of Stimulated emission will be given by
(R21 )i = N2 B21 u(ν) --------------Eq 6
where N2 is the number of atoms in excited state.

7. Relation between Einstein Co-efficient A and B for Spontaneous and
Stimulated emission
The total rate of emission from state 2 to state 1 is given by adding eq 4 and eq 6 we get
)]([ 2121221 uBANR  ------Eq 7
In thermal equilibrium, the rate of emission must be equal to the rate of absorption
)]([)( 21212121
2112
 uBANuBN
RR


Solving for u(ν) we will get
][
)(
)()(
212121
212
212212121
BNBN
AN
u
ANuBNuBN





Dividing up and down right hand side by N2/B21, we get
------Eq 8
1
)(
21
12
2
1
21
21













B
B
N
N
B
A
u 
Now applying Maxwell’s Boltzmann Distribution Law in thermal equilibrium.

8. ,/
2
1
/
21 kThvEE
kTE
ee
N
N
so
eN




1
)(
21
12/
21
21







B
B
e
B
A
u
kThv

The above expression of spectral energy density can be compared with planck’s radiation law,
accordingly
,
1
/8
)( /
33

 kThv
e
ch
u


3
3
21
21 8
c
h
B
A 
When B21 = B12 ------Eq 8
Let us now compare the probability of spontaneous emission to that of stimulated emission
)1(
)()(
)( /
21
21
21
21
 kThv
e
vuB
A
iP
sP
------Eq 9
Now applying two conditions
(i) If hv >>kT, (P21)s>>(P21)I, then spontaneous emission will be much more probable than
induced emission. This is ordinarily true for electronic transitions in atoms and molecules
and in radiative transition in nuclei.
(ii) If hv <<kT, (P21)s<<(P21)I, then the stimulated emissions no more negligible and it even
predominant over spontaneous emission (Whatch This Link Video)
https://www.youtube.com/watch?v=1LmcUaWuYao
https://www.youtube.com/watch?v=-YPixpL6Rnc

9. Threshold Condition for Laser Action
For laser action, two conditions (Threshold Conditions) should be satisfied
(i) The probability of spontaneous emission which produced incoherent radiations
should be much smaller than the probability of stimulated radiations i.e. A21 <<
B21u(v).
(ii) The emission rate should be larger than the absorption rate.
R12 << (R21)s+ (R21)i. This condition can be achieved by following steps.
1. The Situation in which the number of atoms in the higher energy state exceeds with
that in the lower energy state is known as population inversion.
2. The method adopted to achieve population inversion is called pumping. There are
several methods of pumping such as
(A) Optical Pumping, (B) Electron Impact (C)In elastic atom-atom collision (D)
Chemical Pumping (E) Gas Dynamic Pumping.
E2
E1
Ordinary Condition N1>N2 Population Inversion N2>N1

10. The higher probability of stimulated emission is achieved by the following
methods
(1) The spectral energy density of incident radiation must be made very large, means u(ν)
must be large
To increase u(ν), the emitted radiations are reflected coherently again & again between
two parallel mirrors facing each other in a cavity containing active medium.
(2) The life time of excitation τc must be very large
To minimize the value of τc metastable state of higher energy is chosen for stimulated
transition of because transition from metastable state spontaneous emission are not
allowed but life time τc is very large
Metastable states :- There are some energy states whose life times are much longer (10-3
sec) than the life of usual excited states (10-8 sec). Such relatively long lived states are
called metastable states.
Einstein coefficient of spontaneous emission is given by A21=1/ τc
For metastable states the value of τc is very large so A21 is comparatively very small. So
spontaneous emission from metastable states are forbidden. Hence an atom from
metastable state can jump to ground state through by stimulated emission. That’s why in
laser, transition from metastable states are chosen for laser action.

11. Main Component of a LASER
A LASER requires three main components for operation
• It is a material which achieves the condition of population inversion &
through stimulated emission produces LASERs. The active medium may
be solid, liquid and gas.
Active Medium
• It is energy source which is used to raise the system to an excited states to
achieve population inversion in active medium
Pumping
Source
• It consist of two mirror facing each other. The active medium is enclosed
by this cavity. One of the mirror is fully reflecting and another is partially
transparent.
Optical
resonator cavity
Optical Resonator or cavity
Amplifying Medium
Fully Reflected
Mirror
Pumping Source Partially
Transmitted Mirror
Laser Beam

12. He-Ne Laser
Ali Javan, Bennett and Harriott in1961
A Helium Neon LASER, usually called He-Ne laser is a small gas laser generally
used in laboratory to demonstrate optics experiments. Its usual operation
wavelength is 632.8nm, in the red portion of visible spectrum.
Construction:-
He-Ne laser consists of a narrow discharge tube of length 10 to 100cm with a
diameter of order of 5mm to 10mm. The tube is filled with a mixture of helium and
neon gases approximately in the ratio of 85% helium and 15% Ne contained at low
pressure upto 2-3 torr. The positive high voltage connected to the anode and
cathode which provide electrical discharge around 1000Volt. The LASER tube
enclosed in between two mirrors, one will totally reflecting mirror and second is
partially reflecting mirror mounted at the output end. Inside the tube LASER also
has Brewster’s window to produced linearly polarized light.

13. Working:-
Metastable states
Helium Atom Neon Atom
Ground State Ground State
Laser Transition
6328 Å..
Spontaneous
Emission
Radiation less Transition
[by collisions from walls]
Inelastic Collision 20.66eV
18.70eV
20.61eV
He
E1
He
E2
He
E3
Energy
19.81eV
Fast Radioactive Transition
[spontaneous emission]
6000 Å..
Ne
E1
Ne
E2
Ne
E3
Ne
E4
Ne
E5
Ne
E6
33900 Ang.
11500 Å.

14. When the voltage is applied, an electric discharge causes the ionization some of the atoms in
the mixture of He-Ne gases. The electron and ion so created are accelerated towards anode
and cathode respectively. Due to lighter mass of He-atoms, electrons to strike the Helium
atoms and raise them to the excited state E2 and E3 He which lies at 19.81 eV and 20.61eV
respectively above the ground as shown fig. The energy states are metastable states so
transition from these states to ground states by spontaneous emission are forbidden. The
energy excited to He atom to E2 and E3 is almost exactly same as the energy needed to
excite Ne atom in to E4 and E6 energy state. Once the He population is successfully inverted,
excite He atom strikes to Ne atoms and transfer their energy to the Neon atom. This process
is given by the reaction
EHeNeNeHe  **
Where * represent an excited state and ΔE is the small energy difference between the energy
states of two atoms of the order of 0.05eV. This additional 0.05 eV provided by kinetic
energy of He atom. Thus the role of He atom in the LASER is to provide the pumping
medium to neon atom to attain the necessary population inversion
Since E6 and E4 excited state of Ne atom is more stable than other states. The transition from
E6 to E5 and E4 to E3 emission the radiation having wavelength of 33900Å and 11500Å,
but these transition falls in infrared region, However the transition from E6 to E3 generate a
laser beam of red colour of wavelength 6328Å in the visible range. At last Ne aton comes to
ground states though the collision with walls of tube from E2 states.

15. Brewster window in a LASER cavity:- Brewster windows are used in laser
cavities to ensure that the laser light after bouncing back between cavity
mirrors emerged as linearly polarized light.
Advantage:-
(i) It is most common and inexpensive gas laser
(ii) Typical He-Ne laser can generate a few mill watts of output power ranging
fro 1W to 100mW in continuous wave operation.
(iii) Beam quality is usually excellent. It is often used for alignment purpose.

16. Semiconductor Diode Laser
Hall and Its Co-Workers in 1962
• A semiconductor diode laser is a specially fabricated p-n-
junction device that emits coherent light when it is forwarded
biased.

17. Construction:- A schematic diagram of the simplest type of GaAs LASER in shown in
figure. It is a solid state device that consists of two outer semiconductor layers separated by
a middle layer and generated LASER radiation, when charge carriers of opposite polarity,
one to each from the top and bottom layers meets in the middle layer. Such LASER consists
of two basic components and optical amplifier and a resonator. Excitation is provided by
current flow through the device as the top and bottom layers are metalized and metal
contact are provided

19. Principle of operation (Working)
The radioactive recombination of electron-hole pairs can be used for the generation of
electromagnetic radiation by the electric current in a p-n junction. This effect is called
electroluminescence.
When a diode is forward biasd, holes from p-junctions are injected into the n-region, and
the electron from the n-region are injected into the p-region. As electrons and holes are
present in same region, they may radioactively recombine and emits a photon with the
energy of bandgap. This is called spontaneous emission. If before the spontaneous
recombination a photo will interact with them, than stimulated emission will be happened
and as a result we obtained a photon with same frequency, same direction same as the
incident photon.
As the mirror are provided for total and partial reflection to obtained a coherent
electromagnetic radiation with all photon in same phase with each other.
Advantage:-
(i) Small size and weight, low current voltage and power required
(ii) Low intensity.
(iii) Wide angle beam

20. Application of LASER
a) Holography: Holography is a technique to record the complete picture of an object, that
is it will produce the three dimensional picture. The process of holography will be
discussed in detail later on.
b) Measurement of long distance: The beam spreading in the laser light is very small,
laser can travel along distances, without appreciable spreading. The time taken by laser
pulse to travel from laser source to a given target and back is measured. As the velocity of
light is known, the distance of the target can be calculated using the relation 2d = c x t
where d is the distance of the target and c is the velocity of light.
c) Applications in scientific research: Due to the coherent nature of laser light, many new
optical phenomenon have been observed using laser. Using laser light we investigate the
basic laws of interaction of atoms and molecules with electromagnetic waves.
d) Application in communication: In the fibre communication system, laser beam is used.
The rate at which information is transmitted is proportional to the band width of the
information carrier signal.
The bandwidth is proportional to the frequency of the carrier. Since the frequency range of
laser signal is quite high compared to the microwaves, large bandwidth can be obtained
using optical region as compared to the microwave region.
e) Applications in Industry: Due to the high intensity of laser beam, laser can be used in
welding, cutting and in producing very high temperatures. The other advantage of laser is
that the beam can be focused onto a fine spot. The small spot size implies that high energy
densities are possible. Lasers are also found suitable for machining and drilling holes.

21. f) Lasers in Isotope separation: The light emerging from a laser is extremely
monochromatic. When laser light falls on a mixture of two isotopes, the laser
light excites the atoms of only one of the isotopes thus separating it from the
other isotope.
In addition to the high monochromaticity, the high intensity of the laser is also responsible
for its application for isotope separation because with low intensity beams the
separation rate would be too low for practical use.
g) Applications in Medicine:
i) Laser Surgery: The focused laser beam is capable of bloodless surgery, since the beam
not only cuts but also welds blood vessels being cut. Laser surgery is painless because
operations are very fast and there is not enough time for the patient to respond to the
incision and sense pain.
ii) Laser Therapy: He-Ne laser has produced curing effect on trophic ulcers, poorly
healing wounds, and bone fractures. Laser can also be guided through optical fiber into
blood vessels to remove the clothings, in case of heart patients, through heating. It has
also found application in treating the decaying teeth. Laser can replace dental drills.

22. Holography
Holography is a technique that allows the light scattered from an
object to be recorded and later reconstructed so that it appears as if
the object is in the same position relative to the recording medium as
it was when recorded. The image changes as the position and
orientation of the viewing system changes in exactly the same way
as if the object was still present, thus making the recorded image
(hologram) appear three dimensional. Holograms can also be made
using other types of waves.
The technique of holography can also be used to optically store,
retrieve, and process information. While holography is commonly
used to display static 3-D pictures, it is not yet possible to generate
arbitrary scenes by a holographic volumetric display.

23. History of Holography
Holography was invented in 1947 by Hungarian physicist
Dennis Gabor (1900–1979), work for which he received the
Nobel Prize in Physics in 1971.
Gabor's research focused on electron optics, which led him to the invention of
holography. The basic idea was that for perfect optical imaging, the total of all the
information has to be used; not only the amplitude, as in usual optical imaging, but
also the phase. In this manner a complete holo-spatial picture can be obtained.
"Train and Bird" is the first hologram ever made
with a laser using the off-axis technique. This
pioneer image was produced in 1964 by Emmett
Leith and Juris Upatnieks at the University of
Michigan only four years after the invention of the
laser

24. In 1983 MasterCard
International, Inc.
became the first to use
hologram technology in
bank card security.
Dr. Stephen A. Benton,
Massachusetts Institute of
Technology, seen through
"Crystal Beginning," a white
light transmission hologram
produced at the Polaroid
Corporation in 1977.(Photo
by Michael Lutch for
WGBH, Boston)

25. Two photographs of a single hologram taken from
different viewpoints

26. Difference between Holography and Photography
1. In photography, only intensity is recorded so photography produces two
dimensional picture of the object whereas in holography, both intensity as
well as phase of light wave is recorded, thus holography gives three
dimensional picture of the object.
2. Negative is prepared first in photography whereas in holography no negative
is required. The hologram is negative and image it gives is positive.
3. A holographic recording requires a second light beam (the reference beam)
to be directed onto the recording medium.
4. If the hologram is broken into parts, each part is capable of reconstructing
the entire object. But in photography the destruction of even very small
portion of negative or photography results in a irrepareable loss of
information.
5. Holography has high information capacity as compared to photography
6. A photograph can be viewed in a wide range of lighting conditions, whereas
holograms can only be viewed with very specific forms of illumination.

27. Construction of a Hologram

28. https://www.youtube.com/watch?v=lJVhWwI
NovY
Construction of a Hologram

29. Recording of hologram. The recording of hologram is based on
the phenomenon of interference. It requires a laser source, a plane mirror or
beam splitter, an object and a photographic plate. A laser beam from the laser
source is incident on a plane mirror or beam splitter. As the name suggests, the
function of the beam splitter is to split the laser beam. One part of splitted beam,
after reflection from the beam splitter, strikes on the photographic plate.
This beam is called reference beam. While other part of splitted
beam (transmitted from beam splitter) strikes on the photographic plate after
suffering reflection from the various points of object. This beam is called object
beam.
The object beam reflected from the object interferes with the reference beam when
both the beams reach the photographic plate. The superposition of these two
beams produces an interference pattern (in the form of dark and bright fringes)
and this pattern is recorded on the photographic plate. The photographic plate
with recorded interference pattern is called hologram. Photographic plate is also
known as Gabor zone plate in honour of Denis Gabor who developed the
phenomenon of holography.
Each and every part of the hologram receives light from various points of the
object. Thus, even if hologram is broken into parts, each part is capable of
reconstructing the whole object

30. 2. Reconstruction of image.. In the reconstruction process, the hologram is
illuminated by laser beam and this beam is called reconstruction beam. This
beam is identical to reference beam used in construction of hologram. The
hologram acts a diffraction grating. This reconstruction beam will undergo
phenomenon of diffraction during passage through the hologram. The
reconstruction beam after passing through the hologram produces a real as well
as virtual image of the object. One of the diffracted beams emerging from the
hologram appears to diverge from an apparent object when project back. Thus,
virtual image is formed behind the hologram at the original site of the object and
real image in front of the hologram. Thus an observer sees light waves diverging
from the virtual image and the image is identical to the object. If the observer
moves round the virtual image then other sides of the object which were not
noticed earlier would be observed. Therefore, the virtual image exhibits all the
true three dimensional characteristics. The real image can be recorded on a
photographic plate.

32. Holographic Microscopy
Digital Holographic Microscopy (DHM) is digital holography applied to microscopy.
Digital holographic microscopy distinguishes itself from other microscopy methods by
not recording the projected image of the object. Instead, the light wave
front information originating from the object is digitally recorded as a hologram

33. To create the necessary interference pattern, i.e., the hologram,
the illumination needs to be a coherent (monochromatic) light
source, a laser for example. As can be seen in Figure 2, the laser
light is split into an object beam and a reference beam. The
expanded object beam illuminates the sample to create the object
wave front. After the object wave front is collected by
a microscope objective, the object and reference wave fronts are
joined by a beam splitter to interfere and create the hologram.
Using the digitally recorded hologram, a computer acts as
a digital lens and calculates a viewable image of the object wave
front by using a numerical reconstruction algorithm.
Commonly, a microscope objective is used to collect the object
wave front. However, as the microscope objective is only used to
collect light and not to form an image, it may be replaced by a
simple lens. If a slightly lower optical resolution is acceptable,
the microscope objective may be entirely removed.

34. Previous RTU 2014, 2015 Questions

35. Thank You