1.4. LASERS& HOLOGRAPHY Introduction:
LASER is an acronym
Radiation. Maiman demonstrated
for Light Amplification
by Stimulated
the first laser in 1960. The development
rapid and laser action can be demonstrated
with solids, liquids
another
few unique characteristics.
type of light source, which possesses
laser is uptoseveral
MW and size of the advanced
Emission
of
of laser was very
and gases. Laser is just The power of the
laser system is one tenth the diameter
of
a human hair.
1.
Write the Characteristics of the laser
When compared outstanding
with any conventional
characteristics,
1. Directionality:
During the propagation
2. Monochromaticity:
of a laser, its angular
Hence, it possesses
The property
of exhibiting
Due to its high directionality
therefore the intensity 4. Coherence:Laser The property
beam possesses either
waves is known as coherence.
the phase
coherence:
difference
few
will be less and
high degree of directionality. a single
wavelength
by a light is
many beams of light incident
in a small area,
is more. So it is used for the welding.
high degree of both spatial and temporal
zero or constant Coherence
phase difference
is of two types l.Spatial
Consider the wavefront
between
Even it is sent
in the optical spectrum.
is high. Hence its brightness
of existing
coherence.Spatial
spreading
The light from a laser source is highly monochromatic.
through a prism a single line will be appeared 3. Intensity:
light or tube light), laser possesses
they are
occupies less area where it incidents.
monochromaticity.
light(sun
coherence.
between
two or more
coherence
2. Temporal
from a laser propagates
two points xi and Xz along the direction
along x-axis, of propagation
remains constant
at all times. This is called spatial c.oherence. Temporal
coherence:
phase difference
between
a given point at
successive
regular interval of time remains
wavefronts
as they pass through
constant then the wave is said to have temporal
If the
coherence.
Page 1 oflO
2.
Write a short notes on stimulated,
spontaneous,
population
inversion
and pumping process. 1. Stimulated Absorption:
consider
active energy levels are present
a system in which two
whose energies
(
l
are El and
BEFCRE
hv =E2-E1
E2 where El is ground state and Ez is excited state as shown
external
forces are not applied.
hv = £2 - £1 is incident then
it excites
2.Spontaneous
of energy BEFORE
state
Ez. This phenomenon
4FTER
-"- .. -
b
"'P,\(lI""d'" I '111"'1\111 .-A.-..L.L. E.
is
hv =E2-E1
r : _A-IL. •.. __
Absorption.
Emission: Let us assume that the system is in the excited state Ez. After the
life time the atom de-excites hv=Ez-E,
When a photon
on the atom lying in ground stage
to higher
known as stimulated
stage as long as
-"--
\1\""1'("'" E~ •.••
r<: . /~
in figure. Usually atoms are in the ground
·'HER
Ez
as shown
to its ground stage spontaneously
in figure. This phenomenon
photons emitted have various wavelength
emitting
a photon of energy
is known as spontaneous
emission.
The
and they are out of phase. Thus the photons are
incoherent. 3.Stimulated
Emission: Let us assume
that the atom
is in the excited state Ez. If a photon of energy hv=Er-E, is incident
on it before the life time, it stimulates
hv =Ea-Es /:
,,/""\..
4. Life Time: the duration
E: "'1 III Ild.1I,'Ll
l.nu i-.L.L
"llil
E,
photon as shown in figure. These two photons will have
same energy and phase. This phenomenon photons have the single wavelength
-'.FTER
BEFCRE
the
atom from Ez to El then a photon of energy hv releases along with the incident stimulating
-
is known as stimulated
emission.
and they are in phase. Thus the photons
The emitted are coherent.
of time spent by an atom in the excited state is known as life
time of that energy level. :.. Metastable state: The excited state, which has long life time, is known as metastable state. According to Heisenberg's an atomic
system
whose
uncertainty
energy
principle metastable
level width
state is an excited state of
is very small so that the life time of the
eleclrons is very large. Thus it is easy to achieve population
inversion
in metastable
state to
start lasing action. Page 2 of 10
6. Population: population
The number
of atoms per unit volu
e in an energy level is known as
of that energy level and is given by N = Noexp [
-£]
K8T
Where No is the population
in the ground energy level; KB is Boltzmann
constant; T is
temperature.
E3 ••• __
-_ Rapid Deca',' (Spontaneous
E2. ~
Decay)
••, -N2
r'.,letastab'e
state
Boltzmann's Distribution :: Le s er Tr ans rron
Et
Jt Figure(a)
7. Population
Et.
Pooutatton under normal condit ons
Inversion:
Ei, Ez& E3 are present
El is the ground
Flgure(b):
Inversion
in those energy levels are N 1, Nz & N3 respectively.
El Nz
>
N3 as shown in figure(a).
state; its lifetime is unlimited
energy state; its lifetime is very less. Whereas Hence Ez is metastable
Poputaton
Consider a three level system in which three active energy levels
and populations
In normal conditions
~-----------------
and it is most stable
state. E3 is highest
Ez is an excited state and has more lifetime.
state. When suitable form of energy is supplied
to the system in a
suitable way, then the atoms excite from ground state Ei to excited states(Ez&
E3). Due to
instability, excited atoms will come back to ground state after the lifetime of the respective energy states Ez and E3. Because E3 is the most unstable population
in Ez will become
called population 8. Pumping:
population
inversion
The
inversion
state, atoms will fall into Ez immediately.
more than the population
state. This situation
is
and is shown in fig(b).
population suitable
inversion
cannot
be achieved
form of energy must be supplied.
suitable form of energy to a system to achieve population In severalways
in ground
At a stage the
pumping can be done.Most commonly
inversion
thermally.
To achieve
The process
of supplying
is called pumping.
used pumping methods
are
Page 3 of 10
1. Optical
Pumping
(Used in Ruby laser)
2. Electric
discharge
method
(Used in He-Ne laser)
3. Direct conversion
(used
in semiconductor
4. Chemical
(used
in COzlaser)
reaction
laser)
9. Active Medium:A medium
in which
the population
as active
medium
for a laser.
makeup
medium
or the gain
of the gain medium,
which
inversion
Laser
is to be achieved
systems
are named
is called
based
on the
may be a gas, liquid or solid.
3. Explain construction and working of Ruby Laser The Ruby laser was the very first laser,
of energy:
Helical Xenon
Active Medium Optical
by Maiman
in 1960, which
emitted
deep
of the
Ruby
of 6943 Avin the form of pulses.
red light at wavelength Source
developed
flash lamp
Ruby rod
Cavity
Arrangement
of silver
rod.
polished
surfaces
on either
side
Resulting photons are reflected from end 10 end.
LASER BEAM
COOLING SYSTEM
Construction:
The Ruby laser
rod of length
between
Alz03 doped
with
parallel
to each
consists
5 to 20 em and diameter
0.05% of Cr203. other.
of a single crystal
Its two
One of the faces
end
is highly
of ruby in the form of a cylindrical
0.5 to 2 cm. Ruby rod is prepared faces
are
silvered
ground and
and
the other
from
made
absolutely
end
is partially
silvered.
Page 4 of 10
The ruby rod is placed along the axis of a high Intensity xenon flash lamp of helical shape which itself surrounded
by a reflector
to maximize the intensity
of the light incident on the
ruby rod. The lamp gives out flashes of brilliant light when its two ends are connected pulsed
high voltage
source.
During each pulse, the white
light flash lasts
milliseconds.
From the white light, energy in the form of radiation
for exciting
the Chromium
production
enormous
ions to higher
energy
amount of heat is generated.
levels.
H
by the ruby
the course
The ruby rod is protected
enclosing it in a hallow tube through which cold water circulation
for several
is absorbed
During
to a
of flash
from heat by
is maintained.
Level 3. (higher energy level) V\.,·" tust. '·udioUUIr,<.,\S
M ---,------~-__~
trcmst uon
_ e v el ::!. ( met astable star e )
....-L--L G
[e ve l I {~roHn d . rille •.
Fig:The energy l evel diagram
0
fer
3-
in the ruby crystal
Working: When the light from the flash lamp is incident ions absorb return
the radiation
and get excited
back to their ground
metastable
to the (higher
state G in two steps.
energy
level)H and these ions
First step is from higher
state H to
state M, which is a shorter jump and energy emitted in this transition
process to
the crystal lattice as heat. This transition The chromium
atoms returned
The accumulation transitions
on the ruby rod, the Chromium
is called radiationless
transition.
to M level can remain in this state for several milliseconds.
of excited atoms at M level increases
the population
occur from M to G level emitting out the photons
working of flash lamp, the chromium
randomly.
atoms are raised continuously
at M level and then Due to continuous
to higher energy level
H and then to M level.
Page 5 ofl0
At a particular chromium atoms
stage population
of excited
chromium
atoms will be reversed
atoms at M than at G. At this position photons
at level M to a significant
extent.
begin to interact
This results
in stimulated
and more
with chromium
emission
of other
identical photons and a cascade begins. The photons travelling photons travelling meantime,
in any direction
photons
the intensity
parallel to the axis of the ruby rod are used for stimulation
other than this will pass out from the ruby rod. In the
moving back and forth inside the ruby and continue
of radiation
while the
is great enough to come out in bursts
through
to build up until the partial silver
polished surface and it serves as an output laser. The output beam of wavelength
4.
is 6943Ao in the red region of visible spectrum.
Explain construction
The Helium-Neon
and working of Helium-Neon
laser was first built in 1961 by Ali [avan.It is a gaseous laser system and
is used to produce
a continuous
laser. This laser is highly directional,
coherent and stable. But the output power is moderate state laser. It is very useful in making holograms
Arrangement
:
Construction: The Helium-Neon
under a pressure
and interferometric
with the solid
experiments.
of Reflectors. laser system consists of a gas discharge
The tube is made up of quartz
mixture
the length of the discharge
is 10: 1. The power output
tube and pressure
tube are connected
tube which is the
and is filled with the mixture
of 1mm of mercury and Neon under a pressure
ratio of Helium-Neon
of the discharge
when it is compared
Helium and Neon gas mixture
Active Medium
active medium.
monochromatic,
R.F. Oscillator
Source of energy:
Optical Cavity
(He-Ne) Laser
of Helium
of O.lmm of mercury. The
from these lasers depends
of gas mixture. Further electrodes
to ar.f. oscillator to produce
electrical
upon
at the ends
discharge
in the
He-Ne mixture so as to pump the He atoms to higher energy state.
Page 6 of 10
,.,.,... Partially reflecting end plate
Quartz tube Fully reflecting end plate
He:" Ne gas' '----i
~
----. ~----"-V"'J-;~.
-
Laser beam
III t--.f\,/\/\--~
R High voltage source Working:
By the electrical
discharge
in the gas tube, the ground
excited to higher levels+le- and Hez. This process of excitation By resonance
collision transfer
energy to the ground
level Helium atoms are
is called electron
excitation.
method, the Helium atoms at He- give up their excitation
state Neon atoms. Thus the neon atoms are excited to their higher
energy level Nes, Meanwhile
these
Helium atoms
are de-excited
and returned
to their
ground state. Similarly the He atoms at He. give up their excitation atoms and the neon atoms
are excited to another
figure. The He atoms are de-excited
and returned
Since Ne3 and Nes are two meta-stable takes place at these levels. Anyone
wavelength
2. Transition wavelength
3. Transition wavelength
from
higher energy level Ne3 as shown in
energy levels of Neon atoms, population
of the spontaneously
emitted
photons
inversion
will trigger the
take place. They are
Nes level to the Ne4 level which
gives raise
to radiation
of
gives raise
to radiation
of
gives raise
to radiation
of
33912 AOwhich is in infra-red region.
from
the
Nes level to the Ne2 level which
6328 AOwhich is in visii.'e region (Red light). ,
from the
Ne3 level to the
Ne2 level which
11523 Avwhich is in infra-red
Some optical wavelengths
the
state neon
to their ground state.
laser action. Thus three main types of laser transitions 1. Transition
energy to the ground
elements
placed
inside
region.
the laser system
to absorb
the infrared
laser
33912 AUand 11523 AU.
Hence the output of He-Ne laser contains only a single wavelength
of 6328 AO. Page 7 of 10
He2---
.••. --
---P-"!P
Ne5
He-Ne Collision He1- •••••• ~ ••-
33912 AO LASER Transition
Mt Ne3
14e4
VV\ •.. •• '\,/V\,
11523 AO LASER Transition
6328 AO LASER Transition
_-~iiiiIII--Ne2 /
,/
//Fast Transition (Spontaneous Emission)
,/
Excitation By Electron Collision
••o?'o
Ne1
: Decay to Ground State : (Collisions with Tube Walls)
--
••••----.
.
t
Ground State
Helium Ftgure : Energy Level DiagramofHe-Ne
5.
Explain construction
Construction:One
_
Neon
and working
of
the
examples of semiconductor
lasers
Laser
of Semiconductor
Laser
Metal Contact
is gallium
arsenide
heavily doped n-region
semiconductor.
is formed
by
face
Its :1-= :grJf
heavily
-
doping
with
tellurium
in
1018
while its p-region
around
Active medium:The
Laser
Ouiput
r\
I::::::::::;::==:f==!=~pn juncnon
1019 atoms/ems. active medium
said that depletion
thickness of the depletion Pumping
-Oo~-'"
1==========+=7
by doping with zinc in
concentration
commonly
Polished Face ~
5x
of
is formed
3x1018 to
a
concentration atoms/ern!
Parlially
·polis ad
(GaAs). It is
Source:
Forward
in GaAs semi-conductor
region is the active medium
laser is GaAS. But it is also in semiconductor
laser. The
layer is usually very small (0.1 urn]. biasing is used as pumping
forward biased that is p side is connected
source. The p-n junction
to positive terminal
of the battery
negative. Under the influence of forward biased electric field, conduction injected from n side into junction
area, while holes will enter the junction
is made
and n side to
electrons
will be
from the p side. Page 8 of 10
Thus, there will be recombination
of holes and electrons
in depletion
region and thus
depletion region becomes thinner. Optical junction
resonator
system:
The two faces of semiconductor
plane make a resonant
cavity. The top and bottom
parallel to junction plane are metallised back faces are roughned
which are perpendicular
to suppress
so as to make external
the oscillations
to
faces of diode, which are connections.
in unwanted
The front and
direction.
Working When the junction
is forward
cause spontaneous
emission.
carrier
concentration
number of electrons
biased, at low voltage the electron But when the forward
and hole recombine
voltage reaches
a threshold
rises to very high value. As a result the region in the conduction
the valence band. Thus the upper energy level has large number of electrons energy level has large number
of vacancy,
t
~r
inversion
large
of holes in
and the lower
is achieved.
The
d
h>"':
Ec--..--------1.. fJ
thus population
value the
"d" contains
band and at the same time large number
and
/;."
~, tyr-·: '~eef""" ~.,.,~ :::',~ .."""",,~ Fe rrr i i :,'\" I Ef :~e . roBS:.<~'.:.•..• ..,~,.; ...'.... ;,~.. Ec.
Fig.ure(a) Unbiased
Ee-----------~ E~---------------~~I Figure(b) When bias voltage
recombination
of electron
others to emit radiation. 30,000
and hole leads to spontaneous GaAs produces
emission
laser light -of wavelength
is applied
and it stimulate ranging
the
from 7000 to
A.
Page 9 of10
6.
Write the applications
of Lasers
Medical applications
Welding
Garment
Laser nuclear
industry
and Cutting
Surveying
fusion
Laser printing
CDs and optical
Heat treatment
Barcode
Communication
discs
Spectroscopy
scanners
Laser cooling
HOLOGRAPHY The method phenomena
of producing of coherent
idea of holography
was first developed research
in this field. When an object is photographed
So holography
In
stands
object is obtained.
on the photographic
of the light waves
called hologram.
plate is known as holography.
by Dennis Gabber in 1948. The invention
image of 3-dimensional
light wave is recorded amplitude
image of an object due to the interference
light waves on a photographic
during 1960 enhanced a 2-dimensional
the 3-dimensional
are recorded
Greek,
'holo'
for whole
writing.
whole
and
i.e., holography
by a camera, of the
both the phase and the
in the film. The resulting
means
of laser
Here only the amplitude
film. In holography,
The
photograph
'graphy' means
means
"complete
is
writing.
writing"
or
"complete recording". The basic principle
of holography
of light, which interfere photographic for holography
with
is to create the image using two simultaneous
each
other
film. The image developed
to form
1. Recording The experimental
0f
image.
arrangement
is a two-step
ue:tm
.
on a suitable
The light source used
process of 1. Recording of a
I LASER
of a ho Iogram
image
in this way is the hologram.
is usually a laser light.Holography
hologram 2.Reconstruction
a complex
beams
..
a laser
M2';
1 • M1
.,
for the
Splitter
~
2
I ,
I
recording
of a hologram
using
beam is shown below: Reference'
When a laser beam is made to fall on a beam splitter, the laser beam is divided
~ Objeet ! Object .~ Be:am
Be:Jm
Photogr:aphic
PI,.tc
Page 10 of 10
into two parts. A part of the beam is made to fall on the object by means of a plane mirror M1.The scatteredrays the photographic
from the surface of the object, called object beam,
plate. The other part of the beam is made to fall on a mirror M2 and then
to photographic
plate. This beam is called referencebeam.
beam interfere
and the interference
the photographic
wavelength
has no resemblance
the
laser
having
used for recording
images . a two dimensional
The virtual identical
by a process called reconstruction.
is to be4
-, .
the same
results
and
to the object.
GRAM
in two
real image and a
the
'
1"-----
H0
LASER ///
virtual image.
image
on
to the original object. It has in it a coded form
of the hologram.
of the hologram
three dimensional
of the object is recorded
pattern gives hologram.
of the object. The image is reproduced
with
Illumination
is characteristic
interference
In order to view the image, hologram illuminated
The object beam and reference
of image
hologram
of information
pattern
plate. This recorded
2. Reconstruction The recorded
is made to fall on
//
//
real
image
If the observer
are
•
moves
/
vjrtuer jmeqe (3~)
Realim••ge .••• (20) •
round the virtual image then other sides of the object which were not noticed earlier would be observed.
Therefore,
characteristics
of the object.
Applications
the
virtual
image
exhibits
all the
true
three dimensional
of holography
1) In information
storage in computers.
2) In fog droplet camera. 3) In dynamic aerosol camera. 4) In holographic
interferometry.
5) In holographic
cinema.
6) In acoustic holography. 7) In data processing. 8) Hologram can be used as an optical grating.
Page 11 of 10
----------------------
---
--
-
-
--
Questions from previous examinations 1. Distinguish between
spontaneous
2. What are Einstein coefficients emission and stimulated 3. By considering
and stimulated
emission
and explain the concept of absorption,
spontaneous
emission
Einstein coefficients,
show that population
inversion
is requiredf No-Nr]
for laser action. 4. Draw a neat diagram to represent construction
the components
of a Ruby laser and explain its
and working.
5. Explain the construction
and working ofHe-Ne laser
6. Explain the construction
and working of semiconductor
7. Mention few applications 8. Explain the construction 9. What is holography?
laser
of lasers. and reconstruction
of hologram
How is it different from conventional
photography?
Page 12 of 10
