MODULE – 1
Chapter-1
LASER
Introduction
A laser is an electronic optical device that amplifies light and produces a highly
directional,more intense ,coherent light radiation.
The term LASER is an acronym for Light Amplification by
Stimulated Emission of Radiation. The concept of stimulated emission was
proposed by Einstein in 1917.Then Schawlow and Townes gave an explanation
for laser oscillation. In the year 1960,T.HMainmann demonstrated the first
laser the ruby laser.
The laser source has unique characteristic that make it a special light source
for many industrial and medical application.
Characteristics of Laser
The following are the remarkable characteristics of laser by which it is
distinguished from other ordinary light sources.
1.Directionality
The laser is highly directional. It means that the laser light does not
spread in all directions and it is concentrated in only one direction .
Therefore the laser beam is very strong and does not require external
systems such as lens for focusing on to particular point.
The degree of directionality is expressed in terms of divergence. The divergence
tells how rapidly the beam spreads when it is emitted from the laser.
The light beam can travel as a parallel beam up to a distance of
d2
where ‘d’ is

the diameter of the aperture through which the light passing and '  ' is the
d2
wavelength of the light used. After travelling the distance
, the light beam

spreads radially. In ordinary light beam, the angular spread is given by  
. In ordinary light beam, the angular spread is 1mm/1m ,but for an ordinary
1
d

source of light the angular spread is 1m per 1m.For example, the laser beam
can be focused to moon from the earth with an angular spread of a few
kilometers on the surface of the moon.
If a1 and a 2 are the diameters of laser radiations at distances d1 and d 2 from a
laser source respectively then the angle of beam divergence is given by

a 2  a1
2(d 2  d1 )
2. Intensity
The laser beam is more intense beam. As the laser beam is highly
directional, it is more bright such that a 1mW laser is thousand times powerful
than a 100W ordinary light source.
It is surprise that a 1mW He-Ne laser emits a laser beam of intensity 3 10 8
Wm-2.
Where a sun emits light of intensity 2  10 7 Wm-2.
If a person is allowed to observe ordinary light emitted by a 100W bulb at a
distance 1m from the source he can perceive only a few thousands of a watt of
light. While, if the person is allowed to observe a laser beam from the same
distance , the entire laser beam penetrates through his eye. It will damage the
eye of the observer. This shows the high intensity of the laser beam.
3. Monochromatic
The laser beam is monochromatic. It contains one specific wavelength
of light. It means that it has only one colour.
The monochromaticity is related to the wavelength spread of the radiation is
given by
 C 
   2  V
V 
The value of  is in the order of 300nm for white light, 0.01nm
for gas discharge lamp, while it is .0001nm for laser.
5. Coherence
Two sources are said to be coherent sources, if they have same phase Or
constant phase difference,same frequency,same amplitude,same wavelength,
and same direction of emission.
There are two types of coherent sources
(1) Temporal coherence
(2) Spatial coherence
2
If two sources have coherent property only for a short period of time ,
then it is said to be temporal coherence .
The coherence length,
Lc  CTc
Lc Coherence length( Lc ,is used to find out how long the two sources
are
maintaining the coherence property)
C Velocity of light
Coherence time
Tc
Temporal coherence describes correlation b/w signals observed at different
moments in time. The two waves are said to be temporally coherent if they
having same propagation characteristics at different instants of time.
1
Coherent time Tc 

Spatial Coherence
If two sources are having coherent property for the entire period of their
travel,it is said to be spatial coherence.Spatial coherence describes the
correlation b/w signals at different points in space. Thus the spatial coherence
is described as a function of distance.
   
, where '  ' is the wavelength of laser beam and  is the
lc   

   
wavelength spread(line width) of a laser beam.
2
Einstein Theory Of Radiation
Einstein 1917, considered the interaction of photons with assembly of atoms
and showed that excited atoms would produce stimulated emission in addition
to spontaneous emission.
Consider two energy levels E1 and E2 with E1< E2. Let N1 and N2 be
the number of atoms in the energy level E1 and E2 respectively.
When light radiation of energy h is made to incident on these
atoms, the following process takes place.
(1) Stimulated absorption Or absorption.
(2) Spontaneous emission.
(3) Stimulated emission.
3
(1) Absorption Or Stimulated absorption
An atom in the lower energy level E1 can absorb photon of energy h and
goes to higher energy level E2. This process is called stimulated absorption Or
absorption.
Atom + Photon → Atom*
The rate of absorption depends on energy density of incident radiation (  ) and
no. of atoms N1 in E1.
RAB   → (1)
RAB  N1
From (1) & (2)
RAB   N1
→ (2)
RAB = B12 
(1)
N1
Where B12 is called Einstein Coefficient.
(2)Emission
The lifetime of the excited atom is very less. So the excited atom returns
to the ground state by any one of the following.
(a)Spontaneous emission
4
The atom in the excited state returns to the ground state with out
help of any external radiation. During this transition a photon of energy h is
emitted. This transition is called spontaneous emission.
Atom* →Atom + Photon
The rate of emission is proportional to the no. of atoms in the state E2.
RSP  N2
(2)
RSP=
A21N2
Where A21 is called Einstein coefficient.
(b) Stimulated emission
The atom in the excited state returns to the ground state with the
help of external radiation, ie, a photon . During this transition a photon of
energy h equal to the energy of the incident photon is emitted. This process is
called stimulated emission.
Atom* + Photon → Atom + 2 Photon
5
The rate of transition is directed propotional to energy density (  ) of incident
radiation and number of atoms N2 in E2.
RST   →(1)
RST  N2 →(2)
Combining eq(1) &(2)
RST   N2
RST = B21  N2
(3)
Where B21 is called Einstein coefficient.
At thermal equilibrium,
Rate of absorption =Rate of emission
RAB = RSP + RST
B12N1  =A21N2 + B21N2 
B12 N1   B21 N 2   A21 N 2
 ( B21 N 1  B21 N 2 )  A21 N 2

A21 N 2
B12 N 1  B21 N 2
Dividing by B12N2 in all terms,
A21
B21

B12 N 1
1
B21 N 2
(4)
According to Boltzmann distribution law,
Number of atoms in an energy level,
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N  N0e
E
K BT
N1  N 0 e
 E1
K BT
N 2  N 0e
N1
e
N2
(5)
 E2
K BT
E2  E1
K BT
But, E2  E1  h
h
N1
 e K BT
N2
(6)
Substitute eq(6) in eq(4)
Energy density of incident radiation,
A21
1
(7)
B21 B12 KhT
B
e
1
B21
According to Plank’s theory of radiation, the energy distribution is given by,


8h 3
c3
1
h
k BT
(8)
e 1
Comparing eq(7) and(8)
A21 8h 3

B21
c3
(9)
B12  B21
From the above equation it is confer that the rate of stimulated absorption per
atom (B12) is equal to the rate of stimulated emission per atom(B21).
Ratio Of Spontaneous Emission to Stimulated Emission
From plank’s theory , the energy distribution is given by

But
8h 3
c3
, A21
B21
1
e

h
1
k BT
8h 3
c3
7

A21
B21
1
e
A21
e
B21 
h
K BT
h
k BT
1
1
Even for sources operating at higher temperatures and lower frequencies
h  K BT . Therefore A21>> B21 which means that under normal conditions of
thermal equilibrium, spontaneous emission predominates the stimulated
emission.
DIFFERENCE B/W SPONTANEOUS AND STIMULATED EMISSION
S.no Spontaneous emission
Stimulated emission
1
2
3
4
5
6
The atom in the excited state
returns to the ground state by
emitting a photon without any help
of external photon is called
spontaneous emission.
The emitted photon can move
randomly
It emits incoherent photon
The rate of transition is given by
RSP = A21N2
Emission can’t be multiplied with
chain reaction.
Eg. Sodium vapour lamb
An atom in the excited state
returns to ground state by emitting
a photon with the help of an
external photon is called as
stimulated emission.
The emitted photons move in same
direction and highly directional
It emits coherent photon
The rate of transition is given by
RST = B21  N2
Emission can be multiplied with
chain reaction.
Eg. Laser beam
PRINCIPLE OF LASER ACTION
Consider a group of atoms all in the same exited state. When light
radiations is exposed, a photon may cause stimulated emission in one of these
atoms. The result is emission of two photons.
Each of these photons may cause induced emission in two other
excited atoms . This process may continue in a chain reaction. The result will
be an intense beam of photons moving in the same direction (shown in fig.) and
all are coherent. This is the basic principle of operation of lasers.
“ Due to stimulated emission , the photons cause a chain reaction
giving rise to an intense beam of coherent photons, the light get
amplified”
8
POPULATION INVERSION
To obtain the condition for getting more number of stimulated emission than
spontaneous emission, consider the following ratio
Rate of stimulated emission
B21 N 2 
B12 N1 
=
Rate of stimulated absorption
=
As
B21 N 2
.
B12 N1
B21  B12
RST N 2

Rab
N1
(1)
To get more stimulated emission, the value ofN2must be greater than
N1i.e,when the number of atoms in excited state is more than the number of
atoms in lower energy state, the probability of getting stimulated emission will
become more predominamt.
9
“ The situation of getting more number of atoms in the excited state than the
number of atoms in the ground state is called population inversion”.
We know that,
h
N1
 e K BT
N2
In L.H.S of above equation h,KB values are positive and constants . The
frequency will have only positive value. Hence
(I)
For ‘T’ is positive,
N1
 e ve
N2
N1  N 2
(ii)
For ‘T’ is negative
N1
 e ve
N2
N1  N 2
Therefore population inversion can be achived only in the negative temperature
which is not possible practically. There fore an external agency is required.
“ The process of raising more number of atoms from the ground
state to the excited state by an external agency is called pumping”.
PUMPING PROCESS
The most commonly used pumping methods are
a) Optical pumping Or Photon Excitation
b) Direct electron excitation
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c) Direct conversion
d) In elastic atom –atom collision
e)
Chemical bond
a) Optical pumping
In this method ,alight source emits photons and these excite the atoms in
the ground state to the excited state.
A  h  A*
e.g. Ruby laser, Nd YAG laser
b) Direct electron excitation
These electrons are accelerated to high velocities by the electric field and made
to collide with gas atoms. These atoms gain the kinetic energy of the electron
and raised to the excited state.
A  e  A*
Example: He-Ne laser , CO2 laser, Argon laser
c) Direct conversion:-When electric energy is applied to the direct band gap
semiconductors, recombination of electrons and holes take place and light will
be emitted. Hence the electrical energy is converted into light energy directly.
Example: Semiconductor diode laser
d) In elastic atom –atom collision
A combination of two types of gases is used in this method. These two
gases are having same excited states that coincide Or nearly coincide.
First, the gas A is excited by electric diacharge.
A  e  A*
This excited atom collide with another gas atoms B so that the atom B gets
excited
A*  B  A  B *
Example: He-Ne laser
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e)Chemical method
In this method, the molecules involve in chemical reaction and the product
molecule Or atom will be in the excited state.
Example: Hydrogen fluoride
H 2  F2  2HF *
METASTABLE STATE
The energy states are classified into three categories according to their life time.
It is the time an atom spends in one particular state
Stable( ground) state:- It is the state in which an atom exists forever unless
forced upon to leave. Its life time is infinite.
Excited state:-Whenever an atom is excited, it moves to a different state and
spends some time there usually in nanoseconds to picoseconds. Such a state is
referred to as the excited state. Emission from such a state is spontaneous in
nature as the life time is small. Here the atom cannot be made to interact with
external controls.
Metastable state:- The atom excited to this state spends time is about
milliseconds-that is , nearly 1010 times more than in the excited state. Such a
state is known as metastable state
The emission from such a state is known asdelayed emission Or
phosphorescence. Since life time is higher , this state is suitable to bring about
population inversion and the atoms can be made to interact with external
controls so as to get stimulated emission.
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CONDITION FOR LASER ACTION
The two important conditions for laser action are
i)
ii)
Population inversion should be achieved
Stimulated emission should be predominant over spontaneous emission.
LASER ENERGY LEVEL SCHEME
a) Two level system
b) Three level system
c) Four level system
There are mainly Four levels
Ground state level:-The energy level in which more number of atoms are
available in usual condition.
Pump level:- The energy level to which the atoms are excited.
Lower Laser Level(LLL) and Upper Laser Level( ULL):-The energy level where the
transfer of atom to this level from metastable state results laser beam.
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A) Two level system
A two level pumping is not suitable for obtaining population inversion.The life
span t , for which atoms have to stay at upper energy level E2, must be longer
for achieving population inversion condition. According to Heisenberg
uncertainty principle,
E.t  
t will be longer if E is smaller, that is, E2 is narrow.If E is smaller, the
pumping efficiency is smaller, As a consequence of which less number of atom
excited . Though a sharp energy level supports the population inversion ,
enough population cannot be excited to level E2 in the view of small E . The
result is that the upward transitions would be accompanied by premature
downward stimulated transitions and the population in level E2 would not
accumulate to the required extent.
Laser materials cannot be produced at will such that they would have
a three-level Or a four –level pumping scheme . They have to be selected from
hundreds of available materials to suit a specific purpose.
B)Three Energy Level Laser:-The threelevel lasers will have ground start, pump
level, and lower laser level Or upper laser level
If the ground state itself the lower laser level , there will be
upper laser level
Example : Ruby Laser
Otherwise, there are Ground state (GS) , Pump Level(PL) And Lower Laser
Level(LLL) only.
Example : N2 laser
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C)_Four Energy Level Laser:- There will have Ground state , Pump level , Upper
laser level and Lower laser level
Example: Nd YAG laser
In the above four energy levels ,upper laser level Or lower laser level is a
metastable state.
“ A metastable state is an excited state at which the life time of the atoms is
larger”
The large life time increases yhe probability of stimulated emission which is
the important condition for laser action and helps to achieve population
inversion.
Therefore , if metastable states do not exist , there could be no population
inversion and hence no laser action.
COMPONENTS OF LASER
A laser needs three components
15
1. A medium such as gas, liquid Or solid etc. which contains the assembly of
atoms Or molecules . The medium is called as active medium and the atoms Or
molecules are known as active centre
2. A pump Or excitation source such as high voltage circuiteOr flash lamp etc. to
bring about strong excitation and population inversion.
3. An optical cavity Or resonator constitutes an active medium kept b/w a fully
silvered Or partially silvered mirror.
OPTICAL RESONATOR( OPTICAL CAVITY)
The optical resonator consists of a pair of plane Or spherical mirrors placed at
each end of the lasing medium. One of the mirrors has 100% reflection , and
the other is half silvered, acting as a partially reflecting mirror. The optical
resonator is basically a feedback device that directs the photons back and forth
through the laser medium.
As the light is bounced b/w the two mirrors, it increases in strength ,
producing optical feed back which results in the amplification of light. A part of
this light escapes through the partially transparent morror as laser beam,
while some of the light will be trapped inside to continue laser generation.
16
LASER ACTION
The steps involved in the laser action is shown in fig. as flow chart and
explained as follows.
Step 1: Pumping: The atoms in the ground state are excited to the pump level
Step 2: Population inversion: The atoms in the pump level spontaneously
dropped to the meta stable state . As life time of the atoms are greater
compared with pump level , accumulation of atom takes place resulting
population inversion .
Step 3 : Stimulated emission: At this stage, a spontaneously emitted photon
incidents on the excited atoms and triggers the stimulated emission.
Step 4: Amplification: The coherent photons emitted due to stimulated
emission are reflected back by the mirrors and get amplified.
Step 5: Laser: Finally , an intense beam of amplified light is emitted through
partially silvered mirror.
17
TYPES OF LASER
Based on the materials used (i.e) active medium , lasers are classified into five
types.
a) Solid state laser
Example: Ruby laser, Nd YAG laser.
b) Liquid laser Or Dye laser
Examples: Organic dye laser
c) Gas laser
Example: He-Ne, CO2
d) Semiconductor laser
Example: GaAs laser
e) Chemical laser
Example: HF laser
He- Ne Laser
He-Ne laser is a first continuous wave gas laser. It is four level laser. It
was invented by Ali Javan in 1961.
Principle
The population inversion of Ne atoms is achieved by inelastic
collision with excited He atoms. If an external photons triggers the
stimulated emission , then the laser beam of wavelength 6328 A0 is
emitted.
Construction
Active medium: The mixture of helium and neon gases mixed in the
ratio of 10:1 acts as active medium . They are mixed under the pressure
of 1 mm Hg of He and 0.1 mm Hg of Ne. The mixture is filled inside a
quartz discharge tube, which is connected to an electric power supply.
Pumping system
18
Population inversion is achieved by inelastic collision b/w the
excited He atom and lower state Ne atom.
Resonant cavity
The two end faces of the discharge tube are attached Brewster
window. Two concave mirrors , one fully silvered and another one is
partially silvered.
19
Working
1) When the electric power is switched ON , the electrons from the
discharge tube collide with helium atoms and excite them to the
energy levels F1 Or F2.
He  e  He * (since the helium atoms are more readily excitable than
2)
3)
4)
5)
Ne atom because they are lighter)
The excited He atoms inelastically collide with the neon atoms and
raise them to its excited energy levels which are very closer to the
excited energy levels of helium atoms.
He *  Ne  He  Ne*
As excited energy levels of He and Ne are very closer, the energy
transfer is known as Resonant Energy Transfer.
Since E3 and E5 energy levels of Ne atoms are metastable state ,
population inversion takes place at these energy levels.
Any one of the spontaneously emitted photon will trigger the
stimulated emission resulting laser output. The laser beam are
produced by three types of transition.
They are :
a) From E5 to E4,   3.39 m ( infra red region)
b) From E5 to E2,   6328A 0 ( Visible region)
c) From E3 to E2,   1.15m ( Infrared region)
6) The transition from the energy levels E4 to E1 and E2 to E1 in the form
of fast decay giving photons by spontaneous emission.
7) During these transitions , heat energies are produced. As helium has
high thermal conductivity , it helps to conduct heat away through the
walls.
8) Finally, the Ne atoms are returned to the ground state from E1 by
non radiative diffusion and collision , Therefore ,no radiation will be
emitted during this transition.( Neon atoms returns to ground state by
collision with walls of discharge tube, otherwise no. of atoms available
at the ground state will go on diminishing and the laser o/p
decreases. To increase the probability of atomic collisions with the
walls , the discharge tube is made narrow).
9) By suitable optical filter , the laser beam of particular wavelength can
be obtained.
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ADVANTAGES
1)
2)
3)
4)
The output is continuous .
It is highly monochromatic and directional
It has high frequency stability
Cooling system is not required
DISADVANTAGES
1) Power out put is average when compared to that of solid state lasers.
2) It comes in very high power which can even burn skin.
Applications
1)
2)
3)
4)
It is used to produce holograms.
It is used in laser printing , barcoding etc.
It is used to determine the size of tiny particles.
He-Ne laser are used in laser surgery to position the powerful
infrared cutting beams.
5) Surveyors take advantages of He-Ne lasers good beam quality to take
precise measurements over long distances Or across inaccessible
terrain.
RUBY LASER
The Ruby laser is a three level solid state laser and it was first
demonstrated by T.H Mainman in 1960
CONSTRUCTION
The Ruby laser uses a ruby rod as an amplifying medium. The
Ruby Rod is an Al2O3 crystal doped with Cr2O3. Only 0.05 wt% of
Cr3+ is doped with Al2O3. The colour of the ruby rod for 0.05 wt%
of Cr3+ doping is pink.
A ruby laser uses a ruby rod of length 4 c.m and diameter 5
mm. The two ends of the ruby rod are madeas optically plane and
parallel. They are made as highly polished surfaces so that one
end is fully reflecting surface, whereas the other end is partially
reflecting surface. A helical xenon lamp is made to surrounds the
quartz tube . The ruby rod is kept inside a quartz container. Air is
circulated through the apparatus as a coolant. The diagrammatic
representation of a ruby laser apparatus is shown in fig.
21
WORKING
When an electric field is applied to the flash lamp it
produces a flash of light. The flash of light is absorbed by Cr3+
ions, and hence it gets excited to higher energy levels.The flash of
light produced by xenon lamp is a white light and it has a broad
spectrum of wavelength. If Cr3+ absorbs 0.42µm wavelengths it
gets excited into the second excited state 4F1 and if it absorbs a
wavelength of 0.55µm , the Cr3+ gets excited to the first excited
state labeled as 4F2 . The excited Cr3+ atom exists in that excited
state only for a short period of time. After spending a short period
of time in the excited state, it returns to the metastable state.
When it is making a transition from the metastable state to the
ground state , it emits a laser radiation of wavelengths 0.6943 µm
and 0.6927 µm respectively. The emitted light move with in
amplifying medium and get reflected by the resonators and finally
emerge out as a red beam from partially reflecting surface.
22
Advantages
1) The out put power of a Ruby laser is 109 W.
2) The out put laser beam is produced in the form of pulse.
3) The output posses a high degree of coherence.
Drawbacks
1) The Ruby laser is a solid state laser and hence it has
crystalline imperfection, scattering and thermal distortion
in output.
2) Solid state laser has poor monochromaticity and
directionality compared to gaseous lasers.
Difference between Ruby and He-Ne laser
RUBY LASER
He-Ne LASER
It is a solid state laser . Its active
medium is a ruby rod.
It is a gaseous laser. Its active
medium is a mixture of helium and
neon gas
It produces output in the form of a
continuous wave
It produces pulsed output
The output power is in the order of
109 W
It is a three level laser
Its output is red in colour and its
wavelengths are 694.3 nm and
692.7 nm.
The atoms are pumped in to the
higher energy levels by optical
pumping
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The output power is in the order of
10-3W
It is a four level laser
Its output wavelengths are
632.8nm,3.39µm and 1.15 µm
The atoms are excited into the higher
energy levels by atom-atom inelastic
collision
1)
2)
3)
4)
5)
APPLICATIONS
It is used in rangefinding
The Ruby laser was the first laser used to optically pump tunable dye
laser and is particularly well suited to excite laser dyes emitting in the
near infrared.
It is usedin drilling holes through diamond
Ruby laser were used extensively in tattoo and hair removal
Many non-destructive yest labs use ruby laser to create holograms of
large objects such as aircraft tires to look for weakness in lining.
SEMICONDUCTOR DIODE LASER (HOMOJUNCTION)
Introduction
The semiconductor lasers are solidstate lasers. These lasers are
very tint and compact. They are best for optical communication and they
paved the wayfor transition from integrated circuits to integrated optics.
Principle
When forward bias applied to a PN junction diode , recombination of
electron-hole pair takes place which results radiation of energy in the form
of light. This is called recombination radiation. When this light is amplified
produces a laser beam.
Construction
24
Active medium
The active medium is a PN junction diode made from a single
crystalline material GaAs. In this P region is doped with Germanium and N
region is doped with Tellurium.
Pumping source
It is a special type of laser. In this laser population inversion is
achieved by heavily doping the Ptype and Ntype semiconductor. An
externally connected battery is used to pump the charge carriers to
produce laser by direct conversion of optical energy.
Optical resonator
A pair of parallel planes at PN junction is well polished and it acts
as optical cavity for laser action.
Metal contacts are made at P and N region for external battery
connections.
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Working
1. The energy band diagram of heavily doped laser is shown in fig. Due to
high doping the Fermi level of N type semiconductors lies within
conduction band whereas that of P type lies within valence band.
2. When the junction is forward biased by means of an external bettery,
electrons and holes are injucted into the junction. (i.e) the charge
carries are pumped by DC voltage.
3. At very high forward current , the concentration of electrons in the
conduction band of N type and the concentration of holes in the
valence band of P type increases. Hence population inversion is
achieved at the junction. This region is called active region Or inversion
region.
4. Due to forward bias , recombination of electrons and hole results a
spontaneous photon. This photon triggers further recombination of
electrons and holes and hence produce a laser beam.
5. The wavelength of laser depends on the band gap of the material used.

hc
Eg
For e.g. GaAs laser has band gap value 1.44eV and produce laser of
wavelength 8626 A0 in IR region.
Advantages
1. It is easy to manufacture and the size is very small.
2. The cost is low.
3. It can be operated at low power.
4. The out put is modulated by varying the junction current.
5. No external mirror is used for optical cavity.
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Disadvantages
1. The beam has large divergence.
2. Coherence of the beam is very poor.
3. It produces only 1mW power out put.
Applications
1. The semiconductor diode lasers are used for recording and reading of
CDs.
2. It is used for bar coding.
3. Due to small size, they are used in fibre optic communication.
4. It is used in laser printers.
5. It is used to heal the wounds by means of IR radiation.
Semiconductor Diode Laser( Hetero Junction)
If the material on one side of junction differs from that on
the otherside of the junction , it is known as Hetero junction. In
modern GaAs diode lasers, a hetero junction is formed between GaAs
and GaAlAs.
Principle
When forward bias is applied to a PN junction diode, recombination
of electron-hole pair takes place which results radiation of energy in
the form of light. When this is amplified produces a laser beam.
27
Active medium
The heterojunction of laser consists of five lasers . The third layer
GaAs (n-type) has narrow band gap and acts as active region. This
layer is sandwitched between the layer of GaAlAs (p type) and GaAlAs
(n type).
For biasing a contact layer of GaAs p type is made as the top
layer. All these four layers are grown over the substitute made up of
GaAs n type.
Pumping source
In this layer, population inversion achieved by heavily doping the
p and n regions. An externally connected battery is used to pump the
charge carriers to produce the laser beam by direct conversion.
A battery is connected b/w two electrodes.
Working
1) The diode is forward biased with the help of an external battery.
2) Due to this forward biasing , the charge carriers are injected into
the active region from the GaAlAs (p type) and GaAlAs (n type)
layers.
3) The charge carriers are injucted continuously until the
population inversion is achieved.
4) The electrons and holes in the active region recombines and
emits photon spontaneously.
5) These spontaneously emitted photons triggers further
recombination and emit more photons.
28
6) These photons are reflected back and forth at the junction and
finally emit the laser beam.
7) The wavelength of laser beam is depends on the energy gap of
the materials used.
For e.g., if the energy gap Eg =1.55 eV then wavelength
of laser is,
hc

Eg
6.624  10 34  3  108
1.55  1.6  10 19
= 8014 A0
Advantages
1. It produces continuous wave Or pulsed wave output.
2. The output power is high.
3. The laser beam is highly directional and coherent.
Disadvantages
1. Growth of different layers is practically difficult process.
2. It is costly compared with homojunction laser.
Applications
1. Mostly used in fibre optic communications.
2. It is used to read/ write CDs.
3. It is used in laser printer.

Advantages and Disadvantages Of Ruby laser , He- Ne laser and
Semiconductor laser
Advantages
1.
2.
Ruby Laser
He-Ne Laser
Semiconductor Laser
Easy to construct
and operate
Very strong and
intense beam upto
a power of 10 kW
Easy to construct
and operate
Continuous beam
Easy in operation
29
Long life, highly
monochromatic ,
tunable and
continuous beam
3.
Beam diameter as
large as 25 mm and
operation duration
of few hours.
Exceptionally
monochromatic
beam with high
operation
duration.(10,000
hrs)
Excellent efficiency
with very high
operation duration(
20,000 hrs)
Its laser beam is
only pulse like and
its operation
duration is very
less( few hours)
It has got very
low power about
0.5- 5 mW
It has got low power
about 200mW.
Disadvantag
es
Comparison of Lasers
S. Charecteristics Ruby laser
No
1
Type
Solid state laser
2
Active medium
Ruby rod
Semiconductor
laser
Semiconductor
laser
PN junction
He-Ne laser
Active region
where the
recombination
of electrons and
holes takes
place
Direct
conversion
Ne atom
Well polished
PN junction
Concave
mirrors
1mW
Continuous Or
pulsed.
In IR region
0.5-50mW
Continuous
Gas laser
Mixture of HeNE in the ratio
of 10:1
3
Active centre
Cr3+ ions
4
Pumping
method
5
Optical
resonator
6
7
8
Power output
Nature of
output
Wavelength
9
Application
Optical pumping
by Xenon flash
tube
Ends of polished
rod with silvered
mirror
109W
Pulsed
0.6943µm and
0.6927µm
1. it is used
in range
finding.
2. Drilling
1.Reading and
writing of CD.
2.Barcode
reading.
30
Electrical
discharge
1.5µm, 3.39µm
and 6328A0
1. Open air
communication.
2. Hologram
holes
through
diamond.
3. Used in
tattoo and
hair
removal
system
3. Fibre optic
communication
Difference b/w homojunction and heterojunction laser
S.No. Homojunction laser
1
It is made by a single crystalline
material
2
Power output is low
3
Pulsed out put(sometimes
continuous)
4
It has high threshold current
density
5
Cost is less
6
Life time is less
7
E.g.(1)GaAs (ii) InP
Heterojunction laser
It is made by different crystalline
materials
Power out put is high
Continuous output
It has low threshold current
density
Very costly.
More life time
E.g.(i)GaAs/GaAlAs (ii) InP/InAlP
Difference b/w LED and LASER
S.No LED
1
It requires low current density
2
Junction of diode need not be
polished
3
Minority carried injunction
should take place
4
Power output is low
5
Intensity is less
31
LASER
It requires high current density
Junction of the diode should be
highly polished
Stimulated emission will take place
Power output is high
Intensity is very high
APPLICATION OF LASERS
The laser beam has wide applications due to its special charecteristics.
In metrology
1. Laser alignment – Aligning of tubes Or pipes using laser.
2. Measurement of distances- In Michelson interferometer , laser is used
instead of sodium vapour lamp for measuring distance.
3. Remote sensing- As the laser beam is intense , it interact with cloud,
smoke , pollutants, aerosols etc. and therefore it is used for weather
monitoring.
In
1.
2.
3.
Industries
High power lasers are used for welding , drilling, cutting etc.
Lasers are employed in NDT methods to test the quality of the materials.
Using lasers, three dimensionless photography can be constructed and
reconstructed. This photography is called as holography.
In communication
1. As the diode laser is smaller in size, they are used in fibre optic
communication.
2. Lasers are modulated to transmit more number of messages.
3. As the laser waves are not absorbed by water , they can be used for
underwater communication b/w submarines.
In Computer
1. The lasers are used to transfer entire memory of one computer to
another computer , with the help of optical fibres as light guide.
2. Lasers are used in laser printers.
3. Diode lasers are used to read and write a CD.
In Military
1. A powerful laser beam can be used to destroy big size objects like
missiles.
2. Lasers with moderate power is used to disable the enemy weapons
3. Lasers are used to guide the missiles and satallites.
In Chemistry
1. Using lasers , billions of photons of same energy can dump in small
volume.
2. Isotope separation can be done with lasers.
3. It can accelerate some chemical reaction.
32
In Medicine
1. They are used for treatment of detached retina.
2. Lasers are also used for removal of piles, tattoos Or superficial
unwanted.
3. Lasers are used for micro surgery and bloodless operations ti cure
cancer and skin tumors in human beings and animal
A) Medical Application
Treatment of tumours
The malignant tumours absorb the laser beam strongly. This property is
used to remove tumours successfully.
Dental Studies
The decayed area of the teeth has rough surfaces and appears dark and
dull in colour. The decayed portions of our teeth absorbs more amount of
laser beam. This property is used to remove the caries ( decayed portions)
of the teeth. Ruby laser is generally used for this purpose.
Surgical Applications
The laser beam is used to cut the blood vessel and tissue without
producing bleeding. This makes surgery easier. It is used for the
operation like the removal of a portion of liver lobe, which was previously
impossible .
Treatment of retina
If tear develops, it passes from vitreous body through the hole pushing
the retinal cell away from choroids and it produces partial blindness. This
is said to be retinal photocoagulation. The retina is spot welded using a
laser beam and brought back into the original position.
Removal of urinary stone
Laser in conjunction with fibre optic endoscope used to vapourize blood
clots and to remove urinary stones.
33
B) Industrial Application
In industry laser is used for cutting, welding, drilling and
surface hardening.
Cutting
In Order to cut a metal, the laser beam is made to fall on the metal. The
light energy made to fall on the metal gets converted into heat energy.
Due to this heat energy the metal may gets heated. If the heating is
sufficient, the metal melts and then vapourised. Then the metal is cut
into two portions.
Welding
Two different materials are brought together and then a laser beam is
made to fall on them. The two metals are heated together to a compatible
condition and fuse them together.
Drilling
In order to drill a hole in a metal, a laser beam is made to fall on the
required portion of the metal. Due to the intensed heating of laser on the
metal, a fine hole is produced.
Surface hardening
The laser beam is used to increase the hardness of the outer surface of
steel. The laser beam is made to fall on the outer surface of a material
and hence it melts the outer portion of the materials and then it is cooled.
Due to this the hardness of the outer surface is increased. This process is
said to be laser surface hardening.
C) Communication
The laser is used in optical fibre communication. The higher
bandwidth is one of the advantages of the laser beam. The bandwidth is
directly proportional to the rate at which the information can be sent. Due
to the higher bandwidth, larger information can be sent simultaneously.
34
No cross talk , no leakage, safer communication, small in size, light
weight are some of the other advantages of communication using laser
beam
Other Applications
Nuclear fusions
Fusion is the process of producing a heavier nucleus by combining any
two lighter nuclei. To produce the fusion reaction, a very high
temperature in the order of 108 K is required. When a laser beam with
high output power is made to incident on the pellets of the reactants, the
incident laser beam produces sufficient heat energy required for the
fusion reaction.
Separation of isotopes
The separation of isotopes is possible using the laser beam. Consider the
uranium ore. It has two isotopes, namely U235 and U238. These two
isotopes are having different excited energy levels, i.e. the excited states of
U235and U238are different. In order to separate U235 and U238 , a laser
beam having wavelength of  
hc
, is made to incident on the
E 2  E1 
uranium ore, where E1andE2 represent the first and second energy levels
of U235 respectively. The U235 isotope gets excited to the energy level, E2.
Then by exciting the U235 isotope from E2 to E3 , it is brought into the
energy level E3 . Similarly by using suitable laser beams, the U235 isotope
is brought into a detector and hence it is separated
Velocity Of Light
Using laser beam the velocity can be determined accuratel
35
REFERENCE
1.
2.
3.
4.
5.
Engineering Physics By A.Marikani
A text book of Engineering Physics By M.N Avadhanulu,P.GKshirsagar
Engineering Physics By P.K Palanisamy
Engineering Physics By Sivalingam Dinesh
Engineering Physics By S. Gopinath
MODULE-1
Chapter-2
HOLOGRAPHY
Introduction
Holography was discovered by Dennis Gabor in 1947 , due to want of highly
coherent light source this had to wait till 1960 to develop.
The ordinary photography is a process of recording the image of
an object on a photographic film. In the ordinary photography, the reflected
image from the object is passed through the camera lens and it is made to fall
on a photographic film. The photographic film consists of a light sensitive silver
halide chemical, coated on it. When the reflected light is made to fall on the
film, the light reacts with the chemical. The light affected chemical is developed
and fixed using fixer. The developed photographic film is said to be a negative.
We are able to see the inverted image in the negative. In order to get the actual
image, we have to make a positive print.
The word Holography is originated from the Greek word ‘holo’ means whole,
entire, complete , full’graphy’ means recording.Hence holography means
recording of the complete information of an object.
A holography is said to be a lensless three –dimensional photography.
In holography the images are recorded by means of optical codes. We are not
able to see the image ( as in the negative of the photographic film) of the object.
Recording of the complete information of the object, i.e its amplitude and phase
in a photosensitive material is called holography . Such a record is called
Hologram.
Or
36
It is the process of recording the image of an object in a holographic
plate by means of interference of light.
What is mean by complete information? Does a photograph of an object carry
complete information?
It carries information of light intensity distribution on the object only.The
phase which carries the information of the morphology (i.e.relative depth of the
different parts) of the object is not being recorded in the photograph. In
photography , only the intensity distribution of the light is recorded , whereas
in holography the recorded interference pattern has not only the intensity
distribution and phase of the electromagnetic wave.
The holographic process has two stages.
a) Construction
b) Reconstruction
Construction of Hologram
Principle
It is based on the principle of interference of two beams, one beam from the
object and the other from the laser source.
Description
It consists of a laser source, a beam splitter and a photographic plate
37
Recording
1) A laser beam from the source is splitted into two beams A and B by beam
splitter.
2) The beam A (reference beam) is made to fall on the photographic plate
directly.
3) The beam B (object beam) is made to fall on the object and reflected
towards the photographic plate.
4) These two beams interfere with each other and produces interference
pattern.
5) A hologram is only a record of the interference pattern formed by
superposition of two coherent light beams. It does not contain a distinct
image of the object.
6) The interference pattern on a hologram consists of a complex pattern of
alternate dark and bright regions.
7) The hologram is also called as Gabot Zone plate.
Reconstruction of hologram
38
Principle
A beam of light identical to the beam used for construction is made to fall on
the hologram. The beam is diffracted by the hologram resulting the 3D image in
the field of view.
Working
1) First the object is removed and the hologram is placed at P.
2) The reconstruction beam is made to fall on the hologram at the same angle
as the reference beam.
3) The hologram act acts as a diffraction grating.
4) Secondary waves from the hologram interface constructively in certain
directions and destructively in other direction.
5) Therefore two images are produced by the diffracted waves.one image is
real and the other is virtual image.
39
6) On being illuminated by the read-out wave, the hologram miraculously
becomes transparent ,and for an observer looking against the direction of
the divergent diffracted beam, it acts like a window, through which the
observer sees the virtual image , which is an exact three –dimensional
replicaof the original object.
7) The real image is inverted in depth, ie, features farther from the viewer in
the holographed object appearing nearer in the image. Hence this image is
called ‘pseudoscopic’
Advantages of hologram over Photographic images
Photographic images
1)An ordinary photograph of
an object or scene is flat and
lack depth. Details of object
hidden by the image cannot be
retrieved.
2)If a photograph is partially
destroyed, and only a small
portion of it remains intact,the
image is incomplete and much
of the details are permanently
lost.
3)It reveal only the intensity of the
light wave from the object.
4 )It is possible to record
several images on the same
photographic plate.But the
result is the confused mess of
superposed image. Separate
image cannot be restored from
this
Applications of holography.
40
Hologram
1)
The image
constructed from a hologram is
three-dimensional and realistic
and has depth. Looking from
different angles, hidden details
are retrieved.
2) In the case of hologram, even
if it is split in to a number of
pieces, each piece can retrieve
the complete image.
3) It record complete information
of the object wave ie. amplitude
and phase.
4) The images of several objects
can be recorded in the same
hologram .This is done by using
different angle of incidence.
Reconstruction of each image
requires the corresponding angle
of incidence for read-out wave.
1)Holograms for information storage: The possibility of recording a very
large number of images on a single hologram and the ability of even part of
the hologram to reconstruct the complete image point the immense scope
for information storage using holograms.
For eg. A single hologram of 100cm2 size can be store in it the entire data
in one volume of the Encyclopedia Britannia.
2)Acoustic Holography:-Acoustic holography is used to study the
internal properties of opaque objects because high frequency coherent
sound waves can travel long distance through solid materials(opaque)
3)Hologram is used in bank notes , credit cards .. to reduce forgery
4)Ultrasonic Hologram: Hologram generated with the help of ultrasonic
waves are very useful because of the ability of such waves to penetrate
the objects that are opaque to visible light. Hologram formed by
ultrasonic waves are very useful to get 3D image inside the opaque
recordings.
************************************************
REFERENCE
6. Engineering Physics By A.Marikani
7. A text book of Engineering Physics By M.N Avadhanulu,P.GKshirsagar
8. Engineering Physics By P.K Palanisamy
9. Engineering Physics By Sivalingam Dinesh
10. Engineering Physics By S. Gopinath
41