Download UNIT-VII (A) LASER Engineering Physics 1. Introduction:

UNIT-VII (A) LASER
Engineering Physics
1. Introduction:The word laser stands for Light Amplification by Stimulated Emission of Radiation.
It is a device that amplifies light and produces a highly directional, high-intensity beam that
most often has a very pure frequency.
2. Characteristics of laser:The important characteristics of laser are
1.
2.
3.
4.
High directionality
High degree of monochromaticity
High degree of coherence
High intensity
1. High directionality:The conventional light sources emit light in all directions due to spontaneous emission. Laser
other hand emit light in one direction due to stimulated emission. The directionality of laser
beam is expressed in terms of divergence.
In case of laser, the active medium is in a cylindrical cavity which is placed in between two
reflecting resonator mirrors. Stimulated photons travel back and forth between two mirrors
many times and amplify each time. Thus the beam drawn from the output mirrors is highly
parallel and directional.
The degree of directionality is expressed in terms of divergence. The curvature of the mirrors
confines the light within the cavity and causes the beam to narrow down to a radius ( 0 )
called “minimum spot size”.
The beam divergence  is given in terms of the minimum spot size 0

1
1.22
20
UNIT-VII (A) LASER
Engineering Physics
The divergence tells how rapidly the beam spreads when it is emitted from the laser.
At d1 and d 2 distances from the laser window, if the diameters of spot are measured to be
a1 and a2 . Then the angle of divergence is

a2  a1
d 2  d1
2. High monochromaticity:Due to stimulated emission, the light emitted by laser is more monochromatic than that of any
convential monochromatic source.
The degree of monochromaticity will be explained by using line width (or) band width of
source which is the frequency spread of spectral line. Now the degree of nonmonochromaticity  is given by



And for a highly stable gas laser   500Hz and   5 1014 Hz

500
 1012
5  1014
But for a conventional monochromatic source, the degree of non-monochromaticity is 105 .
Therefore the monochromatic source is poorer than the laser source.
3. High degree of coherence:Coherence is related to phenomenon of interference. Interference is observed only with
coherent sources. The property of exciting either zero (or) constant phase difference between
two (or) more waves is known s coherence. The laser beam is temporally and spatially
coherent to an extraordinary degree. Temporal coherence is referred to longitudinal
coherence while the spatial coherence is called lateral coherence.\
Temporal coherence:The case of temporal coherence refers to the relative phase (or) coherence of the two waves at
two separated locations along the propagation direction of two beams. It is sometimes
referred as longitudinal coherence.
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UNIT-VII (A) LASER
Engineering Physics
Assume two points P1 and P2 on a wave in a wave packet which is continuous. They have
correlation, if the phase and amplitude at any point is same for any other point on the same
wave.
This correlation of the amplitude and phase between any two points on a wave is called
temporal coherence. In above fig it is illustrated that the different between coherence and
incoherence.
Spatial coherence:Spatial coherence also referred to as transverse coherence describes how far apart two
sources (or) two portions of the same source can be located in a direction transverse to the
direction of observation and still exhibit coherent properties. This is sometimes also referred
to as the lateral coherence
Assume a wave packet and choose P1 & P2 are two spatial points on two different waves in a
wave packet such that maintaining zero or constant phase and amplitude. Then these waves
are said to be in spatial coherence. In above fig it is illustrated the difference between
spatially coherent waves and incoherent waves.
4. High intensity:The laser beam is highly intense as compared to ordinary source of light. This is because of
high directionality and coherence. Since in laser all photos are in phase with each other, the
amplitude (na) of the resulting wave becomes high and hence the intensity (n2a2) is more.
Where ‘n’ is number of photos. Damp
3. Pumping:The population inversion cannot be achieved thermally. To achieve population inversion
suitable form of energy must be supplied. The process of supplying suitable form of energy to
a system to achieve population inversion is called pumping.
Most commonly used pumping methods are
1.
2.
3.
4.
5.
Optical pumping
Electric discharge
Inelastic atom-atom collision
Direct conversion
Chemical reaction
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UNIT-VII (A) LASER
Engineering Physics
1. Optical pumping:In optical pumping a light source is used to supply luminous energy. Most often this energy
comes in the form of short flashes of light. This method was first used by Maiman in his ruby
laser.
2. Electric discharge method:In this method the atoms are excited by collision with fast electrons in an electric discharge.
This method is preferred in gaseous ion laser. In this method electrons emitted by the cathode
to be accelerated towards the anode. Some of these electrons will collide with the atom of the
active medium, ionize the medium and raise it to the higher level.
3. Inelastic atom- atom collision:This method is used in gas lasers consisting of two species of atoms. Pumping by electrical
discharge raises one type atoms to their excited states. These atoms collide inelastically with
another type of atoms. It is these latter atoms that provide the population inversion need for
the laser emission. An example for this is the helium-neon laser.
4. Direct conversion:This method is used in semiconductor p-n junction lasers. In this laser electrons and holes are
made to combine across the depletion region by applying a forward bias. Electrons and holes
recombine to emit radiation. Thus direct conversion of electrical energy into radiation occurs
in semiconductor laser and in LED’s.
5. Chemical reactions:In this method the energy comes from a chemical reaction without any need for other source.
Hydrogen can react with fluorine to produce hydrogen fluoride according to the reaction
H 2  F2  2HF  heat
This reaction generates enough heat to pump a Co2 laser.
Interaction of radiation with matter:Absorption:Let us consider two energy levels with energies E1 and E2 , where E1 ground state is and E2 is
excited state. Usually atoms are present in ground state E1 . When a photon of energy h is
incident on the atom lying in ground state then it excites to higher state E2 . This phenomenon
is known as “absorption”.
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UNIT-VII (A) LASER
Engineering Physics
Spontaneous emission:Let us consider two energy levels with energies E1 and E2 , where E1 ground state is and E2 is
excited state. Usually atoms are present in ground state E1 .let us assume that the atom is in
the excited state E2 .after life time the atom de-excites to its ground state spontaneously
emitting photon of energy h . This phenomenon is known as “spontaneous emission”.
Stimulated emission:We know that average life time of an atom in the excited state is 108 s. During this short
interval let a photon of energy h is incident on the atom which is in the excited will return
to the ground state within the life time by emitting two photons. This phenomenon is known
as “stimulated emission”.
The direction of propagation, phase and energy emitted photon is exactly same as that of
incident stimulating photon.
6. Main components of laser:The main components of laser are
1. Active medium
2. Energy source
3. Optical resonator
1. Active medium:It is medium in which metastable state is present. In metastable state only the population
inversion takes place. It can be a solid, liquid, gas or semiconductor junction.
2. Energy source:It supplies suitable form of energy to the active medium to achieve population inversion. It
performs pumping process.
3. Optical resonator:It is an enclosure of the active medium and essentially consists of two mirrors facing each
other. One mirror is fully reflective and other one is partially reflective. The function of
resonator is to increase the intensity of laser beam.
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UNIT-VII (A) LASER
Engineering Physics
7. Einstein’s coefficients:Absorption:Let us consider two energy levels 1 and 2 . The probable rate of transition from 1  2
depends upon properties of states 1 and 2 and it is proportional to energy density u ( ) of
radiation of frequency  .
Energy density u ( ) is defined as the radiant energy per unit volume in the frequency interval
 and   d .
The probable rate of occurrence of absorption
P12  u( )
P12  B12u ( ) ------------------------------ (1)
Where B12 is called “Einstein’s coefficient of absorption”.
Spontaneous emission:In spontaneous emission the probable rate of transition from 2  1 is depends upon
properties of states 1 and 2 and it is independent of the energy density.
 P21 spon  A21 ------------------------------ (2)
Stimulated emission:-
In stimulated emission the probable rate of transition from 2  1 is depends upon properties
of states 1 and 2 and it is proportional to energy density u ( ) of the stimulating radiation and
is given by
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UNIT-VII (A) LASER
Engineering Physics
( P21 ) stimu  u ( )
( P21 ) stimu  B12u ( ) ------------------------------ (3)
The total probability for an atom in state 2 to state 1 is therefore
P12  A12  B12u ( ) ---------------------------- (4)
Relation between different Einstein’s coefficients:Let us consider an assembly of atoms in thermal equilibrium at temperature T with radiation
of frequency  and   d and energy density u ( ) . Let N1 and N 2 be the number of atoms
in lower energy state 1 and higher energy state 2 respectively.
The number of atoms in state 1 that absorb a photon and rise to state 2 per unit time is given
by
N1P12  N1B12u ( ) --------------------------------- (5)
The number of atoms in state 2 to state 1, either by spontaneous emission or by stimulated
emission is given by
N1P21  N2  A21  B21u( ) --------------------------------- (6)
Under the condition equilibrium, the number of atoms absorbing radiation per unit time is
equal to the number of atoms emitting radiation per unit time. Hence
N1P12  N1P21
N1B12u( )  N2  A21  B21u( )
N1B12u ( )  N 2 B21u ( )  N 2 A21
[ N1B12  N 2 B21 ]u ( )  N 2 A21
u ( ) 
N 2 A21
N1 B12  N 2 B21
u ( ) 
N 2 A21
N B

N 2 B21  1 12  1
 N 2 B21 
u ( ) 
A21
1
---------------------------------- (7)
B21  N1 B12 
 N B  1
 2 21 
According to Boltzmann distribution law, the ratio of N1 and N 2 is given by
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UNIT-VII (A) LASER
 E
N 0 exp   1
N1
 k BT

N2
 E
N 0 exp   2
 k BT
Engineering Physics






 E  E1 
N1
 exp  2

N2
 kBT 
 h 
N1
 exp 
 ------------------------------------ (8)
N2
 kBT 
Substitute equation (8) in equation (7), we get
A
1
------------------------------- (9)
u ( )  21
B21 
 h  B21 
 1
exp 

 k BT  B21 

According to plank’s radiation law
8 h 3
1
----------------------------------- (10)
u ( ) 
3
c

 h  
exp 
  1
k
T

B
 

Comparing eq (9) and eq(10), we get
A21 8 h 3

B21
c3
--------------------------------------------- (11)
B21
1
B21
Equation (11) shows the relation between Einstein’s coefficients B12 , B21 and A21 .this shows
that the ratio of Einstein’s coefficient of spontaneous emission to Einstein’s coefficient of
absorption is proportional to the cube of frequency.
The second relation shows the rate of probability of induced emission and absorption are
equal, when the system is equilibrium.
8. Types of lasers:On the basis of active medium used systems, lasers are classified into several types and most
popular methods are
1.
2.
3.
4.
5.
Solid-state laser (Ruby laser)
Liquid laser (Europium laser)
Gaseous laser (He-Ne laser)
Dye laser (coumarin dye laser)
Semiconductor laser (GaAs laser)
1. Ruby laser:Ruby laser is a solid state three-level laser system developed by Maimen in 1960.
It produces pulsed laser which is useful for various industrial applications like surface
hardening, hard facing cladding of various industrial products.
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UNIT-VII (A) LASER
Engineering Physics
It is a high power laser which has hundreds of MW. Each pulse will come out in duration of
10 nano seconds. The main components of ruby laser are
Source of energy: - xenon flash lamp
Active medium: - ruby crystal rod
Optical cavity: - arrangement of silver polished surface on either sides of the ruby rod.
Construction:The schematic diagram of ruby laser is is shown in fig.
Ruby is taken in the form of a cylindrical rod of about 4 cm length and 1 cm in diameter.
Ruby crystal is basically Al2O3 crystal containing about 0.05% of chromium atoms.
3
The Al3+ ions in the crystal lattice are replaced Cr ions will play main role in the emission
of laser beam. The two ends of a ruby crystal are grounded and polished and one face is
silvered to achieve 100% reflection while the opposite face is partially silvered to make it
semitransparent. A xenon flash tube is arranged around the ruby rod. Which supplies green
color flash light of wave length 5600A0 to active medium to active population inversion.
Only a part of flash light is used for the pumping the Cr 3 , while the rest heats up the
apparatus. A cooling arrangement is provided to keep the experiment setup at normal
temperature.
Working principle:1. The energy level of Cr 3 ions in the crystal lattice is shown in fig. they form basically
a three level system.
2. The xenon flash lamp generates an intense white light lasting for a few milliseconds.
The green component of the light having wavelength 5600A0 is absorbed by Cr 3
ions raising them from the ground state E1 to the excited state E3 . The excited levels
are highly unstable.
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UNIT-VII (A) LASER
Engineering Physics
3. The Cr 3 ions rapidly lose part of their energy  E3  E2  to the crystal lattice and
undergo non-radiative transition to the E2 is metastable state. Therefore Cr 3 ions
accumulate there.
4. If pumping occurs at a faster rate the population at the level E2 exceeds that of the
ground level E1 in a short time. The state of population inversion gets established
between E2 and E1 level.
5. A spontaneous photon emitted by a Cr 3 ion at E2 level initiates the stimulated
emission by the other Cr 3 ions in the metastable state.
6. Photons traveling along the axial direction are repeatedly reflected and amplified, and
emerge out of the semi-transparent mirror in the form of a strong laser beam.
7. The beam is red in color and corresponds to a wavelength of 6943A0 and
frequency 4.32 1014 Hz .
2. He-Ne laser:The main drawback of ruby laser is that the output beam is not continuous through very
intense. The laser is very highly directional, monochromatic, coherent and stable. But the
output power is moderate when it is compared with solid state laser. It is very useful in
making holograms and interferometric experiments.
Source of energy: - R.F oscillator
Active medium: - helium-neon gas mixture
Optical cavity: - arrangement of fully and partially reflective mirrors on either sides of
quartz tube.
Construction:The gas laser consists of a fused quartz tube with diameter about 1.5 cm and 80 cm long. In
this laser active medium is a mixture of ten parts of helium to one part of neon. The neon
atoms provide the energy level for laser transition. Through helium atoms are not directly
involved in the laser transition, they provide an efficient excitation mechanism for neon
atoms. In He-Ne gas laser electric discharge method is used for pumping process.
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UNIT-VII (A) LASER
Engineering Physics
Working:When an electric discharge is posses the He-Ne gas mixture, helium atoms are excited to
higher levels He2 and He3 through collisions with accelerated electrons.
In this neon atom contain six energy levels Ne1 , Ne2 , Ne3 , Ne4 , Ne5 and Ne6 . Here it should
be noted that Ne4 and He2 have same energy and life time and similarly Ne6 and He3 .
The states He2 and He3 are metastable states from which there are no allowed transitions.
The excited helium atoms then collide inelastically with neon atoms still in ground state and
transfer energy to them. This interaction excites the neon atoms to their metastable states Ne6
and Ne4 . After collision, the helium atoms are returned to ground state He1 .
A population inversion is thus created between Ne6 and ( Ne5 , Ne3 ) group and also between
Ne4 and Ne3 . There are three possible transitions in between Ne6 , Ne5 , Ne4 and Ne3 .
1. Ne6  Ne3 Transition: - This transition generates a laser beam of red colour of wave
length 6328A0 .
2. Ne6  Ne5 Transition: - during this transition electromagnetic radiation of wave
length 3390A0 .
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UNIT-VII (A) LASER
Engineering Physics
3. Ne4  Ne3 Transition: - during this transition an electromagnetic radiation of 1150A0 is
emitted.
Whereas 3390A0 and 1150A0 transitions are in infrared region where as 6328A0 transition is
in visible region. Thus build up of 1150A0 and 3390A0 transitions reduce 6328A0 transition.
To overcome this problem in order to get only 6328A0 output, the laser tube windows are
made up of glass (or) quartz. That absorb strongly 1150A0 and 3390A0 .
When an excited neon atom passes from metastable state Ne6  Ne3 it emits photon. This
photon travels through the gas mixture. If the photon is moving parallel to the axis of tube, it
reflects back and forth by mirror ends until it stimulates an excited neon atom by emitting
photon with same phase and direction.
The stimulated transition is a laser transition. This process continues till a beam of coherent
radiation build up in the tube. When the beam becomes sufficiently intense it escapes through
the partially silvered end.
3.
Co2 Laser:-
The carbon dioxide laser is very useful and efficient to produce high power laser of several
kilowatts. Therefore, it is widely used in medical field, communications and weaponry. It is a
four level laser and emit continuous laser.
The carbon dioxide laser was invented by C.K.N Patel in 1963. The active medium is Co for
2
efficient excitation of Co molecules N molecules are used. Addition of He to the gas
2
2
mixture the increases efficiency. The ratio of pressure of Co : N : He is 1: 4 : 5 .
2 2
Construction:The schematic diagram of Co laser is shown in fig. it is basically a discharge tube having
2
cross-section of about 1.5mm2 and length of about 260mm . The discharge tube is filled with a
mixture of carbon dioxide, nitrogen and helium gases at the ration of 1: 2 : 3 . In Co laser
2
electric discharge method is used for pumping.
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UNIT-VII (A) LASER
Engineering Physics
Energy levels of Co laser:2
Co Molecules have a more complicated structure and have energy levels that correspond to
2
rotation (or) vibration motion of entire molecule structure.
The Co molecule is a composed of two oxygen atoms and a carbon atom between them,
2
undergoes three different types of vibrational oscillations known as the “vibrational modes”.
At any one time, the molecules can be vibrating in any combinations of these fundamental

modes. A set of three quantum numbers 1,2,3

are used to denote the modes of
vibration. Where  represents symmetric modes of vibration  represents bending mode
2
1
of vibration and  represents the asymmetric mode of vibration.
3
For example the set (100) represents a molecule vibrating in pure symmetric mode and (020)
indicates that the molecule is vibrating with pure bending with two units of energy.
In addition to these vibrational the molecule can also rotate and thus it has closely spaced
rotational energy levels associated with each vibrational energy level.
Simplified energy level diagram for Co laser is shown in fig.
2
In Co laser, the excitation is provided by electric discharge. Excited N molecules transfer
2
2
their energy to the Co molecule in resonant collisions exciting them to (001) level which are
2
metastable level with relatively longer life time.
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UNIT-VII (A) LASER
Engineering Physics
With sufficient pumping, a population is produced between (001) state and (100) and (020)
states and laser transition begins. The possible laser transitions are
 The laser transition between  001   020  level will produces 9.6  m wavelength of
radiation
 And the transition from
 001  100
level will produces 10.6 m wavelength of
radiation.
 Since the laser transition from  001  100 has higher gain than from  001   020  ,
so the laser beam usually oscillates at 10.6 m .
This process leads to accumulation of population at (010) level. The presence of helium along
with Co helps to decrease the population density at (010) level. It de-excites the Co
2
2
molecules through inelastic collisions.
The Co laser operates in continuous wave mode and is capable of generating high power of
2
the order of several kilowatts at a relatively high efficiency of about 40%. Therefore it is
mostly used laser.
12. Semiconductor laser:Semiconductor lasers are unique when compared to other types of lasers. They are very
small, they operate with relatively low power input, and they are very efficient. They also
operate in a different way in that they require the merging of two different materials and the
laser action occurs in the interface between those two materials. One of the materials has an
excess of electrons (n-type) and the other material (p-type) has excess of holes. When
forward bias voltage is placed across this junction electrons are forced into the region from
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UNIT-VII (A) LASER
Engineering Physics
the n-type material and holes are forced into junction from the p-type material. These
electrons with a negative charge and the holes with a positive charge are attracted to each
other, and when they collide they neutralize each other and in the process emit radiation this
process is known as recombination.
On the basis of recombination processes, semiconductors are classified into two categories
 Direct band gap semiconductors are those in which conduction electrons recombine direct
with holes
 Indirect band gap semiconductors are those in which conduction electrons recombine
with holes via intermediate energy levels
There is large possibility to emit electromagnetic radiation during the direct recombination
process, but not in case of indirect recombination. Therefore direct band gap semiconductors
are useful to construct semiconductors laser.
Semiconductor lasers are classified into two types, they are
1. Homo-junction diode laser system
2. Hetero-junction diode laser system
A homo-junction laser is formed between n-type and p-type semiconductor of same materials
where as hetro-junction is formed between n-type and p-type semiconductors of different
materials.
Construction of homo-junction GaAs diode laser:In this laser system, the active medium is pn-junction diode formed between n-GaAs and pGaAs. The impurities germanium and tellurium are dopped into GaAs semiconductor to
obtain p- type GaAs and n-type GaAs respectively. The thickness of the pn-junction layer is
very narrow so that the emitted laser radiation has large divergence and poor coherence. At
the junction two sides which are parallel to each other are well polished through which laser
is emitted. And the other sides are roughened to avoid laser emission.
To provide forward bias two metal contacts are provided in the top and bottom of the diode.
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UNIT-VII (A) LASER
Engineering Physics
Construction of hetero-junction diode laser:Hetero-junction means that the material on one side of the junction differs from that on the
other side of the junction. A layer of GaAs is sandwiched between two layers of GaAlAs that
has a wider energy gap than and also lower refractive index. Fig shows a double hetero
structure strip laser diode in which the numbers 1,2,3,4 and 5 are indicating the various
layers. The laser emission takes place between the layers 2 and 4. Where 1, 2 and 4 are
GaAlAs layers and 3, 5 are GaAs layers.
Working principle:The working principle is same for the homo and hetero junction diode laser systems. The
population inversion can be obtained by injecting electrons and holes into the junction from
the n-region and p-region respectively by means of forward bias voltage. When forward bias
is not connected then energy diagram will be shown in fig. i.e. no electrons and holes present
in the depletion region.
When a small forward bias is given to the pn-junction then small number of electrons and
holes will be injected into the depletion region from respective region.
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UNIT-VII (A) LASER
Engineering Physics
When a relatively large current is passed through the junction then large number of electrons
and holes will be injected into depletion region and direct recombination process takes place.
Further the emitted photon increase the rate of recombination. Thus more number of photons
produced. Hence the emitted photons from induced recombination are having the same
frequency as that of original inducing photons.
The wavelength of emitted radiation depends upon the concentration of donor and acceptor
atoms in GaAs.
17