Download LASER Introduction: • The word laser stands for `Light Amplification

LASER
Introduction:
 The word laser stands for ‘Light Amplification by
Stimulated Emission of Radiation.
 Laser is a device which produces light waves all exactly in
phase.
 T.S. Simon discovered laser in 1960.
 Lasers are used to produce very intense, monochromatic,
collimated and completely coherent light beam.
 The laser works on the principle of stimulated emission.
 The working principle of a laser can be understood by
knowing the various processes by which the incident
radiation can interact with the matter on which it is incident
upon.
 Lasing action involves following three processes:
1. Absorption
2. Spontaneous Emission
3. Stimulated Emission
1. Absorption: Let E1 and E2 be the two energy states in
an atom then it can excite the atom from the lower energy
ground state E1 to higher energy state E2 by absorbing a
photon of frequency υ. This frequency is given by:
E  E1
 2
h .
This process is known as Absorption of Radiation This
process is pictorially represented as:
1
E2
E2
E1
E1
hυ
Before
After
The probable rate of transition E1→ E2 depends on the
properties of the energy states E1 and E2 and is proportional to
the energy density u(υ) of the radiation of frequency υ incident
on the atom. Therefore, P12  u( ) .
P12  B12u( )
Here the proportionality constant B12 is known as Einstein’s
coefficient of absorption of radiation. This process is also
known as induced absorption. It may be shown by the
following eq.:
atom  photon  atom*
Where ‘*’ represents the excited state.
2. Spontaneous Emission : An atom from an excited state E2
may jump to a lower state E1 by emitting a photon of
E 2  E1


frequency υ given by:
h . This is known as
spontaneous emission of radiation.
E2
E2
hυ
E1
E1
Before
After
2
The probability of spontaneous emission E 2→E1 depends only
on the properties of the energy states. The probable rate of
spontaneous emission is equal to the Einstein’s coefficient of
spontaneous emission A21. This relation may be given as,
P21  A12
In the form of a equation the spontaneous emission may be
represented as
atom*  atom  photon
3. Stimulated Emission:A photon of appropriate energy when
interacts with an atom in excited state, then it may cause its
de-excitation by the emission of an additional photon of same
frequency as that of incident one. Then the two photons of
same frequency move together. This process is known as
Stimulated emission of radiation. The emitted photon has the
same direction of propagation, phase, energy and state of
polarization as that of incident photon. The probability of
stimulated emission of radiation from E2→E1 is given
by: P21  B21u( )
Where B21 is the Einstein’s coefficient of stimulated emission
of radiation and u(υ) is the energy density of incident
radiation.
Thus a transition from E2→E1 may occur via spontaneous or
stimulated emission process. Therefore, the total probability
for E2→E1 transition may be given by:
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P21  A21  B21u( )
E2
E2
hυ
hυ
hυ
E1
E1
Before
After
This process may be expressed by the following eq.
atom*  photon  atom  2 photon
Difference between Spontaneous and stimulated emission:
 In spontaneous emission the emitted photon has energy hυ
and can move in any random direction whereas in
stimulated emission for energy incident photon, we have
two outgoing photons moving in the same directions.
 In spontaneous emission the photons emitted from various
atoms have no phase relationship between them while in
stimulated emission the emitted photons have same
frequency and are in phase with the incident photon. Thus
we can achieve a unidirectional coherent beam.
 In spontaneous emission the probable rate of transition from
excited level E2 to lower level E1 is proportional to the no.
of atoms in the excited state and the energy density of the
incident radiation.
 The radiations achieved by spontaneous emission are
incoherent while these obtained from stimulated emission
are unidirectional and coherent.
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Relation Between Einstein’s ‘A’ and ‘B’ coefficient: Let
there be an assembly of atoms in thermal equilibrium at
temperature T with radiation of frequency υ. Since the rate of
absorption of radiation, i.e. the transition per unit time per unit
volume from E2→E1 is given by:
N1 P12  N1 B12u( )
Where N1 is the no. of atoms in energy state E1.
Similarly the rate of emission (spontaneous + stimulated) from
state E2 to E1 may be given as:
N 2 P21  N 2 A21  B21u( )
Where N2 is the no. of atoms in energy state E2.
In equilibrium, state the absorption and emission must occur
equally. Therefore,
N1 P12  N 2 P21
or N1 B12u( )  N 2 A21  B21u( )
or
u ( ) 
N 2 A21
A
 21 
N1 B12  N 2 B21  B21 N1
N2
1
 B12  ................(i)

  1
B
 21 
But according to Boltzman’s distribution law the no. of atoms
in energy states E1 and E2 at thermal equilibrium is given by,
N1  N 0 e  E / kT
N 2  N0e E / kT
Where N0 is the total no. of atoms, k is the Boltzman’s
constant and T is the absolute temperature.
N1
 e  E2  E1  / kT  e h / kT
Therefore, N
2
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Thus eq. (i) changes to
u ( ) 
A21

B21
e h / kT
1
..................(ii)
B
 12  1
B21
But according to Plank the energy density of the radiation of
frequency υ at temperature T is given by,
8h 3
1
u ( ) 

c3
e h / kT  1 ...............(iii)
Comparing eq. (ii) and (iii), we get,
A21 8h 3
B12

1
3
and
B21
c
B21
Thus the two conclusions may be drawn:
 The probability of stimulated emission is same as that of
(incident or stimulated) absorption.
A
 The probability of stimulated emission B is proportional
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21
3
to υ .Thus the probability of spontaneous emission
increases with the energy difference between the two
energy states.
Metastable State: An atom can be excited to a higher level by
supplying energy to it. Normally, excited atoms have short
lifetimes and release their energy in a matter of 10-8 sec
through spontaneous emission. It means that atoms do not stay
long enough at the excited state to be stimulated. Population
inversion cannot be achieved in such circumstances. In order
to achieve this excited atoms are required to wait at the upper
energy level till a large no of atoms accumulate at that level.
In other words, it is necessary that the excited state has a
longer lifetime. A metastable state is such a state.
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Active Medium: Those atoms, which cause laser action, are
called active centers. The medium hosting the active centers is
called the active medium. An active medium is a medium
which when excited reaches the state of population inversion
and promotes stimulated emission of leading to light
amplifications.
Pumping and Population Inversion: The no of atoms
occupying an energy state is called Population of that state.
Let N1and N2 be the no of atoms in E1 and E2 state
respectively then in thermal equilibrium the population ratio is
given by:
N2
  E 2  E1  / kT
e
N1
The negative exponent in this eq. indicates that N 2  N1 at
equilibrium which indicates that more atoms are in the lower
energy level E1. This state is called normal state.
In order to achieve Laser action stimulated emission has to be
performed. For stimulated emission there must be more atoms
in upper level than in lower level. Therefore a non-equilibrium
state is to be produced in which the population of upper level
exceeds largely the population of the lower energy level.
When this state occurs the population distribution between the
levels E1 and E2 is said to be inverted and the medium is said
to have gone into the state of Population Inversion.
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N2
E2
N1
E1
N2
N1
E2
E1
Non-equilibrium State N1<<N2
(inverted state)
Normal State N1>>N2
Pumping: For achieving and maintaining the condition of
population inversion, the atoms in lower level must be raised
continuously to the upper level. It requires the energy to be
supplied to the system.
Hence, the process of supplying energy to the laser medium
with a view to transfer it into the state of population inversion
is known as Pumping.
Basic pumping techniques employed are:
 Optical Pumping
 Electrical Pumping
 Direct Conversion
 Chemical
 Inelastic atom-atom collision
Pumping schemes widely employed are
 Three level Pumping
 Four level Pumping
Three level Pumping:
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Pumping Level
E3
E3
Rapid Decay
Upper Lasing Level
E2
E2
Metastable State
E1
Lower Lasing Level
Ground State
Pumping
E1
Lasing Action
Four level Pumping:
Pumping Level
E4
Upper Lasing Level
E3
E4
Rapid Decay
E3
Metastable State
E2
E2
Lower Lasing Level
E1
E1
Ground State
Ground State
Pumping
Lasing Action
Comparison of four level laser with three level laser:
 High pump power is needed in three level laser than four
level laser in order to achieve N2>N1.
 In case of three level pumping scheme, once stimulated
emission commences, the population inversion condition
reverts to normal population condition. Lasing ceases as
soon as the excited atoms drop to the ground state. Lasing
occurs again only when the population inversion is reestablished. The light output therefore is a pulsed output.
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While in case of four level pumping such problem does
not occur hence the output is continuous.
Components of LASER:
1. Energy Source: With the help of energy source, the
system can be raised to an excited state. With the help of
this source the no. of atoms in higher energy state may be
increased and hence the population inversion is achieved.
Therefore, energy source may also be called as pumping
device.
2. Active Medium or working substance: This working
substance must have a metastable state (lifetime ≈10-4
sec). Thus when excited by energy source it achieves
population inversion. This medium may be solid, liquid or
gas.
3. Resonant Cavity: It is a specially designed cylindrical
tube the ends of which are silvered, one end being
completely silvered while the other is partially
silvered. Thus, the light intensity can be increased
by multiple reflections. The intense coherent beam
can emerge out from partially silvered mirror. The
rough structure of resonant cavity is shown below:
ENERGY SOURCE
FULLY
REFLECTING
MIRROR
ACTIVE MEDIUM
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PARTIALLY
REFLECTING
MIRROR
Types of LASER:
 Solid state laser
 Gaseous laser
 Semiconductor laser
 Liquid dye laser
 Chemical laser
Ruby Laser
Introduction:
 It is the first solid state laser developed by T.H. Maiman in
1960.
 It has three energy levels of population inversion i.e. it
contains excited energy level (E3); upper lasing level is the
metastable state (E2) and the lower lasing level is the ground
(E1).
 It consists of the following three parts:
1. The working system in the form of a rod of ruby crystal
(Al2O3).
2. The optical pumping system consisting of a helical xenon
discharge tube.
3. The resonant cavity consisting of two optically plane and
accurately parallel reflecting plates (mirrors). The plate at
the left end is fully silvered while at the right end is
partially silvered.
 The ruby rod is made up of aluminum oxide crystal doped
with 0.05% of Cr2O3.This impurity of Cr3+ ion is
responsible for the pink colour of cylindrical ruby rod of
4.0cm length , 1.0 cm diameter.
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Working:
 It uses three level pumping schemes. The energy levels of
Cr3+ ions in the crystal is shown in the fig below:
Energy (eV)
E’3
Radiation less transition
E3
Green
Blue
Pumping
E2
Stimulated
Emission
Photon 6943 A0
E1
Ground
State
Energy levels and transition in a ruby laser
 The xenon discharge generates an intense burst of white
light lasting for a few milliseconds.
 The cr3+ions are excited to the energy bands E3 and E’3by
the green and blue components of white light.
 From there the Cr3+ ions undergo non-radiative transitions
and quickly drop to the metastable level E2.
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 The metastable state has a lifetime of approximately 1000
times more than the lifetime of E3 level. Therefore Cr3+ ions
accumulate at level E2.
 When more than half of the Cr3+ion population accumulates
at level E2 the state of population inversion is established
between E2 and E1 levels.
 A chance photon emitted spontaneously by a Cr3+ ion
initiates a chain of stimulated emission by other Cr 3+ions in
the metastable state.
 Red photons of wavelength 6943A0 traveling along the axis
of the ruby are repeatedly reflected at the end mirrors and
light amplification takes place.
 A strong intense beam of red light (λ=6943A0) emerges out
of the front end mirror.
Drawbacks of Ruby Laser:
 The ruby laser requires a strong energy source because
more than one-half of the atoms must be pumped to
higher energy state to achieve population inversion.
 In this laser only the green component of the pumping
light is used. Therefore the efficiency decreases to very
low value.
 The defects present in the crystal are also found in the
laser.
 The laser output is not continuous but occurs in the
form of pulses of microsecond duration.
Nd: Yag Laser: Nd:Yag means Neodium Yattrium
Aluminum Garnet. The Nd:Yag is a solid-state material. It has
good thermal and optical properties. So, the Nd: Yag laser is
used in many places instead of Ruby Laser.
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Principle: The photon in the flash tube excites the Nd atoms
to higher energy states and causes stimulated emission. Then
by several reflections in the optical cavity the LASER beam is
formed.
Construction:
 The laser rod is made up of Nd-Yag material is placed along
one focal axis of the optical resonator.
 The elliptical optical resonator is made up of two mirrors,
one partially reflecting mirror and the other completely
reflecting mirror.
 The flash tube is placed along the other focal axis of the
optical cavity.
 The capacitor bank is connected across the power supply.
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Photons From
Completely Reflecting
excited Nd atoms
Optical cavity
Mirror
Partially Reflecting
Mirror
Reflected
Photons
Laser
Beam
Photons from
flash tube
Flash
Tube
Capacitor
bank
Power
supply
Nd-YAG LASER
Working:
 Here the Nd atoms are doped as an impurity in the host
material YAG.
 In a pure Nd, energy levels are at the same level.
 However, when the Nd ions are doped in YAG material,
these energy levels are spread due to the electrostatic field.
 This makes the stimulated emission easier.
 Initially the capacitor bank is charged. When the switch is
closed, this capacitor bank discharges through the flash
tube.
 The ON time of the flash tube is around1ms. The photons
from the flash tube cause the excitation of the Nd atoms to
higher energy levels.
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 The lifetime for Nd atoms at this higher level is long enough
to cause stimulated emission.
 Therefore, as the photons strike these excited atoms,
stimulated emission takes place.
 This increases the no. of photons. These photons travel to
and from in the optical cavity. These oscillations take place
until a resonant wavelength is obtained.
 When a resonant beam is obtained, the LASER light comes
out through the partially reflecting mirror.
Energy Level Diagram:
 The Nd3+ions to upper states is done by a krypton arc lamp.
 The optical pumping with light of wavelength 5000 8000A0 excites the ground state Nd3+ ions to higher states.
 The metastable state E3 is the upper lasing level, while the
E2 forms the lower laser level.
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 The upper laser level E3 will be rapidly populated, as the
excited Nd3+ ions quickly make downward transitions from
the upper levels.
 The lower laser level E2 is far above the ground level and
hence it cannot be populated by Nd3+ ions through
thermal transition from the ground level.
 The population inversion is readily achieved between
the E3 level and E2.
 The laser transition occurs in infrared region at a
wavelength of about 10600 A0.
 As the laser is a four level laser, the population inversion
can be maintained in the face of continuous laser emission.
Hence, it can be operated in CW mode.
Advantages:
 Nd:YAG material has good thermal and optical
properties.
 Can be operated in CW mode.
Disadvantages:
 The LASER beam coherence is poor.
 Output power is less.
 Power efficiency is less.
Applications: LASER welding, soldering, continuous high
power operations.
He-Ne Laser: Ali Javan, Bennett and Harroit made it in 1961.
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-
+
Anode
Cathode
He-Ne
mixture
6328A0
Glass
Window
Mirror
Construction:
 Working Substance: The working substance used in this
laser is a mixture of He and Ne gases taken in the ratio of
10:1at a pressure of 1mm of Hg. The actual lasing atoms or
the active centers are the neon atoms and the helium atoms
are used for selective pumping of the upper laser level of the
neon.
 End Mirrors: End mirrors are used for forming the resonant
cavity, which can be adjusted to a high degree of
parallelism. At one end the mirror is fully silvered and acts
as perfect reflector while at another end, it is partially
silvered and acts as partial reflector. These mirrors form
Fabry-Perot resonator. The distance between these mirrors
is mλ/2. Thus with the help of these mirrors, the coherent
photons obtained by stimulated emission may undergo
multiple reflection resulting a coherent intense laser beam.
 Excitation Source: Excitation source is used to create a
discharge in the gas. It is in the form of high frequency
potential difference. This high potential difference may be
generated with the help of a Tesla coil and is applied by
means of three metal bands around the outside of the tube.
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 Quartz tube or resonant cavity: This tube is made up of
quartz. It is nearly 50 cm long and of 1.0 cm diameter,
contains a mixture of helium and neon.
Working:
 It employs four level pumping scheme.
 When the power is switched on, some of the electrons
present in the mixture of gases are accelerated.
 The energetic electrons excite helium atoms through
collisions.
 The excited state of helium E H1 at 20.61eV is a metastable
state.
 The excited state of neon is 20.66 eV. The excited helium
atoms transfer their energy to neon atoms through the
process of collisions.
 The kinetic energy of helium atoms provide the additional
0.5 eV required for excitation of the neon atoms.
 Helium atoms drop to the ground state after exciting neon
atoms. Thus, the role of helium atoms is to provide the neon
atoms the necessary excitation energy and to cause
population inversion.
 Since He-Ne ratio is 10:1 hence reverse transfer of energy is
not possible.
 The upper state of Ne, EN5 is a metastable state. Therefore,
neon atoms accumulate in this upper state.
 Since state EN3 is sparsely populated at ordinary temp.
hence a state of population inversion is readily achieved
between EN5 and EN3.
 The transition EN5→ EN3 generates a laser beam of red
colour of wavelength 6328 A0 .
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 Other possible transitions are 3.39μm and 1.15μm
respectively.
 From EN3 the neon atoms drop to EN2 level spontaneously.
 EN2 is however, metastable state Ne atoms start
accumulating here.
 In order to maintain laser function it is required to bring the
atoms to ground state, which is accomplished by collision.
 For this discharge tube is made narrow so as to increase the
probability of collision with walls.
helium
2
2
Energy (eV)
2
0
Pumping
(electron
impact)
1
8
neon
atomic
collisions
1
2
S3
2
radiative decays:
~100ns)
3s
2s
S
1s
1
6
3.39μm
543nm
633nm
1.15μm
Fast decay (~10ns)
Collision with
tube walls
ground state
0
CO2 LASER: The carbon laser is a four level molecular laser
and operates at 10.6μm. It operates both in continuous and
pulse mode.
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Water Out
Water In
Brewster
Window
Laser
Beam
Gas Out
Gas In
Construction:
 It is basically a discharge tube having a bore of cross section
of about 1.5 mm2 and a length of about 260 mm.
 The discharge tube is filled with a mixture of carbon
dioxide, nitrogen, and helium gases in 1:2:3 proportions
respectively.
 Other additives such as water vapour are also added.
 The active centers are CO2 molecules lasing on the
transitions between the vibrational levels of the electronic
ground state.
Energy Levels of CO2 molecule: The LASER output for this
LASER depends on the rotational and vibrational motions of
the CO2 molecules. The CO2 molecules has three types of
vibrational energy modes:
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Stretch mode
Bending mode
Asymmetric mode
And the rotational modes are:
Rotational mode
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Working:
Energy transfer
through collision
0.3
E5
10.6µm
E4
9.6µm
Energy
(eV)
0.2
E3
0.1
E2
E1
Nitrogen
Carbon dioxide
 Here the excited state of N2 molecule and it is identical to
CO2 molecule.
 As current passes through the mixture of gases, the N 2
molecules get excited to the metastable state.
 The excited N2 molecules cannot spontaneously lose their
energy and consequently, the number of N2 molecules at the
level keeps on increasing.
 These molecules return back to the ground state by collision
with CO2 molecules and thus CO2 molecules get excited to
E5 level which is the upper lasing level.
 E3 and E4 levels are lower lasing levels.
 Population inversion is achieved between E5 and E3 and E4.
 The laser transition between E5→E3 produces radiations of
9.6μm while E5→E4 produces radiations of 10.6μm.
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 Through inelastic collisions with normal CO2 molecules
excited molecules fall to E2 where they start accumulating.
 Accumulation of molecules at E2 level disturbs the lasing
action which is resolved by the presence of He atoms which
causes the de-excitation.
Advantages:
 High output power.
 High efficiency.
 Mechanically durable.
Disadvantages:
 For high out put power single lined beam cannot be
obtained.
 Requirement of cooling system.
 High cost.
Applications:
 Spectroscopy.
 RADAR systems.
 Cutting and welding of metals.
 Material scribing.
 Heat treating operations.
 Surgery.
Properties of LASER:
 Directionality
 Negligible divergence
Power
 High intensity I  Area
 High degree of coherence
 High monochromaticity
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The end
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