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NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY
Prof. S.M.ASADULLAH 9290233597 NSAKCET
UNIT -IIb
LASER
Syllabus:A) Characteristics of lasers, spontaneous and stimulated
emission of radiation, B)Einstein coefficients, population inversion,C)
ruby laser, helium – neon laser, semi conductor laser,D)applications of
lasers
A) Characteristics of lasers, spontaneous and stimulated emission of
radiation,
1. Explain the terms
a) Absorption (b) Spontaneous emission (2) Stimulated emission (2007,
2011)
radiation can interact with matter in three different ways namely a) absorption
(b) Spontaneous emission (c) Stimulated emission
a) absorption: it is a process in which a photon is absorbed by an atom and an
electron undergoes a transition from lower level E1 to a higher level E2.
Absorption is also called stimulated absorption.
(b) Spontaneous emission: An electron can exist in a higher level E2 for a
very short time. It will come down to the lower level E1 emitting a photon of
energy hν = E2 – E1
(c) Stimulated emission: an electron in a higher level E2 can also undergo
transition under the influence of photon of energy hν. In this process two
photons each of energy hν are emitted at the same time. One of them is
stimulating photon and the other is stimulated photon. These two photons are
in phase.
2. Distinguish between spontaneous emission and stimulated emission.
(2006)
Spontaneous emission Stimulated emission
It is a random process It is not a random process
The light emitted is incoherent The light emitted is coherent
The light emitted is non directional The light emitted is highly
directional
The light emitted is polychromatic The light emitted is monochromatic
The light emitted is unpolarised The light emitted is polarised
There is no light amplification There is light amplification
For N photons each of intensity I,
the total intensity is NI
For N photons each of intensity I,
the total intensity is N2I
2
NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY
Prof. S.M.ASADULLAH 9290233597 NSAKCET
3. What are Einstein’s coefficients A and B in laser? OR
Derive the relation between the probabilities of spontaneous emission and
stimulated emission in terms of Einstein’s coefficient. (May 2003 May
2004, May 2007, J 2011)
Einstein’s coefficients are the constants of proportionality in the three
processes of interaction of radiation with matter.
Let us consider a system containing N1 atoms per unit volume having
energy E1, N2 atoms per unit volume having energy E2 . Suppose ‘n’ photons
per unit volume each of energy hν = E2 – E1 interact with matter. Three
different processes can occur as given below.
1. Absorption or stimulated absorption
The rate of stimulated absorption depends on the number of atoms in the
lower level N1 and the energy density of photons ρ(ν).
Stimulated absorption rate α N1
α ρ(ν)
= B12 N1ρ(ν) ……………(1)
B12 is the Einstein coefficient of stimulated absorption.
2. Spontaneous emission:
The rate of spontaneous emission depends on the number of atoms N2 per unit
volume in the higher level
Spontaneous emission rate α N2
= A21N2 ………….(2)
A21 is the Einstein coefficient of spontaneous emission.
3. Stimulated emission: The rate of stimulated emission depends on the number of atoms N2 per unit
volume in the higher level and the energy density of photons ρ(ν).
Stimulated emission rate α N2
α ρ(ν)
= B21N2ρ(ν) …………….(3)
where B21 is the Einstein coefficient of stimulated emission.
Relation between Einstein’s coefficients
In equilibrium the upward transition rate must be equal to the total downward
transition rate. Hence we have
B12 N1ρ(ν) = A21N2 + B21N2ρ(ν) …………(4)
From equation (4) we get (5)..........BNBN
ANρ(ν)
212121
212
On dividing throughout by N2B21
We get (6)..........
1N2
N
B
B
/BAρ(ν)
1
21
12
2121
According to Boltzmann distribution
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NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY
Prof. S.M.ASADULLAH 9290233597 NSAKCET
N1 = No exp ( – E1 /kT) …………….(7)
and N2 = No exp (– E2 /kT ) …………….(8)
From the above equations
kT
EEexp
N
N 12
2
1
or
kT
hexp
N
N
2
1 …………(9)
On substituting equation(9) in (6)
(10)..........
1kT
hνexp
B
B
/BAρ(ν)
21
12
2121
From Planck’s law of black body radiation, the radiation density is given as
(11)..........
1kT
hνexp
18ρ(ν)
3
3
c
h
On comparing equations (10) and (11) we get
3
3
21
21 8
c
h
B
A ……………(12)
And ...(13)..........BorB1B
B1212
21
12
Equations (12) and (13) are referred to as Einstein’s relations
We have three devices working on the interaction of radiation and matter.
1)Light emitting diode (LED): in this device the spontaneous emission
dominates over absorption and stimulated emission.
The ratio of spontaneous emission rate to the stimulated emission rate is given
by )14.....(..........1kT
hνexp
ρ(ν)B
A
ρ(ν)BN2
ANR
21
21
21
212
Under normal conditions R ≈ 105. This indicates that under normal conditions
spontaneous emission dominates.
2)LASER: in this device the stimulated emission dominates over absorption
and spontaneous emission.
To find the conditions suitable for lasing action let us consider the ratio of
stimulated emission rate to stimulated absorption rate
1
2
121
212
N
N
Bρ(ν)N
Bρ(ν)N
rateabsorptionstimulated
rateemissionstimulated
At thermal equilibrium N2/N1 << 1
At thermal equilibrium stimulated absorption predominates over stimulated
emission. Stimulated emission will dominate if population of the excited state
(N2) is greater than the population of the lower state ( N1). This is called
population inversion.
Let us consider the ratio of stimulated emission rate to spontaneous
emission rate
)(hνρA
B
AN
Bρ(ν)N
rateemission sspontaneou
rateemissionstimulated
21
21
212
212
Hence the conditions suitable for LASER are
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NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY
Prof. S.M.ASADULLAH 9290233597 NSAKCET
1. Population inversion
2. High energy density of interacting radiation
3)PHOTODIODE: In this device stimulated absorption dominates over
spontaneous emission and stimulated emission.
problem 1: Find the relative population of the excited state with respect to the
lower energy state of LASER that produces a light beam of wave length
1.06 μm at 300 K [2.39x10-20]
problem 2: Find the relative population of the excited state with respect to the
lower energy state of LASER that produces a light beam of wave length
6943 Å at 300 K [8x10-31]
Hint:
kT
EE-exp
N
N 12
1
2
where E2 –E1 = hν = hc/λ
4. What are the three important requisites for laser action to take place?
Three important requisites for laser action to take place are
1) suitable active medium ii) creation of population inversion and
iii) proper optical feedback
5. What is the principle of laser? What is meant by laser action?
When population inversion is created in a medium, the number of
photons emitted by stimulated emission process increases in cascade process
resulting in amplification of light. This process of Light Amplification by
Stimulated Emission of Radiation is called Laser action.
6. Explain population inversion in lasers. (June, 2014)
What do you understand by population inversion? How it is achieved?
When the number of atoms (i.e population of atoms) in any excited state
is higher than the number of atoms in the lower energy state, that condition is
called population inversion. This will happen if the life time of higher energy
state is longer (metastable state). Light amplification by stimulated emission
of radiation can occur only when population inversion is achieved.
7.What are the different methods of achieving population inversion? (b)
(b) Explain the various pumping mechanisms that are adopted in lasers
The process of exciting the atoms of the active medium to a higher energy
state by supplying energy in relevant form is called pumping. Different
pumping methods are
i) Optical pumping ii) Electric discharge iii) Chemical reaction and
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NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY
Prof. S.M.ASADULLAH 9290233597 NSAKCET
iv) Injection current through junction
1) Optical pumping: in this a xenon flash is used. This is used in solid state
lasers.
2) Electric discharge pumping: in this process electric discharge is carried
out to supply energy. This used in gaseous lasers like He-Ne, CO2 etc
3) Injection current or direct conversion of electrical energy into light
energy: This method is used in semiconductor diode laser.
8. Explain a) life time of an energy level b) metastable state (J 2006)
a) life time of an energy level: it is the time for which an electron can exist in
a level.
In the ground state the life time is infinity because it is a stable level.
In the excited state an electron can exist for 10-9 to 10-8 seconds because it is
an unstable state.
The life time of a metastable state is 10 -6 to 10 -3 seconds.
9. Explain a) three levels and b) four level scheme.
a) Three level scheme:
a typical three level pumping
scheme is shown in figure
The state E1 is the ground state, E3
is the pump state and E2 is the
upper lasing state which is
metastable state. When the
medium is exposed to pump
frequency radiation a large number of atoms are excited to level E3.they
rapidly undergo transition to E2 level. Transition E2 to E1 is forbidden. Hence
population of E2 increases. As result there will be population inversion
between E1 and E2. When population inversion is high stimulated transition
E2→ E1will take place.
b) Four level scheme:
A Four level scheme is shown the figure. E1 is the ground state and E4 is the
pump state. E3 is the metastable upper lasing state and E2 is the lower lasing
state. When the medium is pumped with suitable energy E4 - E1 the atoms get
excited to level E4. The atoms cannot remain in this state for more than 10-8
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NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY
Prof. S.M.ASADULLAH 9290233597 NSAKCET
seconds. They undergo spontaneous transition to E3. The transition E3→ E2is
forbidden. Hence population inversion develops between E3 and E2 levels.
When this grows sufficiently stimulated emission takes place between E3 and
E2 levels. From E2 to E1 non radiative transitions take place.
10. Explain the important components of a laser
11. Explain the purpose of an active medium in a gas laser. (May 2003,
May 2004)
12. Explain Optical cavity. (May 2007)
13. Explain the need of a cavity resonator in a laser. (June 2009, June
2011)
The important components of a laser are 1) an active medium 2) a pumping
agent 3) an optical resonator.
1) Active medium: a material in
which lasing action can take place
is called active medium. The
atoms which cause laser action are
called active atoms. If a medium
has different species of atoms, all
the atoms of the medium are not
active.
2) The pump: the pump is a
device used to supply energy to
the active medium. The process of supplying energy is called pumping. The
different methods of pumping are
1) Optical pumping: in this a xenon flash is used. This is used in solid state
lasers.
2) Electric discharge pumping: in this process electric discharge is carried out
to supply energy. This used in gaseous lasers like He-Ne, CO2 etc
3) Injection current or direct conversion of electrical energy into light energy.
This method is used in semiconductor diode laser.
Optical resonator : a pair of optically plane parallel mirrors enclosing laser
medium between them is called optical resonant cavity or Fabry-Perot
resonator. It is used to amplify the light and also to select the wave length due
to this we get intense beam of monochromatic light.
14. Describe the construction and working of a Ruby Laser
Ruby laser is a solid-state lasers. Ruby is
basically Al2O3 crystal containing about 0. 05% of
chromium atoms. Cr3+ ions are the actual active
centers while aluminum and oxygen atoms are inert.
Ruby rod is taken in the form of a cylinder of
length 4 cm and diameter 1 cm. Its ends are grounded
and polished such that the end faces are exactly
parallel and are also perpendicular to the axis of the
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NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY
Prof. S.M.ASADULLAH 9290233597 NSAKCET
rod. The end faces of the ruby rod are
silvered so that they form the optical
resonator. The rear face is made totally
reflecting while the front face is made
partially reflecting. The laser rod is
surrounded by a helical xenon flash lamp.
Working: when the xenon lamp is
switched on it produces intense flashes of
white light. Chromium ions have
absorption bands in the blue and green regions. They are excited to E3 and E’3.
The Cr3+ ions undergo non - radiative transitions from these energy levels to
level E2. E2 is a metastable state. So the population of electrons in the E2 level
will become more than that in E1level causing Population Inversion. A chance
photon is produced when a Cr3+ ion makes a spontaneous transition from E2
level to E1 level. This spontaneous photon stimulates another excited ion to
make a downward transition. This stimulated photon and the initial photon
trigger many excited ions to emit photons. Red photons of wavelength 6943Å
travelling along the axis of the ruby rod are repeatedly reflected at the end
mirrors and light amplification takes place. On attaining sufficient photon
energy, the laser beam emerges out through the partially reflecting mirror. The
laser emission occurs in the visible region at a wavelength of 6943 A (694.3
nm). Once stimulated transitions commence, the metastable state gets
depopulated very rapidly and the state of population inversion disappears and
lasing action ceases. The laser becomes active once again when population
inversion state is reestablished. Therefore, the output of the laser is not a
continuous wave but occurs in the form of pulses of microsecond duration.
In short a ruby laser uses three-level pumping scheme . The active
centers are Cr3+ ions . Light from a xenon flash lamp is the pumping agent. It
operates in pulsed mode. Its efficiency is poor.
Uses: the ruby laser is used
1) For measuring distance by pulse echo technique.
2) For pulsed holography.
3) For drilling high quality holes.
4) For trimming resistors and IC masks.
5) In military, used as target designator and range finders.
6) For plasma production and fluorescence spectroscopy.
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NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY
Prof. S.M.ASADULLAH 9290233597 NSAKCET
15. Describe the construction and working of a He-Ne Laser(May 2003, May 2004, Dec. 2010)
It was invented by by Ali Javan, William R. Bennett, and Donald R.
Herriott.
Construction :::
Helium -Neon laser
consists of a long
discharge tube filled with
a mixture of helium
and neon gases it, the
ratio 10:1. Neon atoms are the active
centers while helium atoms help in
exciting neon atoms. Electrodes are
provided in the discharge tube to
produce discharge in the gas. They
are connected to a high voltage
power supply. The tube is sealed by
inclined Brewster’s windows
arranged at its two ends. On the axis
of the tube, two mirrors are arranged
externally, which form the Fabry-
Perot optical resonator. The distance between the mirrors is adjusted to be m
λ/2 such that the resonator supports standing wave pattern
Working:
A high voltage of about 10 kV is applied across the gas mixture. It ionizes the
gas. The electrons and ions produced in the process of discharge are
accelerated towards the anode and cathode respectively. They collide with
helium and neon atoms on the way.
The energetic electrons excite helium atoms more readily, as they are lighter.
One of the excited levels of helium F2(2s) is at 20.61 eV above the ground
level. It is a metastable level. The excited helium atom cannot return to the
ground level through spontaneous emission. However, the excited helium atom
can return to the ground level by transferring its excess energy to a neon atom
through collision. Such an energy transfer can take place when the two
colliding atoms have identical energy levels. Such an energy transfer is known
as resonant energy transfer.
The neon energy level E5(5s) is at 20.66eV, which is close to the excited
energy level F2 of helium atom. Therefore, resonant transfer of energy occurs
between the excited helium atom and ground level neon atom. The kinetic
energy of helium atoms provides the additional 0.05 eV required for excitation
of the neon atoms.
Helium atoms drop to the ground state after exciting neon atoms. This is the
pumping mechanism in He-Ne laser.
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NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY
Prof. S.M.ASADULLAH 9290233597 NSAKCET
The upper state of neon atom E5 is a metastable state. Therefore, neon
atoms accumulate in this upper state.
As the population at the higher energy level E5 is greater than the
population at the lower level E3, a state of population inversion is established
between E5 and E3 levels.
Random photons of red colour of wavelength 6328 Ǻ are emitted
spontaneously by a few of the atoms at the energy level E5.
The spontaneous photons traveling through the gas mixture prompt
stimulated emission of photons of red colour of wavelength 6328 Å.
The photons bounce back and forth between the end mirrors, causing more
and more stimulated emission during each passage. The strength of the
stimulated photons traveling along the axis of the optical cavity (discharge
tube) builds up rapidly while the photons traveling at angles to the axis are
lost.
Thus, the transition E5 → E3 generates a laser beam of wavelength
6328 Å. From the level E3 the neon atoms, drop to E2 level spontaneously.
E2 level is a metastable state. Neon atoms return to the ground state E1
through frequent collisions with the walls of the discharge tube .
The neon atoms are once again available for excitation to higher state and
participate in lasing action. The laser operates in continuous wave mode.
Uses:
1. They are used to read bar code. They are also used for OCR.
2. The are used in recording holograms.
3. They are used in interferometric experiments.
4. They used in survey.
16. What is the role of helium atoms in He-Ne laser.
The role of helium atoms in the laser is to excite neon atoms and to cause
population inversion. The probability of energy transfer from helium atoms to
neon atoms is more, as there are 10 helium atoms per 1 neon atom in the gas
mixture. The probability of reverse transfer of energy from neon to helium
atom is negligible.
17. What is the necessity of narrow glass tube.
During the operation of the laser, it is necessary that the atoms accumulating
at the metastable level E2 are brought to the ground state E1 quickly; otherwise
the number of atoms at the ground state will go on diminishing and the laser
ceases to function. The only way of bringing the atoms to the ground state is
through collisions. Therefore, to increase the probability of atomic collisions
with the tube walls, the discharge tube is made narrow. ‘
18. What are the salient features of He-Ne laser. 1. Uses four-level pumping scheme
2. The active centers are neon atoms
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NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY
Prof. S.M.ASADULLAH 9290233597 NSAKCET
3. Electrical discharge is the pumping agent
4. Low efficiency and low power output
5. Operates in CW mode
19. Distinguish between Ruby laser and He-Ne laser.
Ruby laser He- Ne laser
It is solid state laser It is gaseous laser
The active centers are Cr3+ ions The active centers are Ne atoms
The pumping is done by xenon
flash
The pumping is done by electric
discharge
It works on 3-level system It works on 4-level system
It produces pulses of laser light It produces continuous wave of
laser light
21. What are the roles played by N2 and He in CO2 laser?
During electric discharge N2 molecules get excited and by collision
transfer the energy to CO2 molecules and result in efficient excitation of CO2
molecules. Addition of He to the gas mixture enhances the efficiency by
depopulating lower laser level.
22. Describe the principle, construction and working of semiconductor
diode laser. (May 2003, June 2009,June 2014)
Homojunction Semiconductor Laser
A semiconductor diode laser is a specially fabricated p-n junction
device, which emits, coherent light when it is forward biased. It is made from
Gallium, arsenide (GaAs). Diode lasers are remarkably small in size (0.1mm
long).it was invented by R. N. Hall and his coworkers in 1962.
A homojunction diode laser uses same semiconductor material on both
sides of the junction. Example: Gallium arsenide (GaAs) laser.
Principle: The energy band structure of a semiconductor consists of a valence
band and a conduction band separated by an energy gap, Eg. The conduction
band contains electrons and the valence band contains holes and electrons.
When an electron from the conduction band jumps into a hole in the valence
band, the excess energy, Eg is given out in the form of a photon. Thus, the
electron-hole recombination is the basic
mechanism responsible for emission of
light. The wavelength of the light is
given by the relation λ = hc/Eg
Construction: Figure shows the
schematic of a homojunction diode laser.
Starting with a heavily doped n-type
GaAs material, a p-region is formed on
its top by diffusing zinc atoms into it. A
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NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY
Prof. S.M.ASADULLAH 9290233597 NSAKCET
heavily zinc doped layer constitutes the heavily doped p-region. The diode is
extremely small in size. Typical diode chips are 500 μm long and about 100
μm wide and thick. At the top and bottom metal contacts are provided
working:
pumping mechanism: When the junction is forward-biased, electrons and
holes are injected into the junction region in high concentrations .In other
words, charge carriers are pumped by the dc voltage source. When the diode
current reaches a threshold value, the carrier concentrations in the junction
region will rise to a very high value.
Population Inversion: Thus the upper energy levels in the narrow region are
having a high electron Population while the lower energy levels in the same
region are vacant. Therefore, the condition of population inversion is attained
in the narrow junction region This narrow zone in which population inversion
occurs is called an inversion region or
active region.
Lasing: Chance recombination acts of
electron and hole pairs lead to emission of
spontaneous photons. The spontaneous
photons propagating in the junction plane
stimulate the conduction electrons to jump
into the vacant states of valence
band. This stimulated electron-hole recombination produces coherent
radiation. GaAs laser emits light at a wavelength of 9000 A in IR region.
23. What are advantages and uses of semiconductor lasers.
Semiconductors having a suitable value of Eg emit light in the optical
region. Semiconductor diode laser has efficiency of the order of 40%.
Modulating the biasing current easily modulates the laser output. Because of
the rapid advances in semiconductor technology, diode lasers are mass
produced for use in optical fiber communications, in CD players, CD-ROM
drives, Optical reading, high speed laser printing etc wide variety of
applications.
24. What are the disadvantages of homojunction Semiconductor diode
laser.
1. In homojunction lasers, the active region is not well defined due to the
diffusion length of the carriers.
2. The semiconductor has nearly uniform refractive index throughout.
Therefore, light can diffuse from active layer into the surrounding medium. As
a result the cavity losses increase.‘
3. High threshold currents are required and the laser cannot be operated
continuously at room temperature. '
25. What is heterojunction laser, what are its advantages over is
homojunction laser
A heterojunction laser consists of a thin GaAs layer sandwiched between two
layers of GaAlAs which has wider energy gap and less refractive index.
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NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY
Prof. S.M.ASADULLAH 9290233597 NSAKCET
Working:
The basic principle of working of heterojunction diode is similar to that of a
homojunction diode. When it forward biased the electrons and holes are
injected creating a population inversion. A chance combination of electron and
hole gives rise to photon. This photon stimulates other electron-hole
combinations producing laser light.
Advantages
1. Heterojunction lasers have high efficiency even at room temperature.
2.As a result of reduction in the threshold current density, continuous
operations possible.
3.With operating currents of less than 50 mA, output powers of about 10 mW
can be produced.
26. What are different types of semiconductor lasers. Compare them.
There are two types of semiconductor diode lasers a) homojunction and b)
heterojunction semiconductor lasers.
Homojunction laser Heterojunction lasers
It has less efficiency It has higher efficiency
Threshold current density is high Threshold current density is low
Cannot be operated continuously
for a longer time.
Can be operated continuously for a
longer time.
Output power is less. Output power is more.
27. Discuss briefly the different methods of producing laser light.(June
2006,Dec. 2010)
laser Ruby laser He-Ne laser Semiconductor laser
type solid gseous Semi conductor
Active
center
Cr3+ ions in
ruby
Ne atoms in
He-Ne mixture
GaAs semiconductor
excitation Xenon flash Electric discharge Injection current
type flash Continuous wave Continuous wave
28. State the properties of laser beam. (or) What are the characteristics of
laser light? (June 2005, June 2009)
The important characteristics of laser beam are
i) high monochromaticity (i.e., single wavelength) ii) high degree of coherence
iii) high directionality (i.e. less divergence) and iv) high brightness.
29. What are two types of coherence?
The two types of coherence are
i) Temporal coherence: The maximum separation between any two points on
the wave train emitted by light source which have phase correlation is called
coherent length. This can also be expressed in time as coherent time
(τ = coherent length / velocity of light).
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NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY
Prof. S.M.ASADULLAH 9290233597 NSAKCET
ii) Spatial coherence: The maximum separation between any two points on
the cross section of the wave front which maintain correlation (i.e. in phase
with each other all the time) is called spatial coherence.
30. What are the important characteristics of a laser beam or Discuss the
essential features of a laser beam. (June 2009)
The important characteristics of a laser beam are: (i) directionality (ii)
negligible divergence (iii) high intensity (iv) high degree of coherence and (v)
high monochromaticity.
31. Explain (i) directionality (ii) negligible divergence (iii) high intensity
(iv) high degree of coherence and (v) high monochromaticity.
(i) Directionality: The conventional light sources emit light uniformly in all
directions. In case of laser, the active material is a cylindrical resonant cavity.
the light that is travelling parallel to the axis is selected and reinforced. Light
propagating along the axial direction emerges from the cavity and becomes the
laser beam. Thus, a laser emits light only in one direction.
(ii) Divergence: Light from conventional sources spreads out in the form of
spherical wave fronts and hence it is highly divergent. On the other hand, light
from a laser propagates in the form of plane waves. The light beam remains
essentially as a bundle of parallel rays. The small divergence that exists is due
to the diffraction of the beam at the exit mirror. A typical value of divergence
of a He-Ne laser is 10-3 radians. It means that the diameter of the laser beam
increases by about 1 mm for every meter it travels. The extent of divergence
can be estimated in a simple way as follows:
If the diameters of spot produced by the laser on a screen which is held at two
different distances from the laser are measured, then the angle of divergence is
given by 12
12
ll2
ddφ
where d1 is the spot diameter at the distance l1 and d2 is the spot diameter at the
distance l2.
Problem: Calculate the divergence of light beam issuing out of He-Ne laser,
which produces spot diameters of 4 mm and 6 mm at 1m and 2m distances
respectively. [10-3 radian]
(iii) Intensity: The intensity of light from a conventional source decreases
rapidly with distance as it spreads out in space. Laser emits light in the form of
a narrow beam with its energy concentrated in a small region of space.
Therefore, the beam intensity would be tremendously large and stays constant
with distance. The intensity of a laser beam is approximately given by I =
sectioncrossofarea
beamtheofpower
problem: A 10 mW laser has a beam diameter of 1.6 mm. What is the
intensity of the light assuming that it is uniform across the beam?[4.97
kW/m2]
(iv) Coherence: The light that emerges from a conventional light source is
incoherent. Coherence length is one of the parameters used as a measure of
coherence. It is the distance upto which the beam can maintain coherence.The
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NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY
Prof. S.M.ASADULLAH 9290233597 NSAKCET
coherence length of light from a sodium lamp, which is a traditional
monochromatic source, is of the order of 0.3 mm. On the other hand the
coherence length of light emitted by helium-neon laser is about 100 m.
(v) Monochromaticity: If light coming from a source has only one frequency
(single Wavelength) it is said to be monochromatic and the source a
monochromatic source. Light from traditional monochromatic sources spreads
over a wavelength range of 100 Å to 1000 Å. On the other hand, the light from
lasers is highly monochromatic and contains a very narrow range of a few
angstroms (< 10 Å).
32. What is the advantage of using laser as light sources in CD player?
Because of coherence and directionality lasers can be focussed to much
smaller spot of micron size. Because of this storage capacity increases.
33. State any four applications of lasers in engineering and industry. (Dec.
2010, June 2011)
i) Material processing such as cutting, drilling, welding, etching, surface
hardening etc.(Ruby laser, CO2 laser, Nd: YAG laser)
ii) For pollution monitoring and remote sensing (CO2 laser, Nd: YAG laser)
iii) Non destructive testing of components using holographic interferometry
(He-Ne laser)
iv) In compact discs for storing and retrieving the data.
34. State any four applications of lasers in the field of medicine. (May
2007, Dec. 2010, June 2011)
i) Medical lasers are used as scalpel for bloodless surgery (CO2 laser,
Nd:YAG laser)
ii) Fibre optic endoscopes with lasers as light sources are very useful in
treatment of internal organs (e.g. Nd: YAG lasers in treatment of
gastrointestinal bleeding).
iii) Lasers are used for retinal welding (Argon ion lasers)
iv) Excimer lasers are used for eye lens curvature correction.
v) they are also used in ngioplasty.
35. Mention any two applications of laser each in the field of scientific
research.
1) Lasers are used to find the pollutants in atmosphere.
2) They are also used to separate isotopes.
3) They are also used in causing controlled fusion.
4) They are also used in measuring the distances of the moon and the planets at
different times.
5) They are also used in guiding the missiles as well as in destroying the enemy
missiles.
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NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY
Prof. S.M.ASADULLAH 9290233597 NSAKCET
6) They are also used in optical communication.
36. What are different types of compact discs? How are data stored, read
and rewritten?
Compact discs are classified as 1) audio C.D. 2) video CD 3) text CD.
1) in audio CD sound is stored in digital form 0 or 1
2) in vedio CD picture is stored in digital form 0 or 1
3) 1) in text CD informtion is stored in digital form 0 or 1
All informations are first into binary form 1 or 0.it is then stored as reflecting
and surfaces with the help of laser.
LASERS OBJCTIVE
1. ‘Laser’ is an acronym for Light Amplification by .........Emission of Radiation
(stimulated)
2. In 1954, Charles H. Townes and his group operated microwave device
called...... (MASER)
3. In 1960, Maiman achieved first laser action in ........(Ruby).
4. 1961 Ali Javan developed He-Ne laser.
5. The rate of stimulated absorption is.......(B12N1 ρ(ν))
6. The rate of stimulated emission is.......(B21N2 ρ(ν))
7. The rate of spontaneous emission is.......(A21N2 )
8. In thermal equilibrium the rate of transitions upwards is always .....to the rate of
transitions in the downward direction.( equal)
9. In thermal equilibrium we have B12N1 ρ(ν) = A21N2 + B21N2 ρ(ν)
10. In Einstein’s coefficients we have B12 = B21
11. In Einstein’s coefficients we have B12 = B21= 213
3
Ahνπ8
c
12. The ratio of 3
3
21
21
hνπ8
c
A
B
13. The ratio of the rate of stimulated transitions to spontaneous transitions = ρ(ν)A
B
21
21
14. The ratio of the rate of stimulated transitions to absorption transitions = 1
2
N
N
15. The law governing the distribution of atoms at various energy levels of a system
is called ........(Boltzmann distribution)
16. According to Boltzmann distribution Ni = giNo exp(Ei/kT)
17. In thermal equilibrium N2 << N1
18. In thermal equilibrium kTEEe
N
N /
1
2 12 or kThe
N
N /
1
2
19. At room temperature N2/N1 is of the order ....(10 -33)
20. When the population of the excited state is larger than the population of the
lower state, the condition is called........ (population inversion)
21. The life time of an electron in the excited state is 10 – 9 s
22. The life time of an electron in a metastable state is 10 – 6 s to 10-3s
23. The life time of an electron in a metastable state is 106 to 103 times longer.
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NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY
Prof. S.M.ASADULLAH 9290233597 NSAKCET
24. For lasing action population inversion is necessary.
25. In population inversion N2 >> N1.
26. For population inversion metastable states are necessary.
27. The main parts of a laser are 1)active medium 2) the pump 3) optical resonator
28. The material in which laser action takes place is called active medium.
29. The atoms containing metastable state are called active centers.
30. The process of supplying energy to the active medium is called pumping.
31. The pumping can be optical, electrical discharge, injection current.
32. Optical resonator cavity consists of two mirrors held parallel to each other
33. Optical resonator cavity is also called Fabry- Perot resonator.
34. Optical resonator cavity increases the path of photons and amplifies light.
35. Optical resonator cavity also selects the wavelength of light.
36. For creating population inversion we have three level pumping scheme and four
level pumping schemes.
37. Ruby laser and Nd: YAG are solid state lasers.
38. He-Ne laser and CO2 lasers are gas lasers.
39. GaAs laser and InP lasers are semiconductor diode lasers.
40. In ruby the metastable state is due to Cr+3 impurity atoms.
41. Ruby is Al2O3 containing 0.05 % of chromium ions Cr+3.
42. In ruby laser Cr+3 impurity atoms are active centers.
43. In ruby laser optical pumping is used.
44. In ruby laser the pumping is done by using xenon flash lamp.
45. In ruby laser three level pumping scheme is used.
46. Ruby laser works in pulse mode only.
47. Ruby laser has poor efficiency.
48. Ruby laser produces light of wavelength 6943Å.
49. He – Ne laser uses a mixture of He and Ne in the ratio 10 : 1.
50. In He-Ne laser the active centers are Neon atoms.
51. In He-Ne laser the helium atoms are used to excite Neon atoms.
52. He-Ne laser uses four level pumping scheme.
53. In He-Ne laser the pumping is by electrical discharge.
54. He-Ne laser operates in continuous wave (CW) mode.
55. He – Ne laser produces light of wavelength 6328 Å
56. CO2 laser is a molecular laser.
57. In CO2 the pumping agent is electric discharge.
58. CO2 laser uses four level pumping scheme.
59. CO2 laser produces light of wave length 10.6 μm.
60. CO2 laser operates in continuous wave (CW) mode.
61. In CO2 laser the active centers are CO2 molecules.
62. CO2 laser operates in continuous wave (CW) mode.
63. CO2 laser has high efficiency (40%).
64. CO2 laser has very high power output several kilowatt.
65. R.N. Hall devised semiconductor diode laser.
66. Semiconductor diode laser is made from direct band gap semiconductor.
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NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY
Prof. S.M.ASADULLAH 9290233597 NSAKCET
67. GaAs is a direct band gap semiconductor.
68. Silicon and germanium are indirect band gap semiconductor .
69. Semiconductor diode laser is very small in size (0.1 mm) and has high
efficiency (40%)
70. Semiconductor diode laser can be operated at low temperature.
71. Semiconductor diode lasers are used on fiber optic communication, laser
printing, in CD player, CD-ROM drive, optical reading.
72. If Eg is the energy gap of a semiconductor the wavelength of light emitted is
λ = hc/Eg
73. In semiconductor diode laser the pumping is by forward biasing and injecting
current into the junction region.
74. Homojunction semiconductor diode laser is made of same material on either
side of the junction
75. Heterojunction semiconductor diode laser is made of different materials on
either side of the junction. (GaAs on one side of the junction and GaAlAs on the
other side)
76. When photons pass through a medium under thermal equilibrium condition the
photon density ...........(decreases)
77. When photons pass through a medium under population inversion condition the
photon density________.(increases)
78. Ruby laser is the best example for a ........system (three level)
79. He-Ne laser is a good example for a .......level system.(four level)
80. The mechanism applied to create population inversion in semi conductor lasers
is through (injection current)
81. Resonator mirrors in a laser provide optical ........to the photons(feed back)
82. The degree of directionality of a laser is expressed in ......(divergnce).
83. A predictable correlation of the amplitude and phase at any one point with any
other point is called ......(coherence)
84. The maximum length of a wave train on which any two points can be correlated
is called..........( coherence length)
85. The experiment used for measuring temporal coherence is .....(Michelson
interferometer)
86. The experiment used for measuring spatial coherence is ....(Young’s double slit
experiment)
87. In Ruby laser, the emission is from ......(chromium ions)
88. In He-Ne laser, the emission is from....(neon atoms)
89. Hetero junction lasers could be operated at .....temperature (room)
90. Laser radiation is highly..... (coherent)
91. In He-Ne laser, He: Ne ratio is ..... (10 : 1)
92. Ruby laser output is ...... in colour. (red)
93. The laser beam has high directionality. The directionality is measured in
divergence.
94. The divergence of He-Ne laser is 10-3 radian.
95. The divergence is given by 12
12
ll
dd
18
NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY
Prof. S.M.ASADULLAH 9290233597 NSAKCET
96. Intensity of beam is power per unit area.
97. Since the laser beam is highly narrow it has very high power.
98. CO2lasers are used in welding thin sheets and foils.
99. Pulsed ruby laser is used in drilling holes.
100. CO2lasers are used in the heat treatment of tools.
MULTIPLE CHOICE QUESTIONS
1 Emission of photon when an electron jumps from higher energy state to lower
energy state due to interaction of external energy is called
a) spontaneous emission b) stimulated emission c) induced emission
d) amplified emission
2 The population of the various energy levels of a system in thermal equilibrium
is given by
a) Boltzmann distribution law b) Einstein relations
c) Planck’s law d) Beer’s law
3 A three level laser system will be
a) always CW b) either CW or pulsed
c) always pulsed d) made CW as well as pulsed by temperature control
4 Choose the correct statement
a) Four level laser system will be always a pulsed system
b) Three level laser system will be always a CW system
c) Resonators help in increasing the brightness of laser beam .
d) Resonators act as frequency selectors and also ' directionality to the
output beam. 5 T. Maiman invented
a) H e-Ne laser b) CO2laser c) Ruby laser d) Nd: YAG laser
6 The wavelength of emission of CO2 laser is
a) 632.8nm b) 10.6 μm c) 1.064 μm d) 694.3nm
7 The ratio of pressure of CO2:N2:He used in CO2 laser is
a) 5:4:1 b) 4:5:1 c) 1:4:5 d) 1:5: 8.
8 CO2 laser was invented by
a) T. Maiman b) Ali Jawan c) Einstein d) C.K.N. Patel
9 In Ruby lasing material the percentage of chromium ions in aluminium oxide
is
a) 0.5 b) 0.05 c) 5 d) 0.005
10 The colour of the laser output from a Ruby laser is
a) green b) blue c) red d) violet
11 The wavelength of radiation from Nd : YAG/Nd : glass is
a) 1.064µm b) 10.6µm. c) 694.3µm d) 1.064nm
12 Measurement of variation of divergence of laser beam with distance is used to
determine
a) coherence b) monochromacity
c) brightness d) directionality.
13 Coherence of light is measured from
a) variation in spot size with distance
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NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY
Prof. S.M.ASADULLAH 9290233597 NSAKCET
b) visibility of interference fringes it produces
c) brightness of the beam
d) wavelength of the beam
14 In H e-N e lasers, the ratio of He -N e is in the order
a) 1:10 b) 1:1 c) 100:1 d) 10:1 .
15 Choose the correct answer.
Under population inversion condition intensity of light passing through the
medium
a) decreases b) increases c) remains the same
d) first decreases and then increases
16 Example for creation of population inversion by optical pumping is
a) H e-Ne laser b) Diode laser c) Ruby laser c) CO2 laser
17 Example for creation of population inversion by electric discharge. is
a) Ruby laser b) diode laser c) Nd : YAG laser d) CO2 laser
18 Optical feedback in a laser is to
a) Change the wavelength of output emission
b) increase the lifetime of emission
c) effectively increase the length of the active medium
d) none of the above
19 The wavelength of emission from He-N e laser is
a) 10.64 µm b) 337.1 µm c) 694.3 nm d) 632.8 nm
20 Laser of high efficiency is
a) CO2 b) He-Ne c) Ruby d) Nd : YAG
21 The laser used for pollution monitoring and remote sensing application is
a) He-Ne b) Diode c) CO2 d) semiconductor
22 In homojunction laser threshold current density required for laser action is
a) 40 Am m-2 b) 50 Am m-2 c) 50 mAm m-2 d) 400 Amm-2
23 In hetrojunction laser threshold current density required for laser action is
a) 40 Am m-2 b) 50 Am m-2 c) 10 Am m-2 d) 400 Am m-2
24 Laser radiation is
a) monochromatic b) highly directional.
c) coherent and stimulated
d) highly directional, monochromatic, coherent and stimulated. 25 Population inversion cannot be achieved by
a) chemical reaction b) thermal process
c) electric discharge d) optical Pumping
26 He-Ne gas laser is
a) pulsed laser b) semiconductor laser
c) solid state laser d) continuous laser
27 In ruby lasing material the percentage of chromium ions in aluminium oxide s
a) 0.05 b) 0.5 c) 5 d) 0.005
28 In computer printer laser is used
a) semiconductor b) CO2 c) Ruby d) He-Ne
20
NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY
Prof. S.M.ASADULLAH 9290233597 NSAKCET
29 A direct conversion of electrical energy into light energy occurs
a) CO2 laser b) He-Ne laser c) LEDs d) Ruby laser
30 A lasing action is possible only if there is
a) a set of reflecting mirrors b) a black body
c) population inversion d) oscillation of laser source.
31 The source of excitation in ruby laser is
a) sodium vapour lamp b) xenon flash lamp
c) mercury vapour lamp d) incandescent lamp
32 What is active region in GaAs system ?
a) p-region b) n-region c) p-n junction d) total material
33 A He-Ne laser emits light of wavelength 632.8 nm and has a output power of
2.3 mW; then the number of photons emitted per second is
a) 73.3 x 1014 b) 29.56 x 1014 c) 1173.5 x 1014 d) 23.5 x 1014'
34 The unit of Plancks constant is
a) sec b) watts c) Joule sec d) m-sec
35 Emission of photon when an electron jumps from higher energy state to lower
energy state due to interaction with another photon is called
a) spontaneous emission b) stimulated emission
c) induced emission d) amplified emission.
36 The condition for lasing action is
a) excitation b) absorption c) emission d) population inversion.
37 The source of excitation in He-Ne gas laser is
a) xenon flash lamp b) optical pumping
c) electric discharge d) direct conversion.
38 If Eg is the band gap in a GaAs laser then the frequency of laser beam
produced by it is
a) independent of Eg b) inversely proportional to Eg
c) directly proportional to Eg d) none
39 .Which of the following is coherent
a) spontaneous emission b) stimulated emission
c) both d) none
40 Laser action is found in which of the following semiconductors
a) direct band gap b) indirect band gap
c) germanium d) silicon
41 The Pumping process used in Ruby laser is
a) Optical Pumping b) electric discharge
c) passing forward bias d) chemical reactions
42 The Pumping process involved in CO2 laser is
a) optical Pumping b) electric discharge
c) atom-atom in elastic collision d) chemical reactions
43 Threshold current density is minimum in
a) homojunction diode laser b) heterojunction diode laser.
c) in both (d) none
21
NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY
Prof. S.M.ASADULLAH 9290233597 NSAKCET
44 In He-Ne laser, atoms involved in laser emission are
a) Neon atoms b) Helium atoms c) both (1) none
45 Ruby laser emits light of wavelength
a) 6943Ǻ b) 6328 Ǻ c) 8628 Ǻ d) 8370 Ǻ
46 Rate of stimulated emission is proportional to
a) population of excited state
b) inducement energy
c) properties of energy level
d) population of excited state and inducement energy
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