nawab shah alam khan college of engineering &...

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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

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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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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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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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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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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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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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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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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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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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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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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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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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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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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.

16



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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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



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

19



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



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



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

nawab shah alam khan college of engineering &...

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