1

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:

h

EE 12 .

This process is known as Absorption of Radiation This

process is pictorially represented as:

2

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, )(12 uP . )(1212 uBP

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.: *atomphotonatom

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

frequency υ given by: h

EE 12 . This is known as

spontaneous emission of radiation.

E1

E2

Before

hυ

E1

E2

After

E1

E2

Before

hυ

E1

E2

After

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The probability of spontaneous emission E2→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,

1221 AP

In the form of a equation the spontaneous emission may be

represented as

photonatomatom *

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: )(2121 uBP

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:

4

)(212121 uBAP

This process may be expressed by the following eq.

photonatomphotonatom 2*

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.

E1

E2

Before

hυ

E1

E2

After

hυ hυ

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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:

)(121121 uBNPN

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:

)(21212212 uBANPN

Where N2 is the no. of atoms in energy state E2.

In equilibrium, state the absorption and emission must occur

equally. Therefore,

212121 PNPN

or )()( 21212121 uBANuBN

or 1

1)(

21

12

2

121

21

212121

212

B

B

N

NB

A

BNBN

ANu

................(i)

But according to Boltzman’s distribution law the no. of atoms

in energy states E1 and E2 at thermal equilibrium is given by, kTEeNN /

01

kTEeNN /

02

Where N0 is the total no. of atoms, k is the Boltzman’s

constant and T is the absolute temperature.

Therefore, kThkTEE

eeN

N //

2

1 12

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Thus eq. (i) changes to

1

1)(

21

12/21

21

B

Be

B

Au

kTh

..................(ii)

But according to Plank the energy density of the radiation of

frequency υ at temperature T is given by,

1

18)(

/3

3

kThec

hu

...............(iii)

Comparing eq. (ii) and (iii), we get,

3

3

21

21 8

c

h

B

A and 1

21

12 B

B

Thus the two conclusions may be drawn:

The probability of stimulated emission is same as that of

(incident or stimulated) absorption.

The probability of stimulated emission 21

21

B

Ais proportional

to υ3.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:

kTEEe

N

N /

1

2 12

The negative exponent in this eq. indicates that 12 NN 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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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:

E1

E2

E1

E2

N1

N2 N2

N1

Normal State N1>>N2 Non-equilibrium State N1<<N2

(inverted state)

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Four level Pumping:

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 re-

established. The light output therefore is a pulsed output.

Pumping Level

Rapid Decay

E1

Ground State

E3

Metastable State E2

Pumping

E1

E2

E3

Upper Lasing Level

Lower Lasing Level

Lasing Action

Rapid Decay

Pumping Level

E1

Ground State

E4

Metastable State E3

E2

Pumping

E1

E3

E4

Upper Lasing Level

Lower Lasing Level

E2

Ground State

Lasing Action

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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

ACTIVE MEDIUM

PARTIALLY

REFLECTING

MIRROR

FULLY

REFLECTING

MIRROR

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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:

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.

Green Blue

E1

E2

E3

E’3

Ground State

En

ergy (

eV

)

Stimulated

Emission Photon 6943 A

0

Radiation less transition

Energy levels and transition in a ruby laser

Pumping

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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 Cr3+

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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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.

Power

supply

Capacitor

bank

Flash

Tube

Laser

Beam

Reflected

Photons

Completely Reflecting

Mirror

Partially Reflecting

Mirror

Photons From

excited Nd atoms Optical cavity

Photons from

flash tube

Nd-YAG LASER

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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 Nd

3+ 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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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.

Cathode Anode

Mirror

Glass

Window

He-Ne

mixture

6328A0

+ -

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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 EH1 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.

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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20

18

16

21

S 23

S

helium neon

3s

2s

0

1s

Pumping (electron

impact)

Collision with

tube walls

Fast decay (~10ns)

ground state

3.39μm 543nm 633nm

1.15μm

radiative decays:

~100ns)

En

ergy (

eV)

atomic

collisions

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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:

Brewster

Window

Water In Water Out

Gas In Gas Out

Laser

Beam

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And the rotational modes are:

Stretch mode

Bending mode

Asymmetric mode

Rotational mode

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Working:

Here the excited state of N2 molecule and it is identical to

CO2 molecule.

As current passes through the mixture of gases, the N2

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.

En

erg

y

(eV

)

0.1

0.2

0.3

E1

E2

E3

E4

E5

Energy transfer

through collision

Nitrogen Carbon dioxide

9.6µm

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

High intensity Area

PowerI

High degree of coherence

High monochromaticity

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The end