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L7LASER

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LASER

LLight

AAmplification by

SStimulated

EEmission of

RRadiation

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Properties of laser

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Monochromatic

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Directionality

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Coherence

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Incandescent vs. Laser Light

1. Many wavelengths

2. Multidirectional

3. Incoherent

1. Monochromatic

2. Directional

3. Coherent

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BASIC PRINCIPLE NEEDEDFOR LASER

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Stimulated Absorption

Energy is absorbed by an atom, the electrons are

excited into vacant energy shells.

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Spontaneous Emission

The atom decays from level 2 to level 1 through theemission of a photon with the energy hv. It is acompletely random process.

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Stimulated Emission

atoms in an upper energy level can be triggered or

stimulated in phase by an incoming photon of a specific

energy.

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Stimulated Emission

The stimulated photons have unique properties:

In phase with the incident photon

Same wavelength as the incident photon

Travel in same direction as incident photon

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WHY WE NEED META STABLE

STATE?

ANSWER IS

With having the metastable state above the

ground level. Atom reaches the meta stable

state (after first stimulated emission) can

remain there for longer time period. So the

number of atom increases in the meta stable

state. And when these atoms come back to theoriginal ground level it emits laser beam.

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A state of a medium where a higher-lying electronic level has

a higher population than a lower-lying level

POPULATION INVERSION

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The method particle of raising a particle from lower energy stateto higher energy state is called pumping.

TYPES OF PUMPING :

1. Optical pumping

2. Electrical pumping

3. X-ray pumping

4. Chemical pumping

PUMPING

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All lasers have 3 essential components:

A lasing or "gain" medium

A source of energy to excite electrons in the gain medium to high energy

states, referred to as "pump" energy

An optical path which allows emitted photons to oscillate and interfere

constructively as energy is added or "pumped" into the system, ie, a

resonator

LASER COMPONENTS

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LASER ACTION

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Types of Laser

a.According to their sources:

1.Gas Lasers2.Crystal Lasers

3.Semiconductors Lasers

4.Liquid Lasers

a.According to the nature of emission:

1.Continuous Wave

2.Pulsed Laser

a.According to their wavelength:1.Visible Region

2.Infrared Region

3.Ultraviolet Region

4.Microwave RegionX-Ray Region

d. According to different levels 1. 2-level laser

2. 3-level laser

3. 4-level laser

e. According to mode of pumping

1. optical

2. chemical

3. electric discharge4. electrical

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2- Level Laser

h

E1

E2

AbsorptionE1

E2h

Spontaneous

Emission

E1

E2hh h

Stimulated

Emission

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THREE STEP LASER

Stimulated absorption

Spontaneous emission to the metastable state

Stimulated emission from metastable state to ground state.

E2

E1

E0

E2 E1

E1 E0

META STABLESTATE

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4-Level LASER

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PRACTICAL LASERS

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RUBY LASER

Construction

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ENERGY LEVEL IN RUBY

LASER

Energy

4A2

4T2

4T1

2T2

2E

LASING

rapid decay

Cr+

Al2O3

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LASER ACTION IN RUBY

LASER

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HE-NE LASER

Construction

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.

Energy Level Diagram of He-Ne

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Energy Level Diagram of He-Ne cont

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Combined Laser Action

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Emission wavelengths of common

lasers

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APPLICATION S OF LASER

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04/13/11

Not to be Taken LightlyThe Weighty Implications of Laser Technology

Applications of

Laser

Technology

Medical

Entertainment

Telecommunications

Military

Optical SurgeryGeneral Surgery Tattoo removal

CD Players DVD Players Video Game Systems

Information tech. Holograms

Weapons

Satellites

Radar

Industry

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04/13/11

Can You See the Light?

Dentists use

laser drills

Bad eyesight can be

corrected by optical

surgery using lasers

CD-Audio is

read by a laser

Tattoo removal is

done using lasers

Cd-Rom discs

are read by lasers

Laser pointers can

enhance

presentations Bar codes in

grocery stores are

scanned by lasers

Video game systems such as

PlayStation 2 utilize lasers

DVD players read

DVDs using lasers

Airplanes areequipped with

laser radar

Military and Spaceaircraft are equipped

with laser guns

Laser tech. is used in printers,

copiers, and scanners

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Einsteins Coefficients

Einstein gives a probability that stimulated

emission is same as absorption.

Means that if a stimulated absorption can

occur then there is same probability that

stimulated emission can occur.

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Equilibrium condition

In case of energy states the number of electron

absorbed and emitted should be equal or the

rate of change of numbers of atoms in two

states should be equal.

0=dt

dN

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Plancks Radiation Law

( )

1

18

3

3

= kTh

ec

hE

Planks gives the formula that howa gas radiate energy.

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Spontaneous emission

A21

:- correspond to spontaneous emission

probability per unit time

This particular emission can occur

without the presence of external field

E(v)

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Stimulated Absorption

B12

:- correspond to stimulated absorption

probability per unit time

This type of absorption can occur in

presence of external field E(v) only

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Stimulated Emission

B21

:- correspond to stimulated emission

probability per unit time

This type of emission can occur in presence of

external field E(v) only

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Total Emission Probability

Spontaneous Emission + Stimulated Emission

A21 + B21 E(v)

Number of atoms that can jump from level E2 to E1 is

( )[ ] 22121 NEBA +

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Total Absorption Probability

( )[ ] 112 NEB

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At Equilibrium

Then

02 =dt

dN

( ) ( )[ ] 22121112 NEBANEB +=

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Emission and absorption are same

( )

=

B

B

N

NB

A

E

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Maxwell Bolzman Distribution

KT

E

eNN

1

01

=

KT

E

eNN

2

02

=

In thermal equilibrium

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So the equations become

kT

h

eN

N

=2

1

So equation becomes

( )1

1

21

1221

21

=B

Be

B

A

EkT

h

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Conclusions

Stimulated emission have same probability as

stimulated absorption

Ratio between spontaneous and stimulated

emission varies with v3

All we need is to calculate one of the

probability to find others.

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THANKS SEE YOU IN NEXTLECTURE