LASER

SUBJECT HANDLER

V.REVATHIAMBIKA

LECTURER IN PHYSICS

INTRODUCTION OF LASER

L – LIGHT

A – AMPLIFICATION

S – STIMULATED

E – EMISSION

R - REDIATION

A. L. SCHAWLOW and C. H. TOWNES IN 1958

RUBY LASER by T. H. MAIMANN IN 1960

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DEFINITION OF LASER

A laser is a device that generates light by a process called STIMULATED EMISSION.

The acronym LASER stands for Light Amplification by Stimulated Emission of Radiation

Semiconducting lasers are multilayer semiconductor devices that generates a coherent beam of monochromatic light by laser action. A coherent beam resulted which all of the photons are in phase.

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In 1958, Charles Townes and Arthur Schawlow theorized about a visible laser, an invention that would use infrared and/or visible spectrum light.

Light Amplification by Stimulated Emission of Radiation- (LASER).

THE OPERATION OF THE LASER

BASIC IDEA

Consider a group of atoms exposed stream of photons, each with energy h. Let us assume two energy levels E1

and E2 of an atom.During transition from one energy state to another, the

light is absorbed (or) emitted by particles. Under this action, 3 processes can occur.

They are,Stimulated absorptionSpontaneous emissionStimulated emission

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ABSORPTION

Spontaneous event in which an atom or molecule absorbs a photon from an incident optical field

The asborption of the photon causes the atom or molecule to transition to an excited state

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ABSORPTION

E1

E2

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SPONTANEOUS EMISSION

Statistical process (random phase) – emission by an isolated atom or molecule

Emission into 4π steradians

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SPONTANEOUS EMISSION

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STIMULATED EMISSION

Same phase as “stimulating” optical field

Same polarization

Same direction of propagation

E2

hn

E1

2hn

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STIMULATED EMISSION

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STIMULATED EMISSION

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ABSORPTION, SPONTANEOUS EMISSION & STIMULATED EMISSION

E1

E2

h

(a) Absorption

h

(b) Spontaneous emission

h

(c) Stimulated emission

In hOut

h

E2 E2

E1 E1

Absorption, spontaneous (random photon) emission and stimulatedemission.

© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

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ABSORPTION Light that falls on a piece of material will decrease

exponentially. = (N1-N2)B21(hf) n/c N1 is often more than N2 (N1 < N2)

Example for tungsten

is typically 106m-1 (+ve) If we want implication, must be –ve i.e. N2 > N1

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PUTTING IT ALL TOGETHER…

Assume that we have a two state system in equilibrium with a blackbody radiation field.

E2

E1

Stimulated emission

AbsorptionSpontaneous

emission

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Two level system

absorption Spontaneous emission

Stimulated emission

hn hnhn

E1

E2

E1

E2

hn=E2-E1

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E1

E2

• n1 - the number of electrons of energy E1

• n2 - the number of electrons of energy E2

2 2 1

1

( )exp

n E E

n kT

Boltzmann’s equation

example: T=3000 K E2-E1=2.0 eV

42

1

4.4 10n

n

Einstein’s coefficients 

Probability of stimulated absorption R1-2

R1-2 = (n) B1-2

 Probability of stimulated and spontaneous emission :

R2-1 = (n) B2-1 + A2-1

 assumption: n1 atoms of energy 1 and n2 atoms of energy 2 are in thermal equilibrium at

temperature T with the radiation of spectral density (n): 

n1 R1-2 = n2 R2-1 n1 (n) B1-2 = n2 ( (n) B2-1 + A2-1)

  

2 1 2 1

1 1 2

2 2 1

/ =

1

A Bn Bn B

n

E1

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B1-2/B2-1 = 1

According to Boltzman statistics:    

(n) = =  

 

12 1

2

exp( ) / exp( / )n

E E kT h kTn

n

1)exp(

/

12

21

1212

kT

h

B

BBA

n 1)/exp(

/8 33

kTh

ch

nn

3

3

12

12 8

c

h

B

A n

      

Planck’s law

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The probability of spontaneous emission A2-1 /the probability of stimulated

emission B2-1(n:

  1. Visible photons, energy: 1.6eV – 3.1eV.

2. kT at 300K ~ 0.025eV.

3. stimulated emission dominates solely when hn/kT <<1!(for microwaves: hn <0.0015eV) The frequency of emission acts to the absorption: 

 

if hn/kT <<1.

1)/exp()(12

12

kThB

A nn

1

2

1

2

12

12

211

122122 ])(

1[)(

)(

n

n

n

n

B

A

Bn

BnAnx

nn

n

 x~ n2/n1

   

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S.NO SPONTANEOUS EMISSION STIMULATED EMISSION

1. The atom in the excited state returns to ground state thereby emitting a photon ,without any external inducement .

An atom in the excited state is induced to return to ground state thereby resulting in two photons of same frequency and energy

2. The emitted photons can move randomly The emitted photon move in same direction and is highly directional

3. The photons are not in phase The photons are in phase

4. The rate of transition is given byR sp = A21 N2

The rate of transition is given byR st = B21 N2 ρ

5. Incoherent radiation Coherent radiation

6. Having more angular spread during propagationEx: light from sodium (or) mercury vapour lamp

Having less angular spread during propagationEx: light from laser source

DIFF B/W SPONTANEOUS & STIMULATED EMISSION

POPULATION INVERSION

Therefore we must have a mechanism where N2 > N1 This is called POPULATION INVERSION Population inversion can be created by introducing a so call metastable centre

where electrons can piled up to achieve a situation where more N2 than N1 The process of attaining a population inversion is called pumping and the

objective is to obtain a non-thermal equilibrium. It is not possible to achieve population inversion with a 2-state system. If the radiation flux is made very large the probability of stimulated emission

and absorption can be made far exceed the rate of spontaneous emission. But in 2-state system, the best we can get is N1 = N2. To create population inversion, a 3-state system is required. The system is pumped with radiation of energy E31 then atoms in state 3 relax

to state 2 non radiatively. The electrons from E2 will now jump to E1 to give out radiation.

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Condition for the laser operation

If n1 > n2

• radiation is mostly absorbed absorbowane• spontaneous radiation dominates.

• most atoms occupy level E2, weak absorption

• stimulated emission prevails

• light is amplified

if n2 >> n1 - population inversion

Necessary condition: population inversion

E1

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How to realize the population inversion?

Thermal excitation:

2

1

expn E

n kT

Optically, electrically.

impossible.

The system has to be „pumped”

E1

E2

Three level laser

The laser operation

E1

E3

E2

Fast transition

Laser action

• 13 pumping• spontaneous emission 3 2.• state 2 is a metastable state • population inversion between states 2 and 1. • stimulated emission between 2 i 1.

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t

E1

E3

E2

szybkie przejścia

akcja laserowa

- optical pumping - occupation of E3 of a short life time, 10-8s. It is a band, the metastable and ground states are narrow :

-  electrons are collected on E2: population inversion

-   stimulated emission (one photon emitted spontaneously starts the stimulated radiation )

- Beam of photons moves normally to the mirrors – standing wave.

The laser operation04/10/23

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POPULATION INVERSION

When a sizable population of electrons resides in upper levels, this condition is called a "population inversion", and it sets the stage for stimulated emission of multiple photons. This is the precondition for the light amplification which occurs in a LASER and since the emitted photons have a definite time and phase relation to each other, the light has a high degree of coherence.

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