Laser (Light Amplification by Stimulated Emission of Radiation)

BY SUMITESH MAJUMDER

What is Laser? Light Amplification by Stimulated Emission of

Radiation

• A device produces a coherent beam of optical radiation by stimulating electronic, ionic, or molecular transitions to higher energy levels

• Mainly used in Single Mode Systems• Light Emission range: 5 to 10 degrees• Require Higher complex driver circuitry than LEDs• Laser action occurs from three main processes: photon

absorption, spontaneous emission, and stimulated emission.

Properties of Laser• Monochromatic Concentrate in a narrow range of wavelengths

(one specific colour).

• Coherent All the emitted photons bear a constant phase

relationship with each other in both time and phase

• Directional A very tight beam which is very strong and

concentrated.

Basic concepts for a laser

• Absorption

• Spontaneous Emission

• Stimulated Emission

• Population inversion

Absorption

• Energy is absorbed by an atom, the electrons are excited into vacant energy shells.

Spontaneous Emission

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

Stimulated Emission

atoms in an upper energy level can be triggered or stimulated in phase by an incoming photon of a specific energy.

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

Stimulated Emission

Laser Diode Optical Cavity• One reflecting mirror is at one end while the other end

has a partially reflecting mirror for partial emission• Remaining power reflects through cavity for amplification

of certain wavelengths, a process known as optical feedback.

• Construction very similar to the ELEDs.

Mirror Reflections

The operation of the Laser

The operation of the Laser

1E

2E

3E4E

The operation of the Laser

1E

2E

3E4E

absorption

The operation of the Laser

1E

2E

3E4E

Spontaneous emission

The operation of the Laser

Spontaneous emission

1. Incoherent light

2. Accidental direction

The operation of the Laser

1E

2E

3E4E

The operation of the Laser

1E

2E

3E4E

Stimulated emission

The operation of the Laser

Light: Coherent, polarized

The stimulating and emitted photons have the same:

frequency

phase

direction

How a Laser Works

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

E2

Population Inversion

• A state in which a substance has been energized, or excited to specific energy levels.

• More atoms or molecules are in a higher excited state.

• The process of producing a population inversion is called pumping.

• Examples:

→by lamps of appropriate intensity

→by electrical discharge

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 relation 

Probability of stimulated absorption R1-2

R1-2 = ()n1 B1-2 where spectral density is ()

& Einstein coeff. of absorbtion is B1-2

 

Probability of stimulated and spontaneous emission :

R2-1 = () n2B2-1 + n2A2-1

 Einstein coeff. of stimulated and spontaneous emission are B2-1& A2-1

Assumption : For a system in thermal equilibrium, the upword and downword transition rate must be equal : 

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

1 1 2

2 2 1

/ =

1

A Bn Bn B

E1

E2

B1-2/B2-1 = 1

According to Boltzman statistics:    

() = =  

 

12 1

2

exp( ) / exp( / )n

E E kT h kTn

1)exp(

/

12

21

1212

kT

h

B

BBA

1)/exp(

/8 33

kTh

ch

3

3

12

12 8

c

h

B

A

       Planck’s law

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

emission B2-1(:

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

2. kT at 300K ~ 0.025eV.

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

 

if h/kT <<1.

1)/exp()(12

12

kThB

A

1

2

1

2

12

12

211

122122 ])(

1[)(

)(

n

n

n

n

B

A

Bn

BnAnx

 

x~ n2/n1

   

Two-level Laser System

• Unimaginable

as absorption and stimulated processes neutralize one another.

• The material becomes transparent.

Two level system

absorption Spontaneous emission

Stimulated emission

h hh

E1

E2

E1

E2

h=E2-E1

Three-level Laser System

• Initially excited to a short-lived high-energy state .

• Then quickly decay to the intermediate metastable level.

• Population inversion is created between lower ground state and a higher-energy metastable state.

23 21

1.06 m 4

2τ 2.3 10 s

1 3.39 m 2 0.6328 m

3 1.15 m

100nsτ2 10nsτ

1

Three-level Laser System

Nd:YAG laser

He-Ne laser

Four-level Laser System

• Laser transition takes place between the third and second excited states.

• Rapid depopulation of the lower laser level.

Four-level Laser System

23

m 6943.01

m 6928.02

s7

310τ s3

2103τ

Ruby laser

Multimode Laser Output Spectrum

Longitudinal Modes

ModeSeparation

(Center Wavelength)

g(λ)

Lasing Characteristics• Lasing threshold is

minimum current that must occur for stimulated emission

• Any current produced below threshold will result in spontaneous emission only

• At currents below threshold LDs operate as ELEDs

• LDs need more current to operate and more current means more complex drive circuitry with higher heat dissipation

• Laser diodes are much more temperature sensitive than LEDs

Fabry-Perot Laser (resonator) cavity

Modulation of Optical Sources

• Optical sources can be modulated either directly or externally.

• Direct modulation is done by modulating the driving current according to the message signal (digital or analog)

• In external modulation, the laser is emits continuous wave (CW) light and the modulation is done in the fiber

Types of Optical Modulation

• Direct modulation is done by superimposing the modulating (message) signal on the driving current

• External modulation, is done after the light is generated; the laser is driven by a dc current and the modulation is done after that separately

• Both these schemes can be done with either digital or analog modulating signals

Direct Modulation

• The message signal (ac) is superimposed on the bias current (dc) which modulates the laser

• Robust and simple, hence widely used• Issues: laser resonance frequency, chirp, turn

on delay, clipping and laser nonlinearity

Laser Construction

• A pump source

• A gain medium or laser medium.

• Mirrors forming an optical resonator.

Laser Construction

Pump Source• Provides energy to the laser system

• Examples: electrical discharges, flashlamps, arc lamps and chemical reactions.

• The type of pump source used depends on the gain medium.

→A helium-neon (HeNe) laser uses an electrical discharge in the helium-neon gas mixture.

→Excimer lasers use a chemical reaction.

gain medium

• Major determining factor of the wavelength of operation of the laser.

• Excited by the pump source to produce a population inversion.

• Where spontaneous and stimulated emission of photons takes place.

• Example:

solid, liquid, gas and semiconductor.

Optical Resonator

• Two parallel mirrors placed around the gain medium.

• Light is reflected by the mirrors back into the medium and is amplified .

• The design and alignment of the mirrors with respect to the medium is crucial.

• Spinning mirrors, modulators, filters and absorbers may be added to produce a variety of effects on the laser output.

Laser Types

• According to the active material:

solid-state, liquid, gas, excimer or semiconductor lasers.

• According to the wavelength:

infra-red, visible, ultra-violet (UV) or x-ray lasers.

Solid-state Laser

• Example: Ruby Laser• Operation wavelength: 694.3 nm (IR)• 3 level system: absorbs green/blue

•Gain Medium: crystal of aluminum oxide (Al2O3)

with small part of atoms of aluminum is replaced

with Cr3+ ions.•Pump source: flash lamp •The ends of ruby rod serve as laser mirrors.

Ene

rgy

LASING

rapid decay

Ruby laser

Al2O3Cr+

How a laser works?

Ruby laser

First laser: Ted MaimanHughes Research Labs1960

Liquid Laser

• Example: dye laser• Gain medium: complex organic dyes, such

as rhodamine 6G, in liquid solution or suspension.

• Pump source: other lasers or flashlamp.• Can be used for a wide range of

wavelengths as the tuning range of the laser depends on the exact dye used.

• Suitable for tunable lasers.

Schematic diagram of a dye laser

dye laser

A dye laser can be considered to be basically a four-level system. The energy absorbed by the dye creates a population inversion, moving the electrons into an excited state.

Gas Laser• Example: Helium-neon laser (He-Ne laser)• Operation wavelength: 632.8 nm • Pump source: electrical discharge• Gain medium : ratio 5:1 mixture of helium and neon

gases

μm15.1μm6328.0μm39.3 321

He-Ne laser

Semiconductor laser

Semiconductor laser is a laser in which semiconductor serves as photon source.

Semiconductors (typically direct band-gap semiconductors) can be used as small, highly efficient photon sources.

Applications of laser

• 1. Scientific

a. Spectroscopy b. Lunar laser ranging

c. Photochemistry d. Laser cooling

e. Nuclear fusion

• 2 Military

a. Death ray

b. Defensive applications

c. Strategic defense initiative

d. Laser sight

e. Illuminator

f. Rangefinder

g. Target designator

Applications of laser

• 3. Medical

a. eye surgery

b. cosmetic surgery

Applications of laser

• 4. Industry & Commercial

a. cutting, welding, marking

b. CD player, DVD player

c. Laser printers, laser pointers

d. Photolithography

e. Laser light display

Applications of laser