Laser

LASER

• L L ight

• A A mplification by

• S S timulated

• E E mission of

• R R adiation

Properties of laser

Monochromatic

Directionality

Coherence

Spatial and Temporal Coherence

Beams can be coherent or only partially coherent (indeed, even incoherent)in both space and time.

Spatial andTemporal

Coherence:

TemporalCoherence;

Spatial Incoherence

Spatial Coherence;

TemporalIncoherence

Spatial andTemporal

Incoherence

The temporal coherence time is the time the wave-fronts remain equally spaced. That is, the field remains sinusoidal with one wavelength:

Temporal Coherence

Time, τc

Coherent time

Coherent lengthcoherence length is the propagation distance from a coherent source to a point where a wave (e.g. an electromagnetic wave) maintains a specified degree of coherence. Within this distance, the wave is most similar to a perfect sinusoidal wave. The significance is that wave interference will be strong within a coherence length of the source

Spatial Coherence

Length

cL τ= λλ∆

=2

L

Incandescent vs. Laser Light

1. Many wavelengths

2. Multidirectional

3. Incoherent

1. Monochromatic

2. Directional

3. Coherent

BASIC PRINCIPLE NEEDED FOR LASER

Stimulated Absorption

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

Absorption

E1

E2

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.

Spontaneous Emission

Stimulated Emission

atoms in an upper energy level can be triggered orstimulated in phase by an incoming photon of a specificenergy.

Stimulated Emission

Stimulated EmissionThe stimulated photons have unique properties:

– In phase with the incident photon

– Same wavelength as the incident photon

– Travel in same direction as incident photon

Electron/Photon Interactions

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 the original ground level it emits laser beam.

• A state of a medium where a higher-lying electronic level has a higher population than a lower-lying level

POPULATION INVERSION

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

• TYPES OF PUMPING :

1. Optical pumping

2. Electrical pumping

3. X-ray pumping

4. Chemical pumping

PUMPING

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

LASER ACTION

Types of Laser 

a.According to their sources: 1.Gas Lasers 2.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 Region •X-Ray Region

d. According to different levels 1. 2-level laser 2. 3-level laser 3. 4-level lasere. According to mode of pumping 1. optical 2. chemical 3. electric discharge 4. electrical

2- Level Laser

νh

E1

E2

Absorption

E1

E2νh

Spontaneous   Emission

E1

E2νhνh νh

Stimulated Emission

THREE STEP LASER

• Stimulated absorption• Spontaneous emission to the meta stable

state• Stimulated emission from meta stable state

to ground state. E2

E1

E0

E2 – E1

E1 – E0

META STABLE STATE

4-Level LASER

PRACTICAL LASERS

RUBY LASER

Construction

ENERGY LEVEL IN RUBY LASER

Ene

rgy

4A2

4T2

4T1

2T2

2E

LASING

rapid decay

Cr+

Al2O3

LASER ACTION IN RUBY LASER

HE-NE LASER

Construction

.

Energy Level Diagram of He-Ne

Energy Level Diagram of He-Ne cont…

Combined Laser Action

Emission wavelengths of common lasers

APPLICATION S OF LASER

04/09/15

Not to be Taken LightlyThe Weighty Implications of Laser Technology

Applications of LaserTechnology

Medical

Entertainment

Telecommunications

Military

• Optical Surgery• General Surgery• Tattoo removal

• CD Players• DVD Players• Video Game Systems

• Information tech.• Holograms

• Weapons

• Satellites

• RadarIndustry

04/09/15

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 DVD’s using lasers

Airplanes are equipped with laser radar

Military and Space aircraft are equipped with laser guns

Laser tech. is used in printers, copiers, and scanners

Einstein’s CoefficientsEinstein 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.

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

Planck’s Radiation Law

( )1

183

3

−=

kT

h

ec

hE ν

νπν

Plank’s gives the formula that how a gas radiate energy.

Spontaneous emission

A21 :- correspond to spontaneous emission

probability per unit time

This particular emission can occur without the presence of external field E(v)

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

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

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 ν+

Total Absorption Probability

( )[ ] 112 NEB ν

The rate of change of atoms in E2

It can be given by differentiation

(probability)

or

emissionAbsorptiondt

dN −=2

( )[ ] ( )[ ] 221211122 NEBANEB

dt

dN νν +−=

At Equilibrium

Then

02 =dt

dN

( ) ( )[ ] 22121112 NEBANEB νν +=

Emission and absorption are same

( )

−

=

1

1

21

12

2

121

21

BB

NNB

AE ν

Maxwell Bolzman Distribution

KT

E

eNN1

01

−

=

KT

E

eNN2

02

−

=

In thermal equilibrium

So the equations become

kT

h

eN

N ν=

2

1

So equation becomes

( )1

1

21

1221

21

−

=

BB

eB

AE

kT

hνν

After comparing with Planks Radiation Law

3

38

21

21ch

B

A νπ=

And

121

12 =B

B

Units of Einstein coefficient: Joules-Sec/m3

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.