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LASERS. LASER is an acronym for light amplification by Stimulated Emission of radiation. When radiation interacts with matter we have three processes to generate laser light. (1) Optical Absorption (2) Spontaneous Emission (3) Stimulated Emission. Characteristics of Lasers

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slide2
LASER is an acronym for light amplification by Stimulated Emission of radiation.

When radiation interacts with matter we have three processes to generate laser light.

(1) Optical Absorption

(2) Spontaneous Emission

(3) Stimulated Emission

slide3
Characteristics of Lasers

The most important characteristics of lasers are

1.Directionality → Angular spread 10 micro radians

2.Monochromacity

3.Coherence → (a). Spatial coherence

(b). Temporal coherence

4.Intensity:

The number of photons coming out from a laser per second per unit area is given by

slide4
Spontaneous Emission

Incoherent

Less intensity

Poly chromatic

Less directionality

More angular spread

Stimulated emission

Coherent

High intensity

Mono chromatic

High directionality

Less angular spread

slide5

N2

E2

Stimulated

emission

Spontaneous

emission

absorption

N1

E1

Einstein Co-efficient

  • Consider two energy levels E1 and E2.
  • N1 and N2 be the number of atoms per unit volume present at the levels E1 and E2 respectively.

Supplied energy

slide12

The equation shows ratio of spontaneous

Emission Rate to stimulated emission rate.

slide13

N2

E2

N1

E1

Population inversion

To achieve more stimulated emission population of the excited state N2 should be made larger than the lower state N1 and this condition is called population inversion.

slide14

E

E2

Fast decay

E1

pumping

Laser transition

E0

N

Three level Laser system

Meta stable level

slide15

E

E3

Fast decay

E2

Meta stable state

pumping

Laser transition

E1

E0

N

Four level laser system

slide16
Ruby LASER
  • Maiman in 1960.
  • Solid State Laser.
  • Active Medium: Al2O3 doped with 0.05% Cr3+ ions(10cm long & 0.5cm diameter).
  • Resonant Cavity: Fully reflecting surface & partially reflecting surface.
  • Pumping System: Helical Xenon flash lamp.
  • Three level laser system.
  • Wave Length: 694.3nm.
  • Pulsed Laser.
  • Widely used in Echo technique & Pulsed Holography
slide17

Xenon flash lamp

Xenon flash lamp

Completely

Reflecting

surface

Completely

Reflecting

surface

Partially reflecting

surface

Partially reflecting

surface

Laser

output

Ruby material

Ruby material

cooling

cooling

slide18

E

E2

Fast decay

E1

Meta stable level

pumping

Laser transition

E0

N

Energy level diagram of Ruby laser

Short lived state

slide19
He-Ne LASER

1.Ali Javan in 1961.

2.Gas Laser..

3.Active Medium: Helium & Neon Mixture 10:1 ratio...at 0.1mm of Hg.

4.Resonat Cavity: Fully & partially reflected surfaces…

5.Pumping System: Discharge electrodes…

6.Four level Laser System.

7.Wave Length:632.8nm.

8.Red color Continuous Laser.

9.Widely used in Interferometer Experiments & Holography.

slide20

Fully reflecting

mirror

Partially reflecting

mirror

Discharge tube

He + Ne (10:1)

0.1mm of Hg

Laser out put

Discharge

electrodes

slide21

E

E3

Fast decay

E2

Meta stable state

pumping

Laser transition

E1

E0

N

He

Energy level diagram of He-Ne laser

Ne

slide22
CO2 LASER

1.CKN Patel in 1963.

2.Gas Laser..

3.Active Medium: CO2 , N2 & helium mixture 1:4:5 ratio...at 0.1mm of Hg.

4.Resonat Cavity: Fully & partially reflected surfaces…

5.Pumping system: Discharge electrodes…

6.Large no of energy levels are contributes for out put laser radiation..

7.Wave Length:10.6micro meters.

8.Pulsed & Continuous Laser.

9.Widely used in Material processing such as Cutting , Drilling, Welding.

slide23

Cooling

Partially reflected

surface

Fully reflected

surface

Co2+N2+He

1:4:5

Out put

laser

Discharge electrodes

slide24

Energy level diagram of co2 laser

E

Fast decay

Laser

transition

collisions

collisions

pumping

He

N2

co2

slide25

Vibrational modes of the CO2 molecule

Carbon

Oxygen

Oxygen

Symmetric

mode

Bending

modes

Asymmetric

modes

slide26
Semiconductor Laser

1.Semiconductor Laser is also called as Diode Laser.

2.The wave length of the emitted light depends upon the Energy band gap of the material.

3.Diode Lasers are always operated in forward bias..

4.Working Principle: When we apply forward bias to a PN-Diode, charge carrier recombination takes place.. Then in such a process the energy is emitted in the form of light radiation..

5.Active Medium: GaAlAs diode or GaAsP diode..

6.Out put Wave length:

GaAlAs:750-900nm,.GaAsP:1100-1600nm..

7.Pulsed & Continuous Laser…

slide27

The Energy band gap of a material

Where c is the velocity of light & h is Planck's constant.

slide28

Positive

Metal contact

P

Forward

biasing

Active

region

N

Negative

Metal contact