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 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

Spontaneous Emission Incoherent Less intensity Poly chromatic Less directionality More angular spread Stimulated emission Coherent High intensity Mono chromatic High directionality Less angular spread

absorption Spontaneous emission Stimulated emission E1E1 E2E2 N1N1 N2N2 Supplied energy Einstein Co-efficient Consider two energy levels E 1 and E 2. N 1 and N 2 be the number of atoms per unit volume present at the levels E 1 and E 2 respectively.

For system in a equilibrium

Comparing equations (5) & (6)

The equation shows ratio of spontaneous Emission Rate to stimulated emission rate.

Population inversion To achieve more stimulated emission population of the excited state N 2 should be made larger than the lower state N 1 and this condition is called population inversion. E1E1 E2E2 N1N1 N2N2

Three level Laser system N E E0E0 E1E1 E2E2 Fast decay Laser transition pumping Meta stable level

Four level laser system N E E0E0 E1E1 E2E2 Fast decay Laser transition pumping E3E3 Meta stable state

Ruby LASER 1.Maiman in Solid State Laser. 3.Active Medium: Al 2 O 3 doped with 0.05% Cr 3+ ions(10cm long & 0.5cm diameter). 4.Resonant Cavity: Fully reflecting surface & partially reflecting surface. 5.Pumping System: Helical Xenon flash lamp. 6.Three level laser system. 7.Wave Length: 694.3nm. 8.Pulsed Laser. 9.Widely used in Echo technique & Pulsed Holography

Partially reflecting surface Completely Reflecting surface cooling Xenon flash lamp Ruby material Partially reflecting surface Completely Reflecting surface cooling Xenon flash lamp Ruby material Laser output

N E E0E0 E1E1 E2E2 Fast decay Laser transition pumping Meta stable level Energy level diagram of Ruby laser Short lived state

He-Ne LASER 1.Ali Javan in 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.

Discharge electrodes He + Ne (10:1) 0.1mm of Hg Discharge tube Fully reflecting mirror Partially reflecting mirror Laser out put

N E E0E0 E1E1 E2E2 Fast decay Laser transition pumping E3E3 Meta stable state Ne He Energy level diagram of He-Ne laser

CO 2 LASER 1.CKN Patel in Gas Laser.. 3.Active Medium: CO 2, N 2 & 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.

Fully reflected surface Partially reflected surface Cooling Discharge electrodes Co 2 +N 2 +He 1:4:5 Out put laser

Energy level diagram of co 2 laser He N2N2 E pumping Fast decay Laser transition collisions co 2

Vibrational modes of the CO 2 molecule Oxygen Carbon Symmetric mode Bending modes Asymmetric modes

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: nm,.GaAsP: nm.. 7.Pulsed & Continuous Laser…

The Energy band gap of a material Where c is the velocity of light & h is Planck's constant.

P N Active region Positive Metal contact Negative Metal contact Forward biasing