LASERS

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LASERS
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• 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

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• 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

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• Spontaneous Emission
• Incoherent
• Less intensity
• Poly chromatic
• Less directionality
• More angular spread

• Stimulated emission
• Coherent
• High intensity
• Mono chromatic
• High directionality
• Less angular spread

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• 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
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For system in a equilibrium
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Comparing equations (5) (6)
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The equation shows ratio of spontaneous Emission
Rate to stimulated emission rate.
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• 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.

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• Three level Laser system

Meta stable level
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Four level laser system
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• 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

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Energy level diagram of Ruby laser
Short lived state
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• He-Ne LASER
• 1.Ali Javan in 1961.
• 2.Gas Laser..
• 3.Active Medium Helium Neon Mixture 101
  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 Length632.8nm.
• 8.Red color Continuous Laser.
• 9.Widely used in Interferometer Experiments
  Holography.

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Discharge electrodes
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Energy level diagram of He-Ne laser
Ne
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• CO2 LASER
• 1.CKN Patel in 1963.
• 2.Gas Laser..
• 3.Active Medium CO2 , N2 helium mixture 145
  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 Length10.6micro meters.
• 8.Pulsed Continuous Laser.
• 9.Widely used in Material processing such as
  Cutting , Drilling, Welding.

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Vibrational modes of the CO2 molecule
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• 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
• GaAlAs750-900nm,.GaAsP1100-1600nm..
• 7.Pulsed Continuous Laser

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The Energy band gap of a material
Where c is the velocity of light h is Planck's
constant.
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Positive Metal contact
P
Forward biasing
Active region
N
Negative Metal contact