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LASERS

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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LASERS

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  1. LASERS

  2. 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

  3. 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

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

  5. 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

  6. For system in a equilibrium

  7. Comparing equations (5) & (6)

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

  9. 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.

  10. E E2 Fast decay E1 pumping Laser transition E0 N Three level Laser system Meta stable level

  11. E E3 Fast decay E2 Meta stable state pumping Laser transition E1 E0 N Four level laser system

  12. 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

  13. 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

  14. E E2 Fast decay E1 Meta stable level pumping Laser transition E0 N Energy level diagram of Ruby laser Short lived state

  15. 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.

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

  17. E E3 Fast decay E2 Meta stable state pumping Laser transition E1 E0 N He Energy level diagram of He-Ne laser Ne

  18. 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.

  19. Cooling Partially reflected surface Fully reflected surface Co2+N2+He 1:4:5 Out put laser Discharge electrodes

  20. Energy level diagram of co2 laser E Fast decay Laser transition collisions collisions pumping He N2 co2

  21. Vibrational modes of the CO2 molecule Carbon Oxygen Oxygen Symmetric mode Bending modes Asymmetric modes

  22. 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…

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

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

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