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. 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.

9. For system in a equilibrium

11. Comparing equations (5) & (6)

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

13. 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

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

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

16. Ruby LASER 1.Maiman in 1960. 2.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

17. 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

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

19. 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.

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

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

22. CO 2 LASER 1.CKN Patel in 1963. 2.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.

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

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

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

26. 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…

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

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