Ch4_StimulatedEmissionDevicesLASERS

Ch4_StimulatedEmissionDevicesLASERS - Stimulated Emission...

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1 901 37500 光電導論 Stimulated Emission Devices: LASERS Kasap Chapter 4 2 901 37500 光電導論 Contents 4.1 Stimulated Emission and Photon Amplification 4.2 Stimulated Emission Rate and Einstein Coefficients 4.3 Optical Fiber Amplifiers 4.4 Gas Laser 4.5 The Output Spectrum of Gas Laser 4.6 Laser Oscillation Conditions 4.7 Principle of the Laser Diode 4.8 Heterostructure Laser Diodes 4.9 Elementary Laser Diode Characteristics 4.10 Steady State Semiconductor Rate Equation 4.11. Light Emitters for Optical Fiber Communications 4.12 Single Frequency Solid State Lasers 4.13 Quantum Well Devices 4.14 Vertical Cavity Surface Emitting Lasers (VCSELs) 4.15 Optical Laser Amplifiers 4.16 Holography 3 901 37500 光電導論 4.1 Simulated Emission and Photon Amplification 4 901 37500 光電導論 E 1 E 2 h υ (a) Absorption h (b) Spontaneous emission h (c) Stimulated emission In h Out h E 2 E 2 E 1 E 1 Absorption, spontaneous (random photon) emission and stimulated emission. Spontaneous emission: random direction E 1 is empty As if e - is oscillating in freq. v Stimulated emission: induced Two photons are in phase, same direction same polarization, same energy Incoming e - couples to the e - in E 2 Basis for photon amplification Re-absorbed? => population inversion not in two level systems Fig. 4.1 (in steady state: R 12 = R 21 ) Optical Cavity feedback high optical intensity
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5 901 37500 光電導論 E 1 h υ 13 E 2 Metastable state E 1 E 3 E 2 h 32 E 1 E 3 E 2 E 1 E 3 E 2 h 2 h 21 Coherent photons OUT (a) (b) (c) (d) E 3 The principle of the LASER. (a) Atoms in the ground state are pumped up to the energy level E 3 by incoming photons of energy h 13 = E 3 E 1 . (b) Atoms at E 3 rapidly decay to the metastable state at energy level E 2 by emitting photons or emitting lattice vibrations; h 32 = E 3 E 2 . (c) As the states at E 2 are long-lived, they quickly become populated and there is a population inversion between E 2 and E 1 . (d) A random photon (from a spontaneous decay) of energy h 21 = E 2 E 1 can initiate stimulated emission. Photons from this stimulated emission can themselves further stimulate emissions leading to an avalanche of stimulated emissions and coherent photons being emitted. IN Pumping (optical) e.g. ruby laser: chromium ions Cr 3+ in Al 2 O 3 crystal Feedback: silvered mirror and partially silvered mirror LASER: Light Amplification by Stimulated Emission of Radiation Fig. 4.2 (Note: usually, more efficient in four level systems ) Three level system (long- lived state) (See p.6) 6 901 37500 光電導論 Theodore Harold Maiman was born in 1927 in Los Angeles, son of an electrical engineer. He studied engineering physics at Colorado University, while repairing electrical appliances to pay for college, and then obtained a Ph.D. from Stanford. Theodore Maiman constructed this first laser in 1960 while working at Hughes Research Laboratories (T.H. Maiman, "Stimulated optical radiation in ruby lasers", Nature , 187 , 493, 1960). There is a vertical chromium ion doped ruby rod in the center of a helical xenon flash tube. The ruby rod has mirrored ends. The xenon flash provides optical pumping of the chromium ions in the ruby rod. The output is a pulse of red
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Ch4_StimulatedEmissionDevicesLASERS - Stimulated Emission...

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