Lect. 20: Lasers

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Lect. 20: Lasers Optical Amplifier E2 Pin Pout = G Pin E1 Light source based on stimulated emission? - Use photons produced by spontaneous emission as initial seeds - Recycle output photons as seeds for further stimulated emission - Use mirror for recycling output photons LASER: Light Amplification by Stimulated Emission Radiation Optoelectronics (16/2) W.-Y. Choi Lect. 20: Lasers LASER: Optical Amplifier + Mirror Pump Gain Medium R R L Optical property of gain medium: n, g Imaginary part for k ? g g is due to absorption and stimulated emission knk0 j 2 g depends on material property, , amount of pumping factor of 2 because g is often defined for power Optoelectronics (16/2) W.-Y. Choi Lect. 20: Lasers Pump g Gain Medium knkj 0 2 R R Z=L Assume initially there is one photon moving in z-direction inside gain medium What is the condition that this this photon can be maintained within? No loss after one round trip. jkL jkL Ee00 re r E 11 re22 jkL1 e jkL2 rR2 1 1 ee jnkL2 0 gL eegL and jnkL2 0 1 R R Optoelectronics (16/2) W.-Y. Choi Lect. 20: Lasers 1 Pump eegL and jnkL2 0 1 R Gain Medium gL 111 From eg, th ln R LR R R L ==> Sufficient gain to compensate mirror loss 2L Frome jnkL2 0 1, 22nk L m or Lm 0 nm 2n cavity length should be multiples of half wavelength: mode Photons are in phase after one round trip Optoelectronics (16/2) W.-Y. Choi Lect. 20: Lasers 11 2L Two conditions for lasing: (1) g ln and (2) th L Rnm In real lasers, gain is function of wavelength gth λ λ Lasing spectrum Lasing modes has non-zero linewidth λ Laser length determines mode wavelength Optoelectronics (16/2) W.-Y. Choi Lect. 20: Lasers Any optical gain material with mirrors can form a laser First working laser by Maiman in 1960 at Hughes Aircraft Company Ted Maiman (1927-2007) Optical Gain Material: Cr in Al2O3 Pump: Xenon flash lamp Optoelectronics (16/2) W.-Y. Choi Lect. 20: Lasers 1964 Nobel Prize in Physics for invention of laser Nikolay Basov (1922-2001) (1/4) Gordon Gould Aleksandr Prokhorov (1920-2005) Charles Townes (1916-2002) (1/4) 30 year battle (1915-2015) for laser patent (1/2) Optoelectronics (16/2) W.-Y. Choi Lect. 20: Lasers Gas Laser (HeNe) He Ne 5 1 Flat mirror (Reflectivity = 0.999) (1s12s1) Collisions (2p 5s ) Concave mirror (Reflectivity = 0.98 20.61 eV 20.66 eV 632.8 nm Lasing emission Very thin tube (2p53p1) Fast spontaneous decay Laser beam ~600 nm He-Ne gas mixture (2p53s1) Electron impact Collisions with the walls Current regulated HV power supply A schematic illustration of the He-Ne laser 2 6 0 (1s ) (2p ) Ground states Optoelectronics (16/2) W.-Y. Choi Lect. 20: Lasers Fiber Laser Er3+-doped fiber (10 - 20 m) Wavelength-selective Gratings coupler Gratigns Splice Splice Pump laser diode Termination = 980 nm 1.27 eV E3 Non-radiative decay 980 nm Pump 0.80 eV E2 1550 nm 1550 nm Out In 0 E1 Another popular laser material: semiconductors Optoelectronics (16/2) W.-Y. Choi Lect. 20: Lasers Homework Optoelectronics (16/2) W.-Y. Choi Lect. 20: Lasers Homework (Continued) Optoelectronics (16/2) W.-Y. Choi.

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