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1、Chapter 8 Laser and Fourier transform system激光器和傅立叶变换系统激光器和傅立叶变换系统8-1 Stimulated emission受激辐射受激辐射8-2 the type of laser激光器的类型激光器的类型8-3 Characteristic parameters of laser激光器的特性参数激光器的特性参数8-4 Laser optical system激光光学系统激光光学系统8-5 Fourier Transform System傅立叶变换系统傅立叶变换系统Chapter 8 Laser and Fourier transform
2、systemLaser: Light Amplification by Stimulated emission of radiationLaser is autgrowth of the Maser 1960: The fist laser by T.H.Maiman8-1 Stimulated emission1） Metastable states 2） Optical pumping 3） Flourescence 4） Population inversion 5） Resonance 6） Stimulated emission 7） Polarization 8） Coherenc
3、e 9） Fabry-perot interferometry 10） Cavity oscillation-10 basic principles 1The principle of laser Laser emits light, must satisfy two conditions: population inversion by optical pumping or electric pumping many atoms can be raised from the ground state to excited states. avalanche magnification by
4、resonator According to the resonance principle, as a photon passer by another nearby atom, in the same metastable state, it can immediately stimulate that atom, to radiate a photon of the exact same frequency, direction and polarization and exactly the same phase and speed. 2Resonance In which wave
5、can be used over and over principle: Fabry-perot interferometer photons moving at an appreciable angle to the walls of the cavity, will escape and be lost; moving parallel to the axis will reflect back and forth from end to end. System will oscillate of laser depend upon: high reflectance at the end
6、 mirrors high population density of metastable atoms.8-2 the type of laser1The Ruby LaserTransparent crystal of () doped with 0.05 chromium ions .23lA O23Cr OEnds polished flat and parallel, later beveled at Brewsters angle. High-intensity helical flash lamp provides pumping light to produce a popul
7、ation inversion.For some purposes, both end mirrors are fully silvered, and a center spot on one is left clear to transmit part of the light as a narrow emergent beam. Helium-Neon laser Characters: inexpensive, stable, emits continuously.When a metastable helium atom collides with neon atom in its g
8、round state, excitation energy will be transferred to the neon, each colliding neon atom is excited to the upper level.collision transfer increases the population of the upper level of neon, He-Ne wavelength =6328A Resonator Concave mirrors Brewsters window reflection losses of approximately 4% at e
9、ach of the interface are detrimental to coherence by tilting these plates to the polarizing angle Carbon Dioxide Laser high power molecular laser KWContinuous laser beam CWHigh degree of coherence =10.6mcut through a diamond and thick steel plate in a matter of seconds, applied in optical communicat
10、ion system, optical radar and laser cuttinginfrared light is only stightly scattered or absorbed by the atmosphere.(scattered is proportional to )4 Q switching (or Q spoiling) When the barrier is suddenly removed, output from laser results in a sudden pulse whose peak power is at least a 1000 times
11、larger than the average CW. Elements: Mechanical chopper, a rotating mirror, kerr cell, pockels cell with a rotating mirror arranged in the position show in Fig, light pulse at 10.6 is emitted every time.m100W-CW laser will produce pulsed power of 100 KW in bursts approximately 150ns long at a rate
12、of 400 pulses per second.He-NeGas632.8 (nm)cwArgas457.9 514.5Cw pulsedkubysolid694.3pulsedVisibleKrgas461.9 676.4Cw pulsedcaAlAsSolid (diode)0.85 ()mcwGaAsSolid (diode)0.904CwYAGsolid1.064Cw pulsed2COGas10.6Cw pulsedinfraredHe-NeGas1.15 3.39cwHe-cdGas325cwKrGas350.7 356.4cwultravioletArGas351.1 363.
13、8Cw pulsed8-3 Characteristic parameters of laserStanding waves are set up, when integer m=, (d is the distance between mirrors)d /2frequency: , (v is the speed of the waves in cavity medium)2mmv dfrequence difference between modes: , For Gas laser d=1m, , 2v d150MHz: Transverse electric and magnetic
14、mnTEMThe simplest mode, is the most widely used, flux density over the beam cross section is 00TEMGaussian.The angle spread of the emergent beam is limited by diffraction at the exit aperture. 2.44QDCoherence length is given approximately by .cLc t for low-pressure discharge0.32cLfor high-pressure d
15、ischarge0.11cL(take into the lines actual broadening effect ), c= 103 10/cm s8-4 Laser optical systemThe characters of laser beam Fig: section of Gaussian beam Intensity distribution: Gaussian distribution (: The Amplitude at r=0; r: 220wr eAA 0Aradius of light beam section; : The radius of light be
16、am at amplitude falling back to .eA0r= eAA0 propagation of Gaussian beamFig: propagation of Gaussian beam The radius of section of light beam is the function of Z.( Waist of laser beam, Z=0, ) 1 2202 01,ZZ:00 The radius of wavefront , 22 01ZzZRZ=0, R, Plane wave at the waist of section of light beam, 0,
