激光原理及激光器簡介英文

BriefintroductionoflaserByJiangWeiandZengXiangzePhysicsCollegeShandongUniversity2011.4.21PrincipaloflaserlightTheinteractionbetweenelectronsconsistsofthreebasicprocess:spontaneousemission,stimulatedabsorptionandstimulatedemission.spontaneousemissionistheprocessinwhichexcitedatomscanexperiencespontaneoustransitionfromhighenergyleveltolowenergylevel.Simultaneously,certainphotonisemitted.E2-E1=hμwhereμisthefrequencyofthephotonstimulatedabsorption:Whenthereisanatomatitslowenergylevelandanphotonwithenergyhμcominginside,theatomislikelytoabsorbthephotonandtransfertothehighenergylevelifhμ=E2-E1ThetransitiondoesnothappenspontaneouslybutoccursduetothephotonstimulatingE=C12u(μ21)

N1hμ12dtdvwhereC12isEinsteinCoeffientstimulatedemissionWhenthereisanatomatitshighenergylevelandanphotonwithenergyhμcominginside,theatomislikelytotransfertothelowenergylevelandemitaphotonifhμ=E2-E1.Thephotonissameasthesourcephotoninfrequency,emittingdirection,phaseandpolarization.E=B21u(μ21)

N2hμ21dtdvwhereB21isEinsteinCoeffientEinsteinhasprovedthatB21isequaltoC12.Foranatomicsystem,stimulatedabsorptionandemissionbothhappenifthereexistsopticalsignalwithenergydensityu(μ21).Fromequation1and2,weknow:thenetstimulatedemissionenergyE=B21u(μ21)

N2hμ21dtdv-C12u(μ21)

N1hμ12dtdv=(N2-N1)B21u(μ21)

hμ21dtdvTherefore,thelaserlightemitsonlywhenN2>N1However,undernormalcircumstance,NsatisfiestheBoltzmannDistribution:N2/N1=exp[-(E2-E1)/kT]sinceE2>E1,N2<N1Now,weknowthatthekeypointtogeneratelaserlightis"populationinversion"whichmeanswemustdosomethingtomakeN2>N1tomeettherequiredcondition.Wewilldiscussthislaterindetail.Weuse"Pumping"methodtoachieve"populationinversion".Intermsofdifferentenergylevels,thereare3-levelsystem,4-levelsystemandsoon.stationarystateexcitedstatemetastablestate(oneoftheexcitedstatesinwhichatomscanstaylonger)3-levelsystemParticlesinthestationarystateabsorbenergygivenbythepumpandtransferfromE1toE3.Sinceparticlescan'tstaylonginE3,theytendtomovetoE2andreleasesomeheat.Thisachievethegoal"populationinversion"Howcanwegetlaserlight?He-Nilaser1.ActivematerialIt’sthematerialwhichreceiveenergyfrompumpresourceandthengeneratelaserlight.Itsatomicsystemmusthasthemetastablestate.Itcanbesolid,liquidorgas.2.PumpresourceItisusedtostimulatetheatomicsystem(activematerial)andpromotethecomingofpopulationinversion.Itcanbeopticalenergy,electricenergy,chemicalenergy,etc.3.OpticalresonatorItismadeoftwomirrors,ofwhichoneistotalreflecting,theotherispartialreflecting.Lightofwhichdirectionisparalleltotheaxisoftheopticalresonatorreflectsbetweenthetwomirrors.Inthisprocess,lightintensitygoesup.Whenitreachesacertainlevel,thelaserlightcrossthepartialreflectingmirror.Andtheopticalresonatormustbestablecavity，itmeansthatparaxialraycanreflectmanytimesinit.Itsatisfythiscondition:

ListhedistancebetweenthetwomirrorsofwhichcurvatureradiusesareR1,R2.Foracertainopticalresonator,notalltheelectromagneticoscillationwithanyfrequencycanexistinit.Forexample,inaplaneparallelcavity(平面平行腔),onlywhenthelightform

standingwave,canitbecomelaserlight.Nowitsfrequencysatisfythecondition:namely,

where?istherefractiveindexoftheactivematerial.

Ofcourse,itrequiresomeotherconditionsfordifferentopt