Lecture2:IntroductiontoLaserLightSourceLASER:LightAmplificationbyStimulatedEmissionofRadiationExperimentwithalaserPropertiesofLaserPropertiesofLaserPropertiesofLaserCoherence:LaserlightCannot:beperfectlymonochromaticbeperfectlydirectionalperfectcoherentPropertiesofLaserLaserlightCannot:beperfectlymonochromaticbeperfectlydirectionalbeperfectcoherentHowever,laserlightisfarmorecoherentthanlightfromanyothersource.HistoryoftheLaser•Inventedin1958byCharlesTownes(NobelprizeinPhysics1964)andArthurSchawlowofBellLaboratories•WasbasedonEinstein’sideaofthe“particle-waveduality”oflight,morethan30yearsearlier•OriginallycalledMASER(m=“microwave”)HistoryoftheLaserThefirstpatent(1958)MASER=MicrowaveAmplificationbyStimulatedEmissionofRadiationTheMASERissimilartotheLASERbutproducedonlymicrowavesThewave-particledualityParticleshaveamomentumThemomentumcanbealsoclassifiedbythewavelengthPλLouisdeBroglie(1923):EnergyisquantizedToraiseanelectronfromoneenergyleveltoanother,inputenergyisrequired.Whenfallingfromoneenergyleveltoanother,therewillbeanenergyoutputgivenbythePlanck‘slawAtomictransitionAlmostallelectronictransitionsthatoccurinatomsthatinvolvephotonsfallintooneofthreecategories:1.Stimulatedemission:ΔE=hƒ(orhν)Atomictransition2.SpontaneousemissionEnergyoftheemittedphotonhƒΔE2.Stimulatedemission1PhotonwithproducestwophotonswiththesamepropertiesAtomictransitionThefrequencyoftheemittedphotongoingfromLevel2to1isgivenby:DefiningNiastheelectronpopulationofleveliandconsideringtheBoltzmanequationwhichdescribestherelationbetweentheelectronsinlevel1and2atthermalequilibrium:Toamplifylight,thestimulatedemissionmustbystrongerthantheabsorption(N2>N1)…buthowisthispossible???Anelectromagneticwavewithfrequencyνtravelinginz-directionthroughthemediawith2atomlevelsisnormallyexponentiallyabsorbed:μ>0isthelinealAbsorptioncoefficient.Itcanbedemonstratedthatμ∼N1-N2>0TheamplificationoflightisonlypossibleifN1-N2<0(=>μ<0)AtomictransitionInversionofenergylevelOnesolutiontothisproblemistousethreeenergylevels(exampleHe-Nelaser):InversionofenergylevelNowN2>N1,however…wemustamplifytheintensityofourbeam!However,withthisset-uptheintensitywillgrowuptoinfinite!WhatcanwedotoobtaintheLASERbeam?StimulatedemissionElementsofalaserExcitationmechanismActivemediumGainmedium:gas,liquid,solidorplasmaLightgeneratedbystimulatedemission:similartotheinputsignalwavelength,phase,andpolarizationTheopticalcavity(cavityresonator):acoherentbeamoflightbetweenreflectivesurfacespassesthroughthegainmediummorethanonceTheminimumpumppowerneededtobeginlaseraction(lasingthreshold)gainmediumwillamplifyanyphotonspassingthroughit,regardlessofdirectiononlythephotonsalignedwiththecavitymanagetopassmorethanoncethroughthemediumandsohavesignificantamplificationModesofoperation1.Continuousconstant-amplitudeoutput(knownasCWorcontinuouswave)CWDyelaser(abroadrangeofwavelengths)2.Pulsed,Q-switching(producehighpeakpower),Modelocking(extremelyshortdurationpulse,picoseconds(10-12s)femtoseconds(10-15s))Gain-switching(picoseconds(10-12s))ProducehighpeakpoweratparticularwavelengthTypesandoperatingprinciplesGaslasers:Helium-neonlaser(operationwavelengthis632.8nm,CWoutput)GainMedium:amixtureofheliumandneongasesPumpsource:electricaldischargeofaround1000betweenanodeandcathodeCommonlymadebydopingacrystallinesolidhostwithionsthatprovidetherequiredenergysta...
