1激光基本原理共振腔内放发光材料，用光源对它泵浦，使原子激发．原子停留在激发态的平均时间（称为原子激发态平均寿命）很短，会有不少原子同时发射光子．这些光子又诱导其它在激发态的原子产生受激辐射，腔内“相同”的光子数目加倍；如此循环，腔内“相同”光子数目以指数函数形式增长．当受激辐射产生的光功率，比损失的数量还多，就进入振荡状态．达到振荡状态时，处于高能态的原子数比在低能态原子数目多（称为粒子数反转）。PropertiesofLaserlight属性(1).Monochromaticity单色性erlightisconcentratedinanarrowrangeofwavelengths,i.e.thebandwidthorspectrallinewidthofalaserisverysmall.Monochromaticityisrelatedtocoherence.(2).CoherenceTwotypesofcoherence:temporalorspatial.Temporalcoherencedescribesthephaseconstancyasthewavetravelsthroughspace.Spatialcoherencedescribesthephaseconstancyacrossthewavefront.(3).DirectionalityBeamdivergence（发散）Alltheemittedlaserphotonstravelinthesamedirection.(4).BrightnessAlltheenergyisconcentratedwithinthenarrowspectrumoftheactiveatomictransition.Stimulatedemission,populationinversionandgain(1)Interactionoflightwithmatter(2)SpontaneousEmissionExcitedatomsnormallyemitlightspontaneouslyPhotonsareuncorrelatedandindependentIncoherentlightSpontaneousLifetime原子从激发态返回基态或较低能态，约10-7s（3）Stimulatedemission(A.Einstein,1916)Onephotoninandtwoout—AmplificationofLightExcitedatomscanbestimulatedintoduplicatingpassinglightPhotonsarecorrelatedandidenticalCoherentlightAnincidentphotonstrikesanatomandvibratestheatominphasewithit,thephotonisnotabsorbedbuttriggerstheemissionofaphotoniftheatomisinanexcitedstate.Emittedphoton：isinphasewiththeincidentphoton.hasthesamewavelengthastheincidentphoton.hasthesamepolarizationastheincidentphoton.travelsinsamedirectionasincidentphoton.（4）Essentialingredientsofalaserlightamplification+positiveopticalfeedback?Lightamplificationisachievedbystimulatedemission.?Theamplificationofthemediumisdefinedbythegaincoefficient,whichisdefinedbythefollowingequation:Thustheintensitygrowsexponentiallywithinthegainmedium.Lightinthecavitypassesthroughthegainmediumandisamplified.Itthenbouncesofftheendmirrorsandpassesthroughthegainmediumagain,gettingamplifiedfurther.Theconditionforoscillationisthus:round-tripgain=round-triploss2Takingintoaccountthefactthatthelightpassestwicethroughthegainmediumduringaroundtrip,theconditionforoscillationcanbewritten:(5)Threenecessaryconditionsofalaser1.实现粒子数反—工作物质2.使原子被激发—激励能源3.要实现光放大—光学谐振腔(6)Populationinversion(粒子数反转)WeneedlargenumbersofatomsavailableforstimulatedemissionAnincomingphotoncandooneoftwothings:1)collideswithatominstate1andgetabsorbed,2)collideswithatominstate2andproducesanotherphotonIftherearemoreatomsinstate2thanstate1，thenmorephotonsareproducedthangetabsorbed.Normalpopulationofatomsinstate2(ofhigherenergy)issmallerthaninstate1(Boltzmandistribution!).Maxwell-Boltzmandistributionlaw:ConsideralargenumberofatomsNoradiation,thermalequilibriummostinthegroundstatenoandfewintheexcitedstatenEDistributionsarebasedontheMaxwell-Boltzmandistributionlaw.Toamplifythebeam,werequirethattherateofstimulatedemissiontransitionsexceedstherateofabsorption.ThisimpliesthatN2mustexceedN1.Thisisahighlynon-equilibriumsituation,andiscalledpopulationinversion.Howtogetpopulationinversion？SpontaneousLifetimeTheshorterthespontaneouslifetime,thegreateristheprobabilitythatspontaneousemissionwilloccur.M...
