激光原理课件chapter1

1Prof.&Dr.GuohuaLi（李果华）Office:Room305,QianBuildingGuohua_li55@CellrinciplesandTechnologiesofLasers23TextandReferenceBooksPrinciplesofLasers(4thedition),O.Svelto,PlenumPress1998激光原理，陈钰清王静环编著；浙江大学出版社1992激光原理与激光技术,

俞宽新

等编著；北京工业大学出版社1998激光原理，周炳琨编著；国防工业出版社1980,2001,20044GradingProblemsets:assignedFriday,dueoneweeklaterGrading:10%attendancerate,30%problemsets,60%finalexamTextbookonlaserprinciple@TA：刘桂林，2:00pm~4:00pm,WednesdayRoom328,QianBuildingCell:189617410135根据激光对人体的危险度分类，在光树内观察对眼睛的MPE（maximalpossibleeffect最大可能的影响）做基准，可分为一到四级。激光产品厂商应该把ClassII,III和IV的警示标签贴到相应的激光产品上。LASERSAFETYANDCLASSIFICATIONClassI：低输出激光（功率小于0.4mW），不论何种条件下对眼睛和皮肤，都不会超过MPE值，甚至通过光学系统聚焦后也不会超过MPE值。可以保证设计上的安全，不必特别管理。典型应用如激光教鞭，CD播放机，CD-ROM设备，地质勘探设备和实验室分析仪器等。6ClassII：低输出的可视激光（功率0.4mW-1mW），人闭合眼睛的反应时间为0.25秒，用这段时间算出的曝光量不可以超过ＭＰＥ值。通常1ｍＷ以下的激光，会导致晕眩无法思考，用闭合眼睛来保护，不能说完全安全，不要直接在光束内观察，也不要用ClassII激光直接照射别人的眼睛，避免用远望设备观察ClassII激光。典型应用如课堂演示，激光教鞭，瞄准设备和测距仪等。7ClassIII：中输出激光，光束若直接射入眼睛，会产生伤害，基于某些安全的理由，进一步分为IIIＡ和IIIＢ级。

IIIＡ级为可见光的连续激光，输出为1-5ｍＷ的激光束，光束的能量密度不要超过25Ｗ／ｍ﹣ｍ，避免用远望设备观察IIIＡ激光，这样可能增大危险。IIIＡ的典型应用和ClassII级有很多相同之处，如激光教鞭，激光扫描仪等。

IIIB级为5-500mＷ的连续激光，直接在光束内观察有危险。但最小照射距离为13ｃｍ，最大照射时间十秒以下为安全。IIIB激光的典型应用如光谱测定和娱乐灯光表演等。8ClassIV：高输出连续激光（大于500mW），高过第三级，有火灾的危险，扩散反射也有危险。典型应用如外科手术，研究，切割，焊接和显微机械加工等。9SummaryofCourseContent

IntroductoryconceptsInteractionofradiationwithatomsandionsEnergylevels,radiative,andnonradiativetransitionsinmoleculesandsemiconductorsRayandwavepropagationthroughopticalmediaPassiveopticalresonatorsContinuouswavelaserbehaviorTransientlaserbehaviorGas,Solid-state,dyeandsemiconductorlasers10“LASER”standsforLightAmplificationbytheStimulatedEmissionofRadiation11HistoricalbackgroundofthelaserReadersofH.G.Wells’novelTheWaroftheWorldsmightquitereasonablyconcludethatthefirstlaserdevicetobeoperatedontheearthwasinfactbroughtherebyMartianinvadersacenturyago:“Insomewaythey(theMartian)areabletogenerateanintenseheatinachamberofpracticallyabsolutenonconductivity….Thisintenseheattheyprojectinaparallelbeamagainstanyobjecttheychoose,bymeansofapolishedparabolic(抛物面的)mirrorofunknowncomposition….Howeveritisdone,itiscertainthatabeamofheatistheessence(本质，要素)ofthematerial.Whatiscombustible(易燃的)flashesintoflameatitstouch,leadrunslikewater,itsoftensironcracksandmeltsglass,andwhenitfallsuponwater,thatexplodesintostream.”12(first)Rubylaser,T.H.Maiman,July,1960,attheHughesResearchLaboratoriesTheword‘laser’hasbeengenerallyacceptedsinceabout196513Thecrystallinestructureofrubyissimilartotheoneofcorundum,i.e.acrystalofaluminumoxide(Al2O3),inwhichthesmallpartofatomsofaluminum(about0.05%)isreplacedwithionsCr+++,producinga694.3nmoutput.PhysicalandOpticalPropertiesDensity3.98g/cc

Refractiveindexat700nm1.7638OrdinaryRayMeltingPoint2040°C

1.7556ExtraordinaryRayYoung'sModulus345GpaBirefringence0.008MOR425MPaRefractiveIndexvs.ChromiumConcentration3x10-3(Δn/%Cr2O3)CompressiveStrength2.0GpaFluorescentLifetimeat0.05%Cr2O33msat300KHardness9Mhos,2000KnoopFluorescentLinewidth(R1)5.0Åat300KThermalExpansion20°to50°C5.8x10-6/°COutputWavelength(R1)6.94.3nm20°to200°C7.7x10-6/°CMajorPumpBands404nmand554nmThermalConductivityat0°C46.02W/(m•K)

at100°C25.10W/(m•K)at400°C12.55W/(m•K)Allvaluesarefor60°orientationmaterial14Spontaneousemissionh=E2-E1E2E121Decay:Radiative(emwave)ornonradiative(kineticorinternalenergyofthesurroundingatomsormolecules)1.1.SPONTANEOUSANDSTIMULATEDEMISSION,ABSORPTION

15whereN2

isthenumberofatoms(ormolecules)perunitvolumethatattimetoccupyagivenenergylevel.Itiscalledpopulationofthelevel.AiscalledtherateofspontaneousemissionortheEisteinAcoefficientanditdependsonlyonparticulartransition.(1.1.2)isthespontaneousemission,orradiative,lifetime.Fornonradiativedecay,(1.1.3)16Stimulatedemissionh=E2-E1E2E121h=E2-E1h=E2-E1Sincetheprocessisforcedbytheincidentemwave,theemissionofanyatomaddsinphasetothatoftheincomingwaveandinthesamedirection.(eachprocesscreatesaphoton)17Forstimulatedprocesses(1.1.4)whereW21istherateofstimulatedemission.Itdependsnotonlyonparticulartransitionbutalsoontheintensityoftheincidentemwave.So,foraplanewave,wehave(1.1.5)whereFisthephotonfluxofthewaveands21

isthestimulatedemissioncrosssection.18Absorptionh=E2-E1E2E121Externalstimulus:opticalelectricorchemicalItisobvious,forabsorption,wecanwrite(1.1.6)(annihilatesaphoton)19andwealsohavewhereW12istheabsorptionrateands12

istheabsorptioncrosssection.

Itcanbeshown,fornondegenerateenergylevels,onehasandForg1-foldandg2-folddegenerateenergylevels,and(1.1.7)(1.1.8)(1.1.9)201.2.THELASERIDEAAccordingtoBoltzmannstatistics,atthermalequilibriumSinceE2>E1,andT>0,wealwayshaveN1>N2.E2E1N2N1E2E1N2N1Isitpossible?(1.2.2)21LetFisthephotonfluxofaplaneemwaveinzdirectionofanactivemedium(bothabsorptionandstimulatedemissionexist).Forperunittime,thedifferenceofphotonnumberbetweenoutgoingandincomingintheshadedvolumeisSdF.FF+dFzz+dzSFrom(1.1.4)and(1.1.6)andsowehavethen(1.2.1)22Discussion:Whenthematerialbehavesasanamplifier;Whenthematerialbehavesasanabsorber.ispossibleifonlyT<0(negativetemperaturesystem)E2E1N2N1Thiscaseiscalledpopulationinversion（粒子数反转，布居数反转……）.Thiskindofmaterialsisreferredtoasanactivemedium(激活物质).23ThestructureofalaserThresholdconditionoflasing:gainoftheactivemediumislargerthanlosses(e.g.absorption,output)lasing24Let’sconsidersomekindsoflosses.From(1.2.1),itisthegainperpassintheactivematerial.Makinganintegral,wehave(gainperpass)LetFisthephotonfluxinthecavityleavingmirror1,oneroundtriplater,wehavewhereLiistheinternallossperpass,R1andR2arepowerreflectivitiesofmirrors,respectively.25Atthreshold,wemusthaveF=F’or(critical)populationinversion(1.2.3)Let(1.2.4a)(1.2.4b)(1.2.4c)then(1.2.4c)(1.2.5)where(1.2.6)26g1

and

g2

are

calledlogarithmiclossesofthecavitymirrors,giiscalledthelogarithmiclossofthecavity,and

giscalledthesingle-passlossofthecavity.1.3.PUMPINGSCHEMES

Howtorealizepopulationinversion?Wecannotexpectatwo-levelsystemtorealizeit.E2E1N2N1Thelifetimeofanatomisabout.27Three-levelsystem:Rubycrystalscontain0.01-0.1%of(Chromium铬)in(sapphire蓝宝石)1LasingFastdecay(Nonradiativetransition)Pumping23Level2isameta-stablestate.Generally,it’slifetimeisabout.Forrubylaser,it’s.Sincelevel1isthegroundstate,theefficiencyofathree-levelsystemisnothighdespiteitdoesworkasalaser.Thereasonisthelowerlaserlevel(level1)isthegroundstatethereexistsalotofatoms(ormolecules).Itisnoteasytorealizepopulationinversionbetweenlevels2and1.28Four-levelsystem:Sincelevel1isalmostempty,itismucheasiertobuilduppopulationinversionbetweenlevels2and1.Becauseofthis,theefficiencyoffour-levelsystemsismuchhigherthanthatofthree-levelsystems.ShortlifetimeMetastablestateVeryshortlifetime3210Fastdecay(relaxation)Fastdecay(relaxation)PumpingLasingGroundstateHe-Negaslasers.29Quasi-three-levelsystem:1LasingFastdecay(Nonradiativetransition)Pumping23Nd:YAGlaser.Y3Al5O12(yttriumaluminiumgarnet,钇铝石榴石)crystalcontaining0.1-1%of(neodymium钕),1.06or1.32mm.Thelowerlaserlevelissoclosetothegroundstate,thatanappreciablepopulationinthatleveloccursinthermalequilibriumattheoperatingtemperature.Asaconsequence,theunpumpedgainmediumcausessomelossatthelaserwavelength,andtransparencyisreachedonlyforsomefinitepumpintensity.Forhigherpumpintensities,thereisgain,asrequiredforlaseroperation.301.4.PROPERTIESOFLASERBEAMS

Generally,therearefiveuniqueproperties.1.4.1. Monochromaticity

Onlyonefrequencywhichishn=E2-E1andl=n(l/2)cansurviveinthecavity.311.4.2.Coherence

Spatialcoherence:consideringtwopointsP1andP2that,attimet=0,lieonthesamewavefrontofsomegivenemwaveandletE1(t)andE2(t)bethecorrespondingelectricfieldsatthesetwopoints.Bydefinitionthedifferencebetweenphasesofthetwofieldsattimet=0iszero.Ifthisdifferenceremainszeroatanytimet>0,wesaythatthereisaperfectcoherencebetweenthetwopoints.Ifsuchcoherenceoccursforanytwopointsoftheemwavefront,wethensaythatthewavehasperfectspatialcoherence.Inpractice,foranypointP1,pointP2mustliewithinsomefiniteareaaroundP1tohaveagoodphasecorrelation.Inthiscasewesaythatthewavehaspartialspatialcoherence,and,foranypointP,wecanintroduceasuitablydefinedcoherenceareaSC(P).32Temporalcoherence:Consideringtheelectricfieldoftheemwave,atagivenpointP,attimestandt+t.If,foragiventimedelayt,thephasedifferencebetweenthetwofieldremainsthesameforanytimet,wesaythatthereisatemporalcoherenceoveratimet.Ifthisoccursforanyvalueoft,theemwaveissaidtohaveperfecttemporalcoherence.Ifthisoccursforatimedelaytsuchthat0<t

<t0,thewaveissaidtohavepartialtemporalcoherence,withacoherencetimeequaltot0.33TemporalCoherenceisrelatedtomonochromaticity.

SpatialCoherenceisrelatedtodirectionalityanduniphasewavefronts.

341.4.3.Directionality

laserqdDEvenforthecaseofperfectspatialcoherence,abeamoffiniteaperturehasunavoidabledivergenceduetodiffraction.Thiscanbeunderstoodwiththehelpfromdiffractiontheory,foranarbitraryamplitudedistribution,wecanhave(1.4.1)wherelandDarethewavelengthandthediameterofthebeam,respectively.Thefactorbisanumericalcoefficientoftheorderofunitywhosevaluedependsontheshapeoftheamplitudedistributionandhowboththedivergenceandthebeamdiameteraredefined.35Forthepartialspatialcoherence,itsdivergenceisgreaterthantheminimumvaluesetbydiffraction.Indeed,foranypointP'ofthewavefront,theHuygensargumentinFig.1.6canbeappliedonlyforpointslyingwithinthecoherenceareaScaroundpointP'.Thecoherenceareathusactsasalimitingapertureforthecoherentsuperpositionofelementarywavelets.Thus,thebeamdivergencecannowbewrittenas:(1.4.2)Divergense:Gaslasers-0.001radsolid-statelasers-0.01rad(aTEMoogaussianbeamfromarubylaser(l=694.3nm)istransmittedthrougha1-mdiameterdiffraction-limitedtelescopetoilluminateaspotonthesurfaceofthemoon.Thespotsizeonthemoonis191m)361.4.4.Brightness

ThemaximumbrightnessforabeamofpowerPis(1.4.5)Highbrightnessandhighpowerdensity~highphotondegeneration371.4.5.ShortPulseDuration

UtilizingQ-switchandmode-locktechnologies,onecanobtainfrompstofslaserpulses.Generally,thelaserpulsedurationw