清华激光原理第七讲(4月7日)PPT文件格式下载.ppt

清华激光原理第七讲(4月7日)PPT文件格式下载.ppt

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EinsteinrelationshipRedruby,Er3+dopedfiberw13A31S31S32A21S21w21w12E1E2E3He-Ne,Nd:

YAG较大w03A30S30S32S21A21W21E3E2E1E0W12S101.Fourlevelssystem1）singlemodeoscillation（lthmode,）w03A30S30S32S21A21W21E3E2E1E0W12S10没有包括没有包括A21引起的光子数？

引起的光子数？

（ignoreS30）（4.4.13）RateequationsforpracticallaseraresetupaccordingtothevariousphysicalprocessesDifferentexpressionsofrateequationsforasamelaserR1,R2isatomdensitypumpedpersecondintoE1andE2,respectively.t1,t2islifetimeatE1,E2,respectively;

t21isspontaneousemission（荧光fluorescence）lifetimeforE2E1.E2E1R2R1t2t1t21ConsiderlossonlyPhotonlifetime光子寿命（2.1.14）LasertransitionE2E1R2R1t2t1t21PumptransitionsLasertransitionw03A30S30S32S21A21W21E3E2E1E0W12S102）RateequationsformultimodeoscillationSerialnumberofmodeFrequencyofmodePhotonnumberofserialmodeMethod:

对应每个模式分别建立一个速率方程,序数相应变化SimplificationsLineshapefunctionrectangle（矩形）Sameloss,samelifetimeofphotonforeachmode矩形面积原谱线面积矩形面积原谱线面积ReasonsofsimplificationDonotneedtoconsiderdifferencesofthemodesforstudyingquestionNeglectthedifferenceofdiffractionlossinthemodesAccordingtotheabovesimplifiedmodel,multimoderateequationsforfourlevelssystemasfollowingNtotalphotondensityforallmodes泵浦效率泵浦效率Multimoderateequationsforfourlevelslasersystem荧光量子效率荧光量子效率w03A30S30S32S21A21W21E3E2E1E0W12S10Pumpingefficiency:

thefractionofthetotalpumppowerthatisusefulincreatingthepopulationinversionFluorescence（quantum）efficiencyfromE2E1,dependssolelyoncharacteristicsofmedium,itistheupperlimitforperformanceofanylaser.w03A30S30S32S21A21W21E3E2E1E0W12S10Totalquantumefficiency（总量子效率总量子效率）发射荧光的光子数从光泵吸收的光子数（Photonnumbersofemittingfluorescence）（Photonnumbersabsorbedfrompumping）RateequationsExpressionofgaincoefficient（effectingfactors）behaviorofgainsaturation（增益饱和）（forhomogeneousandinhomogeneousmedium）4.5Gaincoefficientinhomogeneousbroadeningmedium1、Smallsignalgaincoefficientforsteadystate（fourlevels）I=Nhvdz=vdt不计损耗II+dIdzDn0I=Nhvdz=vdt（1.3.6）*1）GaincoefficientisproportionaltopopulationinversionDn（fourlevelssystemasexample）GaincoefficientDiscussingkeyfactorsdependentongaincoefficient稳态稳态*Reason:

opticalintensityinmediumisverysmall,thechangeofnintroducedbySTEcanbeneglectedinsmallsignalcaseInsmallsignalcase,Dn0alsoindependentonopticalintensity,dependentonpumpingrateW03n0稳态阈值附近n2很小Smallsignal（unsaturated）gaincoefficientg0isdependentonn0,independentonopticalintensityreview：

如何理解小信号情况？

小信号2）Gaincurve-g0varyingwithfrequencySmallsignalgaincurvedependsonspectrumlineshape（小信号增益曲线的形状完全取决于谱线线型函数）Smallsignalgaincoefficient=s21STEcrosssectionat0增益线宽荧光线宽F（自发辐射线宽H）3）g0（0）isproportionaltol02,inverselyproportionaltospectralwidth3.39mm632.8nm3S3P2PHe-Ne2、GainSaturation（增益饱和）大信号情况UndertheactionofincidencelightwithI1,at1（consideringSTEprocess）1）SaturationofinversionpopulationWhatisgainsaturation？

GaincoefficientisdecreasedasSTEintensitybecomesstrong.Reasonofgainsaturation：

Whentheintensityisstrongenough稳态=1/Is（）saturationofinversionpopulationSaturationintensityat0大信号情况（4.5.7）小信号情况Populationn2isbarelyaffectedbySTE（即可忽略受激辐射的影响）Incidencelightatsamefrequencywithdifferentintensity,differenteffectonsaturationofnIncidentlightwithsameintensityatdifferentfrequency,differenteffectonsaturationofnFrequencyrangeofsaturationactionwhenif（Stimulatingwave）Atcentralfrequency0,STEprobabilityisbiggest,saturationactionisstrongest.Thefrequencydepartfrom0,STEprobabilityisreduced,saturationactionbecomesweak.2）Saturationgaincoefficientinhomogeneousmedium（均匀加宽介质的饱和（大信号）增益系数）Gaincoefficientofquasi-monochromaticlightatfrequency1,withtheintensityofI1Saturationgaincoefficient（4.5.5）where（4.5.12）Saturationgaincoefficientisdroppedtoonehalfofun-saturatedgainwhenGainandsaturationeffectisveryweak,（canbeneglected）,ifthefrequencyisoutsidetherangeof思考题：

大信号增益曲线宽度与小信号增益曲线宽度是否相等？

思考题：

PhysicalmeaningofsaturationintensityIs：

受激辐射造成n2的减小可以与其它自发辐射和无辐射跃迁造成的衰减可以相比拟由于受激辐射，促使E2能级寿命减小；

增益降低不仅是增益饱和与否的判据，又是决定腔内光强即激光输出功率大小的重要参数其值由激光工作物质本身性质决定He-Ne:

632.8nm0.3w/mm2CO2:

10.6mm2w/mm2Arion:

514.5nm7w/mm2讨论此命题的物理背景:

激光器中某一模式频率首先起振,成为强光；

别的模式刚起振（弱光）,强光模式对刚起振的弱光模式的影响3.在强光强光In1n1作用下的弱光弱光增益系数频率为1,光强为I1强光,同时有一频率为的弱光入射求强光对弱光增益系数的影响，即弱光增益系数会如何变化？

定性分析定量分析强光作用下的反转粒子数强光作用下的反转粒子数强光不仅使自身