会计学§1.2光的受激辐射(fúshè)（StimulatedEmission)受激跃迁(yuèqiān)受激吸收stimulatedabsorption(STA)受激发射stimulatedemission(STE)－单色能量(néngliàng)密度(J·m-3·s)二、爱因斯坦关系(guānxì)式－A21、B21和B12的关系(guānxì)说明：SPE,STA,STE三种过程同时存在，只是有强弱差别两种辐射比较(热平衡情况下)答案(dáàn)：SPE（自发辐射）占绝对优势4.自发辐射光子可作为引起受激辐射的外来光起始阶段(jiēduàn)的受激辐射的外来光来自于介质的自发辐射一、受激辐射(fúshè)占优势的条件－粒子数反转分布二、受激辐射光放大开放式谐振腔（Openresonatororcavity）模选择提高光子简并度(Photondegeneracy)激活介质(jièzhì)中的增益增益系数(gaincoefficient),增益饱和(gainsaturation)增益曲线(gainprofile)激光特性:单色性好、方向性好、光亮度相干光(coherentlight)自发辐射～非相干光－光子简并度很低受激辐射～相干光－光子简并度高什么是光子简并度n－衡量相干光的参量具有相同(xiānɡtónɡ)能量和动量的光子数（也称同态光子数）同一模式中的光子数例：黑体(hēitǐ)辐射源（近似封闭腔）的光子简并度模密度(mìdù)nn增益(zēngyì)系数(GainCoefficient)与增益(zēngyì)曲线若介质(jièzhì)中Dn分布均匀,g(z)常数g0－小信号增益系数增益(zēngyì)饱和g(I)（GainSaturation)增益曲线g(n)－增益系数相对于频率的分布问题的提出：引起(yǐnqǐ)STE的外来光(自发辐射）不是单一频率，有一定的频率分布,(第四章详细讨论)损耗系数a激活介质中不仅存在增益，还有损耗损耗系数－单位(dānwèi)长度光强衰减的百分比负增益系数（2）如何使有限(yǒuxiàn)长增益介质成为“无限长”光放大器d－单程损耗，即单程渡越时腔内平均(píngjūn)光强衰减的比值Note:Thresholdconditionisnecessaryforgenerationoflaser,onlydependentonthelosses,independentontheintensityofinitiativelight.Initiativelightcomesfromspontaneousemissionoflasermedium,evenifthespontaneousemissionissoweakatthebeginning.Finally,theterminalintensitycanbeobtainedbygainsaturationeffect.Ifthelengthofactivemedium(l)isshorterthanthelengthofcavity(L),thethresholdconditionstillisSummaryThreeinteractionsbetweenlightandmediumSPESTESTAEinsteinrelationshipA21,B21andB12Threekeypartsconsistedofaconventionallaserandtheirfunctions,respectively.Someimportantphysicalconceptsandparameterspopulationinversion,gainmedium(activemedium)gain(loss)coefficient,(smallsignal;bigsignal)gain(population)saturation;saturationintensity;terminalintensity;thresholdcondition