清华激光原理第七讲(4月7日)PPT文件格式下载.ppt

1、Einstein relationshipRed ruby,Er3+doped fiberw13A31S31S32A21S21w21w12E1E2E3He-Ne,Nd:YAG较大w03A30S30S32S21A21W21E3E2E1E0W12S101.Four levels system1）single mode oscillation（l th mode,）w03A30S30S32S21A21W21E3E2E1E0W12S10没有包括没有包括A21引起的光子数？引起的光子数？（ignore S30）（4.4.13）Rate equations for practical laser are

2、set up according to the various physical processes Different expressions of rate equations for a same laserR1,R2 is atom density pumped per second into E1 and E2,respectively.t1,t2 is lifetime at E1,E2,respectively;t21 is spontaneous emission（荧光 fluorescence）lifetime for E2 E1.E2E1R2R1t2t1t21Conside

3、r loss onlyPhoton lifetime光子寿命（2.1.14）Laser transitionE2E1R2R1t2t1t21PumptransitionsLaser transitionw03A30S30S32S21A21W21E3E2E1E0W12S102）Rate equations for multimode oscillation Serial number of mode Frequency of mode Photon number of serial mode Method:对应每个模式分别建立一个速率方程,序数相应变化Simplifications Line sh

4、ape function rectangle （矩形）Same loss,same lifetime of photon for each mode矩形面积原谱线面积矩形面积原谱线面积Reasons of simplificationDo not need to consider differences of the modes for studying questionNeglect the difference of diffraction loss in the modesAccording to the above simplified model,multimode rate equ

5、ations for four levels system as followingN total photon density for all modes泵浦效率泵浦效率Multimode rate equations for four levels laser system荧光量子效率荧光量子效率w03A30S30S32S21A21W21E3E2E1E0W12S10Pumping efficiency:the fraction of the total pump power that is useful in creating the population inversionFluores

6、cence（quantum）efficiency from E2E1,depends solely on characteristics of medium,it is the upper limit for performance of any laser.w03A30S30S32S21A21W21E3E2E1E0W12S10Total quantum efficiency（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（Photon numbers of emitting fluorescence）（Photon numbers absorbed from pumping）Rat

7、e equations Expression of gain coefficient（effecting factors）behavior of gain saturation（增益饱和）（for homogeneous and inhomogeneous medium）4.5 Gain coefficient in homogeneous broadening medium 1、Small signal gain coefficient for steady state（four levels）I=Nhvdz=vdt不计损耗II+d IdzDn0 I=Nhv dz=vdt（1.3.6）*1）

8、Gain coefficient is proportional to population inversion Dn （four levels system as example）Gain coefficientDiscussing key factors dependent on gain coefficient稳态稳态*Reason:optical intensity in medium is very small,the change of n introduced by STE can be neglected in small signal caseIn small signal

9、case,Dn0 also independent on optical intensity,dependent on pumping rate W03 n0 稳态阈值附近n2很小Small signal（unsaturated）gain coefficient g0 is dependent on n0,independent on optical intensityreview：如何理解小信号情况？小信号2）Gain curve-g0 varying with frequency Small signal gain curve depends on spectrum line shape（

10、小信号增益曲线的形状完全取决于谱线线型函数）Small signal gain coefficient=s21STE cross section at 0 增益线宽 荧光线宽F （自发辐射线宽H）3）g0（0）is proportional to l02,inversely proportional to spectral width3.39mm632.8nm3S3P2PHe-Ne2、Gain Saturation（增益饱和）大信号情况Under the action of incidence light with I1,at 1（considering STE process）1）Satur

11、ation of inversion populationWhat is gain saturation？Gain coefficient is decreased as STE intensity becomes strong.Reason of gain saturation：When the intensity is strong enough稳态=1/Is（）saturation of inversion population Saturation intensity at 0大信号情况（4.5.7）小信号情况Population n2 is barely affected by ST

12、E（即可忽略受激辐射的影响）Incidence light at same frequency with different intensity,different effect on saturation of n Incident light with same intensity at different frequency,different effect on saturation of nFrequency range of saturation actionwhenif（Stimulating wave）At central frequency 0,STE probability

13、 is biggest,saturation action is strongest.The frequency depart from 0,STE probability is reduced,saturation action becomes weak.2）Saturation gain coefficient in homogeneous medium（均匀加宽介质的饱和（大信号）增益系数）Gain coefficient of quasi-monochromatic light at frequency 1,with the intensity of I1Saturation gain

14、 coefficient（4.5.5）where（4.5.12）Saturation gain coefficient is dropped to one half of un-saturated gain when Gain and saturation effect is very weak,（can be neglected）,if the frequency is outside the range of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：Physical meaning of saturation intensity Is：受激辐射造成n2的减小可以与其它

15、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:632.8nm 0.3 w/mm2 CO2:10.6mm 2w/mm2Ar ion:514.5nm 7w/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