CWlaserbehaviorPPT课件

1、ReviewReviewProperties of LaserInteraction between Material and Radiation Blackbody radiation(2.2)Spontaneous and stimulated emission, absorbtion(1.1)(Einstein Coefficients)The laser idea(1.2-1.3)Lineshape function, Natural Broadening, collision broadening, Doppler broadening)第1页/共14页ReviewReviewRay

2、 and wave propagation through optical media(chapet4)ABCD martix(4.2)Fabry-perot interferometer(Longitudinal modes )Gaussian BeamsPassive optical resonators(5.1-5.5)Typical resonator Stability conditionsStable resonators (eigenmodes and eigenvalus)第2页/共14页Continuous Wave Laser Behavior第3页/共14页Introdu

3、ctionWhat we concern:- Behavior of continuous wave laser-Rate equation approximation( on the basis of a simple notion that there should be a balance between the total number of atoms undergoing a transition and the total number of photons being created or annihilated.)第4页/共14页7.2 Rate EquationsFour-

4、level laser Ionic crystal lasers( with the exception of ruby laser)Gas lasers, such as CO2 , Ar+, He-Ne, Cu vapor, etc.Quasi-three-level laser( the lower level laser level is a sublevel of the ground state)Many rare-earth ions in various crystal or glass hosts, such as Yb, Er and Yb:Er, Ho etc. for

5、its shortest wavelength transition.第5页/共14页典型激光器速率方程典型激光器速率方程表征激光器腔内光子数和工作物质各有关能级上的原子数随时间变化的微分方程组，称为激光器速率方程组(rate equations)。归纳共性，针对一些简化的、具有代表性的模型列出速率方程组，所谓的三能级和四能级系统。激光速率方程理论的出发点是原子的自发辐射、受激辐射和受激吸收几率的基本关系式。第6页/共14页爱因斯坦得到的光和物质相互作用的关系式22122212212111211212321121212321()(),(),8,spstabndNA NdtdNW N WBdtd

6、NW N WBdtAhB gB gBc 第7页/共14页7.2.1Four-Level LaserWe assume that there is only one pump level or band (band 3) and relaxation from the pump band to the upper laser level 2 as well as relaxation from the lower laser level 1 to the ground level proceeds very rapidly.We consider the case of space-indepe

7、ndent rate equations. (oscillating on only one cavity mode and pumping and mode energy densities are uniform within the laser material.013 NN2, N21, N1pumplaserFast decay3, N3Fast decay0, Ng第8页/共14页7.2.1Four-Level Laser222()pdNNRB Ndt013 NN2acdV B NdtRp: pumping rateB: stimulated emission coefficien

8、t per photon per mode: total number of photons oscillating only on one cavity mode:the life time of the upper-laser levelVa: mode volume within the active mediumc:the photon lifetime,accounts for the removal of photons due to cavity loss2, N21, N1pumplaserFast decay3, N3Fast decay0, Ng第9页/共14页7.2.1F

9、our-Level LaserOnly concern TEM00 mode420lwVamedium active theoflength : lsizespot central :0w某一模式的模体积描述该模式在腔内所扩展的空间范围模体积大，对该模式的振荡有贡献的激发态粒子数就多，也就可能获得大的输出功率。第10页/共14页7.2.1Four level laserTwo observations: (1) Level 3 can be considered empty if its decay rate is much faster than the pump rate33pgNRN32

10、NN(2) The term accounting for spontaneous emission has been neglectedWhen spontaneous emission is taken into account, 2aV B N2221aaaV B NV BNV BNFor simplicity, the extra photon will not be introduced 第11页/共14页7.2.1 Four level laserIntensity of one wave:2dIN Idzsection crossn transitio:A resonator o

11、f length L in which an active medium of length l and the refractive index n is inserted.Round-trip intensity change:21221222111exp 21 1(1) (1)iiiITTTNNlIwhereTaL Transmission losses (logarithmic transmission losses per pass):111lnT221lnT1mirror ofsmission power tran :1T2mirror ofsmission power tran :2T122i第12页/共14页7.2.1 Four level laserAssume: 21N l22IN lI 2,(1) (the optical length of the resonator)eeLtLLnlc 2eedIlccNIdtLL V=()eaaeL