6.1-CW-laser-behavior知识讲解.ppt

,单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,Review,Properties of Laser,Interaction between Material and Radiation,Blackbody radiation(2.2),Spontaneous and stimulated emission,absorbtion(1.1)(,Einstein Coefficients),The laser idea(1.2-1.3),Line-Broadening Mechanisms(2.5,Lineshape function,Natural Broadening,collision broadening,Doppler broadening,),Review,Ray and wave propagation through optical media(chapet4),ABCD martix(4.2),Fabry-perot interferometer(,Longitudinal modes,),Gaussian Beams,Passive optical resonators(5.1-5.5),Typical resonator,Stability conditions,Stable resonators(eigenmodes and eigenvalus),Continuous Wave Laser Behavior,Introduction,What 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.),7.2 Rate Equations,Four-level laser,Ionic crystal lasers(with the exception of ruby laser),Gas lasers,such as CO,2,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 its shortest wavelength transition.,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。,爱因斯坦得到的光和物质相互作用的关系式,7.2.1Four-Level Laser,We 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-independent rate equations.(oscillating on only one cavity mode and pumping and mode energy densities are uniform within the laser material.,2,N,2,1,N,1,pump,laser,Fast decay,3,N,3,Fast decay,0,Ng,7.2.1Four-Level Laser,R,p,:pumping rate,B,:stimulated emission coefficient per photon per mode,:total number of photons oscillating only on one cavity mode,:the life time of the upper-laser level,Va:mode volume within the active medium,c,:the photon lifetime,accounts for the removal of photons due to cavity loss,2,N,2,1,N,1,pump,laser,Fast decay,3,N,3,Fast decay,0,Ng,7.2.1Four-Level Laser,Only concern TEM,00,mode,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。,7.2.1Four level laser,Two observations:,(1)Level 3 can be considered empty if its decay rate is much faster than the pump rate,(2)The term accounting for spontaneous emission has been neglected,When spontaneous emission is taken into account,For simplicity,the extra photon will not be introduced,7.2.1 Four level laser,Intensity of one wave:,A resonator of length L in which an active medium of length,l,and the refractive index n is inserted.,Round-trip intensity change:,Transmission losses(logarithmic transmission losses per pass):,7.2.1 Four level laser,Assume:,and,The number,of photons in the cavity is proportional to I.Comparing with,Population inversion:,7.2.1 Four level laser,(1)a laser oscillating with uniform pump and mode energy distribution in the active medium,(2)only single-mode oscillation,(3)the population inversion,during laser action,is independent of the longitudinal z-coordinate.,(4)oscillating on a single frequency,