Radiation Balanced Laser's Theory And Yb: KGW Fluorescence Cooling Laser's Design

New concept solid state lasers with anti-Stokes fluorescence radiation as cooling method is explored in this dissertation, mainly on the theory of radiation balance laser, polarized spectra of laser crystal Yb:KGW on fluorescence cooling, and laser design, and simulation of corner-pumped fluorescence cooling laser.The dissertation includes five parts:1,Macroscopic balance condition of fluorescence cooling laser.Ratio between fluorescence and stimulated photon number is introduced as a variable to to estimate the fluorescence cooling power could eliminate waste heat from laser quantum defect. End-pumped quasi-three-level rate equation model of Beach is applied to analyze the properties of an Yb:KGW slab laser when it satisfies the macroscopic balance condition. Thus macroscopic balance condition of fluorescence cooling laser is proposed to determine whether pump absorption and radiation of laser light and fluorescence reach energy balance in the fluorescence cooling laser.2,Analysis on polarized spectra of Yb:KGW about laser jumping and radiation coolingAnisotropic Yb:KGW has three principal refractive index directions m, p, g, the best laser crystal polarization design for the fluorescence cooling laser is: m should be the pump absorption polarization and normal to the face of fluorescence emission; p should be the laser polarization; g should be in the face of fluorescence emission. Three configurations of the fluoresence spetra measurement are designed to analyze red-shift phenonminon of Yb:KGW, and methods for depressing radiation trapping are suggested: high power pumping, low doping concentration, small dimensions and fusing with undoped KGW.3,Calculation of"athermal direction"of Yb:KGW.The Anisotropic Yb:KGW crystal could probably find certain propagation directions, lasers operated in these directions have no thermal lensing effect. Beam quality of this kind of laser will be improved and the laser is called"thermal effect elimination laser". According to the measured thermal expansion coefficients, thermal expansion tensors of Yb:KGW crystal are obtained by least square and tensor rotation methods. With these thermal expansion tensors, the athermal directions are calculated and the results show that Yb:KGW generally has only two athermal directions: when light is polarized in m direction, directions with around twenties degrees angles clockwise or counterclockwise rotated from p axis in p-g plane are the"athermal directions"(different individual has different values). According to the calculated thermal expansion tensor, the thermo-optic properties of Yb:KGW are consistent with the reported Yb:KGW laser thermal lensing experimental results. 4,Correction of Gaussian model of end-pumped laser and simulation of Yb:KGW thermal effect elimination laser.Traditional Gaussian model of end-pumped laser is revised and verified, and is applied to Yb:KGW thermal effect elimination laser. Total photon flux as a function of ordinate replaces the cavity photon number Q scalar in the traditional model, then modeling results of a published Nd:YAG laser well agreed with the experiment data. Based on the revised Gaussian model, numerical simulation of thermal effect elimination laser, is accomplished with the parameters of Yb:KGW. Modeling results show that the output power and efficiency of the thermal effect elimination laser is similar to the conventional Yb:KGW laser, at the same time it can provide high-quality laser beam without thermal effect, so it is an ideal choice for Yb:KGW laser.5,Design of corner-pumped configuration for fluorescence cooling laser.Corner-pumped configuration is suggested to cope with the poor pump absorption caused by the low doping concentration of fluorescence cooling laser crystal, then with the best Yb:KGW crystal cutting design, optimized parameters of corner-pumped configuration is obtained by ray tracing method. Laser rate equations are integrated with spatial tracking of diffraction field to simulate two kinds of corner-pumped fluorescence cooling lasers, with 1.6kW-pumped plane-convex resonator and 1.2kW-pumped convext-convext resonator respcctively. 3-D simulation results show that laser operation is more stable with convext-convext resonator than with plane-convex resonator. Distribution of stimulated population and waste heat from laser quantum defect are analyzed to get that fluorescence cooling power is uniformily distributed while waste heat increases slightly along longitudinal direction in the laser crystal. Based on macroscopic balance condition, a reasonable higher pump power should be chosed in order to avoid negative thermal effect in the fluorescence cooling lasers.The higher the output power of solid state laser is, the more heat deposit is accumulated in the medium. Cooling problem of laser gain medium has become one of the bottle necks for the development of high power laser such as inertial confinement fusion laser and directed energy weapon. Aiming at the limitations of traditional cooling methods (convection or conduction), research of new thechnique is developed that can instantly and directly removes the waste heat from the gain medium. This dissertation has reference value for the design of low thermal laser and high beam quality solid state lasers.