【作者】 周沐；

【导师】 谭吉春；

【作者基本信息】 国防科学技术大学 ， 光学工程， 2010， 博士

【摘要】 本文对anti-Stokes荧光辐射制冷型新概念固体激光器进行了探索和研究,主要包括辐射平衡激光器原理、Yb:KGW激光晶体的荧光制冷偏振光谱性能和激光器设计,并对角泵浦荧光制冷型激光器作了数值模拟。论文主要内容包括以下五个部分:1、荧光制冷型激光器的宏观平衡条件研究。本文将荧光光子与受激光子数之比作为变量,用来估计荧光制冷量能否抵消激光量子亏损产生的废热。利用Beach的端面泵浦准三能级速率方程数学模型,分析了满足宏观平衡条件的Yb:KGW板条激光器的特性。论文给出了判断荧光制冷型激光器泵浦光的吸收与激光和荧光的辐射是否达到能量平衡的宏观平衡条件。2、Yb:KGW晶体与激光跃迁和辐射制冷相关的偏振光谱性质研究。利用Yb:KGW晶体偏振吸收谱和偏振发射谱在三个折射率主轴m, p, g方向上的差别,分析得到最适合荧光制冷型激光器的Yb:KGW晶体偏振方向选择方案:主轴m应当作为泵浦吸收偏振方向并且垂直于用作荧光辐射的晶体表面;主轴p应当用作荧光制冷型激光器的激光偏振方向;而主轴g应当位于用作荧光辐射的晶体表面内。设计三种荧光测量装置对Yb:KGW样品红移现象进行分析,得到抑制荧光再吸收的主要措施包括:泵浦光足够强且分布均匀;Yb离子掺杂浓度尽量小;晶体尺寸要尽量小;晶体辐射面熔接无掺杂的KGW晶体等。3、Yb:KGW晶体“无热方向”计算。论文对各向异性Yb:KGW晶体中能够抵消热透镜效应的传播方向进行了计算,这种利用晶体“无热方向(athermal direction)”改善输出光束质量的新概念激光器被称作“消热透镜激光器”。根据Yb:KGW晶体热膨胀系数的测量数据,分别采用最小二乘法和二阶张量旋转计算出各自晶体的热膨胀张量,由此计算出晶体的无热方向,得到Yb:KGW晶体只存在两个无热方向:即当光为m偏振时,在p-g平面内与p轴成正负二十多度的两个方向是Yb:KGW的“无热方向”(不同个体具体数值略有不同)。利用论文计算所得的晶体热膨胀张量推算出的Yb:KGW热光特性与文献报道的Yb:KGW激光器热透镜实验结果相吻合。4、端面泵浦激光器高斯数学模型的修正和消热透镜激光器的数值模拟。论文对端面泵浦激光器的传统高斯数学模型进行了修正和验证,用来对Yb:KGW晶体的消热透镜激光器进行数值模拟。用包含纵向分布的受激光子通量代替传统模型中的谐振腔光子数Q标量,并利用文献报道的Nd:YAG晶体端面泵浦固体激光器相关参数对修正后的模型进行验证,计算结果与实验数据吻合得较好。根据Yb:KGW晶体消热透镜激光器相关参数,利用修正后模型作数值模拟,结果表明消热透镜激光器在功率输出、转换效率等方面与传统Yb:KGW激光器类似,同时还能够输出无热透镜效应的优质激光光束,所以消热透镜激光器是Yb:KGW晶体激光器比较理想的选择。5、荧光制冷型激光器角泵浦技术研究。论文为解决荧光制冷型激光器激光晶体掺杂浓度造成的低吸收系数对泵浦系统的特殊要求,提出角泵浦方案,并对平凹腔和双凹腔这两种谐振腔条件下的角泵浦荧光制冷型激光器,进行了仿真和分析。在最佳Yb:KGW晶体切割方式下,利用光线追踪方法给出角泵浦方案的最佳设计参数。采用速率方程和衍射场空间追迹相结合的仿真方法,分别对1.6kW总泵浦功率的平凹腔和1.2kW总泵浦功率的双凹腔,这两种谐振腔条件下的角泵浦荧光制冷型激光器进行了仿真。结果表明双凹腔结构比平凹腔结构的角泵浦方式激光器运行更稳定。分析了激发态粒子数及对应的激光量子亏损废热在激光晶体中的分布情况,得到在激光晶体纵向上荧光制冷量呈均匀分布,而量子亏损废热呈略微增加趋势。利用宏观平衡条件为荧光制冷型激光器合理选择总泵浦功率,以避免出现负温差效应。激光器的输出功率越高,介质内部的热沉积量越多。固体介质的散热已成为发展惯性约束聚变激光器、定向能武器等高功率激光器的瓶颈之一。本文针对固体激光介质传统散热方式(对流、传导等)的局限性,提出从介质内部直接、即时散热的新技术,对设计研制低内热、高光束质量固体激光器有参考价值。更多还原

【Abstract】 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.更多还原