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为了研究高功率激光系统中的内光路热变形累积效应对光束质量的影响,开展了理论和实验研究。首先,选择三种非稳腔输出的典型高功率激光,激光功率分别为10 kW、50 kW和100 kW,通过热动力学瞬态方法的有限元分析对激光照射的反射镜进行热变形分析。然后,利用光线追迹理论得到激光的波前像差。最终,激光远场光束质量中的Strehl比,β因子可以通过Fresnel衍射积分进行计算和对比分析。仿真结果表明,由于内光路反射镜的热变形,远场光束将出现偏心和像散的现象,其中心光强及会聚能力均会下降。随着内光路反射镜的增加,Strehl比减少而β因子增加,远场光斑x方向的倾斜、像散及彗差也会逐渐增加,将成为系统的主要像差。为了和上述理论仿真结果进行对比,建立了一个测量反射镜热变形实验平台,实验中采用10 kW的TEA CO2激光器照射99%反射率的铜反射镜,通过等效放大原理,实验结果可以部分代替50kW和100kW功率水平。镜面热变形的测量精度小于λ/15,并和仿真结果吻合得较好。结果表明,反射镜的热变形在激光功率超过10 kW时不能被忽略,且随着激光功率及反射次数的增加,这种热畸变将变得越发严重。所得结果将对高功率激光系统的热畸变分析过程提供指导,并可应用于激光核聚变及激光武器系统等的设计分析中。
In order to study the effect of cumulative effect of thermal path degeneration inside the high-power laser system on the beam quality, theoretical and experimental studies have been carried out. First of all, the typical high-power laser output from three kinds of unstable cavity was selected. The laser powers were 10 kW, 50 kW and 100 kW respectively. The thermal deformation of laser-irradiated mirrors was analyzed by finite element analysis of thermodynamic transient method. Then, using the ray tracing theory to get the laser wavefront aberration. Finally, the Strehl ratio and β factor in laser far-field beam quality can be calculated and compared by Fresnel diffraction integral. The simulation results show that due to the thermal deformation of the inner optical path mirror, the far-field beam will appear eccentric and astigmatic, and the center light intensity and convergence ability will decrease. With the increase of the inner optical path mirror, the Strehl ratio decreases and the beta factor increases, and the tilt, astigmatism and coma in the x-direction of the far-field light spot also gradually increase and become the main aberrations of the system. In order to compare with the above theoretical simulation results, a thermal deformation experiment platform of measurement mirror was established. In the experiment, a 10% TEA CO2 laser was used to irradiate a 99% reflectance copper mirror. Through the principle of equivalent amplification, the experimental results can partially replace 50kW and 100kW power levels. Mirror thermal deformation measurement accuracy is less than λ / 15, and the simulation results are better. The results show that the thermal distortion of the mirror can not be neglected when the laser power exceeds 10 kW, and the thermal distortion will become more serious as the laser power and the number of reflections increase. The results will provide guidance for the thermal distortion analysis of high power laser systems and may be used in the design analysis of laser fusion and laser weapon systems.