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为了有效改善飞秒拍瓦激光系统的输出波前质量,研究了10Hz前端激光脉冲的波前随泵浦能量的变化特性。结果表明:由于增益介质的热效应,放大光束的斯特尔率(Strehl ratio)随泵浦能量的增加而下降,这说明光束的聚焦性能发生改变。利用波前展开的Zernike系数,进一步分析研究发现,由于热透镜效应,经多通放大的激光脉冲波前不仅离焦量发生了变化,像散量也发生了变化。离焦量和像散畸变是光束经过多程放大后畸变变化的两个主要分量,要改善波前必须予以补偿。在此基础上,用He-Ne光作为探测光,测量了实验装置中的钛宝石在不同泵浦能量下的热焦距,与基于钛宝石晶体参数计算出的理论值符合较好。
In order to effectively improve the output wavefront quality of the femtosecond PAW laser system, the wave front of the 10Hz front laser pulse with pump energy is studied. The results show that due to the thermal effect of the gain medium, the Strehl ratio of the amplified beam decreases with increasing pumping energy, which indicates that the focusing performance of the beam changes. Using the Zernike coefficients developed by the wavefront, the further analysis shows that, due to the thermal lens effect, the multi-pass enlarged laser pulse wavefront not only changes the defocus amount, but also the astigmatism. Defocus and astigmatism are the two major components of a beam that undergoes aberrations after multiple magnifications. To compensate, the wavefront must be compensated for. On this basis, using He-Ne light as the probe light, the thermal focal length of the sapphire under different pumping energies was measured, which is in good agreement with the theoretical value calculated based on the parameters of the sapphire crystal.