非真空传输的高效交叉偏振滤波设计与产生

采用常规透镜设计了适用于非真空环境中交叉偏振波(XPW)产生的双透镜聚焦系统,在相对较短的距离实现了长焦透镜聚焦的效果,并测量了聚焦后的激光脉冲,发现其没有显著的非线性相位积累,保证了激光光束质量.在非真空中采用双BaF_2晶体得到了XPW系统转换效率22%,光谱1.78倍展宽的净化脉冲输出,双透镜组合聚焦形式使得双BaF_2晶体间距在13—22 cm内可保证20%以上的XPW转换效率,双晶体间距的调节冗余度提高了两个量级,极大地降低了双晶转换效率对晶体间距的依赖.这种正负透镜组合聚焦的光路设计在非真空中实现了高效稳定的XPW输出,为后续的放大应用提供了高对比度、宽光谱的高质量种子源.

High efficiency cross-polarized wave filter for non-vacuum transmission

Development of high-peak power laser system encounters difficulties in producing the pulses with high temporal contrast. To increase the pulse temporal contrast ratio, a nonlinear filter based on crossed-polarized wave (XPW) generation is proposed. The XPW generation relies on a third-order nonlinear process occurring in a nonlinear medium, such as barium fluorite (BaF₂) crystal. The XPW process is quite straightforward:a linearly polarized laser pulse is focused on BaF₂ crystal positioned between two orthogonally polarizers, high power main pulses due to nonlinear polarization rotation can pass through the second polarizer, while low power unconverted pre-and post-pulses are filtered by the second polarizer. With the XPW technique, pulse contrast can be enhanced by several orders of magnitude. Furthermore, XPW spectrum can be broaden by a factor with respect to the initial spectrum. This efficient pulse cleaner presents many advantages and has proved to be a simple and reliable pulse filter operating in a double chirped pulse amplification system.Most of previous XPW experiments utilize short focal systems or work off focus due to an intensity limit in the crystal (BaF₂). These drawbacks result in a lower conversion efficiency (lower than 10%) when using a single crystal. Dual crystal setup is capable of achieving efficiency more than 20%, yet the configuration restricts the crystal separation to a millimeter level. The use of long focus lens in the XPW device is capable of reaching higher efficiency, with BaF₂ crystal positioned in the focal plane. Hence for milljoule pulses, the setup distance increases to tens of meters, resulting in a complicated system and cumbersome configuration.Considering these limitations, a compact, highly efficient and stable XPW generation using dual-lens system suitable for non-vacuum transmission is presented. The measured nonlinear accumulated phase shows little deterioration of pulse quality. With a compact dual lens system, we realize an excellent XPW conversion of above 22% (internal efficiency of 30%) with using double BaF₂ crystals, while a femtosecond laser pulse can experience a spectrum broadening up to a factor of 1.78. The dual-lens configuration overcomes the crystal separation limit, and conversion efficiency exceeds 20% for a crystal separation from 13 cm to 22 cm, which is conducible to flexibility and robustness. The stability for the setup to generate shorter pulses with very high contrast or compensate for spectral gain narrowing in the preamplifier is ensured due to the dual-lens focusing system.