We experimentally compare a thick BIBO crystal, a walk-off compensating configuration of thin BBO crystals, and a thin PPLN crystal for the use as a nonlinear medium in SHG FROG, aiming for increased sensitivity required for complete pulse characterization of low-energy (below pJ) pulses a few picoseconds long at wavelengths near 1030 nm. We investigate the dependence of the spectral bandwidth of the frequency-doubled input ultrashort pulse and the conversion efficiency on focal-spot size in the nonlinear crystal. Both numerical and experimental results confirm an increase of the bandwidth with tighter focusing in a thick BIBO and a more-than-threefold increase of conversion efficiency compared to a thin BIBO crystal. A PPLN crystal is found to be the most efficient among the three, but very thin (< 0.3 mm) crystals would be required to extend the measurement capabilities of SHG FROG to sub-ps pulses. A SHG FROG using a PPLN crystal for characterization of picosecond pulse is demonstrated with a sensitivity of $2\times 10^{-3}$ (mW)2.

中文翻译：

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更高灵敏度皮秒脉冲 SHG FROG 的晶体配置注意事项


我们通过实验比较了厚 BIBO 晶体、薄 BBO 晶体的走离补偿配置和薄 PPLN 晶体，用作 SHG FROG 中的非线性介质，旨在提高低能量完整脉冲表征所需的灵敏度（低于pJ）在 1030 nm 附近的波长处发出几皮秒长的脉冲。我们研究了倍频输入超短脉冲的光谱带宽和转换效率对非线性晶体中焦斑尺寸的依赖性。数值和实验结果均证实，与薄 BIBO 晶体相比，厚 BIBO 晶体的带宽随着聚焦更紧密而增加，并且转换效率增加了三倍以上。研究发现 PPLN 晶体是这三种晶体中效率最高的，但需要非常薄 (< 0.3 mm) 的晶体才能将 SHG FROG 的测量能力扩展到亚 ps 脉冲。使用 PPLN 晶体表征皮秒脉冲的 SHG FROG 被证明具有 $2\times 10^{-3}$ (mW)2 的灵敏度。