超短脉冲倍频中三阶非线性效应的影响及补偿

 为了提高超短脉冲二倍频的转换效率和改善出射倍频光的脉冲形状，对超短脉冲倍频中三阶非线性效应的影响进行了理论分析和数值模拟，并采用初始相位失配的方法来补偿三阶非线性效应的影响。结果表明：用KDP晶体二倍频中心波长为800 nm的超短脉冲，当入射功率密度大于100 GW/cm²时，三阶非线性效应是倍频转换效率的主要影响因素。对脉宽为50 fs，入射功率密度为250 GW/cm²的超短脉冲在KDP晶体（2 mm）中的二次谐波变换，当初始相位失谐0.9 mrad时，转换效率提高了10%，同时由三阶非线性效应引起的强度调制得到明显抑制，出射基频光和倍频光的脉冲形状得到明显改善。

Compensation for effect of third-order nonlinearity on second harmonic generation of ultrashort pulses

The effect of third-order nonlinearity on the second harmonic generation of ultrashort pulses in KDP crystal is analyzed theoretically. It is found that the influence of third-order nonlinearity on conversion efficiency becomes dominant when the incident intensity is above 100 GW/cm^2. A novel approach in which an initial phase-mismatch is introduced to compensate for third-order nonlinearity effect is proposed. The calculated results show that for a 50 fs pulse and a 250 GW/cm^2 input intensity, the conversion efficiency is increased by a factor of 1.1 at the condition of 0.9 mrad initial phase-mismatching angle. At the same time, the fluence modulation of the fundamental and second harmonic pulses due to third-order nonlinearity is suppressed effectively and the temporal profiles are improved.