针对脉冲宽度100 fs,带宽25 nm,能量6 mJ左右的超短脉冲基频光(经过透镜缩束后峰值光强为200～900 GW/cm2), 采用单块厚度为1.5 mm的BBO晶体进行了三倍频实验研究。在入射基频光强度约300 GW/cm2时, 得到的三倍频转换效率约0.8%。采用分步傅里叶变换及四阶龙格-库塔算法, 对描述飞秒脉冲单块晶体三倍频的耦合波方程组进行了数值计算。研究结果表明,三倍频光主要是由三阶非线性效应产生的; 基频光带宽较大是限制三倍频转换效率的主要因素之一; 对基频光的入射角度及方位角进行优化, 可较好地补偿非线性相位失配, 提高单块晶体三倍频转换效率。

Third harmonic generation (THG) experiment of supper intense femtosecond laser with a single BBO crystal of 1.5 mm was carried out. The pulse width of the input fundamental field is about 100 fs, the bandwidth is about 25 nm, the energy is about 6 mJ, and the maximum intensity is 200～900 GW/cm2 after focusing by a lens. The conversion efficiency was 0.8% with the intensity of the input fundamental field of about 300 GW/cm2. Based on the numerical techniques such as split-step Fourier transformation and fourth-order Runge-Kutta method, theoretical analysis was presented and the results show that the third harmonic field is mainly produced by the third-order nonlinear effect; the large bandwidth of the fundamental field is one of the chief reasons that limit the conversion efficiency; the nonlinear phase-mismatching and the conversion efficiency could be compensated well after optimizing the incidence angle and the azimuth angle of the fundamental field.