超高强度飞秒脉冲的三次谐波转换

Third Harmonic Generation of Super Intense Femtosecond Laser Pulse

针对超高强度飞秒激光,对利用单块BBO晶体产生三倍频(THG)的过程进行了分析,比较了单块晶体中三阶非线性效应以及级联二阶非线性效应对三倍频转换效率的作用,讨论了入射基频光光强、晶体厚度、自相位调制(SPM)、交叉相位调制(XPM)、群速度失配、失谐角、方位角等因素对三倍频光转换效率、时间波形及光谱分布的影响,在此基础上,提出了提高三倍频转换效率的方法.研究结果表明:入射基频光强一定时,三倍频光的峰值光强、脉冲宽度(FWHM)随晶体厚度变化不明显.通过优化基频光入射角度,可提高单块BBO晶体三倍频光转换效率及峰值光强,并减小三倍频光脉冲宽度.此外,方位角的优化也可在一定程度上提高三倍频转换效率.

Third harmonic generation （THG）of super intense femtosecond laser pulse with one BBO crystal was analyzed. Effects of the third-order and the cascaded second-order nonlinearity in one crystal on the third harmonic conversion efficiency were compared. Influences of the factors, such as input fundamental intensity, length of the crystal, self-phase-modulation （SPM） and cross-phase-modulation （XPM）, group-velocity mismatching, detuned angle, azimuth angle, etc, on the conversion efficiency, pulse shape and spectra of the third harmonic field were discussed in detail. Furthermore, the methods to improve the third harmonic conversion efficiency were proposed. The results show that, the variation of the maximum intensity and the pulse width （full width at half maximum （FWHM） of the third harmonic field with the length of the crystal is not obvious. By justifying the phasemismatching angle of the fundamental field, the conversion efficiency and the maximum intensity of the third harmonic field can be increased, and the pulse width （FWHM） can be reduced. Moreover, it is also helpful to improvement of the conversion efficiency by optimizing the azimuth angle of the crystal.