提出了一种新的两级非共线相位匹配光参量放大方案,两级光参量放大分别采用不同的相位匹配条件,充分利用非线性晶体的相位匹配能力和增益光谱范围,可以实现增益带宽接近一个倍频程的光参量放大.并以4.5 mm厚的偏硼酸钡(BBO)晶体和波长为532 nm、强度为5 GW/cm2的抽运光源为例进行了模拟计算,结果表明,该方案可以实现波长范围在710~1190 nm、增益带宽为480 nm 的信号光放大,信号光光谱宽度达到0.75 个倍频程,再压缩后傅里叶限脉冲的时间宽度约为5 fs,约等于1.6 倍的光振荡周期.计算了光参量放大过程中的相位畸变,以及该相位畸变对再压缩后信号光脉冲时间波形的影响.研究结果可以为实现周期量级光参量放大的设计提供理论依据.

对光参量放大的相位匹配进行了研究，建立了产生超宽增益带宽参量放大相位匹配的一般物理模型，依据这一模型，在只给定抽运光波长和晶体参数的条件下，可以确定超宽带相位匹配结构和信号光匹配波长。以偏硼酸钡(BBO)晶体为例进行了模拟计算，结果表明，非共线相位匹配的两种不同结构有着相差极大的晶体接收角。讨论了如何优化非共线相位匹配结构参数以便得到极大增益带宽和极佳晶体接收角的问题。对BBO晶体光参量放大的光谱相位匹配的计算结果可以为多光束抽运或多阶段光参量放大的设计提供理论依据。

建立了光参量啁啾脉冲传输放大(OPCPA)的物理模型，采用分步傅里叶算法编写了三维OPA+CPA 数值模拟程序，利用该程序可以研究OPCPA 系统中的能量放大、增益窄化、自相位调制、空间自聚焦以及脉冲的时间信噪比等问题，对OPCPA 系统进行优化设计。利用该程序对神光II 第九路拍瓦升级系统(SG-II-U-PW)进行了数值模拟研究，计算结果表明，第九路光路无需作太大改动即可满足拍瓦升级指标，并提出了第九路主放大链的两种运行方案。三维OPA+CPA 模拟程序经过简单修改后也可以对飞秒(fs)OPCPA 系统、以及未来峰值功率为exawatt 级(1018 W)的激光系统进行优化设计。

A two-dimension model is presented to analyze the spatial dependence of pulse contrast in the focal plane. The parameters of the SHENGUANG (SG) II laser system are demonstrated as examples. Comparing with the degradation in the beam centroid, the pulse contrast degrades more seriously in the transverse. This spatial degradation of pulse contrast can be improved, such as by controlling the spatial spectrum clipping.

We theoretically study the temporal contrast degradation by the spectral phase distortion in chirped pulse amplification (CPA) lasers. As an example, we analyze the impact of the surface qualities of the optical elements such as mirrors and grating in the stretcher and compressor on the temporal contrast. The temporal contrast declines fast in the case of a rapidly varying random surface error of the optical elements. When the values of PV, RMS and GRMS of the surface error curve are reduced, the temporal contrast is becoming better and better. And the temporal contrast can be improved after the surface error curve is to be spatial filtering. Those results are helpful for the choice of the surface parameters of the optical elements in the stretcher and compressor.

We theoretically study the temporal contrast degradation by the spectral phase distortion in chirped pulse amplification (CPA) lasers. As an example, we analyze the impact of the surface qualities of the optical elements such as mirrors and grating in the stretcher and compressor on the temporal contrast. The temporal contrast declines fast in the case of a rapidly varying random surface error of the optical elements. When the values of PV, RMS and GRMS of the surface error curve are reduced, the temporal contrast is becoming better and better. And the temporal contrast can be improved after the surface error curve is to be spatial filtering. Those results are helpful for the choice of the surface parameters of the optical elements in the stretcher and compressor.

交叉偏振波(XPW)滤波是一种简单灵活的非线性滤波技术,可以极大地提高超短超强飞秒激光脉冲信噪比。介绍了XPW滤波器原理,通过数值模拟得到以下结论:输入光偏振方向与晶体的[100]轴的夹角β、相位调制(PM)、注入光强和晶体长度以及不同的空间光束分布都会影响XPW的能量转化效率。其中PM是造成能量转化过早饱和,导致最大转化效率降低的主要原因。同时注入光的初始二阶和三阶频谱相位会导致XPW脉冲形状畸变,谱宽窄化,中心频移,能量转化效率减小。为 XPW非线性滤波装置能够有效地提高超强超短脉冲的信噪比提供了理论依据。

A numerical model of optical parametric amplification (OPA) is introduced to investigate the impact of wavefront phase distortion of pump on the beam quality of signal. Numerical results show that the unidentical walk-off directions of the pump and the idler waves are the main factors leading to the transfer of wavefront phase distortions of the pump to the signal, and by reducing the angle between the two directions, the beam quality factor (M²) can be greatly decreased and hence the good beam quality of the signal can be maintained.

数值模拟了拍瓦激光器前端非共线光参变放大(OPA)过程中存在波前畸变的抽运光对信号光光束质量的影响。结果表明,当抽运光光束波前存在严重畸变时,衍射效应和走离效应通过不同的物理机制导致了信号光光束质量的急剧下降。在导致信号光光束质量下降的过程中,走离效应在OPA未饱和放大时起着主导作用,当OPA工作在过饱和状态时,过饱和导致的信号光近场空间分布中心凹陷成为光束质量下降的主要因素。

惯性约束核聚变快点火实验, 要求拍瓦终端输出再压缩的高信噪比超短激光脉冲。在拍瓦前端光参变啁啾脉冲放大(OPCPA)阶段, 抽运激光脉冲中由放大的自发辐射(ASE)噪声调制导致的电场强度振幅和相位的随机扰动, 将会被传递到信号光的光谱上, 使再压缩后主激光脉冲前后出现底座, 降低了信号光的信噪比。数值模拟了抽运激光脉冲中ASE噪声对信噪比以及对时间底座的宽度的影响, 并探讨了有利于提高超短激光脉冲信噪比的途径。