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.

在超强超短激光脉冲与物质相互作用过程中, 良好的激光脉冲对比度是一个非常重要的参数指标。为了扩大对超短激光脉冲对比度的测量范围, 准确地测量超短激光脉冲的对比度, 利用非线性晶体对激光脉冲强度变化十分敏感的特性, 研究了超短激光脉冲三阶相关信号的产生过程, 并在此基础上, 研制了一台三阶相关仪, 对一台飞秒激光振荡器进行了脉冲对比度的测量研究。测量结果表明, 通过在测量光路中加入聚焦透镜进行聚焦准直, 可以有效提高激光脉冲的倍频效率, 从而大幅提高该三阶相关仪的可测动态范围。该测量系统目前可实现的可测动态范围达到约108。

The stretcher system is an important part in the high-engery petawatt laser facility. The stretcher system design has a direct impact on the quality of the output pulses. The stretcher systems in some famous high-energy petawatt laser facility are summarized. And the structure and parameters are compared and discussed.

The impact of the stretcher on the contrast ratio of the out pulse is analyzed making use of the ray -tracing method , which is based on the contrast ratio model of chirped pulse.When the beam size in the stretcher is increased from 2 mm to 40 mm ,the contrast of the output pulse approximately increase two magnitude. When the input pulse is Sech2 ,the contrast of the output pulse is better than what the input pulse is Gassian or Rectangular.

In a stretcher,it is important of spectral clipping. It has a direct impact on the quality of the output pulse. Base on the ray-tracing,an analytical spectral clipping model is present in the ffner stretcher . When the stretcher parameters is determined, the location of the grating placed has a very important influence on the pulse spectral clipping. When the grating placed on the symmetric center about optical axis, the spectral distribution through stretcher will be asymmetric about center wavelength of the laser, that is to say,the spectral clipping will be asymmetric spectral clipping.Therefore,the output pulse contrast will be droped approximately one magnitude in the SGII ninth beam petawatt upgrade laser facility. In order to keeping the spectral symmetry distribution about center wavelength of the laser through the stretcher, the grating must be placed on the asymmetrical position about optical axis.

在展宽器中,光谱的剪切情况非常重要,它直接影响输出脉冲的质量。以ffner展宽器为例,参照以往建立的光线追迹模型,建立了分析展宽器中光谱剪切的模型。发现当展宽器的参数确定好后,光栅放置的位置对激光脉冲光谱剪切有非常重要的影响。当光栅相对于光轴中心对称放置时,展宽器容许通过的光谱相对于激光中心波长将是非对称分布的,即此时在展宽器中发生的光谱剪切为非对称光谱剪切,在神光II第9路拍瓦激光装置中,这种非对称光谱剪切将使输出脉冲对比度下降一个数量级。为了使展宽器中通过的光谱相对于激光中心波长对称分布,需要将光栅相对于光轴非中心对称放置。

展宽系统是高能拍瓦激光装置中的重要组成部分。它的设计好坏直接影响输出脉冲的质量。综述了国内外著名的高能拍瓦激光装置中所用展宽系统的结构及其相应的参数，并加以比较分析和讨论。