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。

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.