基于复合型光栅的光谱色散匀滑新方案

A new scheme of spectral dispersion smoothing based on hybrid grating

针对惯性约束聚变装置对提高靶面辐照均匀性的要求,提出了基于复合型光栅的光谱色散匀滑新方案,即利用复合型光栅中不同的色散区域对激光束进行不同方向的色散,例如,内部色散区域Ⅰ的色散方向为水平或垂直方向,外部色散区域Ⅱ的色散方向为圆周方向,使得焦斑内部散斑在远场的扫动方向是平动和旋转两种方式的混合,因而可以有效减小焦斑内部条纹状强度调制,进而提高靶面辐照的均匀性.本文建立了基于复合型光栅的光谱色散匀滑理论模型,分析了复合型光栅方案的匀滑效果,并与典型的光谱角色散方案和"星"光栅方案进行了比较.在此基础上,针对复合型光栅的色散区域面积比值、色散区域Ⅰ和Ⅱ的刻线密度等关键参数进行了讨论.结果表明,色散区域Ⅰ面积占总色散区域面积比值在0.3—0.5时,复合型光栅方案可有效减小焦斑内条纹状强度调制和沿径向的强度调制;随着色散区域I,Ⅱ的刻线密度在一定范围内增加,焦斑均匀性得到改善,但结合实际的加工情况,应选取合理的光栅刻线密度;与典型一维光谱色散(1D-SSD)匀滑方案相比,该方案的靶面辐照均匀性更好,且可实现与多维光谱色散匀滑类似的效果,而与"星"光栅方案相比,该方案中复合型光栅的加工相对较简单,且其靶面辐照均匀性也更佳.

The irradiance uniformity on target plane is a key issue in laser-driven inertial confinement facilities. In the typical schemes of one-dimensional smoothing by spectral dispersion（1D-SSD） and the ＂star＂ grating, the stripe pattern inside the focal spot appears inevitably, besides, the fabrication of the ＂star＂ grating is relatively difficult. Thus, a new spectral dispersion smoothing scheme based on a hybrid grating is proposed, which not only achieves the better smoothing effect,but also exhibits some specific advantages in the fabrication and the dispersion way. According to the different direction of the spectral dispersion, the hybrid grating is divided into inner and outer dispersion areas. That is, the dispersion direction of the inter dispersion area is in the horizontal or vertical direction, and the dispersion direction of the outer dispersion area is in the azimuthal direction. When the laser beam with the temporal phase modulation propagates through the hybrid grating, the dispersion directions of the laser beam in the inner and the outer dispersion areas are different, leading to the redistribution of the speckles inside the focal spot in the resultant direction of the translation and rotation on the target plane. Consequently, the focal spot on the target plane achieves the beam smoothing in radial and horizontal or vertical direction. In the present paper, the theoretical model of the hybrid grating scheme based on the spectral dispersion smoothing is built up. Using the theoretical model, the smoothing effect of the hybrid grating scheme is analyzed, and compared with those of the typical schemes of 1D-SSD and the ＂star＂ grating. The contrast and the fractional power above the intensity（FOPAI） are used to evaluate the smoothing characteristic of the focal spot.In addition, the influences of the area ratio and groove density in the different dispersion areas of the hybrid grating on beam-smoothing effect are also discussed. Results indicate that when the inner dispersion area accounts for the0.3–0.5 of the total dispersion area, the hybrid grating scheme can effectively suppress the stripe intensity modulation both in the radial direction and the vertical direction. With increasing the groove densities of the inner and the outer dispersion area in a certain range, the irradiance uniformity of the focal spot is further improved. However, considering the actual processing of the hybrid grating, the appropriate groove density should be selected. Compared with the typical scheme of the 1D-SSD, the scheme of the hybrid grating can achieve the better smoothing effect with the multi-direction spectral dispersion smoothing. Furthermore, the fabrication of the hybrid grating is relatively simple and the irradiation uniformity on the target plane is also good compared with those of the ＂star＂ grating scheme.