Based on diffraction optical theory, diffraction of a laser beam with periodic amplitude modulation and phase distortion is derived in 3ω optics system. Influence of defocus distance and focal length of a focusing lens on intensity distribution of diffraction light is investigated by numerical simulation. The results show that appropriate distance away from the focus spot and increase the focal length in final optical systems are beneficial to control the modulation of light intensity fluctuations and reduce the optical components damage caused by small-scale self-focusing effect.

This paper presents a new method for measuring vibration based on interference from twospherical waves. By integrating the two interference arms into a beamsplitter cube by reflectivefilm and dividing the probe beam into two parts with discrete apertures, the interferometer candistinguish that the vibrations are from the monitored optical components or from laser interferometer systemitself. At the same time, because the two interference waves are spherical, it can realize monitoringthe three-dimensional vibrations. The measurement system has advantages of being stable andreliable with an integrated structure. Theoretical analysis and experimental demonstration are performed.The experiment results indicate that the method can monitor three-dimensional vibrationsaccurately.

By means of Collins diffraction integral, the propagation expression of an ultrashort laser pulse passing through a spatial filter is derived. The effects of the magnification of spatial filter on pulse broadening and distortion of pulse-front for the ultrashort optical pulse are analytically deduced and numerically simulated. It is found that pulse broadening and propagation time difference of a laser pulse getting through spatial filter is proportional to the magnification. As a conclusion, in an ultrashort pulse laser system with a large aperture, the effect of pulse broadening and distortion of pulse-front should be considered.

在原有鬼像焦斑空间分布研究的基础上，进一步考虑入射激光脉冲与透镜的鬼像之间发生的干涉叠加现象。参考神光Ⅱ系统，分别推导并计算了3~10 ns的入射脉冲引起最小像差型双凸透镜产生鬼像干涉的最大焦距范围，然后以单位矩形脉冲平面波作为入射光，讨论存在干涉的几种情况。结果表明，由于干涉叠加效应，鬼像焦平面存在较大的调制，焦平面附近的区域也有很明显的干涉调制。干涉后鬼像的功率密度大大地超出了原有鬼像光强的估算值，特别是对于高阶鬼像，问题更加严重。

In high power laser systems, the high-frequency noise generated by nonlinear effect is effectively removed by low-pass spatial filter, and the low-frequency noise passing through pinhole is always considered as very safe to optical devices in downstream. However, in practical applications both modulation contrast ratio and spatial frequency of the low-frequency noise will be changed and possibly become dangerous components, depending on different magnification ratios of spatial filter. In this paper, the evolution of low-frequency noise is theoretically analyzed and numerical simulated depending on different magnification ratios of spatial filter. The analysis results show that both modulation contrast ratio and spatial frequency of the low-frequency noise passing through pinhole will be changed 1/M times, where M is magnification ratio of spatial filter. For M<1, the safe low-frequency noise will be extruded into high-frequency which is the fastest growing components and finally develop into the most dangerous part to the damage of optical devices again. It is significant to consider the evolution of low-frequency noise in practical applications of spatial filter for high power laser system.

作为对高功率激光装置中鬼像的进一步深入研究，在原有鬼像研究的基础上，考虑单透镜鬼像对实际加工、安装中误差的敏感性。运用矩阵光学的方法，推导得到了鬼像点的近似焦距公式，以及由误差引起的鬼像点位置的增加量的一般性表达式。以神光Ⅱ升级装置中终端光学组件的聚焦透镜参数为例对公式作数值分析，结果表明，透镜曲率半径和中心厚度误差对鬼像的影响较小，部分情况下可以忽略；而中心误差对鬼像的影响很明显，在工程设计中应予以重视。

寄生反射是影响高功率激光近场质量的重要因素之一。以Bespalov-Talanov理论为基础, 重点分析了高功率系统中楔形光学元件的寄生反射引起的小尺度自聚焦效应, 综合考虑了各个参数对纹波增长的影响。将反射光束与主光束的干涉叠加场进行了仔细分析, 得出了干涉场在非线性增益介质中小尺度调制增益谱的表达式。针对小信号增益情形, 数值计算了在钕玻璃片状放大器中, 介质增益及传输距离改变的情况下小尺度调制的最大增益与放大器窗口楔角的变化规律。具体分析了放大器窗口的楔角与厚度对增益增长率的影响以及不同脉冲宽度的脉冲通过相同的窗口时, 增益系数随脉冲宽度的变化关系。

Bespalov-Talanov theory on small-scale self-focusing is extended to include medium loss for a divergent beam. Gain spectrum of small-scale perturbation is presented in integral form, and based on the derived equations we find that the cutoff spatial frequency for perturbation keeps a constant value. The larger the medium loss is, the smaller the fastest growing frequency and the maximum gain of perturbation with defined propagation distance are. For a given medium loss the maximum gain of perturbation becomes larger, while the fastest growing frequency becomes smaller as the propagation distance becomes longer. Furthermore, physical explanations for the appearance of these features are given.