“热像”是造成高功率固体激光装置光学元件损伤的重要物理机制。理论分析了相位缺陷经过非线性传输引起的“热像”特性,提出了带通成像提取引起“热像”的相位缺陷关键频谱方法,构建具有典型特征的16个相位缺陷,利用3种光路结构,量化分析了“热像”特性与带通成像特性间的关系,并获得了缺陷带通成像的相对亮度与“热像”强度的线性关系,相关程度与光学构型和带通滤波参数有关。研究结果对“热像”预判有重要意义,为高功率激光装置相位缺陷的检测奠定了技术基础。

The high fluence performance of high-power laser systems is set by optical damage, especially in the final optics assembly (FOA). The flaws on the frequency converter surface can cause optical intensity intensification and, therefore, damage the downstream optical elements, such as the beam sampling grating (BSG), which is an important component in the FOA. Mitigation of BSG damage caused by flaws is discussed. Physical models are established to simulate the optical field enhancement on BSG modulated by the upstream flaw, considering both the linear and nonlinear propagation effects. Numerical calculations suggest that it is important to place the BSG in a properly selected position to mitigate the laser-induced damage. Furthermore, strict controls of flaw size, modulation depth, distance between frequency converter and focusing lens, and the thickness of the focusing lens are also significant to mitigate the BSG damage. The results obtained could also give some suggestions for damage mitigation of optical components and the layout design of the final optics assembly.

The evolution of hot-image formation in consideration of focusing lens effect is theoretically and numer-ically investigated. Through analysis of linear propagation theory with lens, it is shown that the locationof hot image in focusing systems can be predicated through definite functional relation to hot imageposition in non-focusing systems, on condition that the distance between the hot image and the lens isrelatively small. In addition through numerical calculation, specific variation characteristics of maximumintensity with the propagation distance representing hot-image formation process are presented, it isobtained that perturbing scatter size directly affects the location and corresponding peak intensity ofthe hot image. The lens influences the value of scatter size where minimum distance between hot-imageplane and focus lens is generated. Finally, the effects of phase and amplitude modulation of scatter on hotimage are also given. Results in this paper are of fair referenced value for controlling perturbing scatterand restraining the damage risk of optical components in focusing systems.

The evolution of hot image formation through medium with gain in consideration of the effect of spatialfilter is theoretically and numerically investigated. Based on the linear diffraction theory and small-scale self-focusing theory of Bespalov and Talanov, intensity distribution of hot image in conjugate planeis derived analytically. Then, the peak intensity of hot image for different medium gain and differentpinhole sizes are discussed in detail, the results show theoretical analysis is mostly approximate to thenumerical simulations, furthermore, it is found that suppressing effect on peak intensity ratio with smallgain coefficient is larger than that with bigger ones for determined pinhole size.

通过理论分析与数值模拟方法,研究了高功率激光系统中非线性热像位置及光强随介质厚度的变化规律。基于角谱传输和Bespalov-Talanov小尺度自聚焦理论,在不作薄介质近似情况下得到了热像光强表达式,并用数值模拟结果进行了验证。结果表明,在一定的入射功率下,热像光强随介质厚度的增加而增大; 而在一定的B积分下,热像光强随介质厚度的增加先单调增加,达到一极大值后,单调减小。此外,当模糊斑相对于克尔介质的距离的菲涅耳数远大于1时,可采用薄介质近似。数值模拟结果与解析结果吻合得很好。

高功率激光系统中非线性热像的强度可能达到光学元件的损伤阈值,从而危及系统的安全运行。明确并避免引起最强热像大小的模糊斑,可以降低热像的破坏威胁。通过理论分析与数值模拟方法,研究了高功率激光系统中非线性热像光强与模糊斑大小的关系。根据角谱传输和Bespalov-Talanov小尺度自聚焦理论,推导出一定厚度非线性介质的热像光强表达式,得到了热像光强随模糊斑大小的变化规律,并用数值模拟结果进行了验证。结果表明,随着模糊斑尺寸的增大,热像光强先是单调增大,达到一最大值后单调下降;热像光强最大时对应的模糊斑大小基本上与由Bespalov-Talanov小尺度自聚焦理论决定的最快增长小尺度调制尺寸相同。

“热像”效应是制约高功率激光系统,特别是惯性约束核聚变(ICF)激光驱动器安全运行和提高输出功率的瓶颈因素之一。在某些条件下,往往系统运行在典型的损伤阈值功率之下时,某些光学元件也会因为“热像”效应的存在而意外地遭受损伤。总结介绍了近年来关于“热像”效应研究的新进展,包括“热像”形成的衍射理论模型、薄片近似下的解析理论分析结果、厚介质及级联介质情况下激光“热像”的演化规律、多散射点形成“热像”的特点以及“热像”效应的抑制方法等。最后指出理论及实验上尚待进一步解决的问题。

对高功率激光装置中存在的一类非线性自聚焦现象--"热像"效应进行了数值模拟和实验研究.从菲涅耳衍射积分与非线性近轴波动方程出发,采用线性传输和分布傅里叶算法,编制了光传输程序,研究了透明衍射物产生"热像"的形成位置,能流分布及通量随B积分变化关系;并在星光Ⅱ装置上完成了相关实验,与计算结果进行比对,两者吻合较好.

对高功率激光系统中存在的一种类似于小尺度自聚焦的非线性过程--"热像"效应进行了理论分析和模拟计算.当一束经过遮光物调制的激光光束通过非线性介质后,在随后的光路上将产生一个亮点,即"热像"."热像"处的光强比背景光强大几倍,因此位于"热像"附近位置的光学元件很可能遭受损伤.从全息成像的角度对"热像"的形成过程进行了分析.结果表明:"热像"的位置处于以非线性介质为中心与遮光物大约对称的地方,"热像"的光强是原始光强的(1+B)²倍.计算机模拟与理论计算结果吻合较好,验证了上述结果.