Wang Hongchang, Jiao Zhaoyang, Zhang Yanli, Sun Mingying, Zhu Jianqiang
Opt Express. 2020 Aug 17;28(17):25591-25605. doi: 10.1364/OE.399044.
Nonlinear hot image is one of the key elements that limit the output performance of high-power laser systems. In most hot-image researches, only one hot image peak is observed in the conjugate position for a single defect. Generally, multiple hot image peaks occur for multiple defects or cascaded nonlinear media. However, a new phenomenon is found by numerical simulation in our work: one defect can also afford two hot-image peaks near the conjugate position when considering the defect edge steepness. The super-Gaussian defect model is employed to mimic the defect edge steepness. When the super-Gaussian order is higher than one, there could be two hot image peaks under certain conditions. The formation of the double hot image peaks is primarily due to the co-effect of the hard-edge diffraction and the self-focusing effect. The influence of different factors, including the super-Gaussian order, defect size, modulation depth, and Kerr medium thickness, on the double hot image peaks intensity and location is systematically investigated. The results show that with the increase in the super-Gaussian order, the intensity of the double hot image peaks increases gradually. The defect size has a great influence on the position of the two hot image peaks. The modulation depth and thickness of the Kerr medium influence the intensity of the two hot image peaks; however, they have less impact on the peak location. Importantly, the defect edge steepness and size dependences of multiple nonlinear hot-image formation from a single-phase defect are further discussed in this paper. The two hot image peaks are fatal to optical components in high-power laser systems; in particular, the hot image peak behind the conjugate position is totally unexpected for a single defect. This research provides insights into basic physical images and hot-image formation laws. It also provides important guidance for optical defect specification evaluation and optical component layout design, as well as for beam quality control, in high-power laser systems.
非线性热像是限制高功率激光系统输出性能的关键因素之一。在大多数热像研究中，对于单个缺陷，在共轭位置仅观察到一个热像峰值。一般来说，对于多个缺陷或级联非线性介质会出现多个热像峰值。然而，我们的工作通过数值模拟发现了一种新现象：当考虑缺陷边缘陡度时，单个缺陷在共轭位置附近也能产生两个热像峰值。采用超高斯缺陷模型来模拟缺陷边缘陡度。当超高斯阶数大于1时，在一定条件下可能会出现两个热像峰值。双热像峰值的形成主要是硬边衍射和自聚焦效应共同作用的结果。系统地研究了包括超高斯阶数、缺陷尺寸、调制深度和克尔介质厚度等不同因素对双热像峰值强度和位置的影响。结果表明，随着超高斯阶数的增加，双热像峰值的强度逐渐增大。缺陷尺寸对两个热像峰值的位置有很大影响。克尔介质的调制深度和厚度影响两个热像峰值的强度；然而，它们对峰值位置的影响较小。重要的是，本文进一步讨论了单相缺陷形成多个非线性热像时缺陷边缘陡度和尺寸的依赖性。这两个热像峰值对高功率激光系统中的光学元件是致命的；特别是对于单个缺陷，共轭位置后方的热像峰值是完全出乎意料的。这项研究为基本物理图像和热像形成规律提供了见解。它还为高功率激光系统中的光学缺陷规范评估、光学元件布局设计以及光束质量控制提供了重要指导。
