高功率激光光学材料中的光-声相互作用研究

With the developments of the optical materials producing and processing technological, impurity and defects of the optical material is well controlled recently. With powerful laser beams, stimulated Brillouin scattering (SBS) will be excited by electrostriction of the bulk medium and no linear absorption occurs, which becomes more obvious in the regions of high fields. By increasing the local pressure, an excited acoustic wave on which a Stokes wave scatters in the backward direction can damages the front surface of the sample at high laser powers. This may limit the output power of the systems and affect the operation safety negatively. This kind of phenomenon is especially serious in the ICF (inertial confinement fusion) laser devices, and the current theoretical model and numerical simulation method is not accurate. Further experimental study on the relevant theory and the suppression technology are necessary. This project aims to improve the damage threshold of the large-diameter fused silica components. The theoretical models for full space-time (3D+1) transverse stimulated Brillouin scattering (TSBS), as well as the interaction between the SBS and the small-scale self-focusing, will be established. The corresponding physics processes will be simulated numerically. Based on the space-time experiment for the optics, mechanics (acoustics) and damage characteristics, and the online experiment on the SGⅢ Prototype, the mechanism of that the SBS causes damage in laser components will be analyzed, in order to develop methods and techniques to suppress SBS. This study is helpful to the development of new laser materials, and the design of the high power laser systems. Moreover, the result will be of great meaning for super-high power lasers.

随着光学材料生产和元件加工技术的不断改进，光学元件中的杂质和缺陷逐步得到控制，这提高了高功率激光系统的输出能力。但是，随着激光功率增加，由电致伸缩效应引起的受激布里渊散射逐渐增强，严重制约系统输出功率的进一步提升和运行安全。这种现象在ICF驱动激光装置中尤其明显，其理论模型和数值模拟方法并不完善，对其理论的实验验证和有效的抑制方案还研究甚少。本项目围绕提高大口径熔石英元件损伤阈值这一目标，拟建立三维含时横向受激布里渊散射的理论模型，及受激布里渊散射与小尺度自聚焦相互作用的理论模型，并数值模拟其物理过程；结合对该过程的光学、力（声）学和损伤特性的时空分辨实验研究，以及在神光Ⅲ原型装置上开展在线原位实验研究，分析受激布里渊散射对元件破坏的物理机制，寻找抑制受激布里渊散射的方法和措施。本项目的研究可作为新型激光材料研制、高功率激光系统设计的技术支撑，对发展超高功率激光驱动器有重要意义。

本项目以研究高功率激光辐照下光学材料中典型的光声相互作用-受激布里渊散射为目标，主要研究了横向和纵向受激布里渊散射的理论模型并进行了数值模拟，研究了受激布里渊散射与自聚焦相互作用的理论模型并进行了数值模拟，实验研究了典型光学材料中发生受激布里渊散射时，入射光、散射光和声波的时空分布规律，研究了发生受激布里渊散射时材料的损伤特性，分析了受激布里渊散射对激光诱导材料损伤的影响规律，研究了受激布里渊散射的抑制方式和措施。. 利用麦克斯韦电磁方程、光弹效应和电致伸缩效应，分别建立了存在弹光效应时介质内的电磁场方程和存在电致伸缩效应下感应声波的密度波动方程，联立两方程得到代表受激布里渊散射过程的光场-声场耦合方程组。根据横向和纵向受激布里渊散射过程的几何关系和波矢关系，分别得到各自的耦合波方程组；根据克尔效应和非线性Schrödinger方程，结合受激布里渊散射耦合波方程，建立纵向受激布里渊散射与自聚焦相互作用的耦合波方程组。在一些合理且必要的近似下，结合种子光起振模式，数值模拟了激光入射功率、种子光功率、脉冲宽度、激光波长、材料长度等参数对入射光、散射光和声波的时空分布的规律，获得了材料内光强和应力的时间和空间分布。. 研究结果表明，由于脉宽压缩效应、散射光与入射光在材料内的累加效应，会导致材料入射面和材料体内的总光强远大于入射光强的情况，从而导致元件击穿和体内成丝等光致诱导损伤；同时感应声波引起的应力可能超过材料的抗拉强度，导致材料存在力学损坏的可能性；通过对激光参数和材料参数的优化设计，尤其是增加入射激光带宽和采用多纵模激光，可以极大增加受激布里渊散射的发生阈值，从而减小、甚至阻止受激布里渊散射效应发生。