高功率激光驱动器终端光学系统的光束时空特性研究
文献类型：学位论文
| 作者 | 焦兆阳 |
| 学位类别 | 博士 |
| 答辩日期 | 2014 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 朱健强 |
| 关键词 | 终端光学系统 光束传输 光束质量 时空特性 |
| 其他题名 | Research on spatial temporal characteristics of optical beam in final optics system of high power laser driver |
| 中文摘要 | 目前高功率激光驱动器的发展主要受限于系统的负载能力不足，尤其是终端光学系统的负载能力。提升终端负载能力的关键在于提高系统的光束质量，因此有必要研究光束的传输演变以寻找其中的规律来控制光束质量。与此同时，聚变物理实验中面临的另一大难题是对激光与等离子体相互作用(LPI)的理解与控制，LPI严重影响了激光束与靶相互耦合的效率。而靶腔壁上的光场的时空分布是LPI复杂过程的初始条件，为了更好的理解和研究LPI，有必要先研究清楚靶腔壁上光场的时空特性。 本论文围绕高功率激光在终端光学系统会聚过程中的传输演变和焦后的靶腔壁上光场的时空特性展开系统的分析和研究，主要完成了以下几个方面的工作： 1.理论推导并分析了光束在会聚系统和自由空间传输的等价性，结果表明透镜后一定距离处的光场分布与自由空间传输对应等效距离处的分布相似，只是需要按照特定的比例缩放光束尺寸和光场强度，并附加特定的相位因子。这使得自由空间传输中的一些规律和结果可用于分析终端光学系统会聚系统中的情况。并进一步利用等效菲涅尔数的概念对焦前、焦后光场的等价性进行了分析，发现焦后零到无穷远的光场分布只能映射到焦前f/2到f距离内的光场分布。 2.研究了调制光束的在终端光学系统的传输演变规律。首先针对终端光学系统中常见的几种畸变源，建立了各自的物理模型，并对三种畸变光场在终端光学系统中的演化进行模拟计算，分析了它们对近场和远场的影响。其次研究了非理想波前对灰尘、污点等引起调制的影响，发现引入一定程度的正弦位相调制可以降低单个位相扰动物体对后续一定距离内的摆放光学元件处的光场引起的调制。最后研究了非线性传输对光束质量的影响，结果表明，非线性传输不仅增加了近场的调制，而且导致远场的主瓣峰值光强降低、旁瓣增大。 3.分析了终端靶场的结构设计与参数选取的受限因素。首先研究了具有刀纹扰动的光束近场调制随近场位置的变化规律，发现近场调制在特定的位置存在一个极小值，可以选择把光学元件放置在这个位置上，以降低光学元件受到激光损伤的概率。其次研究了焦距的变化对近场光束质量的影响。结果发现，焦后固定位置处的近场调制度随着焦距的增加而减小，近场调制度同时还随着输入光强的增加而增加。为了降低光场调制度，可以增加系统透镜的焦距。对于极限负载系统的设计来说，一个特定的输入光强对应着一个特定的系统极限焦距。最后分析了系统焦距的选取对准远场焦斑特性的影响。研究表明，在满足空间限制和成本允许的情况下，焦距越大越好。 4.研究了靶腔壁上光场的时空特性。首先分析得出在平顶脉冲的上升沿期间靶腔壁光场分布是一个复杂的动态时间演化过程；其次研究发现时空效应对靶腔壁上的台阶脉冲带来畸变，主要表现为对台阶脉冲上升沿的拉伸，并给出了激光入射角度、透镜焦距和输入脉冲的上升沿长度对相对变形度的影响。最后研究了光场存在调制时的脉冲畸变效应。结果表明，空间调制会对时间域上的波形引入额外的畸变。 |
| 英文摘要 | Now the high power laser driver is mainly limited by the load capability of the whole system, especially the final optics system. The key to improve the load capability is to improve the beam quality of the system. So it is necessary to study the propagation characteristics of the beam and find the rules to control the beam quality. Meanwhile, another challenge in the Inertial Confinement Fusion experiment is the understanding and control of the laser plasma interaction process (LPI). The LPI seriously affects the laser beam and target coupling efficiency. The spatial and temporal optical field on the hohlraum wall is the initial condition of LPI. So it is necessary to study the spatial and temporal characteristics of the optical field to better for better understanding and research of the LPI. In this dissertation, propagation and evolution of the high power laser in the final optics system and spatial temporal characteristics of optical field on the hohlraum wall are investigated in the following aspects: 1. The equivalence of laser beam propagation in the focusing system and in the free space is theoretically deduced and analyzed. The results show that the optical field at a location after the focusing lens is similar with the optical filed at equivalent distance in the free space. The difference lies on the beam size and optical intensity and additional phase. Thus the results in the free space propagation can be used to analyze the situation in the focusing system. Furthermore, the equivalence of optical field before and after focal spot is investigated by the equivalence Fresnel number. Results show that the optical field at the location from zero to infinity after focal spot is mapped in to the optical field at the location from f/2 to f before focal spot. 2. The propagation of modulated beam in the final optics system is studied in detail. Firstly, physical models are established for several common distortion sources. And the evolution of three corresponding modulated beams is numerically studied. The influence of distortion on both near field and far field is analyzed. Secondly, the effect of non-ideal wavefront on near field beam quality disturbed by flaw in final target system is analyzed. We find that the introduction of a certain sinusoidal phase can reduce the modulation caused by single phase object at the subsequent position where optical element is placed. Thirdly, the influence of nonlinear propagation on beam quality is analyzed. Results show that, due to nonlinear effect, the near filed modulation is increased, the peak intensity of main lobe is decreased, and side lobes are expanded in far field. 3. The limiting factors of structure design and parameters selection in the final target system are investigated. Firstly, the modulation degree variation with near field position is numerically studied. Results demonstrate that there is a minimum of modulation degree at certain position. So we can put optical element in this specific position to avoid the laser induced damage. Secondly, the influence of focal length on near filed beam quality is studied. We find that near field modulation degree decreases with the increase of focal length and increases with the increase of input optical intensity. In order to reduce the modulation, we can increase the focal length. Meanwhile, for the design of a system with ultimate load, a specific focal length should be chosen for a specific input intensity. Thirdly, the influence of focal length on quasi far field property is also studied. If space and cost is permitted, the longer focal length is better. 4. The spatial and temporal characteristic of optical field on hohlraum wall is investigated. Firstly, we discover that the evolution of optical field at the rising edge of a flat- topped pulse on the hohlraum wall is a dynamic spatio-temporal process. Secondly, we find that spatial temporal effect induces pulse distortion to the stepped pulse on the hohlraum wall. The rising edge is stretched. And the influence of incident angle, focal length and rising edge width on the relative distortion degree is also discussed. At last, the pulse distortion on the hohlraum wall of modulated beam is studied. Results show that modulation in the space domain can induce extra distortion to the time domain. |
| 语种 | 中文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15856]  |
| 专题 | 上海光学精密机械研究所_学位论文 |
| 推荐引用方式 GB/T 7714 | 焦兆阳. 高功率激光驱动器终端光学系统的光束时空特性研究[D]. 中国科学院上海光学精密机械研究所. 2014. |
入库方式： OAI收割
来源：上海光学精密机械研究所
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