高功率激光束中高频位相畸变特性的研究

【作者】 刘红婕；

【导师】 景峰；

【作者基本信息】 中国工程物理研究院 ， 光学工程， 2005， 硕士

【摘要】 高功率激光束焦斑特性是影响聚变物理实验的关键因素之一。焦斑特性主要取决于聚焦前的光束位相分布,其中低频位相畸变主要决定焦斑主瓣,中高频位相畸变主要决定焦斑旁瓣。有关低频位相畸变的研究很多,通过低频位相畸变的研究我们掌握了它的传输规律、与焦斑主瓣的关系,还通过限制光学元件低频位相噪声以及利用变形镜对它采取了主动控制措施。对中高频位相畸变来说,由于其传输性质复杂,还没有掌握它的传输规律,以及它与焦斑旁瓣的关系,更无法对它采取控制措施。 高功率激光束中高频位相畸变主要来源于装置中成百上千个光学元件,如果不加以控制,经过叠加和非线性增长在打靶聚焦时会引起高强度的焦斑旁瓣,高强度的旁瓣打在靶洞边沿会形成等离子体堵孔而导致实验失败。同时高通量运行时中高频位相畸变还有引起自聚焦导致光学元件破坏的风险。因此通过研究高功率激光束中高频位相畸变的传输性质及其对焦斑旁瓣的影响,找到有效控制中高频位相畸变的措施,对实现ICF驱动器打靶是非常有意义的。本文研究的主要内容和主要进步点有以下几个方面: (1) 基于B-T理论,通过高功率激光束位相畸变的空间频率非线性增长特性的研究,以及装置打靶对焦斑的物理要求,建立了高中低频位相畸变的划分方法。并根据这种划分方法,划分了神光Ⅲ原型装置高中低频位相畸变的范围。 (2) 由于实际装置中中高频位相畸变的传输物理过程比较复杂,首先将问题简单化处理,建立了一个简单的局部中高频位相畸变的模型,研究了局部位相畸变对近场调制的影响规律。然后根据神光Ⅲ原型装置的实际情况数值模拟了由光学元件引入的位相畸变对光束近场的影响,并通过分析级间B积分和空间滤波器小孔对近场均匀性的作用,讨论了控制光束近场均匀性的主要措施。 (3) 基于波前畸变PSD的定义和光束近远场之间的傅立叶变换关系,通过对正弦函数波前的解析分析,得到了在中高频位相畸变的幅度满足Φ<<2时,中高频位相畸变PSD与焦斑旁瓣具有非常好的近似关系,并且这个关系只取决于中高频位相畸变的幅度Φ,与空间频率没有关系。还用数值的方法验证了这个结论是正确的。 (4) 基于B-T理论研究了各种空间频率成分位相噪声在频谱面上的非线性增长特性,并研究了表征位相畸变各种空间频率成分分布的PSD的线性传输和非线性传输规律。在满足B-T理论成立的条件时,得到了中高频位相畸变传输前后的PSD满足的解析关系式。还从波前PSD的定义出发,用解析的方法分析了中高频位相畸变PSD的叠加规律,初步研究了用部分相干叠加方法、并基于PSD的光学元件技术指标分解技术。更多还原

【Abstract】 It is well known through many finished physics experiments that the characteristic of focal spot is one of the most crucial factors which determine whether ignition can realize or not. Properties of focal spot are mainly determined by the quality of the beam before focused. In brief, the main of focal spot lies on the low frequency components of wavefront while the tail of it lies on the middle-high frequency components of wavefront. Many researches about the low frequency components of wavefront have come to very useful conclusions, however, for the middle-high components , relatively less work has been done and no useful results have been derived about its nonlinear propagation or its relation with the tail of the focal spot because of its complex transmission. The middle-high frequency components of wavefront in high-power laser system mainly come from hundreds of optics because even extremely small fluctuations on the surface of optics may become negligible by hundreds of addition and nonlinear transmission. The unwanted results from it include pinhole closure and nonlinear small-scale self-focusing which originate from high-intensity tail of the focal spot and nonlinear transmission of the middle-high frequency under high-intensity. So it is very useful to study the transmission of the middle-high frequency and its impact on the tail of the focal spot. The contents are mainly divided into five sections as follows:(1) Dividing the spatial frequency of the phase errors into several districts and making sure the district to study. First, the B-T theory, its assumptions and analysis of phase error’s propagation are introduced. Then, basing on the nonlinear propagation of different spatial frequency and the requirements of ICF for the main focal spot, we divide the spatial frequency components of phase errors into several districts for the SG-Ⅲ system and make sure the emphasis to study.(2) Studying the transmission properties of the middle-high frequency components of phase error on the basis of B-T theory. We first study the transmission properties of a local phase error and draw a conclusion that none of the phase errors undergo apparent nonlinear increase except for those whose main spatial frequency components are in the high-gain district. Then, using numerical simulation, we study the impacts of phase error on the near field of the beam in practice and analyze the relation between the A B and the near-field contrast. Finally, we analyze the impacts of pinholes and different resources of phase error on the near field modulation.(3) Impacts of middle-high frequency components of phase error on the tail of focalspot. First, we discuss the relation between the tail of the focal spot and PSD, which describes themiddle-high frequency phase error, by both analytical and numerical ways. The results show that not spatial frequency but the amplitude of the phase error determines whether the PSD describes the tail of the focal spot completely or not and that the PSD can describe the tail of the focal spot only if the amplitude is small enough.(4) We discuss the regularities of linear and nonlinear transmission of PSD, and using B-T theory, we acquire the analytical expression of PSD after and before the transmission of the phase error.(5) Considering the method by which the low-frequency wavefront RMS is added, we believe that it is useful to add the PSD by the method of partly coherent addition. And basing on the above analysis, we put forward the basic way of decomposing the specifications of optics for the middle-high frequency phase errors.更多还原