【作者】 张巍；

【导师】 李永平；

【作者基本信息】 中国科学技术大学 ， 光学， 2007， 博士

【摘要】 本论文是围绕863-804项目，“用于ICF(惯性约束核聚变：Inertial Confinement Fusion)系统激光束匀滑的大尺寸衍射光学元件的研制”这一主题展开的工程应用型研究工作，在结合“神光”装置需求的基础上，对元件的设计、加工、测试和应用等方面进行了系统的论述，所取得主要成果如下：◆结合搜索优化思想的分步迭代设计方法基于线性变换的迭代方法具有简单、快捷、收敛速度快的特点，因而被广泛的采用。这类方法的关键是迭代过程中的“反馈”是否有效，其直接影响到算法的效率。分步迭代方法基于迭代的框架，在保证迭代方法快速高效的前提下，结合优化的思想对每步迭代反馈中的参数进行有目的的选择，加强对输出结果的控制，提高设计指标。不同设计方法的设计结果对比，以及使用该方法设计所得实际元件的性能指标都证明了该方法的优越性。◆空间频域优化设计方法基于搜索的优化设计方法对于焦面的光强分布控制能力很强，能有效提高焦面光强的均匀性。但是其缺点是搜索效率不高，近乎穷举式的搜索方式需要消耗大量的计算时间。如何设定搜索的路径是提高优化效率的关键。空间频域优化设计方法是在搜索优化的基础上，从空间频域的角度开辟一条新的途径。其意义不在于算法内容本身，而是在于对空间频谱进行优化这一思想的引入。该方法基于对焦面光强的空间频谱分析，在频域内选取优化目标和费用函数，通过控制焦斑中的低频调制成分，提高元件的实际应用性能。与已有空域优化方法的对比证明了该方法的有效性。◆现阶段工艺可行的多台阶型DOE的离子束套刻方案衍射光学元件的加工方法很多，在ICF驱动器均匀照明用DOE的研究过程中，针对DOE的设计和加工我们进行了多种方案的尝试。通过理论和实验论证，相比较于连续位相元件的离子束旋转刻蚀方案和随机位相元件的掩模刻蚀方案，多台阶型DOE的离子束套刻方案是在现有工艺条件下解决元件宽容度和工艺误差的矛盾，缩小设计和实测元件性能指标差异的有效途径。◆两组外径100mm多台阶DOE元件的设计、加工和测试根据不同的设计参数，采用多台阶型位相元件的离子束套刻方案，设计并制作了两组外径100mm(有效尺寸口径分别为80mm和70mm)的位相元件，分别实现了矩形和圆形焦斑的束匀滑。测试结果和数据分析结果显示了元件良好的应用性能，验证了设计方法和工艺可行性。通过对各类工艺误差的数值模拟分析，为设计方法和工艺过程的改进提供了依据。同时误差模拟结果也表明采用改进杂化算法设计的多台阶型DOE具有较强的工艺误差宽容度。◆大尺寸元件在ICF中的应用前景分析相对小尺寸元件试制的成功，标志着用于ICF驱动器束匀滑大尺寸DOE的研制工作已经进入了实用化的阶段。与实际装置使用的列阵透镜系统的性能对比表明大尺寸DOE已经具备了实用化的必要条件。综合大尺寸元件的特殊性和实际应用靶场条件的限制，通过元件的设计和对各类测试误差的模拟，本论文分析了实用化的难点，并提出了相应的改进方案。理论设计结果表明，通过使用分区域设计DOE的方法能够得到在一定照明口径下都保持较高匀滑性能的DOE，能从设计上提高DOE的照明口径宽容性和抗横向对准误差的能力。◆与SSD(谱色散平滑：Smoothing by Spectrum Dispersion)联用的DOE设计方案鉴于ICF系统对靶面光束均匀性的苛刻要求，仅仅依靠DOE单一元件来实现束匀滑是十分困难的。因此，SSD技术的应用受到了广泛的关注，并且初步的联用实验已经验证了DOE与SSD联用的效果。结合联用的SSD参数有针对性的改进DOE的设计方法，成为进一步提高DOE应用性能的有效途径。本论文中，我们从空间频频谱的角度出发，根据SSD对焦面光强频谱调制的特点，本着互补的原则，采用空间频域优化方法设计DOE，使其对焦面光强频谱中，SSD作用较弱区域内的空间调制加以控制。通过两者的共同作用实现更好的束匀滑效果。初步的DOE设计结果和元件的联用效果对比，证明了该方案的可行性。更多还原

【Abstract】 The research devoted to this thesis focuses on the design and applications of large aperture diffractive optical elements (DOE) that will be used to realize uniform illumination on focal plane for inertial confinement fusion (ICF) system. The works are under the support of 863-804 project. The primary achievements obtained are as the following:1. A general iterative Fourier transform algorithm is widely used for designing DOE, because of its brief structure and high rapidity of convergence. The key of this kind of method is the efficiency of feedback obtained from output beam that controls the phase distribution of input beam in each iterative loop. The step iterative optimization (SIO) method is presented, which inserts a quasi-optimum process to optimize the value of feedback coefficients in every loop. The design results and performance of practical elements obtained by using SIO method both prove that the new method is more effective than other previous ones.2. The optimization algorithm can modulate the output results to an ideal profile precisely. However, this method is time-consuming because that lager numbers of configurations should be tried to find a local or global result. The key to the problem of optimization efficiency is the selection of a good searching path along which we can approach the extremum rapidly. A spatial spectrum optimization method for the design of DOE is presented based on the analysis of spatial frequency spectra of intensity distribution on the focal plane. In this new method, the goal and Cost Functions are chosen in spatial spectrum domain to determine the searching path. Compared with existing spatial optimization methods, not only the uniformity of intensity distribution on the focal plane will be improved, but also the modulations, especially low frequency components, will be well controlled by using the new method. The numerical simulation result shows that the new method will help improve the practical performance of DOE. 3. In practice, continuous phase plate (CPP) and random phase plate (RPP) have been tried for high power laser beam shaping. CPP realized by using ion beam rotation etching technique has a relatively high design index, but need for high matching accuracy. Though RPP can easily fabricated by using ion beam mask etching, the low performance index of RPP limits its field of application. The practical performance of CPP and RPP are not satisfied. So a scheme of multi-step phase plate that can be realized with four masks’ etching facility is presented. Both theoretical analysis and experimental result show that this scheme is much better than that of CPP and RPP under the existing technological level.4. Two 100mm external diameter multi-step phase plates used for beam shaping were designed and manufactured. The test results show good performance of these two practical elements. Through the numerical simulation of etching tolerance, some schemes to improve design and processing method are presented.5. With the development of DOE technique, the practical performance of large aperture elements has been achieved or even exceeded the level of lens array. But some adverse factors, such as wave distortion and lateral misalignment of input laser beam, may affect the application of large aperture DOE. Based on the analysis of such factors, several improvement programs are given.6. Smoothing by spectral dispersion (SSD) is an efficient technique to reduce high frequency components in energy distribution. A DOE used with SSD technique will further improve the uniformity of focal spot. A new design method of DOE used with SSD is described. In this method, spatial spectrum optimization is used to design DOE that mainly control the low frequency components which the SSD is not able to smooth in main lobe of focal spot. Preliminary results show the method is effective.更多还原