| 题名 | 二元振幅型面板用于激光空间整形的研制 |
| 作者 | 谢杰 |
| 学位类别 | 硕士 |
| 答辩日期 | 2008 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 范薇 |
| 关键词 | 惯性约束聚变 光束质量 空间整形 填充因子 |
| 其他题名 | The Research of Binary Panels in Spatial Laser Beam Shaping System |
| 中文摘要 | 惯性约束聚变（Inertial Confinement Fusion－ICF）是当今国际上的重大科研领域，有着极其重要的科学意义和应用前景。高功率激光驱动器是探索实现聚变点火的重要技术手段和途径。ICF实验研究对激光光束质量有着十分苛刻和特殊的要求，不仅要求在靶面上有极高的光强均匀性，即远场均匀性，而且在传输过程中也要有好的光束质量，也就是近场均匀性的要求。 在高功率激光传输中，由非线性折射率引起的小尺度自聚焦效应是一重要现象，当非线性折射率系数为正值时，高斯光束中心强度大于边缘强度，从而中心位相落后于边缘位相，产生汇聚，引起自聚焦，使得输出光束质量变坏。而当光束强度分布不均匀时，三阶非线性Kerr效应引起的局部光强急剧增强，导致光束发生分裂成丝等现象，损伤光学元件，因此实际中要求高功率激光束的近场强度分布尽可能均匀。 在高功率激光驱动器的主放大器中，采用的是片状大口径钕玻璃作为增益介质。长的增益距离会带来严重的自发辐射放大（ASE）。片状钕玻璃横向的传输距离较长，边缘自发辐射效应明显，消耗了部分反转粒子数，造成中心部位的增益高，边缘部位的增益低；而在纵向由于激光的传输距离相对较短，增益分布基本一致。ASE效应不仅会降低激光介质的储能效率，使钕玻璃放大器在横向上和纵向上具有不同的增益分布，对增益均匀性产生严重影响。 不言而喻，激光光束质量不仅大大影响激光系统的整体性能，也大大影响激光技术的应用水平，因此为了提高光束质量，需要在空间域对高斯光束进行整形，有效地控制改变光场分布。在钕玻璃放大器ASE效应不明显的情况下，把高斯光束整形成为强度分布尽可能均匀的平顶光束，提高光束的填充因子；而在ASE效应明显的情况下，必须在空间上进行一维的增益预补偿，使放大器的高增益区对应于输入光束的低强度区，最后在高功率激光驱动器的终端近场得到空间强度分布均匀的激光光束。 到目前为止，已经发展了多种激光光束空间整形技术。从目前的发展看，用于ICF高功率激光系统的空间整形技术主要有：可抑制菲涅尔衍射的锯齿软边光阑技术、双折射透镜组空间整形技术、振幅型二元光学面板技术以及液晶空间光调制器技术。 结合目前神光II的工程需求，我们拟采用美国劳伦兹•利弗莫尔国家实验室（LLNL）在国家点火装置（NIF）中使用振幅型二元光学面板技术，将高功率激光驱动器预放系统输出的光束按照具体需求进行精确整形。本论文的主要目的是探讨振幅型二元面板应用于神光II高功率激光系统的可行性，为神光II高功率激光装置提供一种可用的光束整形技术路径。全文共分五个部分： 论文的第一章概述了ICF高功率激光系统对传输光束质量的要求，总结了目前国内外存在的可用于高功率激光系统的光束空间整形技术，并分析了各自的优缺点。确立了用振幅型二元面板空间整形系统作为神光II高功率激光系统的空间整形方案。 第二章阐述了振幅型二元面板的设计原理，分析对比了误差扩散法和随机抖动法的优缺点。基于误差扩散法具有更好的远场特性，根据工程需求，用该算法设计了三种整形面板，分别是把高斯光束整形成为平顶光束的“反高斯”面板、把平顶光束整形成为光强分布为抛物线函数的、中心边缘光强比为1：5的“抛物线”面板、和直接把高斯光束整形成为光强分布为抛物线函数的、中心边缘光强比为1：5的综合面板。在Matlab环境下数值模拟了整形效果，从理论整形效果来看，高斯光束经过“反高斯”面板整形后填充因子提高到98.9%，光场调制度降为0.4%；平顶光束经过“抛物线”面板整形后，中心边缘光强度比变为1：4.97；高斯光束经过综合面板整形后，中心边缘光强比变为1：4.98。三块面板的透过率函数与目标函数的相对误差在各个位置均小于5%。 第三章给出了“反高斯”面板和“抛物线”面板的实验结果。在实验中，“反高斯”面板将经过软边光阑的入射高斯光束的填充因子提高到85%，“抛物线”面板进一步将光束整形成光强分布为抛物线函数，中心边缘光强比变为1：5.2。针对实验和理论的整形结果的偏差，分析了可能引起误差的各种因素，包括面板设计阶段像素线宽的选取，面板加工阶段玻璃基底的实际透过率与1/0的偏差和光刻中的过量刻蚀引起的像素线宽的变化，实验过程中入射光束的口径变化以及空间滤波小孔的直径取值等。在这些因素中，玻璃基底的实际透过率与1/0的偏差是引起误差的主要来源，通过在面板设计时对其进行预补偿，可以很好的消除该项误差。 第四章阐述了利用误差扩散法设计软边光阑的原理，并进行了理论模拟。证实了软边光阑也可以用误差扩散法进行设计。 第五章主要总结了论文工作。本文从理论和实验上利用二元光学元件进行激光光束的空间整形的研制，并且获得了激光光束的空间强度的精确整形，该工作的进行对提高神光II系统的高通量运行提供了一个重要的技术手段，有明显的应用价值。 |
| 英文摘要 | Inertial Confinement Fusion (ICF) is an important scientific research field today, which is of an extremely importance for science and bears a widely applied foreground. High power laser driver is a significant technique and method to achieve the fusion ignition. ICF experiment has a high and special demand for beam quality. The uniform intensity distribution is needed not only at far-field but also at near field. In high power laser system, small-scale self-focusing effect induced by nonlinear refractive index is an important phenomenon. If the coefficient of nonlinear refractive index is positive, the center intensity of Gaussian beam is higher than that of marginal proportion, the phase at center drops behind the marginal phase, and then the self-focusing appears, which will destroy the beam quality. When the light intensity distribution is not uniform, the beam is splited into silks by the 3rd nonlinear Kerr effect, and then damages the optical elements. So the uniform beam intensity distribution is necessary to practical high power laser system. On the other hand, Nd:glass medium is mainly used in high-power laser system, of which the gain is spatially dependant in transverse direction where the amplification distance is relatively longer, under which circumstance the amplified spontaneous emission phenomenon cannot be neglected. Apparently, the beam quality affects not only the whole performance of laser system, but also the application level of laser technology. So, in order to improve the beam quality, it is necessary to shape Gaussian beam and control the intensity distribution effectively in spatial field. When the amplified spontaneous emission of Nd:glass amplifier is obvious, the spatially dependent gain of amplifiers should be precompensated, i.e., the spatial intensity of the laser beam is shaped before amplification so that regions of higher gain in the amplifier correspond to regions of lower intensities in the input beam. A large variety of techniques have been demonstrated to generate a spatially varying transmission, such as photographic plates, mirrors with variable reflectivity, elements with spatially varying birefringence, and elements with spatially varying transmission based on total internal reflection. Because more control can be obtained via binary pixelated arrays of metal pixels, as used on the National Ignition Facility to precompensate for the spatial gain variation of large glass amplifiers, we intend to adopt the binary mask method to spatially shape the Gaussian laser beam in “ShenGuang II” laser driver system. In the first part of the paper, the demand of beam quality in ICF high power laser driver system are expounded, then the main spatial beam shaping technologies used in high power laser system are generalized including the advantages and disadvantages. In chapter 2, the principle of the design of binary masks is introduced in detail. The merits and drawbacks of error diffusion algorithm and random dithering algorithm are analyzed. Three kinds of binary masks, which separately aims to convert Gaussian laser beam into uniform beam(named the Anti-Gaussian mask), to convert uniform beam to parabolic-like beam(named the parabolic mask) and to convert Gaussian beam to parabolic-like beam(named the composite mask) are designed and emulated under Matlab platform. The shaping effects are numerically simulated, and as a result, the fill factor of the incident beam increases to 98.9% and the intensity modulation falls down to 0.4% after the Anti-Gaussian mask, the central to edge intensity ratio of the incident beam converts to 1:4.97 and 1:4.98 after the parabolic mask and the composite mask, of which the relative errors are less than 5% under the whole shaping range. In chapter 3, experimental results are analyzed and the factors, which may influence the shaping effect have been discussed. Fill factor of the incident Gaussian laser beam(after the serrated aperture) increases from 73% to 85% after the Anti-Gaussian mask, and the central to edge intensity ratio of the incident beam converts to 1:5 after the parabolic mask. Amongst all the factors which may cause errors, the transmission bias of the substrate material of the mask to 1/0 is the main source to experimental-theoretical error. Through precompensation in design stage of the mask, we can reduce this kind of error to large extent. In chapter 4, the application of error diffusion method to the design of serrated aperture is represented. Shaping effects are also simulated theoretically. At last, the thesis is summarized briefly. In the thesis, binary optical masks, which aim to shape the intensity of the laser beam, are analyzed theoretically and experimentally. Accurate shaping results are obtained. This work provides a necessary technical method for the operation of the Shenguang II system under high-flux condition, which is of great utility value. |
| 语种 | 中文 |
| 内容类型 | 学位论文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/16394]  |
| 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 | 谢杰. 二元振幅型面板用于激光空间整形的研制[D]. 中国科学院上海光学精密机械研究所. 2008. |
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