双折射透镜组空间整形系统的研制
文献类型：学位论文
| 作者 | 杨向通 |
| 学位类别 | 硕士 |
| 答辩日期 | 2007 |
| 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 波前 空间整形 填充因子 |
| 其他题名 | The Research and Fabrication of Birefringent Lenses Spatial 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，也是限制高功率固体激光驱动器输出能量和系统效率的关键因素之一。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。 作为评价自聚焦程度大小的重要参量B积分，是描述介质非线性折射率横向分布变化引起的光束位相弯曲程度，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。 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，使光束近场强度分布尽可能均匀，从而达到控制B积分值和提高光束填充因子的目的，进而提高放大器介质的孔径利用率，保证尽可能高的能量提取效率。 到目前为止，已经发展了多种激光光束空间整形技术，但是用于高功率激光系统的空间强度整形系统除了具有整形功能外，还要考虑它的引入对激光光束的波前、波面和光束指向的影响，同时所用的光学元件要有较高的破坏阈值。从目前的发展来看，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第九路升级系统的工程需求，我们拟采用美国劳伦兹•利弗莫尔国家实验室（LLNL）在其Beamlet前端使用的一种新型空间光束整形系统—双折射透镜组，将高功率激光驱动器预放系统输出的高斯光束整形为平顶光束。本论文的主要目的是探讨双折射透镜组整形系统应用于神光II九路的可行性，为以后的神光II精密化升级提供一种可用的光束整形技术路径。全文共分五个部分: 论文的第一部分概述了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。 第二章则重点分析了双折射透镜组的工作原理，讨论了双折射透镜组系统的整形效果，以及入射光束的特性对整形效果的影响。并在理论模拟的基础上提出了系统的工程设计参数。 第三章分析了双折射透镜组的设计参数和加工误差对空间整形效果以及对整形光束的波前、波面、光束指向的影响。并根据理论分析，确定了双折射透镜的加工参数和机械设计及装校要求。 第四章则在上述理论分析的基础上，设计制作了一套适用于神光II第九路的双折射透镜组空间整形系统，并已试用于神光Ⅱ第九路中,在近场静态工作条件下,可明显提高激光光束的强度均匀化输出。从测试结果来看，可将激光光束的填充因子从66％提高到80％，能量利用率为30％。对激光光束的指向影响为10urad，双折射透镜组整形系统的焦距为476.8m,波前畸变为0.08 。从实验结果来看，所制作的双折射透镜组空间整形系统不但可以实现将空间高斯分布的光束整形为平顶均匀光束，提高激光光束的填充因子，而且对激光光束的波前、波面以及光束指向都有较小的影响，满足神光II第九路高功率激光系统对空间光束整形系统的要求。 第五章主要总结了论文工作。 |
| 英文摘要 | ABSTRACT 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 implement 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 near field. In high power laser system, small-scale self-focusing effect induced by nonlinear refractive index is an important phenomenon, and it is also one of key factors of limiting the output energy and efficiency of high power solid laser driver. 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. B-integral, as a basic parameter to evaluate the degree of small-scale self-focusing, is used to describe the phase bending produced by transverse distribution change of medium’s nonlinear refractive index. 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. 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 uniform intensity distribution is achieved, the B-integral can be limited and the filling factor can be increased, thus, the extracted energy is improved. Now, there has been developed many spatial beam shaping technologies. In high power laser driver systems, the shaping system should have the function to control the intensity distribution while has little influence on the wavefront and the beam deviation angle. Also, the optical elements should have high damage threshold. So only few technologies can be used in ICF high power laser driver system, such as serrated aperture which was used to eliminate Fresnel diffraction, birefringent lenses, the amplitude binary MASK and spatial light modulator associating with liquid crystal. Considering the practical requirements of the Nineth beam of “ShenGuang II”, We adopt the new spatial beam shaping system- birefringent lenses system, which was used at front end of beamlet of Lawrence Livermore National Laboratory, U.S.A. The birefringent lenses can be used to convert gauss distribution output beam of preamplifier in laser driver into the uniform beam. The purpose of this article is to research the feasibility of applying this beam shaping system into the Nineth beam of “ShenGuang II”, and provide a practical spatial beam shaping technology for the next precision upgrade engineering. There are five parts in this thesis. In chapter 1, 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 birefringent lenses is analyzed. The beam shaping effects are simulated numerically. Then, the characteristics of incident beam, which will affect the shaping effect, are also discussed. Based on the numerical simulations, the engineering parameters of birefingent lenses are confirmed. The designing parameters and fabrication errors of birefringent lenses, which will affect the shaping results and transmitted wavefronts as well as the beam deviation angle, are discussed in detail in Chapter 3. By theoretical analyse, the fabrication parameters of lenses, as well as the demands of mechanical design and calibration, are confirmed. In Chapter 4, base on theoretical analysis, the birefringent lenses spatial beam shaping system is designed and fabricated for the ninth beamline of ‘SG II’ device. When used in the ninth beamline of ‘SG II’ device, the output uniform beam is achieved evidently. The static beam filling factor of near field can be improved from 66% to 80%. The energy utilization is 30% and the beam deviation angle is 10 μrad. The focal length of system is 476.8 meter, as well as the wavefront aberration is 0.08 wavelength. From the results of experiment, the birefringent lenses spatial beam shaping system can not only convert the Gaussian beam into flat-top beam which improve the filling factor but also have small influences on wavefront and beam deviation angle. So, it meets the demands of spatial beam shaping system relative to the ninth beamline of ‘SG II’ device. At last, the thesis is summarized briefly. |
| 语种 | 中文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/16458]  |
| 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 | 杨向通. 双折射透镜组空间整形系统的研制[D]. 中国科学院上海光学精密机械研究所. 2007. |
入库方式： OAI收割
来源：上海光学精密机械研究所
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