高功率激光近场空间周期性调制补偿技术研究
文献类型：学位论文
| 作者 | 高雅茹 |
| 文献子类 | 博士 |
| 导师 | 刘德安 |
| 关键词 | 周期性调制 spatial periodic modulation 空间滤波器 spatial filter 空间光调制器 spatial light modulation 堵孔效应 pinhole closure 光束质量 beam quality |
| 其他题名 | Research on Compensation Technique for Spatial Periodic Modulation of Near-field Beam in High Power Laser |
| 英文摘要 | 在高功率激光系统中，影响其性能的主要因素是系统的增益能力和负载能力，而提高系统的负载能力关键在于提高光学元件的损伤阈值和光束近场分布的均匀性。激光光束在传输和放大过程中，由于光学元件质量，硬边光阑，泵浦噪声，增益饱和，热畸变，灰尘，污染颗粒物，非线性效应，杂物散光和环境噪声等会给近场光束引入振幅调制和相位调制，破坏近场光束分布的均匀性。这些调制又可以归结为单一的遮拦物引起的调制，非周期性随机分布调制和周期性调制三种类型，其中，周期性调制不仅给近场光束引入周期性调制，而且会给远场的焦斑引入旁瓣，此旁瓣的位置对应于周期性调制的空间频率，在高通量运行时，某些频段的周期性调制还会由于非线性效应而迅速增长，引起小尺度自聚焦并对后续光学元件产生损伤威胁。 近场中的周期性调制通常用空间滤波器来滤除，但是空间滤波器在高功率激光系统应用中，还存许多问题，比如等堵孔效应，结构冗长，高真空度的要求等，其中，堵孔效应是影响系统输出能力最重要的因素之一。为了解决这些问题，相关的研究人员提出了对滤波小孔的结构进行改造，滤波小孔历经了垫圈型，四叶型和锥孔型三种类型，其中锥孔型滤波小孔的性能最好，目前已被应用到美国国家点火装置（NIF）的预放系统中。此外，还有研究人员提出了狭缝型空间滤波器，其将传统的凸透镜换成了柱透镜，使焦斑变成了焦线，大大增加了焦面处光场的面积，降低了焦面处的光功率密度。 此外，空间光调制器能调节光场的强度，相位和偏振状态，逐渐被应用到大型激光系统中，不过目前应用最多的是在光束整形方面。考虑到前面提到的空间滤波器在应用中遇到的问题，能不能借助空间光调制器对周期性调制近场光束进行补偿的方法，来避免产生堵孔效应，进而提高近场的光束质量，是我们所要研究的内容。 本论文主要的研究内容如下： 1、利用迭代权重算法对近场中的周期性调制进行补偿，在迭代权重算法中，针对周期性调制补偿的问题，参数的选择以循环前十次后输出光束的对比度进行比较而得的，这种方法比用尝试法确定参数更有针对性和选择性，此外，对光场相位的提取利用MATLAB中的angle函数，可以避免迭代算法中分母出现为零的情况而使迭代终止。这种方法在使用过程中不需要使用空间滤波器的滤波小孔，因而避免了堵孔效应的发生，最后，数值模拟和实验验证了这种方法的可行性。 2、理论分析和数值模拟了矩形相位载波，二元相位载波，闪耀相位载波和正弦相位载波对周期性调制近场光束的补偿，其中，矩形相位载波和正弦相位载波对周期性调制近场光束的补偿效果最好，不论是振幅型周期性调制还是相位型周期性调制，都可以用这种方法进行补偿。这种方法的原理是通过改变矩形相位载波的相位幅度来控制周期性调制空间频率的强度，另外矩形相位载的周期影响振幅型周期性调制的空间频率强度极大值的位置。这种方法使用时可以考虑使滤波小孔的半径设置的大一点，在避免卡边效应发生的同时也有利于缓解滤波小孔堵孔效应的发生。 3、实验验证了矩形相位载波对周期性调制近场光束的补偿，矩形相位载波的相位分布可以控制周期性调制空间频率的强度，其相位幅度差和振幅型周期性的空间频率强度有关，其相位分布平均值和相位型周期性的空间频率强度有关，此外，合理的选取矩形相位载波的周期来确保相位载波调制后的高阶空间频率全部在滤波小孔的截止频率范围之外。通过对实验结果和数值模拟结果中相位载波调制后的空间频率分布进行对比，证明了这个方法的可行性而且分析了两者存在差异的原因。此外，也对未加载矩形相位载波和加载周期为2个像素的矩形相位载波时输出近场光强分布图及对应的一维平均功率谱密度（PSD）曲线进行了比较分析，周期性调制的空间频率峰值下降了一个数量级，并降低到了本底值附近。; In high power laser systems, the main factors that affect the performance of which are the gain capacity and load capacity of the system, and the key to improve the load capacity of the system is to improve the damage threshold of optical elements and the uniformity of the near field distribution of the beam. In the process of transmission and amplification, due to the quality of optical elements, hard-edge aperture, pump noise, gain saturation, heat distortion, dust, particulate matter, nonlinear effect, impurity astigmatism and ambient noise, the near-field beam is introduced the amplitude modulation and phase modulation, which destroy the uniformity of the near-field beam distribution. These modulations can be reduced to three types: modulation caused by a single local obstruction, non-periodic random modulation and spatial periodic modulation. The spatial periodic modulation not only introduces spatial periodic modulation to the intensity of near-field beam, but also introduces the sidelobes to the focal spot in the far field. The position of the sidelobes correspond to the spatial frequency of the spatial periodic modulation. Some spatial frequency of the spatial periodic modulation will increase rapidly due to the nonlinear effect during high throughput, causing small-scale self-focusing and threaten the continued optical elements. The spatial periodic modulation in the near field is usually filtered by spatial filters, but the spatial filters still have many problems in the application of high power laser systems, such as the problem of pinhole closure, long structure, high vacuum, and so on. pinhole closure is one of the most important factors that affect the output capability of the system. In order to solve these problems, the relevant researchers have proposed the reconstruction of the structure of the filter pinhole. The filter pinhole has three types: the washer type, the four-leaf type and the conical hole type. The conical filter pinhole has the best performance, and is has been applied to the pre-amplification system of NIF at present. In addition, the researchers has put forward the slit spatial filter, which changed the traditional convex lens into the column lens, making the focal spot into the focal line, greatly increasing the area of the light field at the focal plane, and reducing the light power density at the focal plane. In addition, spatial light modulator has the ability to adjust the intensity, phase and polarization state of the beam, which is gradually applied to large laser systems, but the most widely used is in laser field is beam shaping. Considering the problems encountered in the application of the spatial filter mentioned above, whether the method of compensationg the spatial periodic modulation of near-field beam by spatial light modulatior is to avoid the problem of pinhole closure and to improve the beam quality of the near-field beam is feasible, which is what we need to study. The main contents of this paper are as follows: 1、The iterative weight-based algorithm is used to compensate the spatial periodic modulation of the near-field beam. In the iterative weight-based algorithm, for the problem of spatial periodic modulation, the selection of the parameter is compared with the contrast of the output beam after ten cycles. This method is more targeted than using the tasting method to determine the parameters. In addition, the extraction of the phase of the beam by using the angle function in MATLAB, which can avoid the denominator appearing zero in the iteration algorithm and terminating the iteration. This method does not need to use the filter pinhole of the spatial filter in the application, thus avoiding the problem of pinhole closure. Finally, the feasibility of this method is verified by numerical simulation and experiment. 2、The theoretical analysis and numerical simulation of the compensation for the spatial periodic modulation in near-field beam by rectangular phase carrier, binary rectangular phase carrier, blazed phase carrier and sinusoidal phase carrier. Rectangular phase carrier and sinusoidal phase carrier have the best compensation effect for the spatial periodic modulation of the near field. This method can be used to compensate for both amplitude-type spatial periodic modulation and phase-type spatial periodic modulation. The principle of this method is to control the intensity of the spatial frequency of spatial periodic modulation by changing the phase amplitude of the rectangular phase carrier, and the period of the rectangular phase carrier affects the the maximum intensity position of spatial frequency. When using this method, the large radius of the filter pinhole can be taken into consideration, which avoids the edge effect of the filter pinhole and helps to alleviate the effect of pinhole closure. 3、The experiment verifies the compensation for the spatial periodic modulation of the near-field beam by a rectangular phase carrier. The phase distribution of the rectangular phase carrier can determine the spatial frequency intensity of the spatial periodic modulation. The phase amplitude difference is related to the intensity of the amplitude-type spatial periodic frequency, and the phase distribution average is related to the intensity of phase-type spatial periodic frequency. In addition, the period of the rectangular phase carrier is reasonably selected to ensure that the higher-order spatial frequency after the phase carrier modulation is all outside the cut-off frequency range of the filtered small holes. By comparing the spatial frequency distribution after phase carrier modulation in the experimental results and the numerical simulation results, the feasibility of the method is proved and the reasons for their difference are analyzed. In addition, a comparative analysis is made of the output near-field beam intensity distribution and the corresponding one-dimension average power spectral density (PSD) curve when there is no rectangular phase carrier and the rectangular phase carrier with a period of 2 pixels. The peak intensity of the spatial frequency of the spatial periodic modulation decreased by an order of magnitude and reduced to the background value. |
| 学科主题 | 光学工程 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/31053]  |
| 专题 | 中国科学院上海光学精密机械研究所 |
| 作者单位 | 中国科学院上海光学精密机械研究所 |
| 推荐引用方式 GB/T 7714 | 高雅茹. 高功率激光近场空间周期性调制补偿技术研究[D]. |
入库方式： OAI收割
来源：上海光学精密机械研究所
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