【作者】 李琨；

【导师】 张彬；

【作者基本信息】 四川大学 ， 光学， 2007， 博士

【摘要】 宽带激光在惯性约束聚变(ICF)激光驱动器靶场系统中的频率转换及传输特性是ICF研究中的一个重要内容，本论文具体针对这方面的问题展开了系统的研究，主要内容包括：1)高强度谐波转换物理模型的建立及数值仿真软件的开发建立了较为全面的高强度谐波转换的物理模型，详细考虑了近轴衍射、离散效应、群速度失配、三阶非线性效应(自相位调制和交叉相位调制效应)、入射基频光的振幅和位相变化、晶体吸收和表面反射等在内的诸多因素的影响，编写了相应的数值模拟程序。利用美国LLNL的理论及实验结果对该程序进行了大量的验证工作，证实其正确性和可靠性。在此基础上，开发了功能较为全面的、具有可视化界面的高强度三倍频数值仿真软件。该软件的开发，为谐波转换实验结果的分析以及谐波转换方案的优化提供了便捷、可靠的模拟平台。将该软件用于中国工程物理研究所神光Ⅲ原型装置2004～2005年的首束达标实验结果的分析，取得了较好的效果。2)高强度纳秒脉冲宽带激光频率转换的研究针对Ⅰ／Ⅱ／Ⅱ类角度失谐匹配方式的晶体级联三倍频，分析了影响三倍频光谱及转换效率的因素；提出了一种新的采用Ⅰ／Ⅱ／Ⅰ类角度失谐匹配方式的晶体级联三倍频方案，分析了入射基频光强度和带宽以及晶体厚度对三倍频转换效率的影响，并对晶体参数进行了优化；分析了采用光谱角色散(ASD)方法实现宽带倍频的原理，指出了传统的ASD方式的局限性，提出了非共线的ASD和频方式，并对光栅角色散率进行了优化设计。采用非共线ASD和频方式的优点在于适用带宽范围大、三倍频转换效率较高并且方案简单。高强度纳秒脉冲宽带激光是发展时间光束匀滑技术的基础，同时，宽带激光可抑制大口径激光器件中的有害非线性效应、避免大口径元件的破坏、改善光束的近场均匀性。因此，系统地研究高强度纳秒脉冲宽带激光的频率转换，对于ICF的研究具有重要的意义。3)高强度飞秒脉冲激光频率转换的理论及实验研究对采用KDP晶体的Ⅰ类及Ⅱ类角度失谐匹配方式的高强度飞秒脉冲二倍频过程进行了理论研究。首次在国内针对脉冲宽度(FWHM)30fs、能量30～150mJ、中心波长800nm的飞秒脉冲，采用厚度为1mm的KDP晶体进行了Ⅰ类二倍频实验，获得了10％左右的二倍频转换效率。对实验结果进行了理论分析，讨论了基频光初始频率啁啾、光强调制、位相畸变以及三阶非线性效应等因素对二倍频光转换效率及频谱宽度的影响。在此基础上，进一步提出了飞秒脉冲二倍频实验的改进措施。此外，还对高强度飞秒脉冲单块BBO晶体产生三倍频的过程进行了理论及实验研究。定量分析比较了单块晶体中三阶非线性效应以及级联二阶非线性效应对三倍频转换效率的作用，讨论了入射基频光光强、晶体厚度、非线性相位调制、群速度失配、失谐角、方位角等因素对三倍频光转换效率、时间波形以及光谱分布的影响。首次在国内针对脉冲宽度100fs、带宽25nm、能量为6mJ左右的超短脉冲基频光，采用单块BBO晶体进行了三倍频实验研究，获得了接近1％的三倍频转换效率。由于超短脉冲紫外光不仅可作为探针光使用，而且，高强度紫外超短激光作为ICF“快点火”的点火驱动器具有独特优势，因此，对高强度飞秒脉冲激光频率转换的研究具有重要意义。4)频率转换过程的逆问题研究首次提出了解决频率转换过程逆问题的数值计算方法。该算法基于分步傅立叶变换和四阶R—K法，通过迭代计算，可在已知输出三倍频光以及谐波转换器参数的情况下，计算得到相应的输入基频光参数。分别针对空间光强分布为高斯、超高斯形状、时间分布为整形脉冲的平面波、有一定空间位相调制的窄带三倍频、有一定时间位相调制及线性频率啁啾的宽带三倍频以及飞秒脉冲二倍频的逆问题进行了数值计算。迭代计算的效率较高并且结果较为准确。频率转换过程逆问题数值计算方法的提出，为ICF系统频率转换过程的优化设计提供一种新的思路和途径。5)靶场系统中的非线性效应研究采用Maxwell-Bloch-Langevin方程来描述强激光的受激拉曼散射效应。针对强激光在空气中长程传输时所产生的受激旋转拉曼散射(SRRS)效应，分析了基频及三倍频泵浦光的光强调制、位相调制以及带宽等因素对阈值条件及光束质量的影响，提出了抑制SRRS效应的方法。同时，针对大口径KDP晶体中三倍频光的横向受激拉曼散射(TSRS)效应，分别讨论了短脉冲(1ns)及长脉冲(3ns)条件下，泵浦光光强、增益系数及带宽对斯托克斯光转换的影响。运用修正的非线性薛定谔方程，针对熔石英介质中强紫外激光自聚焦的强弱、位置与强紫外激光光强调制和位相畸变的关系开展了定量分析，对熔石英介质中强紫外激光产生非线性自聚焦效应的阈值条件定义做了补充。此外，结合实验过程中的实际情况，对靶室的石英窗口及打靶的聚焦透镜出现的破坏现象，采用自聚焦原理进行了分析，并提出了避免产生破坏的方法。对靶场系统中非线性效应的研究，有利于改善三倍频光光束质量、减小高强度激光对靶场光学元件的破坏风险，具有重要的意义。6)宽带倍频光的光束匀滑及聚焦特性的研究对光束匀滑方法进行了总结和比较，针对时间位相调制宽带激光以及啁啾脉冲宽带激光，对采用角谱色散束匀滑(SSD)方案进行了数值模拟和讨论。靶面均匀辐照是实现ICF必不可少的条件，还有利于提高靶丸压缩比，从而降低对激光能量的要求。对宽带倍频光的光束匀滑及聚焦特性的研究，可为实现靶面均匀辐照提供参考。更多还原

【Abstract】 The study on the properties of the harmonic generation and the propagation of the broadband laser in the final optical system is one of the important contents in the research of Inertial Confined Fusion (ICF). Detailed and systematical studies on the above mentioned aspects are carried on in this thesis. The main contents are as follows.1) Establishment of the physical model for harmonic generation of high-intensity laser and development of the numerical simulation code with visual interfaceThe physical model for harmonic generation of high-intensity laser was built up, which included the effect of the paraxial diffraction, the spatial and temporal walk-off, the third-order nonlinear effect including the self- and cross-phase modulation, the amplitude and the phase of the input fundamental field, the absorption and the reflection of the harmonic generation crystals, etc. The corresponding numerical simulation code with visual interface was developed and verified by comparing with the theoretical and experimental results of the Lawrence Livermore National Laboratory (LLNL), showing the validity and reliability of our code.The development of our code provides a convenient and reliable platform for the analyzing of the experimental results and the optimizing of the schemes of harmonic generation experiment. Our code has been used to analysis the experimental results obtained in CEPA and the numerical results achieved good agreement with the experiment results.2) Study on the harmonic generation of the nanosecond high-intensity broadband laserThe factors, which affected the spectrum and the conversion efficiency of the broadband third harmonic field with cascaded crystals of type I/II/II, were analyzed. A new scheme with cascaded crystals of type I/II/I was proposed and optimized, and the effects of the intensity and the bandwidth of the input fundamental field on the conversion efficiency were discussed. The principle of the broadband harmonic generation with the traditional technology of angular spectrum dispersion was analyzed and the limitation of the technology was pointed out. An improved and simpler method of non-collinear angular spectrum dispersion, which is suitable for the larger bandwidth, was presented and optimized.The nanosecond high-intensity broadband laser is the basis of the beam smoothing technology in temporal domain. Moreover, broadband laser has advantages in controlling the harmful nonlinear effects, avoiding the damage of the optical elements with large aperture and improving the uniformity of the laser beam. Therefore, studies on the harmonic conversion of the nanosecond high-intensity broadband laser are of great importance in the ICF research.3) Study on the harmonic generation of the femtosecond high-intensity broadband laserThe second harmonic generation of high-intensity femtosecond broadband laser with a type I KDP crystal or a type II KDP crystal was analyzed theoretically. The experiment of the second harmonic generation of the femtosecond laser with the center wavelength of 800nm, the pulse width (FWHM) of 30fs and the pulse energy of 30～150mJ by using a type I KDP crystal with the thickness of 1mm was carried out for the first time in China. The conversion efficiency of about 10% was achieved. The experimental results were analyzed and the effects of the initial frequency chirp and the modulations of the intensity and the phase of the fundamental field, as well as the third-order nonlinear effect on the conversion efficiency and the bandwidth of the second harmonic field were discussed. Consequently, the method to improve the conversion efficiency and the bandwidth of the second harmonic field was proposed.Theoretical and experimental studies on the third harmonic generation of the high-intensity femtosecond laser with a single BBO crystal were carried out. Comparison of the third-order effect and the cascaded second-order effect in the third harmonic generation was made. The effects of the intensity of the fundamental field, the thickness of the crystal, the nonlinear phase modulation, the temporal walk-off, the detuned angle and the azimuth angle on the conversion efficiency, the pulse shape and the spectrum were discussed. The corresponding experiment was performed for the femtosecond laser with center wavelength of 800nm, the pulse width (FWHM) of 100fs, the bandwidth is 25nm and the pulse energy of about 6mJ by using a single BBO crystal. The conversion efficiency of nearly 1% was achieved.Because the ultra-short UV laser can be used as the probe light and has distinctive advantages in "fast-ignition" in ICF, studies on the frequency conversion of the high-intensity femtosecond laser are very important.4) Study on the inverse problem of harmonic generationA method for solving the inverse problem of harmonic generation was proposed for the first time based on the split Fourier transformation, the forth-order Runge-Kutta method and the iterative method, namely, to determine the characterization of input fundamental field and the requirement for KDP from the required characterization of output tripling field. The inverse problems of third harmonic generation of the narrowband laser and the broadband laser, and of the second harmonic generation of the femtosecond laser were studied. The calculation efficiency of the iteration method and the accuracy of the calculation results are both high.The proposed method for solving the inverse problem of harmonic generation provides a new way of optimizing the frequency conversion process.5) Study on the nonlinear effects in the final optical systemBased on the Maxwell-Bloch-Langevin equations, the stimulated rotational Raman scattering (SRRS) of the high-intensity laser in long air path was discussed. The effects of the modulations of the intensity and the phase and the bandwidth of the fundamental and the third harmonic pump fields on the threshold condition of SRRS effect and the beam quality of laser were analyzed. Consequently, the methods for controlling the SRRS effect were proposed. Furthermore, the transverse stimulated Raman scattering (TSRS) in KDP crystal with large aperture was also studied. The effects of the intensity, the gain coefficient and the bandwidth of the pump field with the pulse width of 1ns and 3ns on the conversion of the Stokes field were discussed.Based on the modified nonlinear Schrodinger equations, the relationship between the intensity and the location of the self-focusing and the modulations of the intensity and the phase of the high-intensity ultraviolet laser in fused silica was analyzed quantitatively. The definition of the threshold condition of the self-focusing effect for ultraviolet laser was supplemented. The theory of the self-focusing was used to explain the damage of the silica window and the focusing lens in the target chamber in the experiment, and the solution was also presented.The study on the nonlinear’ effects in the final optical system has important significance in improving beam quality and diminishing the damage risk of the optical components in the final optical system.6) Study on the beam smoothing and the focusing properties of the broadband harmonic fieldThe beam smoothing techniques were summarized and compared. Numerical simulations of smoothing method for the broadband laser with temporal phase modulation and frequency chirp by using spectrum dispersion were carried out and analyzed.Uniformed irradiation on the target is an essential condition to realize the ICF ignition and has advantages in increasing the compress ratio of the target and decreasing the requirement of laser energy. The study on the beam smoothing and the focusing properties of the broadband harmonic field can provide valuable reference to the experiment.更多还原