| 题名 | 高功率激光前端综合建模与相关实验研究 |
| 作者 | 卢兴华 |
| 学位类别 | 博士 |
| 答辩日期 | 2012 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 李学春 |
| 关键词 | 低温Yb:YAG，再生放大器，ASE，热效应，幅度调制效应 |
| 其他题名 | Integrated Modeling and Experimental Research on High Power Laser Front-end System |
| 中文摘要 | 由于激光二极管(LD)泵浦固体激光装置具有效率高、光束质量好、结构紧凑、寿命长、输出稳定等优点，目前被认为是用于惯性聚变能源激光驱动器的最佳选择之一，因此在惯性聚变能源领域中日益受到重视。 本论文根据目前正在研制中的重复频率低温Yb:YAG激光前端系统的技术需求，建立了前端系统的计算机模拟仿真平台，以前端系统模拟程序为基础，针对前端研制过程中的ASE、热效应、幅度调制等物理问题进行了理论分析，对前端系统的结构、参数的选取提供了理论依据，并开展了相关实验研究，本论文的主要内容如下： 为了研究高功率激光器前端系统中的物理和工程问题，编写了高功率激光器前端系统模拟程序，介绍了前端综合建模的框架结构及实现功能，并结合离轴四通放大器笔形光束的例子，对模型进行了验证，找出了形成笔形光束反射面的位置，提出抑制笔形光束的方法，并进行了实验验证。 在1030nm波段，研制基于全光纤技术和集成波导脉冲整形技术的纳秒整形激光脉冲种子源，为后级低温Yb:YAG再生放大器提供一定能量、一定带宽、高信噪比和高光束质量的种子脉冲信号。 从 离子的能级结构出发，根据准三能级的速率方程并考虑放大的自发辐射影响，采用蒙特卡罗算法研究了脉冲泵浦低温Yb:YAG再生放大器中的储能分布，小信号增益等特性，分析了晶体温度、有效厚度等参数对放大器储能的影响，相应的结论将有助于低温运行Yb:YAG材料参数的选取，泵浦结构的设计。综合考虑泵浦光的吸收与ASE过程，实验中晶体的冷却温度选在183K；在现有的泵浦条件下，考虑到ASE对上能级粒子数的消耗，实验中晶体的厚度选取1.8 mm，泵浦时间1.8ms。通过实验验证，所建立的ASE模型可以比较准确地模拟放大器中的自发辐射放大行为和储能情况。 建立了高功率激光前端系统中增益介质的三维热-应力-光学分析模型，该模型包括三个子模块：有限元热分析、有限元热应力分析、热效应分析程序，从增益介质的热沉积特性和传热的边界条件出发，可以对不同泵浦条件、不同冷却方式下的增益介质内的三维稳态、瞬态温度分布、热应力分布、热致双折射、热退偏、热致波面畸变等一系列热效应问题实现快速的计算模拟。详细分析了热效应对低温Yb:YAG再生放大器的影响。由于低温晶体优良的热力学特性，在泵浦光10Hz抽运时，晶体的温升不大；热透镜的焦距在15m附近，谐振腔仍然为稳定腔；退偏效应不明显，由此带来的腔的损耗可以忽略。 实验中光纤耦合的LD端面泵浦的低温Yb:YAG再生放大器在1030nm得到10Hz，10.2mJ的基横模输出。实验结果和理论分析得到很好的吻合。 分析了高功率激光装置中光纤的色散、光纤的偏振模色散、偏振片与波片等光学器件的弱标准具效应等对幅度调制效应的影响并开展了相关的实验研究。 根据宽带激光在均匀加宽线型增益介质中的放大理论模型，采用短时傅里叶变换算法分析了再生放大器各个参数对幅度调制效应的影响，结果表明激光脉冲中心波长与增益介质中心波长匹配时，光强在时间波形上两倍于调制频率的谐波最为显著。激光脉冲中心波长与增益介质中心波长产生失谐时，光强的时间波形产生基频调制，且幅度调制深度随失谐量的增加而增大。因此在再生放大器中，调节光脉冲中心波长使之与增益介质中心波长的匹配变得尤为重要。双折射滤光片、标准具等滤波器可以有效的补偿再生放大器中的增益窄化效应，从根本上降低甚至消除增益窄化引起的幅度调制效应。此外通过提高注入激光种子脉冲的能量，尽量减小再生放大器峰值能量输出时的放大程数也可以有效的减轻幅度调制深度的大小。 |
| 英文摘要 | Diode pumped solid state laser (DPSSL) system is currently being considered as one of the best choice for the laser driver for the inertial fusion energy (IFE) due to its high efficiency, good beam quality, compact structure, long lifetime, high stable output, and therefore received increasing attention in the field of IFE. The thesis is based on the front-end system of high repetition rate, low temperature Yb:YAG laser system which is currently being developed. A computer simulation platform for the front-end system was established to emulate the physical problems such as the ASE, thermal effects, FM-AM effect, etc. The results provide a theoretical basis for the structure of the front-end system, the selection of the system parameters. Some experimental studies were also carried out. The main content of this thesis is as follows: In order to research the physical and engineering problems in the high power lasers front-end system, a simulation program system was developed based on the physical models of the front-end system. The framework structure and its function of the front-end integrated modeling were introduced. The modeling was verified by the example of pencil beam in the off-axis amplifier. The location of the reflecting surface create the pencil beam was ascertained using the modeling. The method suppresses the pencil beam was suggested. In the 1030nm band, nanosecond laser pulse source based all fiber technology and the integrated waveguides pulse shaping technology was researched. The source provided a certain bandwidth, high signal-to-noise ratio and high beam quality seed pulse signal for the cryogenic Yb:YAG regenerative amplifier. Based on the theory of quasi-three-level rate equations modified by amplified spontaneous emission (ASE), the stored energy density, the small signal gain of the cryogenic Yb:YAG regenerative amplifier for a given geometry for pulsed pumping in three dimensions is theoretically studied by use of Monte Carlo simulation. The influences of crystal temperature and effective thickness on the energy storage were analysed. The present model provides a straightforward procedure to design the Yb:YAG parameters and the optical coupling system for optimization when running at cryogenic temperature. Considering the absorption of the pump and the ASE process, the cooling temperature of the crystal was selected in 183K. The thickness of the crystal in the experiment was selected 1.8mm and the pump duration was selected 1.8ms taking into account of the energy storage consumption by the ASE process in the existing pump conditions. The ASE model can simulate the spontaneous emission, energy storage in the amplifier accurately and was verified by experiment results. The three-dimensional thermal-stress-optical analysis model of the gain medium in high power laser system was established, which consists of three sub-modules: the finite element thermal analysis, finite element thermal stress analysis, thermal effects analysis. Starting from the heat deposition in the gain medium and heat transfer boundary conditions, the three-dimensional steady-state and transient temperature distribution, thermal stress distribution, the thermally induced birefringence, thermal depolarization, thermally induced wave front distortion could been achieved in different pump conditions and different cooling methods. Due to excellent thermal characteristics of the low-temperature crystal, when the gain medium is pumped in 10Hz, the focal distance of the thermally induced lens is about 15m, the cavity is still stable; the depolarization rate could be ignored for the pump and cooling structure which has been designed. A fiber-coupled laser diode end-pumped cryogenic Yb:YAG regenerative amplifier running at 1030 nm has been demonstrated with the maximum output energy 10.2 mJ at a repetition rate of 10Hz. A very good agreement between experiments and the theoretical model was observed. The potential factors that influence FM-AM effect such as group velocity dispersion, polarization mode dispersion, the weak etalon effect of polarizers and wave plates were analysed. Based on the theory of the broadband pulsed laser amplification in the homogeneously broadening medium, a numerical model for the temporal pulse shape propagating through the regenerative amplification has been established by means of the short-time Fourier transform. The results show that when the center wavelength of the laser pulse matches with the gain media center wavelength, the prominent harmonic frequency of the intensity is twice of the modulation frequency, or else the prominent harmonic frequency of the intensity is the modulation frequency and the amplitude modulation depth increases with the increasing detuning. Therefore, in the regenerative amplifier, adjusting the laser pulse center wavelength matching the wavelength of the gain medium becomes particularly important. The birefringent filter and etalon filter can effectively reduce the amplitude modulation depth. In addition, by improving the injected laser seed pulse energy to minimize the number of cycles in the regenerative amplifier can also effectively reduce the depth of the amplitude modulation effect. |
| 语种 | 中文 |
| 内容类型 | 学位论文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15713]  |
| 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 | 卢兴华. 高功率激光前端综合建模与相关实验研究[D]. 中国科学院上海光学精密机械研究所. 2012. |
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