重复频率固体激光放大器热效应研究
文献类型：学位论文
| 作者 | 汪超 |
| 文献子类 | 博士 |
| 导师 | 李学春 |
| 关键词 | 重复频率激光放大器 High-repetition-rate amplifier 热效应 thermal effects 固体激光 solid state laser 高功率激光驱动器 ICF, LD pumping LD泵浦 lamp pumping 氙灯泵浦 |
| 其他题名 | Research on Thermal effects in Solid State Laser Amplifier Operating at high Repetition Rate |
| 英文摘要 | 惯性约束核聚变技术的发展对高功率激光驱动器提出了新的要求，下一代高功率激光驱动器需要具有更高的输出能量、更高的重复频率。高功率激光驱动器中光学元件需要承受高峰值功率的激光辐照，光学元件激光诱导损伤问题成为限制激光驱动器运行效率、增加运行成本的重要因素之一；光学元件的激光诱导损伤与激光脉冲的波长、时间波形、脉冲宽度、光谱、峰值功率、辐照次数、光学元件的疵病等因素有关，因此研究高功率激光驱动器中光学元件的损伤特性需要一台光束性能与驱动器类似、但工作频率更高的激光器。随着重复频率的提高，系统中的热效应成为限制系统性能的主要因素。 由于不均匀泵浦和侧面冷却，传统的棒状增益介质中存在显著的温度梯度，从而产生严重的热效应，显著降低系统的光束质量。但棒状增益介质的技术成熟、容易实现大能量激光输出，在中小口径的重复频率的激光系统中仍具有明显的竞争力。本论文以棒状增益介质为研究对象，从理论和实验上研究系统中的两类热效应：增益介质中的热效应和泵浦源的热效应，其中增益介质中的热效应包括热致波前畸变和热致激光性能变化；针对不同类型的热效应，提出了相应的应对策略；在此基础上研制三套激光系统，实现了高光束质量的激光输出；在激光系统的研发过程中，研发了三款关键器件：氙灯泵浦钕玻璃激光头、LD泵浦钕玻璃激光头和VCSEL泵浦的Nd:YAG激光头。具体内容如下： 1、以钕玻璃和Nd:YAG为例，从微分方程的角度，解析分析了玻璃、晶体两类增益介质在理想近似条件下激光棒中的温度分布、应力分布、热致波前畸变等，讨论了热致波前畸变对激光性能的影响。 2、以Nd:YAG为例，使用有限元方法，分析了不同泵浦分布下棒状增益介质中温度分布、应力分布、热致波前畸变。分析结果显示当泵浦分布不均匀时增益介质的热致波前畸变可以分解为热聚焦、热致双折射、高阶波前畸变三部分，其中高阶畸变会使光束质量显著恶化；高阶波前畸变随泵浦不均匀性增加而增加；在重复频率激光放大系统中，在激光棒横截面上实现均匀泵浦是保证高光束质量输出的前提。 3、以Nd:YAG为例，首先使用半经典理论分析了增益介质中热致激光性能变化产生的原因；其次，从理论和实验上研究了热致激光性能变化。结果显示热致激光性能变化包括三部分：光谱展宽、光谱红移以及受激发射截面减小；在此基础上，讨论了重复频率激光放大器中棒状增益介质在均匀泵浦近似下，增益介质中热致激光性能变化对光束质量的影响，分析结果显示：即使在均匀泵浦条件下，由于热致激光性能变化的影响，也难以获得均匀的放大；通过增加增益介质的饱和深度可以降低热致激光性能变化对光束近场分布的影响。 4、以LD泵浦的钕玻璃激光头为例，使用光线追迹从理论上分析了泵浦源热效应对泵浦分布影响，在此基础上通过求解速率方程分析了泵浦源的热效应对光束近场分布的影响。实验中设计并制造了一个LD泵浦、峰值功率50 KW的钕玻璃激光头，测量了不同温度下激光棒横截面上泵浦分布。 5、针对激光放大系统中不同类型的热效应，提出了相应的应对策略，主要包括减小热效应和补偿热效应两方面。具体包括： 在氙灯泵浦的激光放大器中，提出使用氙灯光谱滤波、均匀泵浦为代表的措施减小增益介质中热效应。研制了一款氙灯泵浦钕玻璃激光头，并成功应用在氙灯泵浦预放大器中。 在LD泵浦的激光放大器中，通过光线追迹分析不同因素对激光棒横截面上泵浦分布的影响，并具体分析了LD泵浦的钕玻璃激光头中泵浦光光谱对激光棒中泵浦分布的影响，在此基础上研制了一个LD泵浦、峰值功率50 KW的钕玻璃激光头。针对LD模块间光谱不一致导致的不均匀泵浦的问题，提出优化模块排序的方法来实现均匀泵浦，避免了严格波长筛选造成的浪费，在实现均匀泵浦的同时兼顾了成本。在此基础上，提出了一套LD泵浦激光头的设计方法，并针对下一代高功率激光驱动器中预放大器的激光头的大规模生产问题，提出了具体的解决方案。 在高重复频率Nd:YAG激光放大系统中，针对增益介质中热致激光性能变化的问题，分析了热致激光性能变化对光斑近场光强分布和提取效率的影响；并针对特定系统提出了相应的优化策略。当激光系统需要输出近场光强均匀分布光斑时，通过选取合适的扩束比并将注入波长调谐在短波方向，可以减小热致激光性能变化对近场光强分布的调制，并保持较高的提取效率；在包含多级放大器的激光系统中，激光头的设计过程中应注意保持各级放大器中激光棒的温度匹配。 6、综合应用各种热效应的应对策略，研制了一台氙灯泵浦钕玻璃激光放大器，工作频率1 Hz，注入能量2.5 mJ的情况下，实现了1 J的激光输出，光斑近场光强调制度小于1.2，95%的能量在720 μrad内，30分钟内能量稳定性（RMS）优于1.1%。在放大器的研制过程中，验证了各种热效应应对策略的有效性，分析了自激振荡、鬼像、泵浦均匀性对系统输出的影响，为下一代高功率激光驱动器的预放大器的研制做准备。 7、结合光学元件预处理的需求，研制了一套LD泵浦Nd:YAG激光系统，系统包括三部分：全光纤种子源、LD泵浦的Nd:YAG再生放大器和LD泵浦的Nd:YAG四程放大器。系统综合应用均匀泵浦技术、热致波前畸变补偿措施、热致激光性能变化的补偿措施，最终实现工作频率300 Hz、单脉冲能量317 mJ、近场光强调制度1.8的激光输出。为了克服泵浦光光谱随温度变化对系统输出的影响，系统中使用带波长锁定功能的VCSEL作为泵浦源。在开发激光系统的过程中，研制了一个VCSEL泵浦Nd:YAG的激光头，峰值泵浦功率达到10 KW，从实验上对比了增益介质掺杂浓度、导流管侧面的粗糙度对泵浦分布的影响。 8、结合光学元件损伤特性研究的需求，研制了一套氙灯泵浦Nd:YAG激光系统，系统包括三部分：全光纤种子源、LD泵浦的Nd:YAG再生放大器和氙灯泵浦的Nd:YAG双程放大器。系统工作频率10 Hz，输出能量1.2 J，能量稳定性（RMS，30分钟内）优于1.4%，近场光强调制度小于1.2；95%的能量在600 μrad内，优于3倍衍射极限；通过光纤种子源调制种子光波形，预补偿放大系统中增益饱和产生的波形畸变，实现了3 ns的近方波激光输出。综合指标达到了光学元件损伤研究的需求。; New requirements for High-Power laser drivers are proposed by the development of the Inertial Confinement Fusion (ICF) technology. The next generation of the High-Power laser driver should fire much powerfully and much frequently. Optics in High-Power lasers withstand extremely intense laser pulses. Optics damage induced by the very high peak power is one of the factors which add experiment cost and decrease operation efficiency. Laser-induced optics damage is related to laser wavelength, laser fluence, pulse width, and pulse shape. Therefore a laser with similar beam characteristics as ICF laser drivers and a much higher repetition is required in the study of laser-induced optic damage in ICF lasers. Thermal effects of the laser system limit its capacity as the repetition rate rises. The combination of the nonuniform absorbed pump radiation and surface cooling required for heat extraction leads to a temperature gradient in the rod. This results in thermal effects, like thermal lensing and stress birefringence, which degrade the beam quality. However the rod amplifier is a mature laser technique and it is easy to get high-energy pulses by rod amplifiers. Therefor in high-repetition-rate (HRR) and high-energy lasers with a moderate beam size the rod is still competitive compared to others types of amplifiers, like the disk and the slab. In this dissertation two types of thermal effect in the rod amplifiers are analyzed, namely thermal effects of the gain medium and thermal effects of the pump source. The thermal effects of the gain medium contain two parts: the thermal-induced wavefront distortion and the thermal-induced spectroscopic properties change. Different strategies are proposed to conquer different types of thermal effects. Based on these three laser systems with excellent beam quality are developed. During the development of these lasers three key devices are made, namely a lamp-side-pumped Nd: glass laser head, a LD-side-pumped Nd: glass laser head and a VCSEL-side-pumped Nd:YAG laser head. This dissertation contains the following 8 parts: 1. The distribution of temperature and stress in the rod of Nd: glass and Nd:YAG is analyzed by solving differential equations with ideal approximations . Further thermally induced wavefront distortion and its effects on laser performances are analyzed. 2. The distribution of temperature and stress in the Nd:YAG rod with different pumping distributions is analyzed by the finite element method. Further the thermal-induced wavefront distortion is analyzed. The wavefront distortion contains three parts, namely thermal lensing, stress birefringence and high-order wavefront distortion. The high-order wavefront distortion degrades beam qualities dramatically and it increases as the pumping inhomogeneity increases. In HHR lasers uniform pumping is the basic requirement for a good beam quality. 3. Thermally induced spectroscopic property change (TISPC) of Nd:YAG is derived with semiclassical theory. Experiment results agree well with the theory. TISPC contains three effects, namely thermal broadening, red shift and temperature dependences of the stimulated emission cross section. Its effect in HRR rod amplifiers is analyzed. The results show that rod amplifiers cannot get uniform gain even with uniform pumping due to TISPC. The effect can be reduced by increasing the saturation of the gain medium. 4. Thermal effects of pumping source on the pumping distribution are analyzed by ray tracing in LD-side-pumped Nd: galss laser heads. Its effect on beam profiles is analyzed by solving rate equations. A LD-side-pumped Nd: galss laser head is designed and manufactured. Pumping distributions under different temperature is measured. 5. The following strategies like reducing and compensating for the thermal effects are proposed. a) In lamp-pumped laser heads techniques like filtering the spectrum of the pump light and uniform pumping are tested and performed to reduce thermal effects. A lamp-side-pumped Nd: Glass laser head is developed and used in the study of the Pre-amplifier in ICF laser drivers. b) In LD-side-pumped laser heads factors those affect the pumping distribution are analyzed by ray tracing, especially the spectrum of the pump light. A LD-side-pumped Nd: galss laser head with 50-KW pump power is designed and manufactured. A uniform pumping distribution with a peak-valley value of 5% in the cross-section of the rod, is gotten by optimizing the permutation of LD bars. This method to optimize the permutation of LD stacks with different spectra rather than to select LD stacks in a narrow spectral range, avoids huge waste, which makes it possible to build this laser head massively. Based on these works a method of how-to-design LD-side-pumped laser head is proposed and a detail solution of the massive production of laser heads in the next-generation ICF laser driver is presented. c) TISPC effects on beam profiles and energy extrication efficiencies in HRR lasers are analyzed. Strategies are proposed to minimum TISPC effects for a specified laser system. In multi-stage amplifiers which are designed to have a uniform beam profile the beam expansion ratio should be carefully selected and the seed wavelength should be tuned to the emission peak of the rod edge. The laser heads in multi-state amplifiers should be carefully designed and the rod of each state has similar temperature to maintain the spectrum matched in each state. 6. A lamp-pumped Nd: phosphate glass four-pass amplifier is demonstrated. The output energy reaches 1 J with a repetition of 1 Hz, an injected energy of 2.5 mJ, and an aperture of 7 mm × 7 mm. The amplifiers system employs a beam-shaping module and a four-pass, lamp-pumped amplifier. The intensity modulation in the near field is less than 1.2, and more than 95% of the energy is in the range of 720 μrads, which corresponds to 2.3 times the diffraction limit expected for a square 6-order-super-Gaussian beam. With these characterizations this amplifier can be a promising alternative configuration to the pre-amplifiers of the ICF lasers. 7. A 95 W Nd:YAG laser system pumped by Vertical Cavity Surface emitting Laser (VCSEL) arrays is developed to pre-process optics in ICF lasers. The laser contains an all-fiber-based seeder, a Nd:YAG regenerative amplifier, and a four-pass amplifier. The laser operates at 300 Hz with energies up to 317 mJ. The beam has a top-hat intensity distribution. The temporal pulse shape is flat in time and the pulse width can be adjusted in the range of 2~6 ns. With these characteristics the laser system can be an effective tool to pre-process optics. 8. A lamp-pumped Nd:YAG laser system is developed to study Laser-Induced Optics Damage (LIOD). The laser system contains three main components, namely the all-fiber-based seeder, the Nd:YAG regenerative amplifier and the lamp-pumped two-pass Nd:YAG amplifier. The laser operates at 10 Hz with pulse energies up to 1.2 J. the temporal profile of the pulse is near-square. The diameter of the beam is 10 mm. the spatial profile of the beam is near top-hat in the near field and the intensity modulation is less than 1.2. More than 95% pulse energy is in the range of 600 μrads. The root mean square (RMS) of the pulse energy is less than 1.4%. With these characters this laser system can be a powerful tool for the study of the LIOD. |
| 学科主题 | 光学工程 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/30912]  |
| 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 | 汪超. 重复频率固体激光放大器热效应研究[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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