【作者】 陈剑华；

【导师】 张彬；

【作者基本信息】 四川大学 ， 光学， 2007， 硕士

【摘要】 在惯性约束聚变(ICF)研究中，由ICF驱动器主放大系统输出的基频光通常需要在空气中传输一段距离后才能到达频率转换器，而经过频率转换器输出的强紫外激光也要在空气中长程传输后才能实现最终的聚焦打靶。然而，强激光在空气中长程传输时会产生受激旋转拉曼散射(SRRS)效应，SRRS效应不仅会造成泵浦光的衰减，还会使强激光的光束质量变差。基频光光束质量变差不仅会导致谐波转换效率降低，而且，还会影响频率转换器输出强紫外激光的光束质量，而强紫外激光在空气中长程传输过程中产生的SRRS效应将进一步导致其光束质量变差，从而对聚焦打靶产生明显影响。因此，对基频光和强紫外激光在空气中长程传输时的SRRS效应研究具有重要的理论参考价值和实用价值。本论文围绕ICF驱动器中基频光和强紫外激光在空气中长程传输过程中产生的SRRS效应，详细讨论了SRRS效应的阈值条件、斯托克斯光的转换效率，以及强激光的光束质量和位相的变化，并分析了强紫外激光的远场聚焦特性。论文的主要内容包括：一．建立了不同波长强激光在空气长程传输过程中产生SRRS效应的物理模型针对ICF驱动器中存在的两种不同波长激光(基频光和紫外光)在空气中的长程传输，建立了较为全面的SRRS效应物理模型。该模型中详细考虑了斯托克斯光和泵浦光的衍射、Langevin(郎兹万)噪声源、介质的非线性极化、泵浦光的衰减、斯托克斯光的非线性放大、稳态拉曼增益系数等诸多因素。采用均方根梯度表征的随机位相屏来描述低频位相扰动，用随机位相扰动描述中、高频位相噪声，建立了波前位相畸变模型。二．强激光在空气中长程传输过程中产生的SRRS效应研究对基频光和紫外光在空气中长程传输时产生的SRRS效应进行了数值模拟分析，详细讨论了强激光的峰值光强和脉冲宽度对SRRS阈值以及转换效率的影响，并分析了波前位相畸变对基频光和紫外光的空间光强分布的影响。三．波前位相畸变对强激光在空气中长程传输后位相的影响研究对不同波长的强激光在空气中长程传输后的位相特性进行了研究，详细讨论了引入不同的波前位相畸变后，基频光和紫外光在空气中长程传输后位相畸变的变化，并定量分析了基频光和紫外光的位相畸变随传输距离的变化规律。四．波前位相畸变对强紫外聚焦特性的影响研究对频率转换器输出的强紫外激光经过空气长程传输后的远场聚焦特性进行研究，详细讨论了引入不同的波前位相畸变后，强紫外激光在空气中长程传输后的焦斑形态，并计算分析了焦斑半径随空间位相畸变和传输距离的变化。更多还原

【Abstract】 In the research of Inertial Confinement Fusion (ICF), the high-power fundamental fields from the main amplifier have a long air path to reach the frequency conversion system, and the high-power ultraviolet laser beams from the frequency conversion system also have a long air path to reach the target. However, the effect of stimulated rotational Raman scattering (SRRS) on high-power laser beams in the process of long air path propagation not only leads laser energy loss, but also the beam quality degrading. The degraded beam quality of fundamental field will influence not only the conversion efficiency of third harmonic generation, but also the beam quality of ultraviolet laser beams from the frequency conversion system, and the SRRS effect of high-power ultraviolet laser beams in long air paths will influence the optimum fuel compression in the target. Therefore, the study on the SRRS effect of high-power fundamental field and tripling field in long air paths would be of great importance in practice.This thesis focuses on the SRRS effect of high-power fundamental field and tripling field through long air paths. The threshold of SRRS effect, the conversion efficiency of Stokes field and the beam quality and phase properties of laser beams were discussed in detailed. Furthermore, the focal characterization of high-power ultraviolet laser beams was studied. The main contents can be summarized as follows:(1) The physical model of SRRS effect of high-power laser of different wavelengths through long air paths has been built upBased on high-power laser thorough long air paths, the physical model of SRRS effects was built up, which considered the diffraction effects of Stokes and pump lasers, Langevin noise sources, the nonlinear polarization of the medium, the depletion of pumped laser, the nonlinear amplification of the Stokes field and the Raman gain coefficient, ect.. The model of the phase perturbation was also given, where the root-mean-squared (RMS) Gradient and random phase perturbation were introduced to describe the low and the high frequency phase noise.(2) SRRS effects of high power laser beams through long air paths depend on several factors has been studiedThe SRRS effect of high-power fundamental field and tripling field through long air paths has been simulated and analyzed. The effects of the peak intensity and the pulse width of the pump laser beams on the threshold of SRRS effect and the conversion efficiency of Stokes field have been discussed in detail. The influence of initial phase perturbations on the intensity distribution of the pump and stokes light has also been analyzed.(3) Influence of phase perturbations on the phase of high-power laser beams has been studiedThe phase characteristics of high-power lasers with different wavelengths through long air paths have been studied. The phase of the high-power fundamental and tripling fields with different initial phase perturbations after long air paths has been discussed in detail. Subsequently, the variations of the phase perturbations of the high-power laser beams with the propagation distances have been analyzed quantitatively(4) Influence of phase perturbations on focal characterization of high-power ultraviolet laser beamsThe focal characterization of high-power ultraviolet laser beams from the frequency conversion system after long air path propagation has been studied. The shape of focal spot of high-power ultraviolet laser beams with different low-frequency phase perturbations through long air paths has been discussed in detail. Furthermore, the variation of the size of focal spot with the spatial phase perturbations and the propagation distances has been analyzed quantitatively.更多还原