| 题名 | 高功率装置中的光束近场研究 |
| 作者 | 李小燕 |
| 学位类别 | 博士 |
| 答辩日期 | 2010 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 朱健强 |
| 关键词 | 小尺度自聚焦，近场，增益，测量，寄生反射，光束质量，空间分布 |
| 其他题名 | Study on near field of the beam in high power laser facility |
| 中文摘要 | 惯性约束核聚变(ICF)实验研究的关键是激光驱动器，此装置要求有非常高的脉冲功率，而且对激光近场的均匀性也有十分苛刻的要求。近场即是菲涅耳数>1的菲涅耳衍射区，对于大功率激光器主要是指具有光束全尺寸的区域。光束近场分布的均匀性决定着光学元件的损伤风险，而光学元件的损伤则限制激光系统的工作负载（或光能量）及运行的稳定性和安全性。因此均匀干净的近场是提高激光驱动器能量负载的必要条件。 本论文着重于高功率激光束近场空间分布的传输特性及光束近场质量的控制。研究了噪声源对光束近场调制的物理过程，分别讨论了衍射场和干涉场的传输特性及其对激光系统的性能、诊断造成的影响，从而为从根本上消除或抑制噪声源提供重要依据，并以此依据来改善光束近场质量，避免小尺度范围的元件破坏，提高激光装置的输出能量。 本论文的主要内容与研究成果如下： 1.阐述了光束近场的均匀性对ICF激光驱动器负载能力的影响，给出了光束质量控制的基本思路。讨论了光束质量主动控制技术的局限性及光束质量被动控制的重要性，并对影响光束近场分布的主要噪声源及噪声源对近场调制的基本物理过程进行了分析，为提高光束质量控制技术提供了理论依据。 2.分别分析模拟了硬边衍射和局部点衍射调制场的空间传输特性，得出了近场分布空间结构的演变规律。硬边光阑衍射场的变化与入射光场分布及光阑形状关系不大，衍射调制都是从边缘开始随着传输距离的变大向中心扩展，最快调制增长都发生在边沿处（矩形光阑的最快调制增长位置在四个顶角处），且衍射纹都是关于光阑中心对称分布的。点衍射场随传输距离的变化与硬边光阑衍射相似，衍射调制从衍射点开始随着传输距离的变大向外扩展，衍射点处一直是最快调制增长点。在非线性介质中传输时，点衍射的调制增益与衍射点的尺寸有关，只有该尺寸所对应的空间频率在非线性增长较快区域内的衍射点才会具有较大的调制增益。 3.在BT理论的基础上，研究了受寄生反射调制的光束近场在非线性传输中的小尺度自聚焦特性。得出了调制场在非线性增益介质中小尺度调制增益谱的表达式。通过数值计算，得到了小信号增益情形在钕玻璃片状放大器中，介质增益及传输距离改变的情况下小尺度调制的最大增益与放大器窗口楔角的变化规律；以及放大器窗口的楔角与厚度对增益增长率的影响以及不同脉冲宽度的脉冲通过相同的窗口时，增益系数随脉冲宽度的变化关系。并据此提出了减小此种调制，改善光束近场质量的相应方法。 4.分析了光束干涉场的线性传输特性及其对光学元件参数和性能的影响，发现寄生反射对主光束的干涉调制会引起衰减器衰减度的振荡，由于光阑口径的剪切限制，寄生反射还会引起叠加光场的起伏，同时还会影响激光系统的稳定性，进而影响近场能量的时间稳定性。 5.通过分析衍射场和干涉场的传输特性，得出了衍射场和干涉场噪声纹理的空间分布规律：衍射场的噪声纹理以光阑中心或衍射点为中心，随着传输距离的增大，中心点位置不变，空间结构关于中心对称分布。而干涉场的纹理与光程差相关，干涉条纹间隔及条纹区域会随距离改变有所变化。 6.讨论了参数诊断系统的取样方式、传输过程和数据采集介质对测量结果准确性的影响，对光束近场的测量与分析都有一定的参考价值。同时还研究了超短脉冲参数测量系统中取样镜楔角的色散效应与所测得的超短脉冲宽度的关系。 |
| 英文摘要 | The pivotal problem for Inertial Confinement Fusion (ICF) experiment is the laser driver, which needs much high pulse power and has a harsh demand for the uniform near-field irradiation. Near-field means the Fresnel diffraction region where the Fresnel number >1, and for high power lasers, it mainly means the field where the beam has hold size. Because the uniformity of Near-field determines the damage risk of optical components, and optical component’s damage threshold limits the work load (or beam energy) of the laser system, the stability and security of operation, a uniform clean near-field is the necessary condition for improving the energy of laser driver. This dissertation focuses on the spatial propagation characteristics of high power laser’s near-field and the control of its near-field quality. The physical processes of the beam near-field modulated by the noise sources, the propagation characteristics of diffraction fields and interference fields, and their impact on the performance of laser system were discussed. We could improve the quality of the beam near-field, avoid small-scale damage and increase the output energy of laser devices according to the results above. We have carried out the theoretical research in the following respects: 1.The impact of the beam near-field’s uniformity on the high power laser’s energy load was represented, and the basic idea about the beam quality control was proposed. We discussed the limitation of the active control and the importance of passive control, analyzed the main noise sources which impacted the distribution in near-field and the basic physical process of near-field modulated by the noise, and provided theoretical basis for the beam quality control. 2.The propagation characteristics of the diffraction field modulated by hard-edged aperture and local spot were simulated respectively, and the evolution law of near-field distribution spatial structure was obtained. The changes of diffraction field modulated by hard edged aperture have little to do with the distribution of incident light and the shape of aperture: diffraction modulation spreads to center from edge as the propagated distance increasing, the fastest growth of modulation happens on the edge (the fastest modulation growth of rectangle aperture happened on the corner), and all the diffraction patterns are symmetrical about the aperture center. The changes of diffraction field modulated by local spot resemble the diffraction field modulated by hard-edged aperture: diffraction modulation spreads from center as the propagated distance increasing, and the fastest growth point of modulation is the local point. But the modulation gain has some connection with the size of the diffracted spot propagating in the nonlinear media. Only when the spatial frequency which the size of the local point corresponds to is within the region of faster nonlinear growth, the local point has a great gain. 3.Based on Bespalov-Talanov theory, the small-scale self-focusing caused by parasitic reflection and the parameter contributing to the growth of ripple in nonlinear medium with gain were detailedly discussed in the paper. By analyzing the interference field caused by the main beam and the reflection beam, the expression of small-scale modulated gain spectrum in no-linear media was found. The laws that the maximum gain of small-scale modulation grows with the window’s wedged angle when the media gain or propagation distance are changed were concluded. In the end, the influence of the length and wedge angle of amplifier window on the gain of perturbation was analyzed. Additionally variation relation between the gain of perturbation and the width of the pulse with fixed window was derived. Methods for reducing this modulation and improving the beam quality were provided. 4.By analyzing the linear propagation characteristics of beam interference field and its impact on the parameter of optical elements, some conclusions were found: the interference modulation by parasitic reflection causes the oscillation of the attenuation and the fluctuation of the light field, and further affects time stability of the near-field energy. 5.By analyzing the linear propagation characteristics of interference field and diffraction field respectively, laws about the distribution of interference field and diffraction field’s noise patterns were obtained. The noise pattern of diffraction field centers around the aperture center or the diffraction point, and position of the center is immovable with the growth of propagation distance. Its spatial structure is symmetrical. While the noise pattern of interference field has relation to the optical path difference. Fringe separation and fringe area will change with the distance change. 6.The impact of sampling methods, propagation and media of data acquisition on the measurement accuracy in diagnostic system was discussed. The conclusion has certain reference value for measurement and analysis of beam near-field. The relationship between the dispersion of sampling mirror’s wedged angle and the measured width of the pulse also was studied. |
| 语种 | 中文 |
| 内容类型 | 学位论文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15624]  |
| 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 | 李小燕. 高功率装置中的光束近场研究[D]. 中国科学院上海光学精密机械研究所. 2010. |
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