三倍频激光薄膜的损伤特性特性研究
文献类型：学位论文
| 作者 | 王虎 |
| 文献子类 | 博士 |
| 导师 | 齐红基 |
| 关键词 | 三倍频激光损伤 3w laser-induced damage 高反膜 high reflective coatings 缺陷 defects |
| 其他题名 | Damage Mechanisms of optical coatings induced by 3w laser |
| 英文摘要 | 全固态高功率激光系统是实现惯性约束聚变的重要途径，为获得更高的功率输出与良好的光束质量控制，需要大量关键激光元器件（诸如透镜、薄膜、晶体、光栅等）在时域、空域和频域上调控激光脉冲的能量密度、脉宽、波长和空间分布等性质。由频率转换晶体与谐波分离系统构成的终端元器件系统是承受三倍频激光的主要部分，其特点是负载激光通量高、谐波成分复杂，终端元器件的三倍频激光损伤成为限制打靶通量提升的重要瓶颈。同传统的楔形透镜与聚焦光栅相比，谐波分离膜制备简单，但是较低的激光损伤阈值限制了谐波分离膜在兆焦激光系统中的应用。大量研究工作表明，元器件承受多波长同时辐照时损伤主要由三倍频激光诱发，而基频或者二倍频更多地表现为在三倍频激光的基础上促进损伤的生长与发展。因此，关于三倍频薄膜的损伤是目前迫切需要攻克的难题。本论文主要针对三倍频激光薄膜的损伤特性进行研究，通过大量的实验与理论分析，开展了以下研究工作： 回顾了损伤概率数据的测试与分析基本原理，总结了概率误差的分析方法，结合缺陷系综模型与蒙特卡洛方法模拟了损伤概率测试过程，模拟得到的损伤概率分布与系综模型理论曲线的吻合说明了模拟算法的合理性。研究表明，利用系综模型可以有效地基于小区域损伤概率测试提取缺陷系综参数，并评估小区域到大区域的损伤特性变换，在获得缺陷系综参数的基础上，使用蒙特卡洛模拟方法可以评估同批样品的实际抗激光损伤能力的分布区间。 基片表面缺陷结构对电场调制作用明显，将缺陷简化为基本的矩形结构，不同高度的矩形结构对电场及能流分布做了严格的计算分析，并将结果与时域有限差分方法做比较，确定了能在保持精度的同时减少计算量的收敛参数。提出了模式密度谱的概念，可以有效地分析基片表面纳米结构的谐振效应。新引入了模式参数，根据模式参数以及水平与垂直界面全内反射条件将总场分解成四个模式区域，研究了不同区域的模式对近场与能流的贡献。研究表明，近场增强明显依赖于侧壁全内反射引起的谐振效应。根据能流分析，尽管电场增强在结构侧壁与根部并不明显，但是入射电场以及低级次散射谐波被矩形结构的侧壁诱导并从结构根部进入结构内部。该特征意味着通过优化结构侧壁尺寸与根部轮廓可以调控与抑制矩形结构内部的电场与能流分布。此外，讨论了结构的高度对近场调制与能流分布的影响，谐振效应会随着结构高度的增加而变得更复杂，进而引起更显著的场与能量的局域化，更容易诱导激光损伤。 对三倍频激光辐照下基片与薄膜耦合损伤问题展开研究，利用缺陷系综模型提取了缺陷密度与损伤阈值等信息，并基于缺陷信息对损伤测试过程进行了蒙特卡洛模拟，分析讨论了当前测试点数对不同样品损伤阈值准确性的影响程度。对比酸洗样品镀膜与未镀膜情况下的损伤形貌差异，总结了不同损伤阈值层次的缺陷种类。最后基于杂质热传导模型讨论了镀膜后阈值降低的可能因素，发现缺陷在镀膜过程中受热向表面迁移凝聚是使阈值降低的主要原因。 总结了薄膜材料的基频与三倍频激光损伤差异，分别讨论了基频与三倍频激光辐照下介质薄膜的烧蚀机制。根据损伤概率曲线的形状与损伤形貌将三倍频高反膜的缺陷分为纳米吸收中心与亚微米节瘤缺陷，前者对于驻波场非常敏感并且容易引起激光损伤，而后者的激光性能取决于节瘤轮廓边界以及氟化物与氧化物的界面结合强度。组合膜可以通过抑制纳米吸收中心而有效提升激光损伤阈值。当前结果指出了氧化物膜层中节瘤缺陷的重要性以及将来进一步提升三倍频高反膜损伤阈值的努力方向。; All-solid-state high power laser system is the most promising method to realize the inertial confinement fusion. Abundant key optical components such as lens, coatings, crystals and gratings are required to control the energy, pulse duration, wavelength and spatial properties of laser beam. The final optical systems containing the frequency crystals and harmonic separation components are the major segment to suffer the 3ω laser. Comparing with the traditional wedged prism and focusing grating, the harmonic separation coatings are relatively simple to fabricate and can arrange the 3ω laser efficiently due to the perpendicular reflection. However, the poor laser-induced damage threshold (LIDT) limits their application in the megajoule system. Abundant investigations indicate that the damage initiation is much more affected by 3ω whereas the 1ω simply adds to the 3ω in enhancing damage growth. Therefore, damage issues about the 3ω mirrors are the prior target to be resolved. In this thesis, various works on the damage mechanisms of optical coatings induced by 3ω laser are implemented with sufficient experimental and theoretical efforts, as stated below: The basic principles on damage probability are reviewed and concluded as well as the probability error. With ensemble model, the Mont-Carlo method is applied to simulate the damage testing process. The simulated damage curves are consistent with the theoretical curved given by the ensemble model, which verifies the reasonability of the Mont-Carlo method. The results indicate that, the precursors inferred from the small-area test can be extrapolated to the large-area of the optics. Based on the ensemble parameters, the distribution of damage threshold of the sample can be assessed by Mont-Carlo method. The near-field and energy flow of rectangle structure on the substrate are analyzed rigorously dependent on the structure height. The parameters are optimized to compare with the Finite-Difference Time-Domain Method so that the consumption of calculation can be decreased. The mode amplitude density spectra are proposed to analyze the resonance due to nano-structure. With the introduce of the new mode parameters, the total field is decomposed into four parts and the contribution of different parts are evaluated. The results indicate that the enhancement of near-field is dependent on the resonance of sidewall. According to the energy analysis, The incident and lower order field is directed into the structure from the root of the rectangle structure although the enhancement of the near-field is not obvious around the sidewall. Moreover, the influence of the height on near field and energy flow is discussed to reveal the localized of near field as well as energy. The coupling mechanisms between the 3ω coatings and substrates are investigated and the defect information is extracted according to ensemble model. Based on the defect information, the damage testing processes are simulated with the Mont-Carlo method. Therefore, the accuracy of damage threshold for different samples can be evaluated. The damage morphologies between the coatings and bare substrates are investigated, so the defect level or category is divided. Various factors reducing the damage threshold are discussed and the defect agglomeration during annealing is the mostly possible reason to reduce the damage threshold. Different damage mechanisms between the 3ω and 1ω laser are discussed as well as the surface ablation. The damage precursors in 3ω high reflective coatings are divided into nano-absorbing defects and sub-micrometer nodules according to the damage probability curve or morphologies. The former is sensitive to the electric field while the latter can destroy the standing wave electric field. The nano-absorbing defects can be suppressed with the design of double stack. The damage resistance of double stack is limited by the mechanical strength of both the nodular and interface between the oxides and fluorides, which predicts a promising method to further improve the damage threshold. |
| 学科主题 | 材料学 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/30940]  |
| 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 | 王虎. 三倍频激光薄膜的损伤特性特性研究[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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