皮秒激光诱致多层介质膜光栅损伤研究
文献类型：学位论文
| 作者 | 郝艳飞 |
| 文献子类 | 博士 |
| 导师 | 朱健强 |
| 关键词 | 多层介质膜光栅 Pulse compressing grating 激光损伤 Laser damage 皮秒激光 Picosecond lasers 损伤增长 Damage growth 污染物 contaminant |
| 其他题名 | Laser induced damage of multilayer dielectric gratings by picosecond pulses |
| 英文摘要 | 自18世纪衍射光栅发现以来，衍射光栅作为色散元件已经被广泛用于通信、天文学、化学、生物传感等领域。全息技术等一系列现代纳米制造工艺的革新推动了大尺寸、纳米级精细光栅发展，为高功率激光系统高性能脉冲压缩光栅的成功研制提供了重要保障。多层介质膜光栅（Multilayer dielectric gratings，MLDGs）具有高衍射效率和优秀的抗激光损伤特性，使其成为啁啾脉冲放大技术（Chirped pulse amplification，CPA）中关键的脉冲压缩光学元件。但是，高能拍瓦激光系统中承受激光功率密度最高的压缩光栅激光损伤问题，成为限制其能量提升的主要因素。膜层和光栅局部缺陷以及表面污染物是大口径光栅初始损伤的主要诱因。因此，本论文实验研究了中心波长1053 nm MLDGs的皮秒激光损伤特性（初始损伤、损伤增长、污染物诱致损伤），结合相应的数值模拟结果分析了激光损伤诱发机理和损伤增长机理。具体开展了以下几个方面的研究工作： 首先，实验研究了MLDGs 的初始损伤特性。S/1损伤阈值随着发次的增加而逐渐降低，其中1/1和20/1 激光损伤阈值（Laser induced damage threshold，LIDT）分别为3.79 J/cm2和3.06 J/cm2。初始损伤形貌主要为针点类型损伤，其位置在光栅脊条的背光面，与入射光方向相反，与光栅近场强区分布相一致。随着激光通量的增加，针点状损伤点数目增加。相同脉冲发次下，针点类型损伤数目和尺寸随着激光脉冲能量增加而增加。与光栅高反膜损伤形貌相比较，初步判断光栅的针点状损伤诱因与顶层膜层的缺陷密切相关。 其次，实验研究了皮秒激光诱致光栅损伤增长行为。实验测试获得损伤增长阈值为2.43J/cm2，明显小于之前测量的20/1损伤阈值3.06J/cm2。当初始损伤发生以后，损伤区域面积随着激光脉冲发次的增加而线性增加，最终达到饱和状态。损伤点区域质心随着激光发次的增加逐渐偏离光斑中心，这说明在激光入射方向上损伤点区域的增长过程是非对称的。在激光入射方向上损伤点面积的增加速率是其在相反方向上增加速率的1.8倍，这与数值模拟结果一致。由此，损伤增长的不对称性是由损伤点对入射光的电场调制不对称性导致。 基于光栅损伤增长的饱和特性，提出了两种基于饱和损伤面积的损伤增长阈值测试方法。将某一损伤增长通量下获得的饱和损伤点轮廓与这一通量的光斑轮廓对应，损伤点边界位置对应的通量位置即为光栅的损伤增长阈值，这种方法称为“单通量饱和损伤尺寸分析方法”。同时，实验中不同通量下，相同初始损伤点最终的饱和损伤点尺寸是不同的。在半对数坐标中，不同通量下饱和损伤点面积与通量对数满足线性关系，线性拟合曲线外延与横轴交点为光栅的损伤增长阈值，这种测试方法称为“多通量饱和损伤尺寸分析方法”。虽然，损伤增长过程是非对称的，且最终饱和损伤点也是非对称的，但是通过以上两种测试方法获得的不同轴上的损伤增长阈值结果基本相同，同时也与采用损伤增长概率方法获得的损伤增长阈值结果完全一致。文章中也对两种测试方法的基本原理与优缺点进行分析，并与损伤寿命阈值进行了对比分析。 最后，实验研究了金属、聚合物、灰尘等运行环境中常见颗粒污染物对于光栅损伤的影响。结果表明污染物显著降低光栅元件的损伤阈值，SEM微观损伤形貌表明其诱发光栅损伤机理主要是热吸收熔融导致材料破坏。获得了不同脉冲通量下，不同尺寸的污染物颗粒诱发光栅损伤规律，绘制出相应的运行负载曲线。真空运行环境下光栅的平均运行通量在1.3J/cm2左右，污染物颗粒尺寸应该控制在50μm以下。 高功率拍瓦激光系统内使用的都是米级光栅，且运行环境为真空。本论文研究了光栅的皮秒损伤规律，初步判断了针点状光栅损伤的诱因，为提高光栅损伤性能提供了重要基础；研究了皮秒激光诱致光栅损伤增长规律和污染物诱致损伤规律，为洁净控制和大口径光栅寿命判断提供了关键数据。总之，本论文研究对于提高高能皮秒拍瓦系统负载能力具有重要的应用意义和价值。; The innovation of a series of modern nano-manufacturing technologies such as holographic technology has promoted the development of large-size, nano-scale fine gratings, and further promoted the development of pulsed compression gratings for high-power laser systems. Due to high diffraction efficiency and excellent laser damage resistance, multilayer dielectric gratings (MLDGs) serve as the key pulse compression optics in chirped pulse amplification (CPA). However, laser damage of large-aperture MLDGs has become the major issue limiting the energy boost of high-power ultrafast laser systems. Local defects and contaminants are the main causes of laser damage of MLDGs. Therefore, the laser damage characteristics of MLDGs were experimentally studied in this thesis. Numerical analysis of the electric field of the grating sample reveals the mechanisms of laser-induced damage and damage growth. The thesis includes the following aspects: The intrinsic damage characteristics of MLDGs were investigated by the 1/1 and S/1 damage test methods. Laser induced damage threshold (LIDT) decreases with the shot number, and the LIDTs for 1/1 and 20/1 were 3.79 J/cm2 and 3.06 J/cm2. The initial damage morphology of the grating sample is mainly the pinpoints, and they locates at the grating ridge opposite to the direction of the incident light, consistent with the intensity distribution on the grating. With the increase of laser fluence, the number of pinpoint damage increases. With the same fluence, the number and size of the damage pinpoint increases with increasing the shot number. By comparing to the damage morphology of the grating coating, it seems that the pinpoint damage of the gratings is casued by the defects on the grating coating. We experimentally investigated the laser damage growth behaviors of MLDGs by the picosecond pulses at 1053nm. The measured damage growth threshold (DGT) of 2.43J/cm2 is significantly lower than the 20/1 damage threshold of 3.06 J/cm2. Once the damage site is initiated, the damage area grows linearly with shot number and saturates after sufficient shots due to the Gaussian spot. The barycenter of the growing damage site deviates from the laser spot center and their distance increases with the shot number, which indicates the asymmetry of the damage growth along the laser propagation axis. The growth rate of the damage site along the laser propagation direction is larger than that in the reverse direction by a factor of ~1.8 for various fluences. The comparison of the experimental and numerical results reveals that the asymmetrical intensity modulation induced by the damage sites causes the asymmetry in growth. The revealed characteristics and mechanisms of the damage growth can be of great significance to predict the lifetime of the MLDGs in high-power laser systems. We propose two efficient methods of determining the DGT based on the saturation damage size analysis (SDSA) for the MLDGs by picosecond pulsed lasers. The damage size at laser fluences above the DGT increases with shot number and finally saturates due to the Gaussian focal spot. The DGT is extracted by mapping the boundary of a saturation damage site obtained at single fluence to the beam profile, which is named as mono-fluence SDSA method. Meanwhile, the saturation damage size decreases when reducing laser fluence. The fitting and extrapolation of the saturation damage sizes at different fluences are also useful to accurately determine the DGT, which is named as multi-fluence SDSA method. Although the saturation damage site is asymmetric, the DGTs measured with two SDSA methods are almost the same and both in very good agreements with those obtained with the standard growth probability method. The underlying mechanisms and advantages of two SDSA methods are extensively discussed. The consistence of two SDSA methods in determining DGT is attributed to the same morphology of the initial damage and the saturation damage boundary, as well as the local damage dynamics. The relation of the lifetime damage threshold and DGT obtained with the SDSA method is also revealed. The influence of common particle contaminants, such metal, polymer, and dust, on the initial damage of the grating was experimentally studied. The contaminants can significantly reduce the damage threshold of the grating materials. The microscopic damage morphology shows that the grating damage mechanism induced by the contaminants is mainly the thermal melting. We obtained the relation between the particle sizes of the contaminants and the grating damage fluence. In the vacuum operating environment, when the average operation fluene of the grating is about 1.3 J/cm2, the particle size of the contaminant should be smaller than 50 μm. The MLDG in the high-power petawatt laser system has a size up to one meter, and the operating environment is vacuum. In this dissertation, the picosecond damage of the grating was studied, and the cause of the pinpoint damage on MLDGs is preliminarily judged, which provides an important information for improving the damage performance of the grating. The growth law of the grating induced by the picosecond laser and the damage induced by the contaminant are studied. The results provide a key data for cleanliness control and lifetime estimation of grating optics. In short, the conclusions of the dissertation are of great significance for improving the damage resistance of high-energy picosecond petawatt system. |
| 学科主题 | 光学工程 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/31061]  |
| 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 | 郝艳飞. 皮秒激光诱致多层介质膜光栅损伤研究[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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