脉冲压缩光栅激光损伤机制研究

The pulse compression gratings (PCG) are one of the key components of the chirped laser pulse amplification and compression, which directly interact with high power density output laser pulses, and its laser damage resistance will directly affect the performance of the entire laser system. The structure of PCG is very complex, which contains the substrate, films and relief. Up to now, lack of dedicated systematical research on laser damage characteristics, mechanism and process of PCG has been performed. This project intends to lay artificial simulated damage source on the surface of the substrate, film and relief of grating, or change the structure of the films stack and relief of the gratings, and then do laser induced damage experiments, and finally sum up damage characteristics and rules of each damage source and grating characteristics. According to the damage characteristics and rules, a reference manual for damage analyzing will be edited. Meanwhile, an effective method for identifying the locations of the damage sources will be work out. This method is based on the damage analyzing reference manual and the laser damage data of practical used PCG, which are obtained using the common used equipments for testing and analyzing materials microstructure, morphology and composition. Using this method, we can know that the damage sources are introduced from which step of the fabrication process of a PCG. And then an effective improvement measure can be suggested to improve the anti-laser damage ability of the PCG. In addition, some theoretical analyzing models will be established to systematically study the laser induced damage mechanism and process of the PCG based on plenty of damage experiment data.

脉冲压缩光栅作为激光脉冲啁啾放大和压缩的核心元件之一，直接与输出的高功率密度激光脉冲作用，其抗激光损伤能力将直接影响整个激光系统的性能。脉冲压缩光栅结构复杂，包含基底、膜层以及浮雕等结构区域，其激光损伤特征、机理与过程缺乏专门的系统的研究。本项目拟在光栅基底、膜层及浮雕表面人为铺设模拟损伤源，或改变光栅的膜系与浮雕结构，进行激光辐照损伤实验，总结出各损伤源及光栅结构相关的损伤特征与规律，整理成损伤分析参考手册。同时，制订依据损伤分析参考手册和光栅损伤数据，有效分离出引起光栅损伤的损伤源位置的分析方法。采用此方法可以分析出光栅制作过程中引入损伤源的工艺环节，有助于提出有针对性的工艺改进措施，提高脉冲压缩光栅的抗激光损伤能力。该方法所需损伤数据由常规显微结构、形貌与成分测试仪器获得，易于推广。此外，项目还将以大量损伤实验数据为基础，建立理论分析模型，系统研究脉冲压缩光栅的激光损伤机理与过程。

衍射光栅尤其是脉冲压缩光栅作为激光脉冲啁啾放大和压缩的核心元件之一，直接和输出的高功率密度激光脉冲作用，其抗激光损伤性能对整个激光系统至关重要。本项目实施中设计和制备了镀金光栅、介质膜光栅和金属介质光栅等三种脉冲压缩光栅，分析了其激光损伤特性与规律。同时，建立有限元分析模型，计算了不同结构光栅中的电磁场、温度场和应力场分布情况，研究了所制备光栅的激光损伤机理。三种结构光栅中，镀金光栅具有最宽的高衍射谱区，介质膜光栅具有最高的衍射效率峰值，金属介质的衍射效率峰值和带宽介于镀金光栅和介质膜光栅之间。.对于镀金光栅，由于金膜自身吸收较高，损伤阈值较低，缺陷杂质对损伤阈值影响不大，光栅损伤形式主要是金膜熔融或剥落。通过添加保护层、增强金膜附着力以及采用较软的基层光刻胶光栅材料，可以适当提高光栅的激光损伤阈值。对于介质膜光栅，纳秒激光辐照时由于材料自身吸收较低，光栅损伤阈值较高。光栅表面存在污染或缺陷会明显降低其损伤阈值，损伤形式主要为光栅脊或槽表面的损伤微坑。因此光栅表面保持清洁和控制缺陷很重要。皮秒激光辐照时，损伤主要以光栅脊的开裂和倒伏为主，缺陷杂质的影响随激光脉宽变窄越来越不明显。对于金属介质光栅，损伤阈值介于前两者之间，皮秒激光辐照时光栅损伤形式主要为光栅脊中氧化铪层的热烧蚀。飞秒激光辐照时光栅损伤位置不变，但是损伤形式主要为超热电子引起的材料喷发，具有明显的应力损伤特征。.对于损伤源位置的判断，实验上以FIB-SEM测试方法最为有效，该方法可以精确定位损伤斑位置，获得精细的损伤斑剖面形貌，以便直观地判断损伤源位置、形态和损伤机理。三种结构光栅在近阈值能量密度激光辐照下，损伤均发生在光栅表面，影响光栅激光损伤阈值的是分布于光栅表面的缺陷或杂质。因此，本项目着重研究了光栅表面损伤的特征和规律。本项目研究所取得的结论对提高脉冲压缩光栅的激光损伤阈值具有指导性意义。