熔石英元件超精密制造中强激光低损伤阈值诱因的形成及去除机制研究

Laser-induced damage threshold of fused silica surface is an issue of great importance in high power laser facility. The limitation to enhancing laser damage threshold of fused silica is the defects that are induced during the processing of fused silica, inclusive of micro-cracks by grinding process, contaminants due to polishing and wet chemical etching, and so forth. The micro-cracks and chemical contaminants, two factors of paramount importance in laser induced damage, will be investigated thoroughly in the proposed research plan. Firstly, the generation mechanism of micro-cracks in grinding will be studied by correlating the external processing parameters (macroscopic) to the fracture of fused silica due to a single abrasive (microscopic). The influence of micro-cracks/scratches on the laser damage will be researched based on the electromagnetics theory and thermodynamics. Secondly, the roles of contaminants in deteriorating laser damage threshold will be examined from the viewpoint of thermal absorption and scattering. The proposed plan is aimed at elucidating the dependence of damage threshold on the size, optical and thermal properties of contaminants. Lastly, some potential physic-chemical processing methods that may diminish the defects and ameliorate the damage threshold of fused silica will be explored tentatively. The proposed research, if successful, will be instrumental in raising the damage threshold of fused silica and thus in facilitating the construction of giant laser system.

熔石英光学元件加工中形成的各种缺陷决定了光学元件激光损伤阈值的高低，研究影响损伤的各种缺陷对提高熔石英元件的损伤阈值具有重要的科学意义。本研究以熔石英光学元件超精密加工过程中产生的极大限制损伤阈值的两类缺陷（物理结构缺陷和化学异质缺陷）为对象，建立微观力学模型、化学反应动力学模型和电磁学模型，研究加工中的物理结构缺陷和化学异质缺陷的产生机理。利用电磁学有限差分法和光学散射吸收理论探究物理结构缺陷与化学异质缺陷对熔石英激光损伤阈值的影响规律，研究物理化学改性处理对物理缺陷和化学缺陷的影响以及对熔石英元件激光损伤性能的影响，为得到具有高损伤阈值的熔石英元件提供理论基础和参考数据。本项目研究可为高功率激光系统的建设和稳定运行提供有益参考，在强激光系统和光电子通信等领域具有重要的理论意义和实用价值。

熔石英玻璃由于其光学性能良好、热学性能稳定，已成为高功率激光聚变装置中应用最广泛的一种光学元件。但在紫外纳秒激光(351/355nm)的辐照下，熔石英表面很容易产生激光损伤点，且在后续激光的连续辐照下，损伤点尺寸可能呈现指数型增长，对元件造成灾难性的破坏。激光损伤已成为限制激光系统输出能量水平的“瓶颈”，因此，抑制元件激光损伤的产生和增长，是提升整个激光系统的性能和寿命的关键。光学元件的损伤分为本征损伤和非本征损伤，完美无缺陷的元件在光场强度足够高时也会诱发损伤，称之为本征损伤，反之如果损伤是由于缺陷诱导而在光学元件表面或体内形成的损伤则为非本征损伤。熔石英玻璃体内的纳秒紫外损伤阈值可以达到 100J/cm2以上，接近本征损伤，但表面损伤阈值只有提损伤阈值的十分之一不到。根本原因是表面含有大量的缺陷，这些缺陷大大降低了光学元件的损伤阈值。这些缺陷包括元件制造过程中引入的表面/亚表面裂纹等物理缺陷，表面杂质污染等化学缺陷以及氧空位(ODC)、非桥氧键(NBOHC)等电子结构缺陷。如何有效地提升元件激光损伤阈值(Laser-induced damage threshold, LIDT)已成为光学领域必须解决的关键问题。本研究基于光学元件加工缺陷的形成机制和激光损伤的诱导机理，研究了亚表面缺陷的形成和特性，结合仿真与实验研究了机械缺陷对光学元件损伤性能的影响，采用不同后处理技术对诱发激光损伤的缺陷进行了处理获得不同处理技术的处理效果，优选出合适的处理工艺，将划痕处的损伤阈值恢复到接近无损伤处的阈值。本项目研究成果已经应用于大型激光装置的光学元件生产上，利用本项目研究成果加工的光学元件已能供装置在负载6J/cm2下>50发次运行。