Evolution mechanism of subsurface damage during laser machining process of fused silica

The machining-induced subsurface damage (SSD) on fused silica optics would incur damage when irradiated by intense lasers, which severely restricts the service life of fused silica optics. The high absorption of fused silica to 10.6 μm makes it possible to utilize pulsed CO2 laser to remove and characterize SSD by layer-by-layer ablation, which improves its laser-induced damage threshold. However, thermal stress during the laser ablation process may have an impact on SSD, leading to extension. Still, the law of SSD morphology evolution mechanism has not been clearly revealed. In this work, a multi-physics simulated model considering light field modulation is established to reveal the evolution law of radial SSD during the laser layer-by-layer ablation process. Based on the simulation of different characteristic structural parameters, two evolution mechanisms of radial SSD are revealed, and the influence of characteristic structural parameters on SSD is also elaborated. By prefabricating the SSD by femtosecond laser, the measurements of SSD during CO2 laser layer-by-layer ablation experiments are consistent with the simulated results, and three stages of SSD depth variation under two evolution processes are further proposed. The findings of this study provide theoretical guidance for effectively characterizing SSD based on laser layer-by-layer ablation strategies on fused silica optics.

熔融石英光学元件的加工诱导亚表面损伤（machining-induced subsurface damage, SSD）在强激光辐照下会引发损伤，严重限制其服役寿命。熔融石英对10.6 μm波段具有高吸收特性，使得可利用脉冲CO₂激光通过逐层烧蚀工艺实现亚表面损伤的去除与表征，进而提升其激光诱导损伤阈值。然而，激光烧蚀过程中产生的热应力会对亚表面损伤产生影响，导致其扩展，目前亚表面损伤形貌演化机制的规律尚未被明确揭示。本研究建立了考虑光场调制的多物理场仿真模型，以揭示激光逐层烧蚀过程中径向亚表面损伤的演化规律。通过对不同特征结构参数的仿真分析，阐明了径向亚表面损伤的两种演化机制，并详细论述了特征结构参数对亚表面损伤的影响。通过飞秒激光预制亚表面损伤，CO₂激光逐层烧蚀实验中的亚表面损伤测量结果与仿真结果相符，进一步提出了两种演化过程下亚表面损伤深度变化的三个阶段。本研究成果为基于激光逐层烧蚀策略有效表征熔融石英光学元件的亚表面损伤提供了理论指导。