熔石英表面紫外损伤点的飞秒激光修复技术

Mitigation of Ultraviolet Laser Damage on Fused Silica Surface with Femtosecond Laser System

熔石英表面激光损伤发展问题一直制约着高功率激光系统的运行通量,采用飞秒激光修复损伤点抑制损伤发展并探索修复机理。首先采用时域有限差分方法(FDTD)分析不同形状修复点的电场分布,优化修复点结构。通过改变飞秒激光脉冲能量、样品台移动参数控制修复点的形状、尺寸与深度,实现最优化修复结构。结果表明矩形修复结构降低了局部区域光强分布,经飞秒激光修复后,修复点的损伤发展阈值远高于修复前损伤点的发展阈值。采用微区电子能谱仪(EDS)分析修复点的化学成分发现飞秒修复能减少氧缺陷含量,从而降低吸收系数。因此,减少吸收性缺陷以及降低局部光强是抑制损伤发展的关键因素。

Growth of laser induced damage on the surface of fused silica plays a major role in determining optics lifetime in high power laser systems. In this paper, a femtosecond laser system is applied to create benign mitigation pits to replace growing damage sites; the mitigation mechanism is also explored. The electric filed distribution around mitigation pit is modeled with the finite difference time domain （FDTD） method to determine the optimal mitigation geometry. The shape, size and depth of mitigation pit are controlled by varying energy of femtosecond laser, changing the movement mode of sample stage, in order to achieve the optimal mitigation geometry. The results of laser damage growth test indicate that the rectangular mitigation structure can reduce the light intensity distribution and the damage threshold of mitigation pits is much higher than damaged sites. Furthermore, Energy dispersive spectrometers （EDS） microanalysis technique is used to detect the chemical composition of mitigation pits, and results shows that ultraviolet absorbing defects are removed after mitigation. Reduces of the ultraviolet absorbing defects as well as the local light intensity are key factors to succeed in mitigating growing damage sites.