One of the challenges faced by large-scale high-power laser facilities is that the triple frequency laser operates above the laser damage initiation and growth thresholds of fused silica optics. Thus, the surface damage of optics is inevitable. Once the laser damage occurs, the damage increases exponentially in the subsequent shots, resulting in serious light scattering and light modulation phenomena, which quickly leads to the failure of fused silica optics. At present, the most effective method is local melting or evaporation by CO2 laser to remove the damage. However, after CO2 laser removal of the damages, serious thermal residual stresses are introduced on the surface of fused silica optics, which may lead to the risk of cracking of the optics. For the engineering application of CO2 laser damage mitigation, the introduced residual stress must be eliminated. The mechanism of residual stress in the process of mitigated fused silica damage by CO2 laser was systematically studied. The influence of CO2 laser parameters on the local fictive temperature variation was studied systematically by coupling simulation of laser-heating temperature field and fictive temperature field. It was found that that the formation mechanism of residual stress is local fictive temperature increase of fused silica. By effective control of CO2 laser parameters, the depth of local fictive temperature-increased fused silica can be reduced to several microns, which results in nearly no thermal residual stress at the mitigated damage sites. At the same time, by controlling the local increase of fictive temperature, the edge bulge produced by Marangoni effect during damage mitigation is also effectively suppressed, and the damage of downstream optics, caused by light modulation enhancement of edge bulge of mitigated damage sites, can be avoided. The engineering application of the developed fused silica damage mitigation technology effectively prolongs the service life of large-aperture fused silica optics, and greatly reduces the decommissioning rate, ensuring the economic and stable operation of large-scale high-power solid-state laser facilities.
 

laser-induced damage,fused silica,high power lasers,threshold