Fused Silica Vacuum Damage And Damage Growth

Laser induced damage of large fused silica optics in high power laser system limits output of the laser system used for inertial confined fuse (ICF); meanwhile, growth of initiated damages have become one of important causes of high operational cost. To enhance resistance of optics to laser damage, inhibit the growth of initiated damage and prolong the lifetime of optics are the problems needed to be solved immediately.In this paper we discussed the mechanisms of formation of damage initiated by defects; tested thresholds of fused silica in different atmospheres at different vacuum; tested the growth of initiated damages located on input and output surfaces of fused silica; pre-conditioned fused silica surface with ultraviolet and CO2 laser.Main jobs we have finished and their corresponding results as follows:1) Thresholds of fused silica sample have been tested in air, N2 and O2 at vacuum ranged from 10-3 to 105 Pa with different test methods respectively,the experimental results show that environments of different atmospheres have no conspicuous effects on laser-induced damage threshold (LIDT) and lifetime of fused silica, although partial pressures of oxygen are different. The resistance to multi-pulse irradiation of fused silica in 103 Pa can be comparable with in 105 Pa, whereas this resistance in high vacuum and low vacuum tend to decline.2) Growth of initiated damages located on fused silica input and output surfaces have been tested. Experimental results show exponential growth in the lateral size of damage sites located on exit surface with shot number, whereas linear growth in the lateral site of damage sites located on input surface.3) In order to have more insight into the relation between growth of fused silica initiated damage and numbers of laser pulse, the damage sites located on exit surface of conditioned fused silica have been irradiated with 355nm pulse laser. By comparison with the case of growth of unconditioned fused silica damage sites, experimental results show exponential growth in the lateral size of damage sites located on exit surface with shot number in both the case of conditioned and the case of unconditioned, whereas the growth of conditioned silica damage sites are more rapid than the growth of unconditioned silica damage sites. Laser condition can effectively improve laser damage threshold, but it can reduce the resistance of fused silica to damage growth.4) Fused silica sample have been preconditioned with ultraviolet laser and CO2 laser respectively via raster scanning method. Results shows that two preconditioning techniques can enhance the resistance to 3ωlaser damage; In comparison, only using ultraviolet laser, there are no negative effects on optical properties for fused silica sample, but magnitude of the enhanced LIDT is not obvious. Only using CO2 laser, the enhancement of the resistance to 3ωlaser damage is more prominent, while the thermal stress produced in preconditioning process would transform the surface, degrade the surface quality of optical elements. Optimal effects on enhancement of LIDT and surface quality can be gotten by the combination of two preconditioning methods.