The influence of CO2 laser local irradiation on the laser damage resistance of fused silica

The influence of CO2 laser local irradiation (CLLI) on the laser damage resistance of fused silica was studied. It is found that CLLI has no evident effect on the laser-induced damage threshold (LIDT) of fused silica when the temperature of irradiation center is low (1139 K). However, the influence of CLLI on LIDT becomes important when the temperature of irradiation center is higher (1638 K). AT first, LIDT decreases with r increasing from zero to r(M), where r is the distance to irradiation center, and r(M) is the distance at which the residual stress-induced phase retardance reaches the maximum. Then, LIDT increases a little when r is larger than r(M). The origin of this phenomenon is discussed. Due to the residual stress, re-initiated damage in irradiation region can result in the formation of fractures. The fractures due to the re-initiated damage at irradiation center transport along radial direction firstly, and then transport along the tangential direction near r(M). This may be due to the maximum tensile hoop stress and radial stress dependence of radius. Cares should be taken for the optical cleaning when thermal oven annealing is used to eliminate the residual stress. Otherwise, crystallization can be induced by contamination during annealing. The contamination also has negative impact on the light transmission and LIDT.