An Improved Method of Mitigating Laser Induced Surface Damage Growth in Fused Silica Using a Rastered, Pulsed CO2 Laser

A new method of mitigating (arresting) the growth of large (> 200 m diameter and depth) laser induced surface damage on fused silica has been developed that successfully addresses several issues encountered with our previously-reported(5,6) large site mitigation technique. As in the previous work, a tightly-focused 10.6 mu m CO2 laser spot is scanned over the damage site by galvanometer steering mirrors. In contrast to the previous work, the laser is pulsed instead of CW, with the pulse length and repetition frequency chosen to allow substantial cooling between pulses. This cooling has the important effect of reducing the heat-affected zone capable of supporting thermo-capillary flow from scale lengths on the order of the overall scan pattern to scale lengths on the order of the focused laser spot, thus preventing the formation of a raised rim around the final mitigation site and its consequent down-stream intensification. Other advantages of the new method include lower residual stresses, and improved damage threshold associated with reduced amounts of re-deposited material. The raster patterns can be designed to produce specific shapes of the mitigation pit including cones and pyramids. Details of the new technique and its comparison with the previous technique will be presented.