Mitigation of laser damage growth in fused silica with a galvanometer scanned CO2 laser

At the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory (LLNL), mitigation of laser surface damage growth on fused silica using single and multiple CO2 laser pulses has been consistently successful for damage sites whose lateral dimensions are less than 100 μm, but has not been for larger sites. Cracks would often radiate outward from the damage when a CO2 pulse was applied to the larger sites. An investigation was conducted to mitigate large surface damage sites using galvanometer scanning of a tightly focused CO2 laser spot over an area encompassing the laser damage. It was thought that by initially scanning the CO2 spot outside the damage site, radiating crack propagation would be inhibited. Scan patterns were typically inward moving spirals starting at radii somewhat larger than that of the damage site. The duration of the mitigation spiral pattern was ~110 ms during which a total of ~1.3 J of energy was delivered to the sample. The CO2 laser spot had a 1/e2-diameter of ~200 μm. Thus, there was general heating of a large area around the damage site while rapid evaporation occurred locally at the laser spot position in the spiral. A 30 to 40 μm deep crater was typically generated by this spiral with a diameter of ~600 μm. The spiral would be repeated until there was no evidence of the original damage in microscope images. Using this technique, damage sites as large as 300 μm in size did not display new damage after mitigation when exposed to fluences exceeding 22 J/cm2 at 355 nm, 7.5 ns. It was found necessary to use a vacuum nozzle during the mitigation process to reduce the amount of re-deposited fused silica. In addition, curing spiral patterns at lower laser powers were used to presumably "re-melt" any re-deposited fused silica. A compact, shearing interferometer microscope was developed to permit in situ measurement of the depth of mitigation sites.