Mitigation of laser damage growth in fused silica with a galvanometer scanned CO 2 laser

Abstract

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 CO 2 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 CO 2 pulse was applied to the larger sites. An investigation was conducted to mitigate large surface damage sites using galvanometer scanning of a tightly focused CO 2 laser spot over an area encompassing the laser damage. It was thought that by initially scanning the CO 2 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 CO 2 laser spot had a 1/e 2 -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/cm 2 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.

Investigation of stress induced by CO2 laser processing of fused silica optics for laser damage growth mitigation

“…For instance, the influence of irradiation parameters with models taking into account heating, evaporation and stress generation has been studied [5,6]. Some parametric studies have been conducted in order to determine optimum irradiation conditions [7][8][9] and different protocols have been developed to increase the efficiency of the technique [10,11]. Dedicated tools have also been developed to characterize the damages sites and the mitigated area [12][13][14].…”

Simulations of CO2laser interaction with silica and comparison to experiments

“…Thus, theoretical and experimental works have been completed to improve the CO 2 laser mitigation process like parametric studies in order to determine the optimum irradiation conditions [3][4][5] or works on different protocols 6,7 . The material structural changes, especially stress generation induced by laser heating, have also been investigated 8 .…”

Method of mitigation laser-damage growth on fused silica surface

“…The growth of the damage sites, which is the key factor in determining the lifetime of the optics, increases the output power loss and enhances the beam obscuration. The method of CO 2 laser mitigation is commonly used to resolve the problem of damage growth [4][5][6][7][8][9][10][11][12]. But the downstream intensification [5], generation of redeposition material [6], and thermal stress [7,10] are accompanied by the CO 2 laser mitigation process, which greatly limits the development of laser mitigation technology.…”