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 m CO 2 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 redeposited 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.

Localized infrared (IR) laser heating of fused silica optics has proven highly effective in reducing or removing surface flaws, which tend to limit performance in high power laser systems. Here, we present both simulation and experimental results to examine the use of IR laser light to polish, anneal, and micro-shape fused silica surfaces used in high power laser systems. We show how the resulting material response can be tuned by considering the temperature-dependent optical constants of the material and choosing the appropriate laser parameter set. For example, nonevaporative laser polishing of glass surfaces to heal crack networks is shown most effective when using mid-IR lasers, which lead to laser energy coupling up to %1 mm in depth. In contrast, longwave IR light tuned to the Restrahlen frequency of the material is shown to evaporate material most efficiently with penetration depths of <1 mm. Through calibrated, time-resolved thermal imaging we are able to monitor the laser polishing process, to control material response. The results of our studies can be applied beyond the practical application of damage mitigation in high energy pulsed laser systems to any which require laser-smoothing and shaping of silica surfaces.

Mitigation of 351nm laser-induced damage sites on fused silica exit surfaces by selective CO 2 treatment has been shown to effectively arrest the exponential growth responsible for limiting the lifetime of optics in high-fluence laser systems. However, the perturbation to the optical surface profile following the mitigation process introduces phase contrast to the beam, causing some amount of downstream intensification with the potential to damage downstream optics. Control of the laser treatment process and measurement of the associated phase modulation is essential to preventing downstream 'fratricide' in damage-mitigated optical systems. In this work we present measurements of the surface morphology, intensification patterns and damage associated with various CO 2 mitigation treatments on fused silica surfaces. Specifically, two components of intensification pattern, one on-axis and another off-axis can lead to damage of downstream optics and are related to rims around the ablation pit left from the mitigation process. It is shown that control of the rim structure around the edge of typical mitigation sites is crucial in preventing damage to downstream optics.

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.

No abstract