A process whereby laser-initiated surface damage on KDP/DKDP optics is removed by spot micro-machining using a high-speed drill and a single-crystal diamond bit, is shown to mitigate damage growth for subsequent laser shots. Our tests show that machined dimples on both surfaces of an AR coated doubler (KDP) crystal are stable, for 526 nm, ~3.2 ns pulses at ~12 J/cm<sup>2</sup> fluences. Other tests also confirmed that the machined dimples on both surfaces of an AR coated tripler (DKDP) crystal are stable, for 351 nm, ~3 ns pulses at ~8 J/cm<sup>2</sup>. We have demonstrated successful mitigation of laser-initiated surface damage sites as large as 0.14 mm diameter on DKDP, for up to 1000 shots at 351 nm, 13 J/cm<sup>2</sup>, ~11 ns pulse length, and up to 10 shots at 351 nm, 8 J/cm<sup>2</sup>, 3 ns. Details of the method are presented, including estimates for the heat generated during micromachining and a plan to implement this method to treat pre-initiated or retrieved-from-service, large-scale optics for use in high-peak-power laser applications.
We report an experimental investigation of mitigating surface damage growth at 351 nm for machine-finished DKDP optics. The objective was to determine which methods could be applied to pre-initiated or retrieved-from-service optics, in order to stop further damage growth for large aperture DKDP optics used in high-peak-power laser applications. The test results, and the evaluation thereof, are presented for several mitigation methods applied to DKDP surface damage. The mitigation methods tested were CW-CO<sub>2</sub> laser processing, aqueous wet-etching, short-pulse laser ablation, and micro-machining. We found that micro-machining, using a single crystal diamond tool to completely remove the damage pit, produces the most consistent results to halt the growth of surface damage on DKDP. We obtained the successful mitigation of laser-initiated surface damage sites as large as 0.14 mm diameter, for up to 1000 shots at 351 nm and fluences in the range of 2 to 13 J/cm<sup>2</sup>, ≈ 11 ns pulse length. Data obtained to-date indicates that micro-machining is the preferred method to process large-aperture optics.
We report a summary of the surface damage, growth mitigation effort at 3(omega) for fused silica optics at LLNL. The objective was to experimentally validate selected methods that could be applied to pre-initiated or retrieved-from- service optics, to stop further damage growth. A specific goal was to obtain sufficient data and information of successful methods for fused silica optics to select a single approach for processing NIF optics. This paper includes the test results and the evaluation thereof, for several mitigation methods for fused silica. The mitigation methods tested in this study are wet chemical etching, cold plasma etching, CO<SUB>2</SUB> laser processing, and micro-flame torch processing. We found that CO<SUB>2</SUB> laser processing produces the most significant and consistent results to halt laser-induced surface damage growth on fused silica. We recorded successful mitigation of the growth of laser-induced surface damage sites as large as 0.5-mm diameter, for 1000 shots at fluences in the range of 8 to 13 J/cm<SUP>2</SUP>. We obtained sufficient data for elimination of damage growth using CO<SUB>2</SUB> laser processing on sub-aperture representative optics, to proceed with application to full- scale NIF optics.
A program to identify and eliminate the causes of UV laser- induced damage and growth in fused silica and DKDP has developed methods to extend optics lifetimes for large- aperture, high-peak-power, UV lasers such as the National Ignition Facility (NIF). Issues included polish-related surface damage initiation and growth on fused silica and DKDP, bulk inclusions in fused silica, pinpoint bulk damage in DKDP, and UV-induced surface degradation in fused silica and DKDP in a vacuum. Approaches included an understanding of the mechanism of the damage, incremental improvements to existing fabrication technology, and feasibility studies of non-traditional fabrication technologies. Status and success of these various approaches are reviewed. Improvements were made in reducing surface damage initiation and eliminating growth for fused silica by improved polishing and post- processing steps, and improved analytical techniques are providing insights into mechanisms of DKDP damage. The NIF final optics hardware has been designed to enable easy retrieval, surface-damage mitigation, and recycling of optics.
A technique for inhibiting the growth of laser-induced surface damage on fused silica, initiated and propagated at the 351-nm laser wavelength, has been investigated. The technique exposes the damage sites to single pulses of a CO<SUB>2</SUB> laser operating at the 10.6 micrometers wavelength at or near beam focus. This method results in a very localized treatment of the laser damage site and modifies the site such that laser damage does not propagate further. A laser damage site initiated with a single pulse of 355-nm laser light at approximately 45 J cm<SUP>-2</SUP> and 7.5-ns pulse duration grows rapidly upon further illumination at 8 J cm<SUP>-2</SUP> with 100% probability. Treatment of these sites with single pulses of 10.6 micrometers laser light for one second at a power level of between 17 and 37 Watts with a beam diameter of 5 mm alters the damage site such that it does not grow with subsequent 351-nm laser illumination at 8 J cm<SUP>-2</SUP> 10-ns pulse duration for > 1000 shots. The technique has been found to be 100% effective at stopping the growth of the laser damage.
We investigated chemical etching as a possible means to mitigate the growth of UV laser-induced surface damage on fused silica. The intent of this work is to examine the growth behavior of existing damage sites that have been processed to remove the UV absorbing, thermo-chemically modified material within the affected area. The study involved chemical etching of laser-induced surface damage sites on fused silica substrates, characterizing the etched sites using scanning electron microscopy (SEM) and laser fluorescence, and testing the growth behavior of the etched sites upon illumination with multiple pulses of 351- nm laser light. The results show that damage sites that have been etched to depths greater than about 9 micrometers have about a 40% chance for zero growth with 1000 shots at fluences of 6.8-9.4 J/cm<SUP>2</SUP>. For the etched sites that grow, the growth rates are consistent with those for non-etched sites. There is a weak dependence of the total fluorescence emission with the etch depth of a site, but the total fluorescence intensity from an etched site is not well correlated with the propensity of the site to grow. Deep wet etching shows some promise for mitigating damage growth in fused silica, but fluorescence does not seem to be a good indicator of successful mitigation.
The effective lifetime of optics in the UV is limited both by laser induced damage and the subsequent growth of laser initiated damage sites. We have measured the growth rate of laser induced damage in fused silica in both air and vacuum. The data shows exponential growth in the lateral size of the damage site with shot number above threshold fluence. The concurrent growth in depth follows a linear dependence with shot number. The size of the initial damage influences the threshold for growth; the morphology of the initial site depends strongly on the initiating fluence. We have found only a weak dependence on pulse length for growth rate. Low fluence conditioning in air may delay the onset of growth. Most of the work has been on bare substrates but the presence of a sol-gel AR coating has no significant effect.
General physical relations connect the expected size and depth of laser damage induced craters to absorbed laser energy and to the strength of the material. In general, for small absorbers and instantaneous energy release, one expects three regions of interest. First is an inner region in which material is subjected to high pressure and temperature, pulverized and ejected. The resultant crater morphology will appear melted. A second region, outside the first, exhibits material removal due to spallation, which occurs when a shock wave is reflected at the free surface. The crater surface in this region will appear fractured. Finally, there is an outermost region where stresses are strong enough to crack material, but not to eject it. These regions are described theoretically and compared to representative observed craters in fused silica.