Methods for mitigating growth of laser-initiated surface damage on DKDP optics at 351 nm

Abstract

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-CO₂ 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/cm², ≈ 11 ns pulse length. Data obtained to-date indicates that micro-machining is the preferred method to process large-aperture optics.