Producing fused silica optics with high UV-damage resistance to nanosecond pulsed lasers

Abstract

The laser induced damage to optics has been an issue of paramount importance in laser research community. The low damage threshold of fused silica surfaces predominantly restricts the development of high power and high energy systems. This paper is aimed at improving the surface damage threshold of fused silica substrates by researching the effect of mechanical and chemical defects on laser damage: cracks/scratches and metallic impurities. The cracks were found to close, at least in part, after thermal processing and the damage threshold of the indented region was little affected by the thermal processing. In contrast, the cracks were enlarged after chemical etching and the damage threshold was improved slightly. Concerning scratches, the damage threshold can be recovered significantly after different HF-based etching. The metallic contamination can be removed by HF-based etching and acid leaching. The etched surface shows that the damage threshold increased first to ~30J/cm 2 and then decreased with etching time while the damage threshold stabilized at ~30J/cm 2 for leaching >45min. The surface roughness may degrade after etching, from <1nm to 3~5nm RMS, but that is ~1nm after leaching. The leaching may be a potential method for dissolving metallic contaminants on the glass surface in order to get a smooth surface with high damage resistance.

Ultrasonic‐assisted wet chemical etching of fused silica for high‐power laser systems

“…The damage resistance of scratches vs etched depth was investigated, and the average results of LIDT at the specific etched depth were obtained as the diamond data in Figure show. Although the damage characteristics of scratches are affected by multiple factors in addition to the size of scratches, the bluntness of cracks, etc., our results suggest that deep etching (~25 μm) benefits the LIDT improvement of brittle scratches in that the scratches are blunted and the lateral cracks are vanished by deep etching that is highly deleterious to damage resistance of fused silica. The average of LIDT for 1, 1.5, and 2 N scratches is increased to ~9.1, 6.9, and 6.4 J/cm 2 , respectively, when the material removal amount is ~25 μm.…”

“…Surface/subsurface defects like metallic impurities, mechanical fractures (scratches, cracks, etc. ), and organic contaminants are found to be potential laser damage initiators . Among them, scratches can reduce laser‐induced damage threshold (LIDT) of optics for multiple reasons, including the weakening of surface due to existing mechanical fractures, thermal effects triggered by the UV‐absorptive impurities embedded within the cracks, and/or the light filed intensification caused by the scratches/cracks .…”

“…Therefore, mitigating and eliminating surface scratches contributes to the improvement of LIDT. Reports have indicated that HF‐based etching is a comprehensive treatment to raise the LIDT of fused silica due to the removal of metallic impurities and the bluntness of cracks . Suratwala et al used optimized etching method to process the scratches on fused silica optics, and the average damage initiation threshold at scratches was increased from 7 to 41 J/cm 2 under 3 ns pulsed UV laser, and the damage initiation probability is decreased from ~100 mm −1 of scratch length to ~0.001 mm −1 at laser fluence of 12 J/cm 2 after etching process .…”

Characterization of the polishing induced contamination of fused silica optics

“…First, it has been shown that the surface layer modification is dependent on the processing parameters applied. Suratwala et al 4 and Wang et al 17 explained that the cerium depth penetration is consistent with a mechanism which depends on the relative rate of hydrolyze between Ce-OCe and Si-O-Si bonds, where the ratio increases with the interface temperature which increases with polishing removal rate. However, the process parameters cannot totally explain the huge differences in depth and concentration.…”

Laser induced damage due to scratches in the surface of nonlinear optical crystals KH2PO4 (KDP)

“…On the other hand, soft but brittle properties render KDP susceptible to scratches. Scratches generally have significant impact on damage resistance to lasers, which has been demonstrated on fused silica sample [6][7][8][9]. Thus it is of great importance to carry out investigations into the damage characteristics, in particular, the effect of scratches on laser-induced damage of KDP.…”