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

Brittle scratches (microcracks) are important damage precursors to fused silica optics under 351/355-nm laser irradiation. HF-based etching is an economical way to mitigate the influence of scratches on laser-induced damage. We here employed ultrasonic/megasonic-assisted HF etching to alleviate the damage precursors. Samples were treated by being submerged into 6 wt% HF solution under various conditions, that is, without acoustic power and with acoustic power at different power density, to etch various material away from fused silica surfaces. The evolution of scratch morphologies was monitored and damage performance was tested against the surfaces without brittle scratches (control samples). The results show that the scratches can be enlarged and the microcracks can be opened and blunted by means of wet chemical etching. Approximately 25 μm material removal is sufficient to remove the lateral cracks that accompany the brittle scratches. It is shown that wet chemical etching can increase the laser-induced damage threshold (LIDT) at the scratches up to a factor of 1.77, while the addition of acoustics (~0.6 W/cm² and/or ~2 W/cm²) has marginal effects on the LIDT. However, the acoustic-assisted etching can increase slightly the etching rate of fused silica and more powerful acoustics can lead to a little higher etching rate.