KOH-based shallow etching for exposing subsurface damage and increasing laser damage resistance of fused silica optical surface

Abstract HF-based wet deep etching is an effective method for improving the laser damage resistance of fused silica optics. However, the fact that the etching process involves HF reaction makes the operation very unsafe. Potassium hydroxide (KOH) has recently shown great potential to outperform the more commonly used HF-acid as an etchant for the damage resistance enhancement of fused silica optical surface. In this paper, we present a detailed investigation into how the KOH-based shallow etching process affects the polished surface of fused silica. The results show that KOH-based etching does a good job at exposing the subsurface damage and increasing the laser damage threshold of fused silica while keeping the surface roughness nearly unchanged. Potential factors that limit the further increase in surface damage threshold of fused silica are also delved. The study offers an exciting option for fabricating high-quality fused silica optics with shallow wet etching under higher safety conditions that cannot be achieved with conventional HF-based wet etching approaches.