Fused silica is widely used in high-power laser systems because of its good optical performance
and mechanical properties. However, laser damage initiation and growth induced by 355 nm laser illumination
in optical elements have become a bottleneck in the development of high energy laser system. In order to
improve the laser-induced damage threshold (LIDT), the fused silica optics were treated by two types of
HF-based etchants: 1.7%wt. HF acid and buffer oxide etchant (BOE: the mixture of 0.4%wt. HF and 12%wt.
NH4F), respectively, for varied etching time. Damage testing shows that both the etchants increase the damage
threshold at a certain depth of material removal, but further removal of material lowers the LIDT markedly.
The etching rates of both etchants keep steady in our processing procedure, ~58 μg/min and ~85 μg/min,
respectively. The micro-surface roughness (RMS and PV) increases as etching time extends. The hardness (H)
and Young’s modulus (E) of the fused silica etched for diverse time, measured by nano-indenter, show no
solid evidence that LIDT can be related to hardness or Young’s modulus.
Detection of the subsurface damage depth in optical elements has significance on the subsequent material removal amount and improving element surface quality. The paper focuses on the subsurface damage of chemical-mechanical polished K9 specimen, and analyses the chemical-mechanical polishing mechanism and the cause of subsurface damage. A most suitable etchant is chosen and the step-by-step etching method is applied to measure the subsurface damage depth. A microscope is used to detect the damage morphology and the variation trend at different depth. Research shows that the subsurface damage caused by chemical-mechanical polishing is Hertz scratch, and the scratch quantity below surface presents a variation of zero-more-less-disappeared. The K9 specimen is polished for 3 min under the pressure of 2.5 Kgf and the spindle speed of 43139 r/min, thus resulting in a subsurface damage depth 15.3μm.
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