Laser-induced damage characteristics in fused silica surface due to mechanical and chemical defects during manufacturing processes

Mechanical and chemical defects incurred by grinding and polishing as well as post-processing have been recognized as the most influential culprits that hamper the elevation of laser power/energy in high peak power/energy laser systems. In order to find out the causes for limiting the operational power of laser systems, the effects of these defects on laser damage and removal and mitigation of the defects were investigated in detail in the article. Cracks and scratches were created, annealed, etched and damaged so as to reveal the likely effects of mechanical defects on damage and potential techniques to reduce their influence. The results show that HF-based etching can open and smooth cracks/scratches, improving laser-induced damage threshold (LIDT) at scratches by up to >250%. Thermal annealing did heal, to some extent, cracks but the LIDT is little improved. Both HF-etching and leaching proves to be effective in removing metallic contamination during polishing process and handling of optics, which can “contribute” to damage/damage density in fused silica. However, HF-based etching may degrade surface roughness, from <1 nm to >20 nm under some conditions when >20 µm material was etched away while the surface roughness was perceptibly altered by leaching (<1 nm to 1-2 nm). Although the LIDT might not be directly correlated to each individual kind of metallic contaminants or surface roughness, it is found that the surfaces with the highest LIDT's have some distinguished characteristics: clean surface (almost no metallic contamination) plus very smooth surface (RMS surface roughness: <5 nm). By removing metallic contamination and scratches, surface damage threshold of fused silica can exceed >30 J/cm2 (355 nm @3 ns, beam diameter ~400 µm @1/e2), a significant progress.