Analysis for damaged surface of Fused Silica with enhanced damage resistance after Magnetic Field-Assisted Finishing

Effect of polishing contamination and surface defects for the LIDT has already been extensively studied in fused silica based transparent optics. The presence of contamination and damage layers on the surface of polished fused silica contributes to a large reduction in LIDT at ultraviolet wavelengths. The magnetic field-assisted finishing (MAF) technique has been shown to be successful in the fine finishing of optical components such as transparent Nd:YAG ceramics. Magnetic field-assisted finishing (MAF) with several tools has previously been applied to fused silica and was shown to improved surface LIDT at 266 nm. In this paper, the damaged surface of fused silica with enhanced damage resistance after MAF was analyzed to classify the MAF processed condition. Irradiated energy density and damaged volume calculated from depth geometry were measured with a white color interference microscope (Zygo: Zegage). Fused silica substrates polished with CeO2 compounds were prepared as workpieces, and the surface roughness was about 0.3 nm Sq after optical polishing. Material removal over 100 nm occurred with the MAF process, however the final surface roughness did not change. The LIDT at a laser wavelength of 266 nm of processed surfaces was about 1.4 times higher than for the as-polished surface. The damaged volume of as-polished surface was linearly increased as increase in the irradiated energy density. In contrast, MAF processed surface showed little change for the damaged volume. The damage morphology will be also discussed.