Formation mechanism and suppression method of surface deterioration in CO2 laser polishing for fused silica

Laser polishing is a highly effective technique for achieving a super smooth fused silica surface. However, significant temperature fluctuations lead to scanning ripple and further destroy surface quality. The underlying mechanism behind the formation of this ripple remains unelucidated. In this letter, a multi-field coupling model is proposed to reveal the deterioration process of polished surface and the formation mechanism of scanning ripple. The findings reveal that optimizing the polishing time can minimize surface roughness while simultaneously preventing the generation of scanning ripple due to recoil pressure and the Marangoni effect. However, the fluctuation of power increases the difficulty of controlling polishing time. Thus, a double-side polishing method is proposed as a solution to suppress the scanning ripple and decrease roughness. Compared to single-side polishing, form error Root Mean Square (RMS) reduces from 0.433 λ (λ=632.8 nm) to 0.194 λ, and surface roughness decreases from 0.371 nm to 0.270 nm by double-side polishing with a power density of 4.244 kW/cm2 and a scanning speed of 1.3 mm/s. Plus, the laser-induced damage thresholds (LIDT) after double-side polishing at 0% and 100% probability demonstrated a marked improvement, rising from 19.4 J/cm² and 39.2 J/cm² to 21.2 J/cm² and 41.5 J/cm², respectively. The enhancement of the form error, surface roughness and LIDT is attributed to the offset of residual thermal stress and secondary surface melting flow. This research provides valuable insights into enhancing the processing quality and performance of fused silica.