Micro-defect suppression on fused silica via dual femtosecond and CO₂ lasers to improve laser damage resistance

Laser ultra-precision machining is one of the most promising methods for suppression of surface defects (≥300 μm) on fused silica optics, thereby extending the service life of these optics applied in high-power laser facilities. This work proposes an innovative dual laser repairing method that combines the capabilities of femtosecond and CO₂ lasers to address much smaller micro-defects (≤20 μm) on fused silica optics. The proposed method consists of a two-step process, in which a femtosecond laser is first used to create high-quality optical structures in the micro-defect regions of fused silica, and then a CO₂ laser performs melt polishing to obtain an ultra-smooth surface. The repaired structure has surface roughness 20 nm of the dual femtosecond and CO₂ lasers, a striking promotion compared to 54 nm surface roughness for a single femtosecond laser. Laser-induced damage experiments are carried out, and the results show that laser-induced damage threshold (LIDT) is improved from 8 J/cm² to 20 J/cm² (LIDT for defect-free surface is 22 J/cm²). Photoluminescence (PL) spectroscopy of the micro-defects and their repaired structures was analyzed to uncover the underlying reasons for the improvement in LIDT. PL spectroscopy shows fluorescence intensity of 3500 counts in the micro-defect region, and 106 counts in the repaired structure obtained by dual femtosecond and CO₂ lasers. PL spectrum indicates that the concentration of point defects in fused silica surfaces treated with dual laser repair is comparable to perfect surfaces. This work paves the way for advancements in the durability of fused silica optics, potentially allowing them to be utilized in high-power laser facilities that demand higher output.