Suppression of undesirable microstructure generated by femtosecond laser repair on the surface of fused silica optics

Fused silica is a critical optics in high-power laser facilities. To extend the service life of fused silica optics, femtosecond laser repair is employed to mitigate surface defects. This technique can rapidly fabricate optically benign conical structures over the defects. However, the laser-induced damage resistance of the repaired site is highly dependent on the surface quality of these conical structures. The primary challenge lies in suppressing the complex undesirable microstructures that are inevitably introduced during the femtosecond laser ablation process. The observed surface anomalies are attributed to three primary mechanisms. First, the femtosecond laser ablation process induces localized plasma explosions, which disrupt the material. Second, the chosen laser scanning pattern leads to non-uniform material accumulation and redeposition, forming a characteristic “knife-pattern” texture. Third, this newly formed textured surface itself possesses an inherent micron-scale roughness. To address this issue, this study optimizes key femtosecond laser processing parameters, specifically the trajectory interval and scanning speed, which are identified as the most influential factors on the formation of undesirable microstructures. The optimized treatment significantly improves the surface quality of the repaired region. The processed area achieves nanoscale surface roughness and exhibits a substantially reduced concentration of atomic-scale defects. Crucially, unlike the grinding process, this laser-based repair method does not introduce new impurities or contaminants. The mechanism behind this improvement was elucidated by establishing an internal light field transmission model for fused silica under high-energy laser irradiation. The model reveals that suppressing undesirable microstructures reduces both the distribution density and the intensity of localized light field concentrations within the optic, thereby enhancing its laser damage resistance. We innovatively propose surface roughness and concentration of atomic-scale defects as key metrics for evaluating the laser damage resistance of the ablated region. The analysis confirms that the complex microstructures generated by femtosecond laser ablation are the root cause of increased fluorescence intensity and atomic defect concentrations. Notably, after the suppression of undesirable microstructures, the photoluminescence (PL) intensity of the ablated area drastically decreases. Characterization reveals a substantial reduction in the concentration of several typical atomic-scale defects. Two specific defects—the STE and the E’ center—were completely eliminated following the repair process. In conclusion, this work successfully demonstrates a parameter optimization strategy for suppressing undesirable microstructures during femtosecond laser repair, which significantly improves the laser damage resistance of fused silica optics. The findings are of great significance for achieving high-quality femtosecond laser processing of optics.