Repaired morphology of CO₂ laser rapid ablation mitigation of fused silica and its influence on downstream light modulation

The threat of cascading damage to downstream components caused by the light modulation intensification of laser repaired morphology on the surface of fused silica optics cannot be ignored in high-power laser systems. This paper uses the angular spectrum diffraction theory based on the analysis of repaired surface morphology of CO₂ laser rapid ablation mitigation to study the influence of different repaired morphologies on the downstream 355 nm laser transmission. Studies show that the arc-shaped laser processing lines on the repaired surface are formed by the residual height superposition of the material after laser scanning of two adjacent layers, and the short-pulse laser can substantially suppress the heat-affected zone of the repaired area. The off-axis ring caustic and on-axis hotspot are sequentially generated in the downstream modulated light fields of the conical repaired sites with different diameters. A secondary peak with modulation larger than 3 emerges downstream of the modulation curve. Meanwhile, the maximum modulation and the secondary peak increase with the diameter and cone angle of the repaired site, and the position of the secondary peak appears farther away from the rear surface. The modulations of three repaired sites with cone angles of 15°, 20°, and 25° can finally be stabilized below 3. Overall, the downstream optics should be installed far away from the positions where the maximum modulation and the secondary peak emerge. Additionally, the maximum value and the secondary peak of the downstream light modulation of double repaired sites are larger than that of the single repaired site, and both rise as the repaired sizes increase. Thus, large-scale and large-size repairing of multiple surface damages in the same area should be avoided in the laser repairing of fused silica.