Ultra-precision optical processing technology for large-aperture laser optics: Ripple structure removal and laser damage resistance enhancement of fused silica optics

In AMRF (arrayed magnetorheological finishing) process of fused silica laser optics, ripple structures would generate on the optical surface. The ripple structures could cause nonlinear self-focusing and localized energy deposition, finally induced laser damage. In this work, the removal of ripple structures and the improvement of anti-laser damage characteristics were studied. First, the heat deposition and light-field enhancement induced by ripple structures were analyzed by finite element simulation method, and the negative impact of ripple structures was clarified. Then, the ripple-structure surface was polished by a combined technique of low-stress CCOS (computer control optical surfacing) and ion beam finishing (IBF). After the combined polishing process, the ripple structures and the hydrolyzed layer on the surface were removed, the photo-thermal absorption decreased from 0.736 ppm to 0.124 ppm, and the laser induced damage threshold (LIDT) increased from 6.3 J/cm2 to 7.4 J/cm2. In this work, the combined technique was able to remove the ripple structures while maintaining the processing efficiency, and it also could improve the anti-laser damage characteristics of the optics. The relative research results had important reference value for ultra-precision manufacturing of large-aperture laser optics.