Metrology guided laser micromachining of SiC for mirrors

Silicon carbide mirrors are sought for a variety of aerospace applications. While optical polishing techniques are straight forward for flat and spherical surfaces, material removal rates for this hard, brittle material are too low for affordable processing of conventionally machined, ground, or as-produced surfaces. The problem is more severe for aspheres. This paper reports on the use of picosecond pulsed laser ablation, combined with iterative metrology, to shape the SiC in a manner that will reduce cost and lead time for mirror fabrication. The goal is to exploit relatively gentle, non-thermal ablation to produce arbitrary surface shapes in SiC that are damage free and that minimize subsequent polishing time. To apply the technology, detailed data must be developed to characterize laser-material interaction, the threshold for ablation, and the dependence of the effective "tool shape" on laser operating parameters and firing patterns. An algorithm can then be developed to calculate optimum laser guidance and firing commands for removal of the required amount of material from the ceramic surface, with reference to metrology data previously collected on the mirror blank. Recent results of machining quality, material removal rates, residual surface roughness, and suitability of surface for subsequent polishing are reported for various types of SiC and paradigms of laser micromachining.