Pulsed laser damage characteristics of vapor-deposited copper mirrors on silicon carbide substrates

Pulsed 100-nsec 10.6-microm laser damage characteristics of composite bare copper mirrors were determined. The mirrors were prepared by vapor deposition of a copper film on polished silicon carbide substrates formed by chemical vapor deposition, Comparisons are made with a bulk oxygen-free high-conductivity (OFHC) copper mirror prepared by precision diamond turning. Vapor-deposited Cu on SiC was found to be inherently superior in resisting surface damage related to defects and to thermal stress. However, melt thresholds are comparatively lower. The overheating problem was analyzed by a thermal modeling of thresholds measured on mirrors having different film thicknesses. Results are consistent with reduced thermal conductivity in the SiC substrate as compared with bulk Cu. The effect of the interface was evaluated from samples that had been ion polished prior to deposition and on others where an intermediate chromium layer was deposited. The scattering and figure of the mirrors are also reported.