Effect of surface preparation methods on laser-induced pitting of copper mirrors at the 10.6 µm wavelength

Ten precision Cu laser mirrors prepared by eight different methods were evaluated at 10.6 μm for resistance to spatially selective laser-induced damage (pitting). Surfaces essentially free of pitting and with melt thresholds close to that of intrinsic Cu were produced by electron-beam melting bulk Cu, followed by diamond turning; by diamond turning with a 20° negative top rake angle; by chemical vapor deposition of pure Cu on a diamond-turned Cu substrate; and by polishing bulk Cu on a hard lap with high pressure. The electron-beam melted surface was the most damage resistant, but residual stresses left by the melting process degraded the optical surface figure. A special cleaning method was developed to remove optically absorbing contaminants trapped by surface defects.