Optical scatter characteristics of high-reflectance dielectric coatings and fused-silica substrates

Optical scatter characteristics for high-reflectance dielectric coatings and polished fused-silica substrates are measured. The coating materials are tantala/silica, titania/silica, and zirconia/silica. The coatings are deposited on substrates that are conventionally polished, superpolished, and irradiated with a CO(2) laser before deposition. The measurements are made at the design wavelengths of the coatings, i.e., 633 and 1320 nm, and consist of bidirectional-reflectance distribution-function angle scans and spatial mappings. The substrate scatter is lower for superpolished surfaces, and the coating scatter is reduced by the use of superpolished substrates. Scatter levels are strongly influenced by coating design and materials.