Surface Evaluation Techniques for the Optics of the Future

A key to improving the performance of optics and optical systems is to reduce scattering and absorption losses in thin film coatings and in the underlying substrates. Scattering can be measured by collecting the light scattered into a hemisphere (total integrated scattering, or TIS) or as a function of angle. The scattering can be related via scalar or vector scattering theories to the root mean square roughness and other statistical properties of the samples. Surface roughness can be measured by a noncontact heterodyne interferometric technique or a diamond stylus profiling instrument. If the optics are too large to fit into the measuring instruments, replicas can be made of parts of the surface and analyzed using one of the above techniques. Subsurface damage produced by the polishing process can in some cases be determined by a light scattering technique. Absorption in substrates and coatings can be distinguished from absorption at air-film or film-substrate interfaces using adiabatic calorimetry. The techniques to be described can measure absorption in parts per thousand, scattered light in parts per million, and surface roughness down to nearly atomic dimensions. Examples of some of the higher performance optics made possible by the sensitive evaluation techniques will also be given.