Super-smooth optical fabrication controlling high-spatial frequency surface irregularity

Abstract

Modern advanced optical systems often require challenging high spatial frequency surface error control during their optical fabrication processes. While the large scale surface figure error can be controlled by directed material removal processes such as small tool figuring, surface finish (<<1mm scales) is controlled with the polishing process. For large aspheric optical systems, surface shape irregularities of a few millimeters in scale may cause serious performance degradation in terms of scattered light background noise and high contrast imaging capability. The conventional surface micro roughness concept in Root Mean Square (RMS) over a very high spatial frequency range (e.g. RMS of 0.5 by 0.5 mm local surface map with 500 by 500 pixels) is not sufficient to describe or specify these surface characteristics. For various experimental polishing conditions, we investigate the process control for high frequency surface errors with periods up to ~2-3mm. The Power Spectral Density of the finished optical surfaces has been measured and analyzed to relate various computer controlled optical surfacing parameters (e.g. polishing interface materials) with the high spatial frequency errors on the surface. The experiment-based optimal polishing conditions and processes producing a super smooth optical surface while controlling surface irregularity at the millimeter range are presented.