Fabrication of flat and supersmooth surfaces with bowl-feed polishing process

Low-scatter supersmooth optical components are wildly used for critical applications such as high power laser systems, mirrors for ring laser gyros, and low-scatter windows. In this paper, a bowl-feed polishing process with fine abrasives to get flat and extremely smooth surfaces is presented. We consider the machining of super-smooth surfaces as a chain consisted of some key nodes, not merely a polishing process. So after the samples of fused silica have been grinded, fine grinded and lapping previously, each step is controlled strictly, a bowl-feed polishing process is adopted to produce the supersmooth surfaces. During the whole polishing process, in order to achieve consistently reliable and meaningful values of both roughness and flatness, every effort is made to get the proper polishing parameters. Abrasives with different grain size distribution and the temperature are taken into account particularly. Two characters of the samples, roughness and flatness, are concerned about in this study. Process controlling by interferometer implies that surface shape is maintained to better than lambda/15 RMS (lambda=632.8nm), 100 mm in diameter for an extended periods of time, of the order of days. The surface roughness is tested by an interference microscopy. In best cases, the final test result of surface roughness is better than 0.8nm RMS.