Advanced Techniques for Improving Laser Optical Surfaces

Abstract

/ Unclassified SECURITY CLASSIFICATION OF THIS PAGEfHTien Dmlm Enlermd)During the first six months of the program, experimental studies of all polishing techniques began. Acceptable conventional polishing techniques for polycrystalline calcium fluoride and zinc selenide were developed, and preliminary grinding steps of the process were identified as requiring further study. Bowl-feed " superpolishing" was shown to improve the surface quality of conventionally polished zinc selenide and calcium fluoride substantially.During the second report period, grinding damage was shown to be effectively removed from polycrystalline fluoride and selenide surfaces by removal by means of polishing a layer somewhat thicker than the diameter of the abrasive used to generate the ground surface.Zinc sulfide powders were evaluated as infrared-transparent abrasives for potassium chloride and found to be inferior to alumina. Since chemical polishing has been found to remove abrasives from the surfaces, transparent abrasives no longer appear to be required and thic part of the experimental program was terminated.Light scattering measurements demonstrated that ion milling could be used to "lean surfaces prior to deposition of optical coatings without seriously degrading them, but that more extensive material removal sharply increased scattering at angles close to the specular direction.Chemical polishing studies of all materials were begun. An ammoniurr chloride solution was found to be a promising chemical-mechanical polish for calcium fluoride. An approximately 0.2 molar alkaline solution of potassium ferricyanide was chosen as an optimum chemical polishing agent for zinc selenide. Etch rates of single and polycrystalline potassium chloride in hydrochloric acid were found to be en the order of 10 micrometers per minute This is likely to be too rapid for controlled etching of large halide optics and attempts to decrease the etch rate by additions to the acid have begun. Profilometer traces of a polycrystalline surface from which approximately 10 micrometers had been removed by hydrochloric acid etching showed typical grain-to-grain variations in height of only approximately 500 angstroms. This sort of variation should be quite acceptable on the optical surfaces. During the first report period, preliminary experimentation in all the project areas began and techniques for conventional polishing and bowlfeed " superpolishing" of calcium fluoride and zinc selenide were developed. Cleavage damage produced during grinding of the polycrystalline materials was identified as a potential source of defects in conventionally polishing surfaces.During this report period, systematic studies of the polishing of ground selenide and fluoride surfaces demonstrated that the removal of a surface layer somewhat greater in thickness than the particle size of the abrasive originally used to grind the surface essentially eliminated grinding damage as a source of defects in conventionally polished material.Glycerol triacetate (triacetin), a compound first ut...