Round-Robin Testing of Low-Scatter Optics

For high quality laser optics it is important to compare measurements of surface quality made in different laboratories, determine how scattering levels of silver coatings produced by different groups compare, and what effects, if any, are introduced by stripping the silver coatings and by handling and transporting samples between laboratories. A round-robin study attempted to answer these questions. Participating laboratories included the Avionics Laboratory at the Air Force Wright Aeronautical Laboratories (AFWAL), Dayton, Ohio, Michelson Laboratory at the Naval Weapons Center (NWC), China Lake, California, and the Air Force Weapons Laboratory (AFWL), Albuquerque, New Mexico. Fourteen very low-scatter, optically polished synthetic fused silica (Suprasil) and natural fused quartz (Homosil) samples were purchased from Robert M. Silva of VTI, Dayton, Ohio. Angular scattering, i.e., bidirectional reflectance distribution function (BRDF), was measured on all the uncoated samples and three silver-coated samples at AFWAL using a variable angle scatterometer. Eleven additional samples were silver coated at NWC, and total integrated scattering (TIS) was measured on all silver-coated samples. Transmission electron micrographs were made of the surfaces (silvered and also stripped) of two samples, and selected coated and uncoated samples were profiled. TIS was then measured on the instrument at AFWL. At AFWAL BRDF measurements were made on all the samples, both silver coated and then stripped, and TIS measurements were repeated on selected samples at NWC. It was found that the TIS measurements made at NWC and AFWL were in good agreement. Scattering from bare substrates increased after silver coating and stripping, and there was a large variability in the silver coatings, particularly those made at NWC. Critical factors in obtaining low-scatter silver surfaces appeared to be the cleaning and coating of the samples. The results of the round-robin measurements are discussed in detail in this paper.