Innovative Aspheric Surface Generation of Silicon Carbide Optics

The superior material properties of Silicon Carbide (SiC) have made it an attractive optical material candidate for a number Air Force and Missile Defense Agency applications. Missions like Miniature Kill Vehicle (MKV), Space Based Infrared System (SBIRS), and Airborne Laser (ABL) all require aspheric optics which are lightweight and thermally stable, with low cost production being critical for multiple unit, operational optical systems. SSG has developed and demonstrated a low cost, near-net-shape, SiC mirror substrate fabrication process, and Tinsley Laboratories, an SSG subsidiary, has demonstrated the ability of the Tinsley proprietary Computer Controlled Optical Surfacing (CCOS) process to deterministically polish these SiC materials. There is a strong need for a low-cost aspheric surface generation process which can be used in conjunction with these other steps in order to define an end-to-end, low cost SiC mirror manufacturing approach. This intermediate aspheric surface generation process, the steps between obtaining a mirror substrate and being ready for optical polishing, has proved to be problematic for very hard optical materials like SiC. Aspheric surface generation techniques like single point diamond turning are routinely applied to softer optical materials, however, these approaches are not suitable for SiC due to the material's hardness. SSG proposes to evaluate two potential solutions to this problem: (1) single point turning of SiC with Carbon Nano Tube (CNT) structures, and (2) precision grinding with in-process dressing. Both of these approaches have demonstrated the potential to provide aspheric surface generation of very hard materials. Carbon Nano Tubes have been used to lathe cut Al2O3 materials with good success (ORNL), while in-process dressing has been successfully applied to precision grinding of BeO (LLNL). Like SiC, both of these hard materials cannot be machined using conventional approaches. During the Phase I activity proposed SSG plans to investigate the suitability of these two approaches in surfacing SiC materials. The optimal process will be defined and a program plan for modifying/developing this approach, specifically for aspheric surface generation of SiC optics, will be developed.