HEL quality polishing of large sapphire substrates

II-VI Aerospace & Defense currently supports the defense industry on several aerospace platforms by providing high quality, large, single crystal sapphire substrates and windows. There continues to be a rapidly increasing interest in large substrates capable of use in high energy laser (HEL) systems. Such applications require materials with high thermal conductivity and high thermal shock resistance, high strength, and near-zero defect levels. Laser damage-inducing defects include those which are incorporated in the substrate during crystal growth, as well as defects incurred during routine optical fabrication and finishing processes. In the following paper, II-VI A&D presents data for a-plane and cplane sapphire windows of sufficient size for projected HEL applications exhibiting laser-grade surface quality and surface roughness, while still maintaining low rms transmitted wavefront error. Included is a discussion evaluating the benefits of sapphire for envisioned HEL windows, as well as engineering challenges that should be considered.