Toward an athermal HEL optical window: Is oxyfluoride glass the way to go?

摘要

Recently, a new glass composition - oxyfluoride glass (OFGTM) - has been promoted as an ideal solution of the Airborne Laser (ABL) window problem in the sense that it will allow obtaining large "athermal" windows for high-energy lasers (HEL) operating at the chemical oxygen-iodine laser (COIL) wavelength [K. Billman, et al., Proc. SPIE 5647, 207 (2005)]. In this context, we emphazise that the performance of a laser-window material candidate must be assessed not only in terms of its ability to transmit high-power beams without generating undue optical distortion but also in terms of the constraints imposed by stress-related failure modes. Here, we provide the tools to carry out an analysis of both pressure- and beam-induced stresses, in addition to thermally induced aberrations, and illustrate the procedure through model windows made of (111)-oriented CaF2, fusion-cast CaF2, and OFGTM glass. Regarding OFG, we find that (a) this material will be vulnerable to surface compressive stresses on account of its poor thermal conductivity; (b) the stress-birefringence contribution to optical distortions cannot be ignored, which rules out creating a zero-distortion ABL window; and (c) based on Strehl ratios, and in the absence of stress-driven failure modes, OFG outperforms Si02 but does not match the performance of fusion-cast CaF2. Regarding CaF2, we find that (a) fusion-cast CaF2 exhibits substantial stress-induced birefringence, which prohibits using this material if depolarization is an issue; (b) highly-oriented (111)CaF2 exhibits no stress-birefringence at the COIL wavelength, in accord with previous investigations at HF/DF frequencies; and (c) in principle, (111) CaF2 windows may transmit aberration-free beam fluences in the 1-MJ/cm2 range but will require improvements in strength to achieve reliable designs.