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

Claude A. Klein

doi:10.1117/12.637454

7 February 2006 Toward an athermal HEL optical window: Is oxyfluoride glass the way to go?

Claude A. Klein

Author Affiliations +

Proceedings Volume 5991, Laser-Induced Damage in Optical Materials: 2005; 59910S (2006) https://doi.org/10.1117/12.637454
Event: Boulder Damage Symposium XXXVII: Annual Symposium on Optical Materials for High Power Lasers, 2005, Boulder, CO, United States

Abstract

Recently, a new glass composition - oxyfluoride glass (OFG^TM) - 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 OFG^TM 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.

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Claude A. Klein "Toward an athermal HEL optical window: Is oxyfluoride glass the way to go?", Proc. SPIE 5991, Laser-Induced Damage in Optical Materials: 2005, 59910S (7 February 2006); https://doi.org/10.1117/12.637454

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