27 June 1997 Frequency and temperature dependence of the refractive index of sapphire
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Abstract
Because the index of refraction is temperature dependent, a temperature gradient across a window causes image blur and boresight error. Accurate temperature and frequency dependent refractive index models can now be constructed from visible measurements of the refractive index, far-infrared reflectance measurements, thermo-optic coefficient measurements, and infrared measurements of the absorption coefficient. Visible measurements determine the contribution to the refractive index from electronic transitions. Measurements of dn/dT are reported on the ordinary ray of sapphire in the 4 micrometer region. Far-infrared measurements determine the contributions from fundamental lattice vibrations (phonons). Infrared absorption data are used to determine parameters in a multiphonon sum band model of the refractive index. Two- and three-phonon contributions to the refractive index are important for an accurate model that includes frequency and temperature dependence. Results for the ordinary- and extraordinary-rays are obtained.
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Michael E. Thomas, Stefan K. Andersson, Raymond M. Sova, Richard I. Joseph, "Frequency and temperature dependence of the refractive index of sapphire", Proc. SPIE 3060, Window and Dome Technologies and Materials V, (27 June 1997); doi: 10.1117/12.277051; https://doi.org/10.1117/12.277051
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