27 June 1997 Frequency and temperature dependence of the refractive index of sapphire
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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, Michael E. Thomas, Stefan K. Andersson, Stefan K. Andersson, Raymond M. Sova, Raymond M. Sova, Richard I. Joseph, 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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