Sellmeier coefficients and dispersion of thermo-optic coefficients for some optical glasses.
The refractive index and its variation with temperature, the thermo-optic coefficient (d n/dT), are analyzed with two separate physically meaningful models for more than a dozen of some important Schott and Ohara optical glasses to find the refractive index at any operating temperature for any wavelength throughout the transmission region. The room-temperature catalog values of refractive indices are fitted with a two-pole Sellmeier equation. Both the average electronic absorption band gap and the lattice absorption frequency, lying in the vacuum UV and IR regions, respectively, contribute to the refractive indices and their dispersion. The estimated absorption band gaps are at 8.5-11.9 eV, and these values agree with the measured values at 8.8-11.6 eV satisfactorily for normal optical glasses. The higher-index glasses have electronic absorption in the region of 5.6-6.3 eV, and the estimated band gap of SF6 glass is 6.0 eV. The dispersion of thermo-optic coefficients is accounted for satisfactorily with a model, based on three physical parameters, the thermal expansion coefficient and excitonic and isentropic optical band gaps that are in the vacuum UV region. These optical constants are used to compute refractive indices at any operating temperature and wavelength. The Abbé number and the chromatic dispersion characteristics of these glasses are evaluated from the computed optical constants; the values of the chromatic dispersions are evaluated particularly at the three optical windows of the optical fiber communication systems and femtosecond technology.
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