Temperature-dependent refractive index of CaF 2 and Infrasil 301

Abstract

In order to enable high quality lens designs using calcium fluoride (CaF₂) and Heraeus Infrasil 301 (Infrasil) for cryogenic operating temperatures, we have measured the absolute refractive index of these two materials as a function of both wavelength and temperature using the Cryogenic, High-Accuracy Refraction Measuring System (CHARMS) at NASA's Goddard Space Flight Center. For CaF₂, we report absolute refractive index and thermo-optic coefficient (dn/dT) at temperatures ranging from 25 to 300 K at wavelengths from 0.4 to 5.6 μm, while for Infrasil, we cover temperatures ranging from 35 to 300 K and wavelengths from 0.4 to 3.6 μm. For CaF₂, we compare our index measurements to measurements of other investigators. For Infrasil, we compare our measurements to the material manufacturer's data at room temperature and to cryogenic measurements for fused silica from previous investigations including one of our own. Finally, we provide temperature-dependent Sellmeier coefficients based on our measured data to allow accurate interpolation of index to other wavelengths and temperatures.