Cryogenic Refractive Index and Coefficient of Thermal Expansion for the S-TIH1 Glass

The S-TIH1 glass from Ohara Inc. is a type of material that exhibits a high room-temperature refractive index as well as a wide variation in dispersion as a function of wavelength. Because of these properties, this material could be a suitable candidate for use in a refractive system based on a prism design. In order to broaden its applicable uses, this paper reports on the results from a temperature-dependent refractive index measurement program performed on this type of glass to enable a high-fidelity refractive system design that would operate at cryogenic temperatures. These measurements were performed using the Cryogenic High Accuracy Refraction Measuring System (CHARMS) facility at the Goddard Space Flight Center (GSFC). We report on the absolute refractive index and thermo-optic coefficient (dn/dT ) at temperatures ranging from 100 to 300 K and at wavelengths from 0.455 to 2.536 μm. We compare our index of refraction measurements to the material manufacturer’s data at room temperature. We also provide temperature-dependent Sellmeier coefficients based on our measured data to allow accurate interpolation of index as a function of wavelength and temperature. These studies are also complemented with measurements of the coefficient of thermal expansion (CTE) to further validate the use of this type of glass in a cryogenic optical systems.