Temperature-dependent absolute refractive index measurements of synthetic fused silica

TLDR

The study presents temperature‑dependent Sellmeier coefficients for fused silica to enable accurate interpolation of refractive index across wavelengths and temperatures. Using NASA’s CHARMS system, the authors measured refractive index and compared the results to an independent interferometric study and sparse cryogenic literature data. Absolute refractive index of Corning 7980 fused silica was measured from 30 to 310 K at 0.4–2.6 µm with an absolute uncertainty of ±1×10⁻⁵, and the thermo‑optic coefficient varied at ±2×10⁻⁸ K⁻¹, with data and uncertainties tabulated.

Abstract

Using the Cryogenic, High-Accuracy Refraction Measuring System (CHARMS) at NASA's Goddard Space Flight Center, we have measured the absolute refractive index of five specimens taken from a very large boule of Corning 7980 fused silica from temperatures ranging from 30 to 310 K at wavelengths from 0.4 to 2.6 microns with an absolute uncertainty of ±1 ×10^-5. Statistical variations in derived values of the thermo-optic coefficient (dn/dT) are at the ±2 × 10^-8/K level. Graphical and tabulated data for absolute refractive index, dispersion, and thermo-optic coefficient are presented for selected wavelengths and temperatures along with estimates of uncertainty in index. Coefficients for temperature-dependent Sellmeier fits of measured refractive index are also presented to allow accurate interpolation of index to other wavelengths and temperatures. We compare our results to those from an independent investigation (which used an interferometric technique for measuring index changes as a function of temperature) whose samples were prepared from the same slugs of material from which our prisms were prepared in support of the Kepler mission. We also compare our results with sparse cryogenic index data from measurements of this material from the literature.