The Cryogenic Refractive Indices of S‐FTM16, a Unique Optical Glass for Near‐Infrared Instruments

The Ohara glass S‐FTM16 is of considerable interest for near‐infrared optical designs because it transmits well through the K band and because negative S‐FTM16 elements can be used to accurately achromatize positive calcium fluoride elements in refractive collimators and cameras. Glass manufacturers have sophisticated equipment to measure the refractive index at room temperature, but cannot typically measure the refractive index at cryogenic temperatures. Near‐infrared optics, however, operate at cryogenic temperatures to reduce thermal background. Thus, we need to know the temperature dependence of S‐FTM16’s refractive index. We report here our measurements of the thermal dependence of S‐FTM16’s refractive index between room temperature and ∼77 K. Within our measurement errors we find no evidence for a wavelength dependence or a nonlinear temperature term, so our series of measurements can be reduced to a single number. We find that Δnabs/ΔT = -2.4 × 10-6 K−1 between 298 and ∼77 K and in the wavelength range 0.6–2.6 μm. We estimate that the systematic error (which dominates the measurement error) in our measurement is 10%, sufficiently low for most purposes. We also find the integrated linear thermal expansion of S‐FTM16 between 298 and 77 K is −0.00167 m m−1.