Temperature-dependent refractive index of CaF 2 and Infrasil 301

Abstract

In order to enable high quality lens designs using calcium fluoride (CaF 2 ) 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 CaF 2 , 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 CaF 2 , 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.

Polarization modeling and predictions for DKIST part 3: focal ratio and thermal dependencies of spectral polarization fringes and optic retardance

“…For temperature dependence of the refractive index n, several articles show dn/dT in the 10's of parts per million range for various materials. [53][54][55][56][57] For most crystals, this term has opposite sign from the CTE (α). As the crystal heats, the part becomes physically thicker but the refractive index drops.…”

Temperature-dependent refractive index measurements of CaF2, Suprasil 3001, and S-FTM16 for the Euclid near-infrared spectrometer and photometer

“…While we know of no previous studies of Suprasil 3001 at cryogenic temperatures, we have measured refractive index of three other kinds of synthetic fused silica which makes for an interesting comparison as fused silica is often thought of as "if you've seen one, you've seen them all" material. In Figure 9, we compare our measurements with 1) the catalog room temperature (293 K or 20 C) dispersion relation for the "Suprasil family" 18 ; 2) the catalog room temperature dispersion relation for Homosil and Infrasil 18 ; 3) CHARMS Sellmeier fit for Homosil (VIS/NIR only); 4) CHARMS Sellmeier fit for Infrasil 301 19 ; 5) a dispersion relation for Suprasil 3001 at room temperature obtained from an optical designer affiliated with the NISP team in the form of Schott index coefficients in a Zemax private glass library (derived through means unknown to us); and 6) CHARMS Sellmeier fit for Corning 7980. 20 In all cases other than for absolute measurements made in CHARMS, we first adjust catalog or library indices to vacuum by multiplying by the spectral index of refraction of air.…”

“…Since the mid-1990's, the refractive index of CaF2 has been measured at cryogenic temperatures by several investigators including our group. 17,19,23 Since index measurements of CaF2 with CHARMS are currently 3-5 times more accurate than and are based on more recently grown material than any of those other investigations, in Figure 11 we compare our Sellmeier fit for the three batches of CaF2 for NISP with that from our own previous investigation for the ESO VLT Xshooter instrument. 8,19 The combined uncertainty for the two sets of measurements is ≥ 2E-5 for the wavelength range shown.…”

“…17,19,23 Since index measurements of CaF2 with CHARMS are currently 3-5 times more accurate than and are based on more recently grown material than any of those other investigations, in Figure 11 we compare our Sellmeier fit for the three batches of CaF2 for NISP with that from our own previous investigation for the ESO VLT Xshooter instrument. 8,19 The combined uncertainty for the two sets of measurements is ≥ 2E-5 for the wavelength range shown. Thus, to within our ability to tell, the absolute spectral index of the NISP CaF2 is practically the same as that for X-shooter.…”

The art of cryogenic lens design: the Gemini Planet Imager

“…Glasses are mainly limited to calcium and barium fluoride (CaF 2 , BaF 2 ), zinc sulfide and selenide (ZnS, ZnSe), Cleartran TM , fused silica, and S-FTM16 (Ohara). [3][4][5] At the preliminary design stage, it is acceptable to adopt room-temperature conditions (and an atmospheric pressure of 1atm), although with the selected set of cryogenic-glass specimens. Upon acceptance of the preliminary design, it must be modified with cold data.…”