The temperature variation of the thermo-optic coefficient of CaF2

We report measurements of dn/dT, the thermo-optic coefficient, i.e., the temperature derivative of the refractive index, for single-crystal CaF2 at temperatures ranging from 95 to 175 °K. Over this temperature range dn/dT shows a strong variation, with a substantial reduction in magnitude from that at room temperature. These data along with those obtained previously at room temperature indicate that dn/dT for this crystal is proportional to the thermal expansion coefficient over a wide temperature range and that the proportionality constant can be accurately obtained from the pressure dependence of the refractive index.