Temperature dependence of the refractive index of alkaline earth fluorides

The temperature coefficient of the refractive index dn/dT of CaF(2), SrF(2), and BaF(2) single crystals is measured by a laser interferometric technique at a number of frequencies over temperatures ranging from 20 degrees C to 85 degrees C. Although dn/dT is found to display little dispersion between 0.6328 microm and 3.39 mum, its magnitude shows a slight increase with temperature. A possible origin of the latter effect is discussed.