Interferometric method for concurrent measurement of thermo-optic and thermal expansion coefficients.

A Fabry-Perot-type interferometer is described that permits the concurrent measurement of the coefficient of thermal expansion (alpha) and the thermooptic coefficient (dn/dT) of transparent materials. Measurements of the a and the dn/dT of vitreous silica and a heavy-metal fluoride glass show that the technique is accurate, reproducible, and easily performed. In addition, the technique is used to show that the dn/dT of heavy-metal fluoride glasses is negative and temperature dependent. The magnitude of dn/dT is found to increase with increasing temperature.