AN INTERFEROMETRIC DILATOMETER FOR THE DETERMINATION OF THERMOOPTIC COEFFICIENTS OF NLO MATERIALS

An interferometric dilatometer has been designed for determination of thermo-optic coefficients of millimetric nonlinear optical (NLO) materials. The apparatus is a combination of a modified Mach-Zehnder arrangement for dilation measurements and a Fizeau technique for determination of changes in optical length. Its configuration allows measurements under known conditions of applied electric field. Analysis of the linear thermal expansion coefficient alpha(b) of a triglycine sulphate (TGS) single crystal around its second-order phase transition is used to evaluate the sensitivity of the interferometer, and measurements performed on a LiNbO3 single crystal exhibit step-like variations of changes in optical length from 20-degrees-C to 122-degrees-C. These experiments show that an accuracy better than 1 x 10(-6) K-1 is achievable for both dilation and thermo-optic coefficients with this dilatometer, provided that they are not highly temperature-dependent and are under the condition of suitably controlled applied electric field.