Measurement of the thermal expansion coefficients of ferroelectric crystals by moiré interferometry

A moire´ interferometer is used to measure the thermal expansion of two ferroelectric crystals, LiNbO3 and KTiOPO4. The crystal samples are patterned with a chromium reflective grating and used as a diffractive component in a reflective grating interferometer. The thermal expansion of all the three axes of congruent LiNbO3 and of x and y axes of the flux-grown KTiOPO4 were measured from room temperature to 200 °C. For this temperature range the thermal expansion coefficient has been modeled by a second-order polynomial and its coefficients have been estimated by accurate analysis of the resulting moire´ fringe pattern.