Simultaneous determination of thermo- and electro-optic coefficients by Fabry-Perot thermal scanning interferometry

A proper implementation of electro-optic materials in laser systems requires an accurate knowledge of their electro-optic coefficients, along with their temperature dependence which could be of importance at high power level. A new technique has been developed for this purpose, which takes advantage of the thermodynamic equivalence of two intensive parameters, namely the temperature and applied electric field. A suitably oriented parallelepipedic shaped sample is exposed to a laser beam and acts as a Fabry-Perot interferometer which is submitted to a linear ramp of temperature. The interference pattern is observed by reflection and the shift of interference fringes generated by the thermo-optic effect is detected through amplitude modulation of the light beam and recorded as a function of temperature. We then switch from amplitude- to phase- modulation by applying a suitable electric field to the crystal: the signal features now the derivative of the thickness fringes generated by the electro-optic effect. The thermo- and electro-optic coefficients are obtained from the fringe shift recorded respectively through amplitude- and phase- modulated operating modes. The study of both KTiOPO 4 and LiInS 2 single crystals is given as an example to illustrate the so-called Fabry-Perot Thermal Scanning Interferometric (FPTSI) method.