Accurate measurements of the electro-optic coefficients and birefringence changes using an external modulation signal

We propose a new method to measure within a high accuracy both the electro-optic properties and the birefringence changes in bulk crystals. This technique is based upon the frequency doubling electro-optic modulation (FDEOM) method in which an external modulator is inserted in the experimental Sénarmont setup. We present a complete analysis of the optical response of a Sénarmont setup within Jones formulation. The FDEOM method possesses as main advantages a high accuracy and sensitivity in the measurements due to a direct determination of the electric field-induced phase retardation in the crystal under study. The measurements are recorded with an adjustment of the shape of the output beam at a frequency double the frequency of the ac applied voltage. The external modulation version of this method allows us to perform measurements with a higher modulation depth and to avoid any additional dynamical perturbation in the electro-optic crystal under study. Therefore, even small values of the electro-optic coefficients can be accurately measured. Moreover, the new method is able to measure with a high accuracy the variations of the birefringence in any crystal, even if it does not present an electro-optic effect. Some significative results in LiNbO3 point out the large advantages of the method.