The paper presents a study of Faraday Rotation angle (FR), and Cotton-Mouton phase shift (CM) measurements to determine their mutual interaction and the validity of the linear models presently used in equilibrium codes. Comparison between time traces of measurements and model calculations leads to the result that only exact numerical solution of Stokes equations reproduces all the experimental data. As consequence, approximated linear models can be applied only in a limited range of plasma parameters. In general the non-linear coupling between FR and CM is important for the evaluation of polarimetry parameters.