The non-linearities in a heterodyne laser interferometer system occurring from the phase measurement system of the interferometer andfrom non-ideal polarization effects of the optics are modeled into one analytical expression which includes the initial polarization state ofthe laser source, the rotational alignment of the beam splitter along with different transmission coefficients for polarization states and therotational misalignment of the receiving polarizer. The model is verified using a Babinet Soleil Compensator allowing a common path forboth polarization states and thereby reducing the influence of the refractive index of air. The verification shows an agreement of the modelwith measurements with a standard deviation of 0.2 nm. With the use of the model it is confirmed that the mean of two polarizer receiverscan reduce the effect of non-linearity. However, depending on the accuracy of the polarizer angles, a second-order non-linearity remains.Also the effect of rotational misalignment of the beam splitter can not be reduced in this way.