Abstract: The nonlinearity of heterodyne interferometers is caused not only by polarization ellipticity or nonorthogonality of the laser beams, but also the nonideal optical parts, in which the beam splitter is an important error source. The nonorthogonality of the polarization splitting directions of polarizing beam splitter is also studied besides the well known polarized light leakage. Their different influences on the nonlinearity of heterodyne interferometer (including other reasons suchas polarization ellipticity or nonorthogonality of the laser beams) are described. It shows how to examine and distinguish experimentally between two typical frequency mixing errors which could often be mistaken. It shows that the frequency mixing due to the leakage of the polarizing beam splitter can be generally neglected. In addition, an important fact confirmed by the experiments is that the nonpolarizing beam splitter can affect the nonlinearity of heterodyne interferometers more seriously than a polarizing beam splitter used in the interferometer.

Key words: Beam splitter, Heterodyne interferometer, Nonlinearity, Nonorthogonality