Optical parts and the nonlinearity in heterodyne interferometers

The nonlinearity of heterodyne interferometers is caused not only by polarization ellipticity or nonorthogonality of the laser beams. The imperfect optical parts can also contribute. Our experiments show that the nonlinearity of heterodyne interferometers can be serious even when perfect optical parts are used. Also it will be shown how to examine and distinguish between the errors experimentally. Moreover, an interesting approach was found to decrease the nonlinearity for heterodyne interferometers.