Optical sources of non-linearity in heterodyne interferometers

Non-linearity of the two-frequency Michelson interferometer is studied by measurement of the modulation in phase and amplitude of the interference signal at the receiver. An important conclusion is that frequency mixing is not a major cause of non-linearity unless the mixing is asymmetric between the two arms of the interferometer. One source of asymmetry is non-orthogonality of the frequency states produced by the laser. Differential transmission between the reference and measurement paths can cause polarization states that are initially orthogonal to become non-orthogonal at the interferometer output. The influence of beam-splitter leakage, misorientation of the polarization axes of the source, and imperfect waveplates are also considered. It is not widely known that typical retroreflectors cause about a 7° rotation of the polarization state. However, polarization mixing induced by the retroreflectors is small in most, but not all, common configurations of the interferometer. © 1990.