Nonlinear errors induced by intermodulation in heterodyne laser interferometers.

A striking abnormal harmonic is detected in the measurement signal spectrum of a heterodyne laser interferometer, which deviates from the conventional frequency-domain model. A nonlinearity model based on intermodulation is proposed to explain this abnormality and to analyze its influence. Analysis from this model demonstrates that the harmonics arising from intermodulation can result in not only nonlinear errors opposite in sign to the traditional first- and second-order nonlinearities, but also new third- and fourth-order nonlinearities; moreover, these periodic nonlinearities cannot be fully compensated anymore by the Heydemann correction.