In this study, the non-linear errors in a commercial heterodyne interferometer are investigated. There are two types of cyclic nonlinearities present in heterodyne interferometers and it is desirable to be able to measure these nonlinearities in order to quantify the uncertainty of the interferometer setup. The current study investigates whether the nonlinearities can be detected by measuring the optical power of the interferometers output signal as a function of its phase. In theory, the optical power can be described as a perfect circle in polar coordinates in the absence of cyclic errors. The cyclic errors present, then manifest themselves as ellipticity of this circle and a translation of its centre. In this study large cyclic nonlinearities were deliberately introduced into a standard heterodyne interferometer setup, making them large enough to measure directly from the displacement data. Comparison with predicted nonlinearities calculated from the optical power data showed a good fit, indicating that it is possible to predict cyclic nonlinearities by reading the optical power from the measurement board.
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