Displacement measuring interferometry provides high resolution and accuracy for dimensional metrology and is used in a number of precision applications. Heterodyne interferometers use a two-frequency laser source that separates the two optical frequencies (typically via polarization) into reference and measurement arms. The reference arm is stationary, while the measurement arm includes the moving target. An interference signal is obtained by the recombination of the reference and measurement arms, resulting in a measurement signal at the heterodyne frequency of the laser source. This measurement signal is compared to an optical reference signal. Motion in the measurement arm causes a Doppler shift of the heterodyne frequency which is measured as a continuous phase shift that is proportional to displacement. In practice, undesirable frequency mixing occurs due to misalignment of optical components, component imperfections, and elliptical polarization of the input; this leads to periodic errors [1-3]. Typically, both 1st and 2nd order periodic errors occur, which correspond to the number of periods (one or two) per fringe displaced, as shown in Figure 1. The periodic errors can limit the linearity of the heterodyne interferometer to approximately the nanometer level.
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