A straightness heterodyne interferometer is proposed for measuring the straightness of a linear motion stage along its moving direction. The interferometer uses a Koster prism as the beam splitter and combiner, a wedge prism as the straightness sensor, and a wedge mirror as the reflector. The interferometric beams have a completely common optic path and space symmetrical measurement structure. The cross-talk errors, from other degrees of freedom, are eliminated due to the spatial symmetry optical path structure. However, the periodic nonlinearities due to the mixed frequency cannot be eliminated by the optical configuration. To overcome this problem, we derived a periodic nonlinearities correction model. The coefficients of the model are previously estimated by splitting the straightness measurement beam into two beams through a polarization beam splitter placed at 45 degrees. Afterwards, the model can be used to compensate the nonlinearities of the straightness measurement. The results show that the nonlinear error is reduced to the resolution limitation of the straightness interferometer. (C) 2022 Optica Publishing Group
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