Shear Phase-Shifting and Common-Mode Suppression for Displacement Interferometry

High-end equipment development (e.g., CNC machine tools, lithography machines) drives demand for miniaturized, embedded, high-precision multi-degree-of-freedom motion measurement technologies. While laser interferometry provides non-contact, long-range precision measurements, conventional homodyne interferometry is environmentally sensitive, which limits its applications. Traditional methods usually use multiple beam-split elements to achieve spatial phase-shifting and collect interference signals with fixed phase differences through four incompletely co-pathed photodetectors to achieve common-mode suppression. However, it fails to eliminate path-specific noise and increasing system bulk. This paper proposes a novel method of shear phase-shifting and common-mode suppression for displacement interferometry. Polarization shearing is innovatively used to realize interferometric phase-shifting, and a Birefringent Crystal (BC) takes the place of the traditional complicated splitting and phase delay components. The new optical structure combined with the linear array photodetector can easily collect the interference signal with a fixed phase difference. Then, by parallel differential operation, the common-mode suppression can be realized in real time. The correction for DC error, AC amplitude inequality error, and non-orthogonality error is implemented by embedded digital processing. Compared with the traditional method, the proposed method has obvious anti-interference advantages and greatly simplifies the opto-mechanical structure. A prototype device was tested on a commercial linear stage, demonstrating 0.028 μm stability over 60 minutes and repeatability errors of ±0.032 μm, ±0.063 μm, ±0.046 μm, and ±0.110 μm at 25 mm, 50 mm, 75 mm, and 100 mm ranges, respectively,, which verifies the effectiveness of the proposed method and provides a new scheme with a simple structure and high precision for displacement interferometric measurement. The subsequent instrumentation design is capable of fulfilling the requirements of miniaturized embedded measurement.