Cycle-to-cycle analysis for high-repeatability optical-heterodyne interferometry

Optical-heterodyne interferometry enables high-precision measurement of displacement, surface topography, and retardation via the introduction of an optical frequency shift. However, certain types of frequency-shifters including rotating half-waveplates may induce repetitive intensity variation, resulting in precision degradation. To address this issue, the heterodyne signals are split at the local minima during analysis. Using this approach, a single-shot retardation repeatability of λ/380, 000 is achieved at 80 Hz sampling. The proposed method applies to other types of optical-heterodyne interferometry to address challenges such as residual amplitude modulation of an electro-optic modulator to facilitate more precise measurement.