Azimuthal Phase-Shifting Demodulation Method for Orbital-Angular-Momentum Interferometer

The orbital-angular-momentum (OAM) beams- based interferometer (OAMI) has shown its potential in high-precision metrology. However, most phase-demodulation methods for OAMIs require the analysis of interferograms captured by charge-coupled device (CCD) cameras, resulting in limited performance in terms of demodulation rate and resolution. In this study, an azimuthal phase-shifting demodulation method has been proposed for extracting the phase from interferograms produced by a Michelson structure-based OAMI (MOAMI). In contrast to other phase-demodulation methods for OAMIs, the proposed method requires only four-spot light intensity located in four specific azimuthal directions of the interferograms, which can be sampled by four photodetectors (PDs) instead of CCD cameras. To our knowledge, this is the first demonstration of four-spot light intensity-based demodulation method for OAMIs, which may greatly improve the measurement speed. Using the proposed MOAMI and demodulation method, the displacements ranging from 0 to 3100 nm were precisely measured with a relative error less than 1%, demonstrating the validity of the proposed method.