Phase-modulated dual-homodyne interferometer without periodic nonlinearity

A novel dual-homodyne interferometer, in which one interferometer acts as the reference interferometer and the other as the measurement one, is proposed to eliminate periodic nonlinearity for nanometer displacement measurement. By using one electro-optic phase modulator to modulate the common reference arm of the two interferometers, the DC interference signals of homodyne interferometers are modulated to AC signals. The measured displacement is carried on the phase difference change of the two AC interference signals. To address the influence of unequal fluctuations of the DC offsets of interference signals on the zero-crossing phase detection, a new phase difference detection method is proposed by counting the sample numbers in positive and negative half periods of interference signals. The merits of this interferometer are not only eliminating the nonlinear errors inherent to heterodyne and homodyne interferometers but also having the abilities of strong anti-interference, insensitivity to laser power drift and the unequal gain of detectors, as well as not demanding quadrature signals. An experiment of nanometer displacement measurement was performed to verify the feasibility of the interferometer, and the results show that sub-nanometer accuracy can be realized without periodic nonlinearity. The large displacement experiment shows that the results obtained with the proposed interferometer are in good accordance with those obtained from a comparison interferometer. These results show that the proposed interferometer can realize large displacement measurement with nanometer accuracy. (paper)