Dynamic displacement measurement beyond half-wavelength in phase-modulated optical interferometer.

We demonstrate an interference signal-processing method that extends the measurement range of dynamic displacement beyond half the wavelength of light without deteriorating the measurement accuracy in a phase-modulated optical interferometer. The measurement range extension is realized by the algorithm focusing on peak direction of the interference signal waveform. Deterioration of measurement accuracy is avoided by the data processing that removes noisy data sets. Performance of the proposed system is evaluated by the experiments using a pseudo-vibrator composed of a phase modulator. In the experiments using a lead zirconate titanate (PZT) device as a sample, we successfully measure the dynamic displacement of up to 3127 nm, which is larger than the light source wavelength of 1537 nm. Proof-of-principle simulations, including the evaluation of the measurement error, are also conducted, the results of which show that the measurement error of the proposed method is small in principle.