Quadrature phase-shifted optical demodulator for low-coherence fiber-optic Fabry-Perot interferometric sensors.

We demonstrate a quadrature phase-shifted optical demodulation scheme for low-coherence fiber-optic Fabry-Perot interferometric sensors. The main part of the demodulator is a fiber-optic Mach-Zehnder interferometer (MZI) constructed with a 1 × 2 fiber coupler, a 3 × 3 fiber coupler, and fiber delay lines. This configuration allows us to generate quadrature phase-shifted interference signals simultaneously, thus eliminating the problem of ambiguous phase discrimination. The path length difference of the MZI is adjustable to adapt to sensors with arbitrary gaps. A great phase stability of the MZI has been obtained by using identical optical components in its two optical paths. The demodulator has been applied to a fiber-optic vibrometer. Measurement of vibrations with amplitudes from 0.67 nm to 12.3 µm has been demonstrated.