Phase demodulator for the measurement of extrinsic fabry-perot interferometric sensors with arbitrary initial cavity length

A phase demodulator for fiber optic extrinsic Fabry-Perot interferometric (EFPI) sensors is proposed for dynamic signal (vibration, acoustic, etc.) measurements. Three interferometric signals with phase differences are introduced by three lasers with different wavelengths. The tangent of the desired dynamic signal is extracted from the three interferometric signals. Then, the phase modulation is recovered by the arctangent algorithm and phase unwrapping. For the technique, constant phase differences are not required. Theoretically, the phase demodulator can interrogate EFPIs with arbitrary initial cavity length. The phase demodulator can perform regardless of whether the EFPI is modulated with a phase shift >2 π or not. Dynamic signals are recovered successfully from EFPIs with cavity lengths ranging from 23.065 μm to 1094.703 μm by the same demodulator. As a passive demodulation method, the proposed demodulator is adapted to the detection of high frequency signals. A sinewave with a frequency of 20 kHz is extracted successfully. The algorithm of the technique is simple, robust, and provides great liberty with respect to choosing different sensors in different applications.