Improved Dual-Wavelength Demodulation Technique Based on Prior Information for Fiber Optic Fabry-Pérot Ultrasonic Sensors

To achieve high-precision demodulation of fiber optic Fabry-Pérot (F-P) ultrasonic sensors over a wide dynamic range, the improved dual-wavelength demodulation technique is proposed. The reference sound signal is introduced to construct the Lissajous figure of the system, and the parameters of the interference signals are obtained using the Ellipse Fitting (EF) algorithm. The Local Outlier Factor (LOF) algorithm is utilized to eliminate outliers from Lissajous figure, obtaining more accurate EF parameters. The proposed dual-wavelength demodulation technique based on prior information overcomes the drawback of the traditional ellipse fitting and differential cross multiplication (EF-DCM) algorithm, which is not not suitable for small signals demodulation. Moreover, it also reduces the influence of system noise and external environment on demodulation results. The experimental results demonstrate that the proposed demodulation technique is suitable for demodulating sensors with arbitrary cavity lengths, and the demodulated phase amplitude and sound pressure exhibit excellent linearity of 0.9989 within the range of 0.20 Pa to 13.36 Pa. The phase sensitivity of the sensor calculated by the improved algorithm is 0.142 rad/Pa.