A Dual Modulation Technique Combine PGC Demodulation Scheme for Long Cavity Fabry–Perot Acoustic Wave Sensors

In this article, we propose a modified phase generated carrier (PGC) demodulation scheme combining dual modulation techniques for acoustic wave detection by long-cavity Fabry–Perot sensors. The scheme is effective in solving the weakness of the ellipse fitting algorithm (EFA) that cannot fit small signals and meanwhile achieves the desired phase demodulation results. The high-frequency current modulation adds a carrier signal to the PGC demodulation scheme, and the low-frequency temperature modulation adds a large-value phase shift to the interferometric system, which can support normal ellipse fitting, even in the case of weak acoustic signals. The modified signal processing scheme can eliminate the effects of carrier phase delay, phase modulation depth, and the accompanying amplitude modulation of the light source simultaneously. The experimental results show that the modified scheme can guarantee the calculation of EFA using temperature modulation adding an additional phase shift signal of more than 1 rad. The linear detection for small amplitude acoustic signals is 0.995, and the minimum detected phase signal is 0.016 rad. Meanwhile, the modified signal processing scheme guarantees excellent demodulation results with different phase delays and phase modulation depths, with a maximum value of SINAD of 26.79 dB and a THD always below −37 dB. This scheme further promotes the practical application of fiber optic sensors.