A High-Accuracy Demodulation Algorithm Based on the Frequency Shift Technique for Interrogating Fiber-Optic Dual-Cavity Fabry–Perot Sensors

A high-accuracy white light interferometry (WLI) demodulation algorithm based on the frequency shift technique is proposed and demonstrated for interrogating fiber-optic dual-cavity Fabry-Perot interferometric (DFPI) sensors, in which the intensity of the reflection spectrum generated by the short FP cavity is relatively weak. The long FP cavity’s optical path difference (OPD) is demodulated directly from the DPFI’s reflection spectrum using the maximum likelihood estimation (MLE) algorithm. A stronger reflection spectrum of the short FP cavity is acquired by shifting the frequency of the combined FP cavity’s reflection spectrum toward a lower frequency direction, and the shift value is equal to the frequency of the long FP cavity’s reflection spectrum. Finally, the short FP cavity’s OPD is demodulated by the multi-extrema-tracing technique. The frequency shift technique can effectively improve the demodulation accuracy of the short FP cavity’s OPD by obtaining a reflection spectrum with a higher signal-to-noise ratio (SNR). Simulation results show that the demodulation errors for both cavities’ OPDs are within ±0.5 nm. Furthermore, this proposed algorithm has demonstrated excellent demodulation performance for a sapphire DFPI fiber-optic temperature and pressure hybrid sensor. The proposed demodulation algorithm has the advantages of high accuracy, resistance to demodulation jumps, large dynamic range, and absolute measurement, which has a wide application potential for interrogating DFPI sensors with a weak reflection component of the shot FP cavity.