High-Precision Demodulation Method for Multi-FP Cavity Sensors Based on Filter Optimization

Abstract

Fibre-optic Fabry–Pérot (FP) multi-cavity sensors hold significant promise for multi-parameter measurements in extreme environments. However, precisely demodulating signals from each cavity within the composite interference spectrum remains a key bottleneck that limits measurement accuracy. To address this issue, this paper proposes a high-precision demodulation method that combines digital filtering with distortion region suppression. This method first uses digital bandpass filters to separate the interference signals from each sub-cavity from the composite spectrum. It then removes the spectral regions distorted by the filters, retaining only the stable central spectral band for interference order fitting and cavity length calculation. To comprehensively evaluate the method, we systematically compared the distortion characteristics of six finite impulse response (FIR) window-function filters and three infinite impulse response (IIR) filters during spectral separation, along with their impact on demodulation accuracy. Simulation results show that, within the 490–510 μm cavity-length range, FIR filtering combined with distortion suppression reduces the demodulation error to below 0.04 nm, approaching the theoretical limit. In contrast, the best-performing IIR filter still yields an error of 0.58 nm after the same processing. Furthermore, in simulations covering a wide range of 200–800 μm, the maximum demodulation error of this method is less than 0.53 nm, demonstrating its excellent robustness. In dual-cavity temperature sensing experiments, the partial-spectrum demodulation strategy with distortion suppression reduced the standard deviation of cavity-length fluctuations from 0.112 nm to 0.072 nm, and the maximum adjacent point jump from 2.158 nm to 0.464 nm, compared with full-spectrum demodulation. This significantly enhances the continuity and stability of demodulation. This study not only provides a high-precision, highly robust demodulation scheme for multi-cavity FP sensors but also offers clear theoretical and experimental grounds for selecting and optimising digital filters in practical engineering applications.