Sensitivity-Enhanced Displacement Sensing System Based on a Microwave Photonic Filter Incorporating a Figure-Eight Loop Interferometer

This work introduces a novel displacement sensing system with enhanced sensitivity and resolution by integrating a figure-eight loop interferometer (FELI) into a microwave photonic (MWP) filter (MPF) architecture. Unlike conventional Mach-Zehnder interferometer (MZI)-based fiber sensors that rely on optical spectrum analyzers (OSAs) with limited resolution (~0.02 nm), the proposed system leverages the distinct interferometric properties of the FELI and the high spectral resolution of MPF-based electrical demodulation. This innovation enables substantial performance improvement. The FELI-based MPF sensor achieves a displacement sensitivity of 0.207 MHz/ $\mu $ m, which is nearly twice that of the MZI-based MPF counterpart (0.113 MHz/ $\mu $ m). Analysis indicates a theoretical displacement resolution as fine as 4.83 pm, with practical accuracy reaching $0.3~\mu $ m, far surpassing the 7.168 mm resolution achievable via typical optical wavelength demodulation. This four-order-of-magnitude improvement highlights the FELI-MPF system as a breakthrough platform for ultrahigh-resolution sensing, offering strong potential for compact, high-performance sensing in biometrics, on-chip diagnostics, and IoT applications.