Distributed Directional Force Sensing in Few-Mode Polarization-Maintaining Fibers via Low-Coherence Interferometry

We propose a simple and cost-effective distributed force sensing technique based on intermodal coupling in polarization-maintaining few-mode fibers. A broadband linearly polarized optical source is injected into a single-mode-few-mode-single-mode fiber structure, and the resulting interferometric signal is analyzed using an optical spectrum analyzer. Localized external perturbations are detected via the induced coupling to higher-order modes, with spatial information extracted through Fourier-domain analysis of the spectrum, in a manner similar to optical frequency domain reflectometry. Experimental results demonstrate a spatial resolution of 8 cm of a maximum of 38 m sensing length, directional sensitivity, and the capability to resolve multiple force points along meter-scale fiber lengths. Transverse forces ranging from 0.65 N to 3.8 N have been tested, yielding a measurement precision of approximately 0.03 N. This technique offers a compact, temperature-independent method for real-time monitoring, providing an alternative to conventional distributed sensing methods.