Directional Magnetic Field Sensing Based on Few-Mode Fiber Long-Period Grating With Spatial Refractive Index Modulation

We present a highly sensitive directional magnetic field sensor based on a long-period fiber grating (LPFG) inscribed in few-mode fiber (FMF) and combined with magnetic fluid (MF). The unique spatial refractive index modulation (RIM) profile of the FMF-LPFG allows for the simultaneous measurement of magnetic field intensity, direction, and temperature. The interaction between the anisotropic refractive index of the MF and the LPFG is exploited to achieve directional magnetic field sensitivity, with a maximum intensity sensitivity of 717.5 pm/mT at 0° and directional sensitivity of 163.3 pm/° at 10.7 mT. Temperature crosstalk is mitigated by monitoring independent interference dips, enabling reliable dual-parameter sensing. The proposed FMF-LPFG sensor demonstrates excellent intensity sensitivity, precise directional detection, and effective temperature monitoring. This work offers a robust and versatile solution for high-precision magnetic field monitoring in applications ranging from industrial sensing to biomedical diagnostics.