U-shaped seven-core photonic crystal fiber–based surface plasmon resonance magnetic field sensor

This paper proposes and experimentally demonstrates a surface plasmon resonance (SPR)–based vector magnetic-field sensor using a U-shaped seven-core photonic crystal fiber (7C-PCF). The device employs a multimode fiber–7C-PCF–multimode fiber (MMF–7C-PCF–MMF) cascaded configuration, in which a gold film on the 7C-PCF forms the SPR-active region and a magnetic fluid serves as the magneto-sensitive medium, enabling magnetic-field-induced modulation of the SPR resonance wavelength. Finite-element simulations show that U-shaped bending enhances evanescent-field interaction at the gold interface, thereby strengthening the SPR response. Experimentally, the sensor achieves wavelength sensitivities of 3180 nm/RIU and 6680 nm/RIU over refractive-index ranges of 1.33–1.37 and 1.37–1.40, respectively. Magnetic-field measurements reveal a pronounced orientation-dependent magnetic field intensity response. In the weak-field regime (0–5 mT), the magnetic field intensity sensitivity reaches 10.3 nm/mT when the bending plane is perpendicular to the field (θ = 90°), but drops to 0.23 nm/mT when the sensor is aligned with the field (θ = 180°). For directional sensing, a maximum angular sensitivity of 0.5 nm/° is obtained within specific angular intervals. These results indicate that the proposed U-shaped 7C-PCF-SPR sensor enables high-sensitivity, direction-resolvable magnetic-field sensing in weak fields, offering a compact, fabrication-feasible platform for fiber-optic vector magnetometry.