High-Sensitivity Magnetic Field Detection Using Whispering Gallery Modes in a Bent Optical Fiber Loop

This work presents a compact and sensitive magnetic field sensor based on whispering gallery mode (WGM) excitation in a coated single-mode fiber (SMF) loop. The sensor is formed by bending the coated SMF around a magnetostrictive nickel (Ni) rod, where curvature-induced WGMs interfere with the core mode to produce measurable spectral fringes. Under an applied magnetic field, the Ni rod undergoes axial elongation, modulating the fiber loop curvature and inducing a resonance shift. Among tested configurations, the 10-mm Ni rod achieved a peak sensitivity of 249.43 pm/mT, while the 8-mm rod exhibited excellent stability and repeatability over time with a sensitivity of 117.88 pm/mT. Beam propagation simulations and experimental spectra both confirm enhanced mode coupling and increased bending loss at smaller loop radii. This simple, single-loop configuration offers high performance with minimal complexity, paving the way for scalable, high-resolution magnetic field sensing based on WGM–magnetostrictive interaction.