Highly Sensitive Magnetic Field Sensor Based on Few-Mode Fiber Interferometry

This article proposes an all-fiber Mach–Zehnder interferometer (MZI) magnetic field sensor based on few-mode fiber (FMF) and hollow-core fiber (HCF). The proposed SMF-FMF-HCF-SMF (SFHS) structure is achieved by fusion splicing FMF and HCF between two single-mode fibers (SMFs), with the offset fusion splicing technique implemented between SMF and FMF. Through the offset fusion splicing, high-order modes are more easily excited when light propagates in the FMF, while the structural characteristics of the FMF reduce the number of excited high-order modes, thus avoiding excessive interferences and spectra overlay effects caused by them. In the HCF region, light is coupled and confined within the cladding of the HCF. Low-order mode and high-order modes propagate in the cladding and form a multimode interference. This phenomenon enhances the sensor’s sensitivity to environmental changes. The sensing structure is enclosed within a capillary tube filled with magnetic fluid (MF), which exhibits tunable refractive index (RI) properties. By subjecting the MF-filled SFHS structure to varying magnetic fields, the RI modulation of the MF facilitates the measurement of magnetic field intensities. Moreover, due to the circular asymmetry of the sensor structure, it exhibits sensitivity to the direction of the magnetic field. The experimental results indicate that the proposed sensing structure exhibits a magnetic field sensitivity of −0.28 dB/Oe at a wavelength of 1583 nm and can be applied as a magnetic field vector sensor.