An ultra-compact and highly sensitive magnetic field sensor using a gradient-tapered SMF–TMMF–SMF structure

This study proposes an ultra-compact magnetic field sensor based on a single-mode–tapered multimode–single-mode (STMS) structure using a gradient-tapered multimode fiber (TMMF). The sensor employs an axially graded tapering process to finely tune the microstructure of the multimode fiber, which not only enhances modal phase accumulation but also effectively extends the penetration depth of the evanescent field. This leads to a high degree of spatial overlap between the interference phase modulation region and the external magnetic field interaction zone, significantly improving the optical field’s sensitivity to magnetic fluid perturbations and its nonlinear response efficiency. Theoretical modeling and simulation analysis further reveal the critical role of the tapered structure in high-order mode excitation, interference phase evolution, and optical field energy distribution. With a compact length of just 2.843 mm, the sensor exhibits a peak sensitivity of 2.16 nm/mT in the 10–16 mT magnetic field range. The repeatability error is as low as ±0.0136 %, demonstrating outstanding stability and sensing performance.