Fig. 1. Basic structure of MF and the behavior of the magnetic nanoparticles under magnetic field

Fig. 2. Typical schemes of MF fiber magnetic field sensors based on MZI. (a) Side-polished optical fiber^[7]; (b) core-offset fusion splicing^[11]; (c) fiber tapering^[15]

Fig. 3. Interferometric fiber optic sensor. (a) Micro-FPI structure with waveguide^[24]; (b) SI structure with MF filled hollow waveguide^[33]

Fig. 4. Optical fiber magnetic field sensors based on SPR and MF. (a) Dual-channel wedge-shaped fiber coated by gold film structure^[39]; (b) side-polished few-mode fiber gold-coated structure^[41]; (c) half-side gold-coated hetero-core fiber structure^[42]

Fig. 5. Illustration of the portable smartphone-based vector magnetometer system^[46]. (a) Smartphone part; (b) schematic diagram of the side-polished fiber sensing unit

Fig. 6. Some other demodulation methods. (a) Fiber cavity ring-down technique^[50]; (b) optoelectronic oscillator^[53]; (c) fiber laser resonator technique^[54]

Fig. 7. Structure of NV color centers. (a) Lattice structure; (b) energy level structure

Fig. 8. Magnetic field sensors by the integration of NDs with optical fiber. (a) Fiber taper covered with NDs^[60]; (b) NDs chemically bonded to the tip of a tapered fiber^[63]; (c) NDs chemically bonded onto the fiber end-face ^[64]; (d) NDs-doped in tellurite glass fiber^[65]

Fig. 9. Optical-fiber magnetic field sensors integrated with micro/bulk diamond. (a) Microscale diamond integrated on the end-face of tapered fiber^[71]; (b) bulk diamond integrated on the fiber end-face with a silver-coated film^[73]; (c) microscale diamond sensor enhanced by magnetic flux concentrators^[74]; (d) diamond-tapered fiber sensor applied for fluid environment sensing^[75]; (e) micro-fabricated fiber-integrated diamond sensor^[76]; (f) distributed diamond magnetic sensing optical fiber^[77]

Fig. 10. Applications of diamond optical fiber magnetic field sensors in biological detection. (a) Diamond SPR fiber sensor for bovine serum albumin detection^[83]; (b)fiber probe based on T₁ longitudinal relaxometry for pH and Gd³⁺ detection^[84]

Fig. 11. Applications of diamond optical fiber magnetic field sensors in surface magnetic scanning imaging. (a) Magnetic scanning of coplanar waveguides using diamond-tapered fiber probe^[92]; (b) magnetic imaging of electrodes with diamond nanopillar structures on fiber end-faces^[93]; (c) magnetic imaging results of Ω-shaped current line^[95]