An Optical Fiber Magnetic Field Sensor Based on Mach–Zehnder Interferometer Composed of Two Peanut-Shaped Structures and Tapered No-Core Fiber

An optical fiber magnetic field sensor based on magnetic fluid (MF) and a Mach–Zehnder interferometer (MZI) composed of tapered no-core fiber (TNCF) spliced between two peanut-shaped structures is fabricated. MF is used as the cladding of TNCF. Because the dip wavelengths in the transmission spectra have different sensitivities to the magnetic field and temperature of MF, simultaneous measurement of the magnetic field and temperature can be achieved using the transfer matrix method. Two peanut-shaped structures with fiber mode field mutation characteristics are fabricated to achieve excitation and coupling between the fundamental mode and higher order mode. The diameter of TNCF between the peanut-shaped structures is reduced to $20 \mu \text{m}$ to enhance evanescent waves. Due to the excited higher order modes and the enhanced evanescent waves, the sensitivities of the proposed sensor are greatly improved. The experimental results indicate that within the range from 0 to 16 mT, the magnetic field sensitivity is 0.49 nm/mT. In the range from 20 °C to 62 °C, the temperature sensitivity is 88.9 pm/°C. The sensor has the advantages of simple operation, low cost, and good stability.