High-precision magnetic field sensor via optoelectronic oscillator having taper-based in-line modal interferometer

A magnetic field sensor with high precision based on a dual-loop optoelectronic oscillator (OEO) in series with a compact in-line modal interferometer (MI) is designed and experimentally demonstrated. According to the sensing principles of the OEO-based sensor, the mapping relationship between the wavelength sensitivity of the designed MI and the microwave frequency sensitivity of the OEO is established, revealing that the frequency sensitivity of the OEO enlarges with the increment in the wavelength sensitivity. Given to this, we design a kind of high-sensitivity fiber magnetic field probe comprised of a tapered two-mode fiber (TMF) coated with magnetic fluid (MF), which is sandwiched between two segments of single-mode fiber (SMF) with unilateral core-offset. Compared to the bilateral core-offset, the unilateral core-offset reduces the mode mismatching loss further. At the same time, it can make the cladding mode have high intensity and interference spectrum generate large contrast. The above treatment ensures high sensing performance of the proposed OEO-based sensor. In the experiment, the in-line MI with a core-offset distance of 5 µm and a waist diameter of ∼9.2 µm is prepared. Firstly, we use Fourier phase unwrapping method to characterize the magnetic field in order to overcome the extraction problem of inaccurate dip wavelength due to the raw interference spectra with irregular burrs, showing maximum wavelength sensitivity of 691.5 pm/mT (655.4 pm/mT) for the light wave vector parallel (perpendicular) to the magnetic field. In order to enhance the measurement accuracy further, by means of the designed dual-loop OEO with high signal-to-noise ratio (SNR), we directly put the designed in-line MI into the above OEO system, obtaining the frequency sensitivity of − 1.508 kHz/mT (−1.429 kHz/mT) in the range of 6.69–8.35 mT (1.96–3.17 mT) for the light wave vector parallel (perpendicular) to the magnetic field, corresponding to the measurement error of within ± 0.089 mT ( ± 0.01 mT). The designed device can have potential in the fields of high-accuracy magnetic field measurement in a confined space.