A quasi-distributed fiber magnetic field sensor based on frequency-shifted interferometry fiber cavity ringdown technique

Abstract

A quasi-distributed fiber magnetic field sensing system based on frequency-shifted interferometry fiber cavity ringdown (FSI-FCRD) technique is proposed and demonstrated. In this scheme, multiple sensing units are connected in series in a frequency-shifted interferometer, and share one continuous-wave light source, one slow detector and one low-frequency data collector to lower the cost. In each unit, a side-polished single-mode fiber coated with the diluted water-based ferrofluid serves as the sensing element for enhancing sensitivity and mechanical strength. The cavity loss of each unit varies with the change of applied magnetic field, due to the tunable refractive index of the ferrofluid and the evanescent wave effect of the polished fiber. Therefore, the measurement of magnetic field can be realized by monitoring the cavity loss change. A dual-unit FSI-FCRD magnetic field sensing system was experimentally investigated. The cavity losses and locations of the two sensing units about 2500 m apart were synchronously achieved from a spatial domain ringdown signal of the system. The measured cavity loss of each sensing unit exhibited a good linear relationship with the magnetic field over a range of 0 to 250 Oe. The sensitivities were separately 6.7894 × 10−4 and 7.4980 × 10−4 dB/Oe, with the corresponding detection limits of 2.56 and 3.63 Oe. Through repeated measurements, the obtained baseline stabilities of the two units were 0.15% and 0.18%, respectively. By theoretical analysis, the maximum multiplexing number of sensing units can reach 30 over a 15.8-km distance under the same experimental settings. It indicates that multi-unit FSI-FCRD provides an alternative technique with low cost, high sensitivity and high stability for quasi-distributed magnetic field detection in many fields such as smart grids, biomedicine and national defense.