Multipoint space-domain active fiber cavity ringdown technique for quasi-distributed magnetic field monitoring

Abstract

A multipoint space-domain active fiber cavity ringdown (FCRD) sensing technique based on frequency-shifted interferometry (FSI) is proposed and investigated for quasi-distributed magnetic field measurements. Multiple FSI-based fiber ringdown cavities (RDCs) are cascaded together to constitute a sensor array, share one continuous-wave optical source and one slow balanced detector, and thus the system is economical and highly sensitive. In each RDC a bidirectional erbium-doped fiber amplifier (Bi-EDFA) is inserted to compensate the inherent cavity loss for achieving higher sensitivity. Compared to the time-domain active FCRD techniques, the amplifier has lower gain fluctuation because the continuous light (instead of pulse light) coupled into the cavity enables more stable optical power, and has lower amplified spontaneous emission noise due to the use of two narrow bandpass filters and differential detection. Magnetic fields in different locations are synchronously resolved in the space domain by measuring the ringdown distances of continuous light, rather than measuring the ringdown times of pulse light in the time domain as in conventional multipoint FCRD techniques. A dual-point active FSI-FCRD magnetic field sensor was experimentally built, and two side-polished fibers coated with magnetic fluid were incorporated inside the fiber cavities as the sensing probes. By applying the external magnetic field from 0 to 230 Gs, the sensitivities of 6.68 × 10−3 and 4.45 × 10−3/(km Gs) were respectively achieved, which are at least one order of magnitude higher than the conventional passive FCRD techniques. After repeated measurements, the obtained ringdown baseline stabilities of the two units were 0.91% and 2.00%, which were much higher than those of time-domain active FCRD sensing systems. Using power budget analysis, the maximum number of identical sensing units can be expected to be 48 over a 25.89-km distance under the nearly same experimental conditions. The proposed multipoint active FCRD sensing scheme has the merits of low cost, high sensitivity, good stability, and strong multiplexing ability, which would find good application prospects at electric grids, national defense, biology, and medicine for monitoring quasi-distributed magnetic field.