A Novel Long-Tail Fiber Current Sensor Based on Fiber Loop Ring-Down Spectroscopy and Fabry-Perot Cavity Filled With Magnetic Fluid

A novel long-tail optical fiber current sensor based on fiber loop ring-down spectroscopy (FLRDS) and magnetic-tunable refractive index properties and magnetictunable transmission properties of magnetic fluid is proposed and demonstrated for the first time. In the sensing system, a Fabry-Perot (F-P) cavity filled with magnetic fluid is used as the sensing head, and the sensitivity of current-sensing has been enhanced significantly. Relations between magnetic field and refractive index of magnetic fluid are measured. Refractive index of magnetic fluid increases linearly as the magnetic field increases and according to the F-P interferometer principle, the best gap length of F-P cavity is obtained. In this paper, the FLRDS sensing system has been theoretically modeled, and a new mathematical model developed for optimizing the coupling ratio of couplers has been presented, and the best coupling ratio has been obtained. The new mathematical model could also be used in the similar ring-down systems. Finally, the performances of the current-sensing system are tested by applying different measured currents. The final results indicate that a sensitivity of 14.88 mA/μs is achieved.