Modulation-free broadband fiber optic current sensor based on a linear resonant cavity

A modulation-free, low-cost fiber optic current sensor based on a broadband source and a single-mode linear resonant cavity is presented. The Faraday effect of the current under test results in a phase difference between the two beams with orthogonal circular polarization states. The out-cavity beams propagating in different circles continuously superimpose and are demodulated together. The utilization of a 3×3 coupler and balanced photodetector enables the acquisition of current information without active modulation. In the direct current measurement, a sensitivity of 1.749 mV/A and a mean relative error below 0.2% within 0-200 A are achieved. The sensor exhibits a good capability for detecting the alternating current. In addition, the short-term and long-term noise performance of this system is analyzed.