All fiber optic current sensor based on phase-shift fiber loop ringdown structure.

An all fiber optic current sensor (AFOCS) utilizing ordinary optical fiber is proposed and demonstrated, which is implemented with a phase-shift fiber loop ringdown (PS-FLRD) structure. The current-induced rotation angle is converted into a minute change in transmittance of the fiber loop, which can be obtained by measuring the phase shift. The current sensitivity is improved by allowing optical signals to traverse the sensing fiber repeatedly. The relationship between the current sensitivity, intrinsic phase shift, and initial transmittance of the fiber loop is numerically analyzed, and the tunable sensitivity is experimentally verified by adjusting the modulation frequency. An optimal current sensitivity of 0.8158°/A is experimentally obtained for the proposed sensor, and the minimum detectable current is at least 100 mA. The proposed sensor requires fewer polarization elements compared with the common type of fiber optic current sensor (FOCS) and has the characteristics of simple structure, high sensitivity, and ease of operation; it will be a promising approach in practical applications.