Optical fibre Faraday rotation current sensor with closed-loop operation

A novel closed-loop fibre-optic current sensor based on the Faraday effect is described which maintains a constant sensitivity to AC currents in the presence of large quasi-DC and low-frequency current fluctuations. A Verdet constant of 2.55×10−6 rad/A (at 0.83 μm) is reported for the fibre used.