Current sensing utilizing heterodyne detection of the Faraday effect in single-mode optical fiber

A fiber-optic current sensor based on the Faraday effect is described which utilizes laser-diode frequency modulation in order to synthesize a heterodyne-type sensor output. The relative phase of the output carrier is directly modulated by the magnetically induced circular polarization mode-phase delay. Current sensing over three orders of magnitude is demonstrated, and a Verdet constant of
2.57 \times 10^{-6}
rad/A (at 0.83μm) is reported for the fiber used.