We demonstrate a novel Er3+-doped fibre laser operating in orthogonally
polarized dual-frequency modes and its application to electric current sensing with
frequency read-out. A Faraday rotating mirror and spatial hole burning effects in a
gain medium and in a saturable absorber are utilized to control the lasing mode and
polarization. The polarization mode beat frequency changes linearly in response to
the non-reciprocal circular birefringence induced by an external magnetic field.
When the laser was applied to sensing an alternating electric current with a simple
phase-locked loop signal processing scheme, the slope coefficient of 8.0 kHz A−1
per turn and the noise equivalent current of 460 Arms Hz−1=2 per turn were
obtained. The new current sensor is immune to perturbations in the lead fibres.