Patent ID: 6891622

Claim:
A current sensor comprising: a) a source of linearly polarized light; b) a coil of single-mode optical fiber having a first end, a second end, and one or more turns, the coil being deployed around a conductor which is carrying an electric current to be sensed; c) a first transformer of polarized light and a second transformer of polarized light for transforming light between linearly polarized and circularly polarized states, said transformers each comprising a birefringent fiber having a first end, a second end, and a central axis, the birefringent fiber comprising a twist through an angle about the central axis, said twist being at a distance from the first end of the birefringent fiber, the angle and distance chosen so that linearly polarized light entering said second end of the birefringent fiber exits the first end of the birefringent fiber effectively circularly polarized, the first end of said first transformer being coupled to the first end of the coil of single-mode fiber, and the first end of said second transformer being coupled to the second end of the coil; d) a first directional coupler for optically coupling the linearly polarized light from the source to the second ends of the transformers; e) an optical detector for receiving the light beams which have traversed the coil and producing an output signal indicative of a magnetic field produced by the electric current; f) a second directional coupler for optically coupling to the optical detector light which has emerged from the coil, passed through the polarization transformers, and been rejoined by the first directional coupler; wherein the angle is approximately equal to an odd multiple of Π/4 radians, and the distance is approximately an odd multiple of one quarter of a beatlength.