Patent ID: 8542366

Claim:
A sensing coil for a Sagnac interferometer current sensor, the sensing coil being composed of an optical fibre that is arranged in use to transmit a single elliptical polarisation state and the sensing coil comprising at least two interconnected loops, a first of the loops being arranged in use to enclose a current conductor and the loops being interconnected such that light propagating in a first direction in the first loop will propagate in a second, opposite, direction in the other or, if more than one, in at least one other loop, wherein the sensing coil is formed with two only loops and wherein the respective loops are wound with turns and enclosed areas that satisfy the relationship N 1 ×A 1 =N 2 ×A 2 , where N 1 =number of turns in the first loop, A 1 =area enclosed by the first loop, N 2 =number of turns in the second loop and A 2 =area enclosed by the second loop; and wherein both the first and the second interconnected loops are wound in opposite directions about a common axis and are arranged in use to enclose portions of a current conductor.