A sensing coil for a Sagnac interferometer current sensor is disclosed, the sensing coil (14) being composed of an optical fiber (for example a spun polarizing Hi-Bi fiber) that is arranged in use to transmit a single elliptical polarization state and the sensing coil comprising at least two interconnected loops (15 and 16 or 26 and 27 or 28 and 29). At least one of the loops is arranged in use to enclose a current conductor (11, 12 or 25) and the loops are interconnected such that light propagating in a first direction (23 or 30) in the first loop will propagate in a second, opposite, direction (24 or 31) in the other or, if more than one, in each other loop, whereby the sensing coil provides minimal sensitivity to rotational movement. Also disclosed are a sensing unit (FIG. 5) that incorporates the sensing coil and a current sensor (FIG. 1) that incorporates the sensing unit.