An interferometric sensor having an optic fiber which forms a sensing loop optical path and a second loop that is wound in the opposite direction of the sensor loop. Circularly polarized light propagates along the optical path of the sensing loop. A differential phase shift is induced in the light waves by a magnetic field of the current in a conductor. The light waves with their phase relationship are detected by a photo detector which outputs an electrical signal that is processed by loop closure electronics into a signal indicative of the current's magnitude and direction. If the phase shift due to rotation is sensed in the sensing loop, then that phase shift is canceled by an opposite phase shift from the compensating loop. The compensating loop or coil may be reversed in connections, or flipped over, for calibrating the current sensor. Also, with a small sensor coil, delay line compensating coils may be used in the optical circuit, so that the bias modulation frequency can be made low enough for low drift loop closure electronics.