Abstract: A general-purpose magnetic field sensor capable of sensitively measuring uniform magnetic fields includes a magneto-optic (Faraday-effect) sensor and a flux concentrator. Both the magneto-optic sensor and the flux concentrator have high magnetic permeability. The magneto-optic sensor is positioned in close proximity to the flux concentrator. The combination of the high-permeability magneto-optic material and the flux concentrator creates a magnetic circuit which efficiently concentrates the magnetic field into the magneto-optic sensor, and greatly enhances the measurement sensitivity. The preferred embodiment uses a pair of cylindrical flux concentrators in axial alignment. A sensing beam passes through a central axis hole in the cylindrical flux concentrators. The Faraday effect sensor is co-axially aligned with the holes in the cylindrical flux concentrators. The concentrators and Faraday effect sensor create a continuous flux concentrating magnetic circuit.