Sensitivity improvement of an optical current sensor with enhanced Faraday rotation

A sensitivity improvement technique is proposed for a class of bulk-glass optical current sensors that employ a ferromagnetic field concentrator. The total effective optical path length is demonstrated theoretically to be an invariant regardless of the bulk glass thickness and consequently independent of the size of the concentrator gap opening. Thus, if the magnetic field is increased by reducing the gap size, the eventual Faraday rotation for a given electric current can be increased proportionally, leading to an improved device sensitivity. The dependence of the gap magnetic field on gap size is calculated with an equivalent circuit model, and this analytical treatment is compared with a dedicated finite element computer package. By taking account of various types of optical power losses present in the bulk glass, the above formulated gap dependence of magnetic field is used to aid a realistic assessment of device sensitivity and this serves as a tool to design and analyze practical bulk-glass optical current sensors. A detailed experimental study to confirm the proposed sensitivity improvement technique is also reported.