Some 175 years ago Michael Faraday discovered magnetic circular birefringence, now commonly known as the Faraday effect. Sensing the magnetic field through the influence that the field has on light within the fiber optic sensor offers several advantages, one of them fundamental. These advantages find application in the measurement of electric current at high voltages by measuring the induced magnetic field, thus warranting application for this kind of fiber optic sensor (FOS) in future smart grids. Difficulties in designing and manufacturing high-performance FOSs were greatly alleviated by developments in optical telecommunication technology, thus giving new impetus to magnetometry based on the Faraday effect. Some of the major problems in the processing of optical signals and temperature dependence have been resolved, yet much effort is still needed to implement all solutions into a single commercial device. Artificial structures with giant Faraday rotation, reported in the literature in the 21st century, will further improve the performance of FOSs based on the Faraday effect. This paper will consider obstacles and limits imposed by the available technology and review solutions proposed so far for fiber optic sensors based on the Faraday effect.

We propose and experimentally prove an algorithm for demodulation of interferometric signals, modified for use in a totally implantable hearing aid device. A fiber optic configuration, which generates two quasi-quadrature signals by a passive 3x3 coupler, for a non-contact detection of the middle ear ossicle vibration is employed. We simulated the ossicle vibration and large movements and demonstrated the effectiveness of the algorithm to compensate changes of the signal DC values and the phase shift introduced by the coupler. Applying the proposed algorithm we obtained the output signal stability better than 0.5 dB, and the system equivalent input noise of about 31 dB (A) SPL @ 1 kHz.

This paper presents a robust fiber-optic magnetic field, or electric current sensor with temperature compensation. The compensation is based on the optical activity temperature dependence of a Bi₁₂GeO₂₀ crystal. A novel measuring head for this extrinsic fiber-optic sensor is developed. Optical path for temperature measurement passes between optical paths, used to sense the magnetic field, thus ensuring proper temperature sampling. Measurement accuracy is improved from 0.8% to 0.2%.