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FOCS measuring principle

The Faraday effect

The ABB FOCS is an unique Fiber-Optic Current measurement product using the Faraday effect in an optical fibre. The sensing fiber encloses the current-carrying bus bars.

The Faraday effect is the phenomenon that in a medium such as glass right and left circularly polarized light waves travel at different speeds if a magnetic field is applied along the propagation direction. As a result the waves accumulate a phase difference.

Two light waves with orthogonal linear polarizations travel from the optoelectronics module, which includes a semiconductor light source, via an interconnecting fiber to the single-ended sensing fiber looped around the bus bar. A single loop is commonly sufficient for high dc currents. At the entrance of the sensing fiber a fiber-optic phase retarder converts the orthogonal linear waves into left and right circularly polarized light. The circular waves travel through the coil of sensing fiber, are reflected at the end of the fiber and then retrace their optical path back to the coil entrance. Here, they are again converted into orthogonal linearly polarized waves.

Since the circular waves travel at somewhat different speeds through the sensing fiber if a dc current, I, is flowing the two returning light waves have accumulated a phase difference. The phase difference is proportional to the line integral of the magnetic field along the sensing fiber and is therefore a direct measure for the current.

The returning waves are brought to interference in the optoelectronics module. The signal processor then converts the optical phase difference into a digital signal. A particular advantage of operating the sensing coil in reflection is, besides the simplicity of the arrangement, the fact that the sensor signal is largely immune to mechanical perturbations such as shock and vibration. While the non-reciprocal Faraday optical phase shifts double on the ways forward and backward, phase shifts caused by mechanical disturbances are reciprocal and cancel each other.

The technology of the optoelectronic detection circuit is the same as used in fiber gyroscopes and has been well proven in demanding navigation systems. Fiber gyroscopes use the Sagnac effect seen by two counter-propagating light waves in a fiber coil to measure rotation velocities.

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