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In recent years, a considerable research effort has been expended on the field of optical current measurement in order to
replace the conventional curient transformer (CT) used in power distribution 1 Although current measurement
schemes based upon the use of an optical fibre as the sensing element were proposed as early as 1976, and a number of new
methods which employ optical fibre, or bulk glass, or even electro-optic hybrid sensing elements have been explored and
aviable commercial product has not yet emerged, despite the considerable level of research that has been devoted to
the concept.
An opticai current sensor consists of an optical current sensing element which measures the integral of the magnetic
field along a closed (or nearly closed) optical loop around the current to be measured, an optical fibre link which connects the
sensing element to an opto-eletronic control/processing unit, used to launch a light beam into the optical fibre and
detect/demodulate the modulated optical signal collected from the fibre. According to the sensing mechanism employed and the
sensing materials used, the optical current measurement devices may be categorized into three main groups: (i) optical current
sensors (OCS) employing optical fibre as their sensing elements, (ii) the OCS type using bulk glass to sense the current, and
(iii) the OCS type using electro-optic hybrid devices. In general, the principle ofthe operation ofthe first two types of sensors
is based upon the so-called "Faraday magneto-optic effect", in which the polarization azimuth, 1F' a linearly polarized light
beam propagating inside an optical material is rotated under the influence of a magnetic field, H, generated by the electrical
current to be measured and is given by
(Formula available in paper) (1)
where V is the Verdet constant ofthe fibre core and 1 is the interaction length. Ifthe medium is subject to the magnetic field
within a long solenoid this then becomes
(Formula available in paper) (2)
where N1 and NC the number of passes of the light through the solenoid and the number of turns of wire in the solenoid
respectively, and I is the electric current. The electro-optic hybrid current sensors are often constructed from a conventional
current Iransfonner interrogated with different electro-optic or fibre optic sensing schemes.
Among these groups ofsensor, optical fibre current sensors have been discussed and reviewed by a number of scientists 1
3. Hence, in this paper, we will summarize the possible solutions for dealing with the stability problems associated with bendinduced
linear birefringence in optical fibre current sensors and review the recent development in the area of current measurement
using optic bulk glasses and electro-optic hybrid sensing techniques.
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