Performance and stability of a field fibre optics current sensor

Performance and stability of a field fibre optics interferometric current sensor are presented, showing a ±0.5% accuracy over a broad temperature range and an excellent insensitivity to the sensing fibre positioning. INTRODUCTION This paper presents the latest evaluations performed on an interferometric Sagnac fibre optics current sensor developed by the Metrology Laboratory in the Swiss Federal Institute of Technology, in collaboration with Trench Switzerland AG. This sensor is designed to measure high DC and AC currents up to ±500kA. The objective of this sensor is to obtain a ±0.5% accuracy over the full ±500kA range, for a temperature range between 0 and 50 degC (-20 to 80 degC for the sensing fibre), to comply with the end user requirements. Figure 1: Sagnac fibre optics current sensor ready for field measurements. The sensing cable that must enclose the probed electrical conductor is partially shown on top-left of the figure. As a result of the developments on optical gyroscopes [1,2] it is now well-known that the Sagnac interferometer has the key advantage to be only sensitive to nonreciprocal effects, such as rotation and Faraday effect. As a result, this interferometer is intrinsically insensitive to reciprocal effects such as fibre elongation due to thermal dilatation or mechanical solicitations. The main issue that prevented the development of a reliable instrument so far is related to the fibre birefringence. It turns out that such a sensor can only give unbiased measurements, provided that the light polarisation is maintained circular all along the sensing fibre loop. Many solutions have been proposed to compensate the effect of birefringence, either optically or through a set of measurements and some calculations [3,4,5]. But these techniques are limited to homogeneous linear birefringence along the sensing fibre, so that they turn out to be widely inapplicable in actual conditions in which the fibre birefringence is basically random. Fortunately the detrimental effect of polarisation mode dispersion in telecommunication systems had led to a big effort for manufacturing very low birefringence fibres at low cost. The remaining birefringence of such fibres turns out to be still too large for the proper operation of a current sensor. But it can now be widely rendered negligible by annealing [6] or mechanically twisting the fibre [7], so that a circular or freely-rotating linear polarisation is maintained over the entire fibre length. The aim of this paper is in particular to demonstrate that fibre birefringence is a solved problem as far as the fibre current sensor is concerned, so that a flexible instrument can be realised with a simple handling for the end user. F. Briffod & al. Performance and stability of a field fibre optics current sensor 2 SETUP The diagram of the realised fibre current sensor is shown in Figure 2. The configuration is very similar to that used in fibre optics gyroscopes, with the particularity to propagate the light through the sensing loop using a circular polarisation. This particularity requires a proper preparation of the light polarisation. For current sensing the fibre loop must enclose the probed electrical conductor. The circulation of the current magnetic field along the fibre causes a phase lag between circularly polarised waves propagating in opposite directions, as a result of the Faraday effect. After propagation into the sensor head, the two waves recombine, resulting in an interference signal to be processed. In absence of any electrical current the interferometer is fully balanced and its sensitivity to a differential phase shift is zero. An electro-optic phase modulator is thus used to bias the response of the interferometer, so that a maximum sensitivity for small electrical current is obtained, technique similar to that used in fibre gyros [8]. A standard lock-in detection at fm and 2fm followed by a suitable signal processing provides a value for the electrical current independent on any change in the light source intensity. The electro-optic phase modulation turns out to be temperature-dependent to an unacceptable extent, so that the Gyro-type modulator has to be temperature-controlled. The electro-optic modulator also achieves the splitting-recombination functions of the interferometer and transmits the light only in a well defined linear polarisation state.