[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - N-Matrix Jones Analysis of a Fiber-Optic Current Sensor

摘要： Fiber-optic current sensors (FOCS) based on Faraday effect could potentially be exploited in several high voltage applications where the electromagnetic immunity is a mandatory requirement. Nevertheless their actual in-field exploitation has always been limited due to critical issues such as fiber residual birefringence that degrades the current measurement accuracy. To this purpose special low-birefringence fibers and spun fibers featuring minimal residual birefringence have been developed. Yet these solutions do not prevent the birefringence contribution induced by the curvature arising during the realization of the current sensing coil. In the present work a novel numerical simulator based on the N-matrix Jones formalism [1] has been developed to analyse the detrimental effect of the birefringence on the Faraday-induced rotation of the state of polarization (SOP) of the light inside a FOCS due to the presence of an electrical current. In particular, the simulator takes into account both the intrinsic fiber birefringence and the bending-induced birefringence [2], combined with the circular birefringence caused by the Faraday effect. Simulation results have evidenced interesting behaviors related to the dependence of the effective Verdet constant Veff on both the number N of turns of the sensing fiber coil and the coil curvature radius r. Indeed Veff represents the FOCS effectiveness in inducing a polarization rotation theta, which is directly proportional to the electrical current I according to: ? = 2Veff (N ,r) ? N ? I . (1) In particular, simulations have shown that increasing N the values of Veff reduce due to the presence of intrinsic and bending-induced birefringence. This implies that the FOCS effectiveness does not grow linearly with N. A same trend also occurs when the coil radius r is progressively reduced. The simulation results have also been experimentally validated with the polarimetric setup shown in Fig. 1(a). The FOCS has been realized by wrapping a standard single-mode ?bre around an electrical cable supplied with an AC current up to a rms value of 35 A at the frequency of 50 Hz. A linearly polarized laser at 1.55 μm is coupled to the fiber coil through a circulator and reflected back to the receiver by a Faraday rotator mirror (FRM). This FRM cancels out all the reciprocal birefringence effect in the retracing path, while maintaining the Faraday rotation to be measured [3,4]. At the receiver side the orthogonal components of the SOP of the light are analyzed by means of a polarizing-beam splitter (PBS) together with a pair of photodiodes (PD1, PD2) and the Faraday-induced rotation is recovered.