Calibrating ITER-fibre optic current sensor

A polarimetric-Fibre Optic Current Sensor (FOCS) is part of the plasma current diagnostic system at ITER. Its operating principle relies on the Faraday effect in optical fibres. When the fibre is looped around a section of the Vacuum Vessel (VV), the Faraday effect causes the polarization plane of the light to rotate proportionally to the current enclosed by the fibre. The rotation angle strength is defined by the Verdet constant <i>V</i>, a material-specific proportionality constant. However, the inevitable linear birefringence in the sensing fibres reduces the Faraday rotation angle, necessitating the calibration of the effective Verdet constant <i>V_eff</i> of the sensing fibre before operation to accurately deduce the current from the measured Faraday rotation angle. This paper presents and experimentally demonstrates an approach for <i>V_eff</i> calibrating with ITER-relevant calibration currents.