Accurate ellipsometric magnetic-field sensor used to align the watt balance magnetic circuit of the French National Metrology Institute

Magnetic-field sensors based on the Faraday effect in a Tb3Ga5O12 crystal are investigated in terms of sensitivity, accuracy and directionality. The possibility of an increased sensitivity due to the application of multipass sensitive elements is considered. Signal-to-noise measurements on a 5 mm diameter × 20 mm long sample yield to a magnetic fields equivalent noise of 300 nT/Hz1/2 (τ = 1 s). The sensor has a sensitivity of 5.457(1) rad/T. Using a spinning quarter waveplate ellipsometer, we can measure magnetic fields up to 1 T. The data acquisition rate could be up to 80 Hz. This sensor is used to check the homogeneity and the orientation of the field provided by the French watt balance magnetic circuit whose aim is to redefine the SI unit of mass in the near future.