A fiber optic current or magnetic field sensor uses a sensing fiber in a coil for measuring a current or a magnetic field and has a retarder for converting between linearly polarized light and elliptically polarized light. The retardation of the retarder, its temperature dependence as well as its azimuth angle in respect to the plane of the fiber coil are optimized in dependence of the birefringence in the sensing fiber in order to minimize the influence of temperature variations and manufacturing tolerances on the overall scale factor of the sensor.