Optical fibre interferometer and method for measuring a magnetic field or an electrical current based on said interferometer

A fiber-optic interferometer comprising a light source (20) capable of generating a source beam (100), a differential phase modulator (16), a fiber-optic device (400), a detection system (18), a signal-processing system (900), the fiber-optic device (400) comprising a detection optical fiber (73) having a Verdet constant capable of inducing a non-reciprocal magneto-optic Faraday effect, the detection optical fiber (73) being arranged in a magnetic field or forming at least one turn around an electric conductor (120), the fiber-optic interferometer being capable of detecting a phase difference of an interferometric beam (300) formed by interferences between two polarized light waves (111, 112) having travelled simultaneously through the detection optical fiber (73) along a closed optical path, the two polarized light waves (111, 112) being modulated by the differential phase modulator, and to deduce therefrom, by dividing the phase difference by a scaling factor, a value of the magnetic field integrated along the closed optical path or a value of an electric current flowing in the electric conductor (120), the scaling factor being proportional to the Verdet constant of the optical fiber (73), wherein the signal-processing system (900) is adapted and configured to measure a variation of power contrast of part of the interferometric beam modulated by the differential phase modulator and to deduce from the contrast variation a measurement of variation of the scaling factor.