Abstract: The invention relates to an optical fiber measuring device of the type in which variation of a measured parameter causes a difference of progression of light waves in the optical fiber. Such a device permits measurement of speed of rotation, or of current and magnetic field. The device includes an electronic device for digitally processing a signal indicative of phase shift of one light wave relative to another, the light waves propagating through a preferably monomode optical fiber in a SAGNAC ring interferometer, modulated by a phase modulator.