The invention relates to a method and a device for controlling the refractive index in the core of an optical fiber. According to the invention, an optical fiber is provided with a longitudinal electrode running along the core of the fiber. An electric current is passed through the electrode to induce ohmic heating thereof, causing thermal expansion and consequently a compressing force upon the core of the fiber. This compression of the core leads to induced changes in the refractive index in the direction of the compressing force, and hence induces or alters birefringence in the core.