Abstract: 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.

Abstract: PROBLEM TO BE SOLVED: To provide a polarization control element which may be easily produced and miniaturized and has high reliability. SOLUTION: A core region 11 is a region of a circular shape in cross section disposed at the center of an optical fiber 10 and has a refractive index n1. A clad region 12 is a region disposed around this core region 11 and has a refractive index n2 smaller than the refractive index n1 of the core region 11. A pair of conductive parts 13a and 13b are respectively symmetrical with each other with respect to the optical axis center and are disposed within the clad region 12 over a specified range in a longitudinal direction. When current is passed to a pair of the conductive parts 13a and 13b, respectively, a stress is generated therein and a strain is generated in the core region 11 and the clad region 12. The propagation light propagated in the optical fiber 10 is controlled in the polarization state according to this strain.