Optical fiber polarizer

A fiber-optic polarizer comprising the combination of

• an optical fiber having a core and cladding with different refractive indices and forming a single-mode guiding region,
• said core having a non-circular cross-section defining two transverse orthogonal axes which, in combination with said different refractive indices, decouples waves polarized along said axes,
• said guiding region being offset from the center of gravity of the fiber and located sufficiently close to the surface of one side of the fiber, along a selected length of the fiber, to expose the evanescent field of the guiding region at the surface of said one side to allow coupling to a contiguous medium,
• the outer surface of the fiber having a non-circular cross-section with a predetermined geometric relationship to said guiding region and said orthogonal transverse axes so that the location of said guiding region and the orientation of said axes can be ascertained from the geometry of said outer surface and
• polarizing means extending longitudinally along a selected length of said fiber on said one side thereof and coupled to the evanescent field exposed at the fiber surface on said one side, said polarizing means having different indices of refraction along axes aligned with the transverse orthogonal axes of said core, the index of refraction that is aligned with the axis of polarization of a desired wave in said fiber being less than the effective index of refraction of said desired wave in said guiding region of said fiber, and the other index of refraction of said polarizing means being greater than the effective index of refraction of an unwanted wave in said guiding region of said fiber, said unwanted wave having an axis of polarization orthogonal to the axis of polarization of said desired wave.

A method of polarizing signals in an elongated optical fiber comprising a core and cladding having different refractive indices and forming a single-mode guiding region,

• said core having a non-circular cross-section defining two transverse orthogonal axes which, in combination with said different refractive indices, decouple waves polarized along said axes,
• said guiding region being offset from the center of gravity of the fiber and located sufficiently close to the surface of one side of the fiber, along a selected length of the fiber to expose the evanescent field of the guiding region at the surface of said one side to allow coupling to a contiguous medium,
• the outer surface of the fiber having a non-circular cross-section with a predetermined geometric relationship to said guiding region and said orthogonal transverse axes so that the location of said guiding region and the orientation of said axes can be ascertained from the geometry of said outer surface, said method comprising the steps of
• stretching a polymeric film along a first axis disposed in the plane of the film to increase the index of refraction of the film along said first axis to a value above the effective index of refraction of an unwanted wave in said guiding region, said film having a second index of refraction along a second axis that is perpendicular to said first axis, said second index of refraction being below the effective index of refraction of a desired wave in said guiding region, and
• locating the film with one edge thereof extending longitudinally of the fiber and coupled to the evanescent field exposed at the fiber surface on said one side of the fiber, with said first axis disposed generally parallel to the minor axis of the elliptical corss section of said core and with said second axis disposed generally parallel to the major axis of the elliptical cross-section of said core.