Single mode, single polarization optical fiber with accessible guiding region and method of forming directional coupler using same

A method for forming a directional coupler from a pair of continuously drawn optical fibers, each of which comprises a single 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, said core having a longer transvere dimension along one of said orthogonal axes than along the other of said axes for guiding two fundamental modes, one of said modes having an electric field parallel to the axis of the longer transverse dimension and the other of said modes having an electric field parallel to the axis of the shroter transverse dimension,
• the difference in the core dimension along said orthogonal transverse axes and the difference between the refractive indices of said core and cladding being sufficiently large to de-couple the fundamental modes so that the polarization of said modes is preserved within the fiber,
• said guiding region being offset from the geometric center of the fiber and located sufficinetly close to one side of the surface of the fiber, to allow coupling to a guided wave through said one side by exposure or expansion of the field of the guiding region,
• the outer surface of the fiber having a non-circular cross-section forming an indexing surface 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 indexing surface on the exterior of the fiber,
• said method comprising joining said fibers along selected lengths thereof with said guiding regions aligned with and coupled to each other along said selected lengths, and with the fiber surfaces that are closest to the respective guiding regions facing each other.

A method forming a directional coupler from a pair of optical fibers each of which comprises a core and cladding having different refractive indices and forming a single-mode guiding region,

• said core hving a non-circular cross-section defining two transverse orthogonal axes, said core having a longer transverse dimension along one of said orthogonal axes than along the other of said axes for guiding two fundamental modes, one of said modes having an electric field parallel to the axis of the longer transverse dimension and the other of said modes having an electric field parallel to the axis of the shorter transverse dimension,
• the difference in the core dimensions along said orthogonal transverse axes and the difference between the refractive indices of said core and cladding being sufficiently large to de-couple the fundamental modes so that the polarization of said modes is preserved within the fiber,
• said guiding region being offset from the geometric center of the fiber and located sufficiently close to one side of the surface of the fiber to allow coupling to a guided wave through said one side by exposure or expansion of the field of the guiding region,
• the outer surface of the fiber having a non-circular cross-section forming an indexing surface 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 indexing surface on the exterior of the fiber,
• said method comprising the steps of
• inserting said fibers into a tube with the guiding regions of the two fibers aligned with each other and with the fiber surfaces closest to the respective guiding regions facing each other within said tube, and
• drawing the tube while heating both the tube and the fibers so that the transverse dimensions of both fibers are reduced to expand the fields of the respective guiding regions of the two fibers, while at the same time fusing together the tube and the fibers.

A continuously drawn optical fiber comprising a single 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, said core having a longer transverse dimension along one of said orthogonal axes than along the other of said axes for guiding two fundamental modes, one of said modes having an electric field parallel to the axis of the longer transverse dimension and the other of said modes having an electric field parallel to the axis of the shorter transverse dimension,
• the difference in the core dimensions along said orthogonal transverse axes and the difference between the refractive indices of said core and cladding being sufficiently large to de-couple the fundametnal modes so that the polarization of said modes is preserved within the fiber,
• said guiding region being offset from the geometric center of the fiber and located sufficiently close to one side of the surface of the fiber to expose the field of the guiding region along a selected length of the fiber, to allow coupling to a guide wave through said one side,
• the outer surface of the fiber having a non-circular cross-section forming an indexing surface with a predetermined geometric relationship to said guiding region and said orthgonal transverse axes so that the location of said guiding region and the orientation of said axes can be ascertained from the geometry of said indexing surface on the exterior of the fiber.

A continuously drawn optical fiber comprising a single 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, said core having a longer transverse dimension along one of said orthogonal axes than along the other of said axes for guiding two fundamental modes, one of said modes having an electric field parallel to the axis of the longer transverse dimension and the other of said modes having an electric field parallel to the axis of the shorter transverse dimension,
• the difference in the core dimensions along said orthogonal transverse axes and the difference between the refractive indices of said core and cladding being sufficiently large to de-couple the fundamental modes so that the polarization of said modes is preserved within the fiber,
• said guiding region having reduced transverse dimensions along a selected length of the fiber to expand the field of the guiding region along that selected length,
• said guiding region also being offset from the geometric center of the fiber and located sufficiently close to one side of the surface of the fiber to allow coupling to a guided wave through said one side via said expanded field along said selected length,
• the outer surface of the fiber having a non-circular cross-section forming an indexing surface 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 indexing surface on the exterior of the fiber.