A multicore fiber includes a plurality of unit multicore fibers each including: a plurality of core portions; and a clad portion which is formed in an outer circumference of the core portions and has a refractive index lower than a maximum refractive index of the core portions. The plurality of the core portions have substantially same refractive index profile and different group delays at same wavelength in same propagation mode. The core portions of the multicore fiber are configured so that the core portions of the plurality of the unit multicore fibers are connected in cascade, a maximum value of differential group delays between the core portions of the multicore fiber is smaller than a reduced value of a maximum value of differential group delays between the core portions of each unit multicore fiber as a value in terms of a length of the multicore fiber.

A method for producing a multicore optical fiber includes: forming a common cladding tube by providing a first glass rod with a plurality of first holes and one or more second holes passing through the first glass rod in an axial direction, the one or more second holes having a diameter different from a diameter of the plurality of first holes; subjecting inner surfaces of the plurality of first holes and inner surfaces of the one or more second holes to a gas-phase process; and inserting a plurality of core rods into the plurality of first holes subjected to the gas-phase process, respectively, inserting a refractive index changing part rod into each of the one or more second holes subjected to the gas-phase process, and performing thermal integration to form a second glass rod. The common cladding tube is formed such that the diameter of the plurality of first holes is no more than 4 times the diameter of the one or more second holes.

The present invention relates to a method of manufacturing an optical fiber preform for obtaining an optical fiber with low transmission loss. A core preform included in the optical fiber preform comprises three or more core portions, which are each produced by a rod-in-collapse method, and in which both their alkali metal element concentration and chlorine concentration are independently controlled. In two or more manufacturing steps of the manufacturing steps for each of the three or more core portions, an alkali metal element is added. As a result, the mean alkali metal element concentration in the whole core preform is controlled to 7 atomic ppm or more and 70 atomic ppm or less.

The present disclosure provides optical fiber preforms formed from core canes having large core-clad ratio, intermediate core-cladding assemblies, and methods for making the preforms and core cladding assemblies. The preforms are made with capped core canes. The capping material has a coefficient of thermal expansion less than the coefficient of thermal expansion of the core cane and more closely matched to or lower than the coefficient of thermal expansion of the surrounding cladding monolith in a cane-in-soot process. Presence of the cap reduces stresses that arise from differential thermal expansion of the core cane and cladding materials and leads to preforms having low defect concentration and low probability of failure during subsequent thermal processing steps.

A preform manufacturing method of the present invention has a hole forming step of forming a plurality of holes in a glass body to produce a glass pipe, and a heating integration step of heating the glass pipe with core rods including core portions being inserted in the respective holes, thereby to implement integration of the core rods and the glass pipe. In the hole forming step, a peripheral hole out of the holes to be formed in the glass body is formed at a position determined in consideration of positional variation of the core portion before and after the integration.

A plurality of clad rods, and a clad tube, an arrangement process for arranging the plurality of core rods and the plurality of clad rods in a tube of the clad tube, in a state in which distances between center axes of the adjacent core rods become equal to each other and a state in which parts of outer circumferential surfaces in the adjacent rods contact, and an integration process for integrating the clad tube and the plurality of core rods and the plurality of clad rods arranged in the tube, wherein a ratio of a total cross-sectional area of a direction orthogonal to a length direction in the plurality of core rods and the plurality of clad rods with respect to an internal cross-sectional area of the tube of a direction orthogonal to a length direction in the clad tube is 0.84 or more.

Methods for producing a multicore fiber comprise a method step in which a component group is reshaped to form the multicore fiber or a pre-form for the multicore fiber, which comprises a hollow cylinder comprising a central bore and a hollow cylinder longitudinal axis, which hollow cylinder comprises a cladding glass region made of cladding glass and a plurality of core glass regions occupied by a core glass, wherein at least part of the central bore is occupied by a glass filling material. In order to provide a method for producing multicore fibers without central signal core, in which the risk of rejects during the completion of the hollow glass cladding cylinder is reduced, a marker element made of marker glass adjacent to the glass filling material is used, which extends along the longitudinal axis of the central bore.

Optical waveguide cores having refractive index profiles that vary angularly about a propagation axis of the core can provide single-mode operation with larger core diameters than conventional waveguides. An optical waveguide includes a core that extends along a propagation axis and has a refractive index profile that varies angularly about the propagation axis. The optical waveguide also includes a cladding disposed about the core and extending along the propagation axis. The refractive index profile of the core varies angularly along a length of the propagation axis.

A multicore fiber 1 includes a plurality of cores 3 disposed at predetermined intervals and surrounded by a cladding 5. The multicore fiber 1 also includes a marker 7 formed apart from the cores 3. The refractive index of the marker 7 is different from those of the cores 3 and the cladding 5. For example, the marker 7 may be made of a material having lower refractive index than that of the cladding 5. In this case, for example, the cores 3 may be made of germanium-doped quartz. The cladding 5 may be made of pure quartz. The marker 7 may be made of fluorine-doped quartz. Further, the marker 7 may be an empty hole.

A known method for producing a multicore fiber having a marker zone close to the edge comprises a method step in which a component group is reshaped to form the multicore fiber or to form a pre-form for the multicore fiber. In order to provide a method on this basis for producing multicore fibers having a marker zone close to the edge, in which the risk of rejects is reduced, the marker element is arranged on the an outer lateral surface of a glass cladding cylinder, wherein a longitudinal groove is created in the outer lateral surface of the glass cladding region and extending in the direction of a cylinder longitudinal axis, and the marker element is melted in the longitudinal groove before the reshaping to form the pre-form or the multicore fiber.