A multicore fiber communicates using light up to an xth-order LP mode (where x is an integer of 1 or more) in a communication band. The multicore fiber includes: a plurality of cores; a clad that surrounds the plurality of cores and has a refractive index lower than refractive indexes of the plurality of cores; and a cover layer that covers the clad and has a refractive index higher than the refractive index of the clad. Each of the plurality of cores propagates light up to an (x+1)th-order LP mode. A core pitch is set to a distance where crosstalk of the light up to the xth-order LP mode becomes less than or equal to 40 dB/km and crosstalk of light of the (x+1)th-order LP mode becomes greater than or equal to 30 dB/km.

A dispersion shifted optical fiber where a radius r.sub.0 of a first center segment is 0.5 m to 2.8 m, and a relative refractive index difference .sub.0 is 0.4% or more and 0.9% or less. A radius r.sub.1 of a first segment is 1.8 m or more and 4.5 m or less. A radius r.sub.2 of a second segment is 4.0 m or more and 8.0 m or less, and a relative refractive index difference .sub.2 is 0.00% or more and 0.07% or less. A radius r.sub.3 of a third segment is 4.5 m or more and 8.5 m or less, and a relative refractive index difference .sub.3 is 0.285% or more and 0.5% or less. A radius r.sub.4 of a fourth segment is 8.0 m or more and 16.0 m or less, and a relative refractive index difference .sub.4 is 0.005% or more and 0.04% or less.

Disclosed herein are optical waveguide fibers comprising: (I) a core comprising an outer radius r.sub.1, a maximum refractive index delta percent .sub.1max and core alpha, , of larger than 5; and (II) a cladding surrounding the core, the cladding comprising: (i) an inner cladding region having outer radius r.sub.2 and refractive index delta percent .sub.2, wherein .sub.1max>.sub.2; (ii) a trench region surrounding the inner cladding region, the trench region having an outer radius, r.sub.3 where r.sub.310 microns and refractive index delta percent .sub.3; and (iii) an outer cladding region having chlorine concentration of 1.2 wt. % surrounding the trench region and comprising refractive index delta percent .sub.4, wherein .sub.1max>.sub.4 and .sub.2>.sub.3, and .sub.4>.sub.3 and wherein the difference between .sub.4 and .sub.3 is 0.12 percent.

An optical fiber having both low macrobend loss and low microbend loss. The fiber has a central core region, a first (inner) cladding region surrounding the central core region and having an outer radius r.sub.2>16 microns and relative refractive index .sub.2, and a second (outer) cladding region surrounding the first cladding region having relative refractive index, .sub.3, wherein .sub.1>.sub.3>.sub.2. The difference between .sub.3 and .sub.2 is greater than 0.12 percent. The fiber exhibits a 22 m cable cutoff less than or equal to 1260 nm, and r.sub.1/r.sub.2 is greater or equal to 0.24 and bend loss at 1550 nm for a 15 mm diameter mandrel of less than 0.5 dB/turn.

A multicore fiber for communication 10 which allows propagation of an optical signal includes: a clad 12; a core 11a which is arranged in a center of the clad 12; and seven to ten cores 11b which are arranged at equal intervals surrounding the core 11a, and the cladding diameter is 230 m, distances between centers of the mutually neighboring cores 11a and 11b are 30 m or more, distances between the centers of the cores 11b and an outer peripheral surface of the clad 12 are 35 m or more and a mode field diameter of light propagating in the cores 11a and 11b is 9 m to 13 m.

The present embodiment relates to a CMCF including a structure to achieve more efficient reduction in transmission loss by suppressing decrease in concentration of alkali metal due to diffusion of alkali metal. In the CMCF including a plurality of cores, a power coupling coefficient h between adjacent cores is set to 110.sup.3/m or more, to maintain an optical coupling state between the adjacent cores. In addition, alkali metal contributing to reduction in transmission loss is added to each of the cores such that a stress maximum value _.sub.max between adjacent cores has a negative value.

A present embodiment relates to a MDL measurement method and the like including a structure for enabling MDL measurement without increasing a processing load. The present embodiment sequentially executes, for N (2) spatial modes, light-input operation of inputting light of a predetermined intensity to an arbitrary spatial mode, and intensity measurement operation of measuring an output light intensity of each of the N spatial modes including the arbitrary spatial mode, to generate a transfer matrix relating to transmission loss in an optical fiber as a measurement target, and determine at least a linear value of MDL per unit fiber length by using each component value of the generated transfer matrix.

An optical fiber comprising: (i) a core region comprising an outer radius r.sub.1, and 3.0r.sub.17.0 microns and a relative refractive index .sub.1max and 0.32%.sub.1max0.5%; (b) a depressed index cladding region surrounding the core region comprising an outer radius r.sub.3 and a relative refractive index .sub.3 less than 0.2%, and trench volume V.sub.3 wherein 45% -micron.sup.2|V.sub.3|200% -micron.sup.2; (c) a first outer cladding region surrounding the depressed index cladding region and comprising a relative refractive index .sub.4 and an outer radius r.sub.4; and (d) a second outer cladding layer comprising 5 wt %-20 wt % titania, a relative refractive index .sub.5, and a thickness T.sub.M, wherein 3 micronT.sub.M30 microns, and outer radius r.sub.565 microns; the optical fiber has a mode field diameter MFD.sub.1550 and 8 micronsMFD.sub.155010.5 microns, a cutoff wavelength 1550 nm when bent 1 turn around a 2.5 mm radius mandrel, and a bending loss at 1550 nm when using a mandrel comprising a radius of 2.5 mm of 1.0 dB/turn.

An optical fiber comprising: (i) a core region comprising an outer radius r.sub.1, and 3.0r.sub.17.0 microns and a relative refractive index .sub.1max and 0.32%.sub.1max0.5%; (b) a depressed index cladding region surrounding the core region comprising an outer radius r.sub.3 and a relative refractive index .sub.3 less than 0.2%, and trench volume V.sub.3 wherein 45% -micron.sup.2|V.sub.3|200% -micron.sup.2; (c) a first outer cladding region surrounding the depressed index cladding region and comprising a relative refractive index .sub.4 and an outer radius r.sub.4; and (d) a second outer cladding layer comprising 5 wt %-20 wt % titania, a relative refractive index .sub.5, and a thickness T.sub.M, wherein 3 micronT.sub.M30 microns, and outer radius r.sub.565 microns; the optical fiber has a mode field diameter MFD.sub.1550 and 8 micronsMFD.sub.155010.5 microns, a cutoff wavelength <1550 nm when bent 1 turn around a 2.5 mm radius mandrel, and a bending loss at 1550 nm when using a mandrel comprising a radius of 2.5 mm of 10 dB/turn.

A multicore fiber communicates using light up to an xth-order LP mode (where x is an integer of 1 or more) in a communication band. The multicore fiber includes: a plurality of cores; a clad that surrounds the plurality of cores and has a refractive index lower than refractive indexes of the plurality of cores; and a cover layer that covers the clad and has a refractive index higher than the refractive index of the clad. Each of the plurality of cores propagates light up to an (x+1)th-order LP mode. A core pitch is set to a distance where crosstalk of the light up to the xth-order LP mode becomes less than or equal to 40 dB/km and crosstalk of light of the (x+1)th-order LP mode becomes greater than or equal to 30 dB/km.