Cascaded core multicore fiber and manufacturing method for same

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.

BACKGROUND

The present disclosure relates to a multicore fiber and a manufacturing method of the multicore fiber.

In recent years, in order to increase transmission capacity, optical fibers suitable for space division multiplexing (SDM) systems have been developed. A multicore fiber is an example of such an optical fiber. In the multicore fiber, a plurality of independent cores are arranged in one optical fiber, so that the SDM systems may be implemented. On the other hand, in a multimode fiber capable of propagating signal light in a plurality of propagation modes (hereinafter, simply referred to as “modes”) in a single core, there is a capability of realizing an increase in communication capacity by mode multiplexing transmission (refer to Lars Gruner-Nielsen, et al. “Few Mode Transmission Fiber With Low DGD, Low Mode Coupling, and Low Loss”, J. Lightwave Technol. Vol. 30, No. 23 (2012), pp. 3693-3698.). Herein, since a normal multimode fiber includes a large number of modes, it is difficult to individually control the modes. However, a few-mode fiber where the number of modes is limited to a small number such as about 10 or less has been studied as a new axis of mode multiplexing transmission because all modes may be controlled to be used for propagation (refer to Lars Gruner-Nielsen, et al. “Few Mode Transmission Fiber With Low DGD, Low Mode Coupling, and Low Loss”, J. Lightwave Technol. Vol. 30, No. 23 (2012), pp. 3693-3698.). In addition, at present, few-mode propagating multicore fibers for simultaneously realizing space division multiplexing and mode multiplexing have been reported.

The most serious problem in the case of performing the mode multiplexing transmission is treatment of crosstalk signals generated between the modes. For the crosstalk treatment, a MIMO (Multiple Input, Multiple Output) technology has been utilized, where signal light in each mode is separated to be treated.

However, in the case of performing the mode multiplexing transmission, since group velocities in the optical fiber between modes are different, there is a problem in that the amount of signal processing during the MIMO process is increased. In order to solve the problem, efforts are also made to match the group velocities between modes (for example, a base mode and a higher order mode) in a few-mode fiber.

On the other hand, in the case of performing the SDM systems by using a multicore fiber, even if the refractive index profile of each core is designed to be the same so as to realize the same light propagation characteristics in each core, there occurs a difference between the refractive index profiles which are substantially the same, and thus, a difference in group velocity occurs between the cores. In a case where such a multicore fiber is used, if an MIMO process is performed, there may be a problem in that an amount of signal processing during an MIMO process is increased. In addition, in the case of processing signal light coupled with other cores caused by crosstalk, it is necessary to strictly control the group velocity difference between the cores.

There is a need for a multicore fiber and a method of manufacturing the multicore fiber where a differential group delay between core portions is reduced.

SUMMARY

In some embodiments, 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, wherein the plurality of the core portions have substantially same refractive index profile and different group delays at same wavelength in same propagation mode, and 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.

In some embodiments, a method of manufacturing a multicore fiber includes: preparing a plurality of unit multicore fibers 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 having substantially the same refractive index profile and different group delays at the same wavelength in the same propagation mode; and manufacturing the multicore fiber by connecting the core portions of the plurality of the unit multicore fibers in cascade, and the core portions of the plurality of the unit multicore fibers are connected in cascade so that 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.

The above and other objects, features, advantages and technical and industrial significance of this disclosure will be better understood by reading the following detailed description of presently preferred embodiments of the disclosure, when considered in connection with the accompanying drawings.

DETAILED DESCRIPTION

Hereinafter, embodiments of a multicore fiber and a method of manufacturing the multicore fiber according to the present disclosure will be described in detail with reference to the drawings. The present disclosure is not limited to the embodiments. In addition, in each figure, the same or corresponding elements are appropriately denoted by the same reference numerals. Furthermore, it should be noted that the figures are schematic ones and, in some cases, relationships or the like among dimensions of the elements may be different from the actual ones. In some cases, between the figures, portions having different relationships or ratios of dimensions may be included. In addition, terms which are not particularly defined in this specification are in accordance with definitions and measurement methods in ITU-T (International Telecommunication Union Telecommunication Standardization Sector) G. 650.1.

First Embodiment

FIG. 1Ais a diagram illustrating a configuration of a multicore fiber according to a first embodiment, andFIG. 1Bis a diagram illustrating a refractive index profile of the multicore fiber according to the first embodiment. As illustrated inFIG. 1A, a multicore fiber10is configured to include a plurality (six in the first embodiment) of core portions10a,10b,10c,10d,10e, and10f, a clad portion10gwhich is formed in the outer circumference of the core portions10ato10fand has a refractive index lower than the maximum refractive index of the core portions10ato10f, and a marker10gawhich is formed in the clad portion10gand is arranged at a position capable of identifying positions of the core portions10ato10f. The core portions10ato10fare arranged at positions in an equal distance r from the central axis O1of the clad portion10gat an equal angle to form a circle centered on the central axis O1. Namely, the core portions10ato10fare arranged to have 6-fold rotational symmetry. The center distance (core pitch) between the adjacent core portions is Λ.

As illustrated inFIGS. 1A and 1B, the core portion10ais configured to include a center core portion10aawhich has the maximum refractive index of the core portion10aand a refractive index profile P1of α power, an inner core layer10abwhich is formed in the outer circumference of the center core portion10aaand has a refractive index profile P2where the refractive index is substantially the same as the refractive index of the clad portion10g, and an outer core layer10acwhich is formed in the outer circumference of the inner core layer10aband has a refractive index profile P3where the refractive index is lower than the refractive index of the clad portion10g. A refractive index profile P4denotes the refractive index profile of the clad portion10g. In addition, the radius of the center core portion10aais denoted by a1(namely, the diameter is denoted by2a1), and the maximum value of the relative refractive-index difference with respect to the clad portion10gis denoted by Δ1. The radius of the inner core layer10abis denoted by a2(namely, the diameter is denoted by2a2), and the average value of the relative refractive-index difference with respect to the clad portion10gis denoted by Δ2. The radius of the outer core layer10acis denoted by a3(namely, the diameter is denoted by2a3), and the average value of the relative refractive-index difference with respect to the clad portion10gis denoted by Δ3. In this manner, the multicore fiber10has a trench-type refractive index profile.

In addition, when the maximum refractive index of the center core portion10aais denoted by n1, the refractive index of the clad portion10gis denoted by n0, and the radial distance from the center of the center core portion10aais denoted by r, the α-powered refractive index profile of the center core portion10aais expressed by the following Mathematical Formula. In addition, “^” is a symbol indicating exponentiation.
n2(r)=n12−(n12−n02)·(r/a1)^α

For example, a refractive index distribution profile of which value of α is 3 may be referred to as an α-powered refractive index distribution profile of which value of α is 3.

The center core portion10aais made of, for example, a quartz glass doped with germanium (Ge) as dopants increasing the refractive index. The inner core layer10abis made of, for example, a quartz glass which does not almost contain dopants for adjusting the refractive index or a pure quartz glass which does not contain dopants for adjusting the refractive index. The outer core layer10acis made of, for example, a quartz glass doped with, for example, fluorine (F) as dopants decreasing the refractive index. The clad portion10gis made of, for example, a pure quartz glass which does not contain dopants for adjusting the refractive index. The marker10gais made of a glass, a colored glass, or the like having a refractive index different from that of the clad portion10g. In addition, a coat may be formed on the outer circumference of the clad portion10g.

The other core portions10bto10fare also made of the same material as that of the core portion10aand has the same configuration. Namely, each of the core portions is configured to include a center core portion which has the maximum refractive index and has a refractive index distribution profile of α power, an inner core layer which is formed on the outer circumference of the center core portion and has a refractive index being substantially the same as the refractive index of the clad portion, and an outer core layer which is formed on the outer circumference of the inner core layer and has a refractive index being lower than the refractive index of the clad portion.

FIGS. 2A and 2Bare diagram illustrating the configuration of the multicore fiber10illustrated inFIG. 1A. As illustrated inFIG. 2A, the multicore fiber10is configured so that a plurality (six in the first embodiment) of unit multicore fibers11,12,13,14,15, and16are connected in cascade.

The unit multicore fibers11,12,13,14,15, and16are manufactured by cutting an original multicore fiber which is manufactured with lines being continuously drawn and has a length substantially the same as or larger than that of the multicore fiber10into six equal-length fibers and have configurations similar to that of the multicore fiber10. Namely, as illustrated inFIG. 2B, for example, the unit multicore fiber11is configured to include a plurality (six in the first embodiment) of core portions11a,11b,11c,11d,11e, and11f, a clad portion11gwhich is formed in the outer circumference of the core portions11ato11fand has a refractive index lower than the maximum refractive index of the core portions11ato11f, and a marker (not illustrated) which is formed in the clad portion11gand is arranged at a position capable of identifying positions of the core portions11ato11f. The core portions11ato11fare arranged at positions in a substantially equal distance from the central axis of the clad portion11gto form a circle centered on the central axis. In addition, each of the core portions11ato11fis configured to include a center core portion which has the maximum refractive index of each core portion and has a α-powered refractive index distribution profile, an inner core layer which is formed on the outer circumference of the center core portion and has a refractive index being substantially the same as that of the clad portion, and an outer core layer which is formed on the outer circumference of the inner core layer and has a refractive index being lower than the refractive index of the clad portion.

The other unit multicore fibers12to16also have similar configurations. Namely, the unit multicore fibers12to16are configured to include core portions12ato12f, a clad portion12g, and a marker, core portions13ato13f, a clad portion13g, and a marker, core portions14ato14f, a clad portion14g, and a marker, core portions15ato15f, a clad portion15g, and a marker, and core portions16ato16f, a clad portion16g, and a marker, respectively. Herein, for example, the core portions11a,12a,13a,14a,15a, and16aare continuous with each other before cutting into the unit multicore fibers. The core portions11bto16b(the core portions11cto16c, the core portions11dto16d, the core portions11eto16e, the core portions11fto16f) are continuous with each other before cutting into the unit multicore fibers.

All the core portions of each of the unit multicore fibers11to16have refractive index profiles of which shapes are substantially the same as that illustrated inFIG. 1B, and the parameters a1, a2, a3, α, Δ1, Δ2, and Δ3indicating the refractive index profile are substantially the same as those illustrate in the figure. Such parameters are set so that, in a wavelength of input light (for example, light of C band (1530 nm to 1565 nm) or L band (1565 nm to 1610 nm) as a wavelength band used for optical communication), propagation modes of two modes (LP01mode and LP11mode) exist in each core portion. In addition, the value of α is set so that group velocity difference between the two modes is small, and Δ3is set so that bending loss in a higher-order mode is suppressed.

Herein, the situation where the refractive index profiles are substantially the same denotes that, for example, among the parameters, Δ1, Δ2, and Δ3are in a range of ±0.05%, a1, a2, and a3are in a range of ±0.5 μm, and the value of α is in a range of ±0.3. Such a situation occurs in a case where, although an original multicore fiber is manufactured by setting each of the core portions so as to have the same refractive index profile, the parameters of the original multicore fiber have deviation due to manufacturing errors or the like. In some cases, such deviation may occur between the core portions, or even in a continuous core portion, such deviation may occur in the longitudinal direction.

Herein, the configuration of the multicore fiber10will be described more in detail. As illustrated in the exploded diagram ofFIG. 2B, each core portion of the multicore fiber10is configured so that the core portions of the unit multicore fiber are connected in cascade. Specifically, the core portion10aof the multicore fiber10is configured so that the core portion11aof the unit multicore fiber11, the core portion12fof the unit multicore fiber12, the core portion13eof the unit multicore fiber13, the core portion14dof the unit multicore fiber14, the core portion15cof the unit multicore fiber15, and the core portion16bof the unit multicore fiber16are connected in cascade.

In addition, the core portion10bof the multicore fiber10is configured so that the core portion11b, the core portion12a, the core portion13f, the core portion14e, the core portion15d, and the core portion16care connected in cascade. The core portion10cof the multicore fiber10is configured so that the core portion11c, the core portion12b, the core portion13a, the core portion14f, the core portion15e, and the core portion16dare connected in cascade. The core portion10dof the multicore fiber10is configured so that the core portion11d, the core portion12c, the core portion13b, the core portion14a, the core portion15f, and the core portion16eare connected in cascade. The core portion10eof the multicore fiber10is configured so that the core portion11e, the core portion12d, the core portion13c, the core portion14b, the core portion15a, and the core portion16fare connected in cascade. The core portion10fof the multicore fiber10is configured so that the core portion11f, the core portion12e, the core portion13d, the core portion14c, the core portion15b, and the core portion16aare connected in cascade.

In addition, the clad portion10gof the multicore fiber10is configured so that the clad portions11g,12g,13g,14g,15g, and16gare connected in cascade.

The multicore fiber10may be manufactured by manufacturing the unit multicore fibers11,12,13,14,15, and16by cutting the original multicore fiber which is manufactured with lines being continuously drawn as described above and by rotating the unit multicore fibers by 60° around the axis in the rotational direction indicated by arrow A inFIG. 2Band fusion-splicing these unit multicore fibers. In the original multicore fiber, the markers are continuous over the entire length. However, in the multicore fiber10obtained by manufacturing the unit multicore fibers11,12,13,14,15, and16by cutting and by rotating the unit multicore fibers around the axis and fusion-splicing the unit multicore fibers, the markers of the unit multicore fibers11,12,13,14,15, and16are located at different positions rotated around the axis.

Herein, as described above, all the core portions of each of the unit multicore fibers11to16have substantially the same refractive index profiles and substantially the same optical characteristics (for example, effective area, group delay, and the like of each propagation mode), but since these are not completely the same, for example, if the unit multicore fiber11is described, the core portions11ato11fhave different group delays at the same wavelength in the same propagation mode, namely, there is the differential group delay between the core portions. In addition, the differential group delay between the different propagation modes at the same wavelength is also different among the core portions11ato11f. In the other unit multicore fibers, similar configuration is also applied. As described above, in a case where there is the differential group delay between the core portions in this manner, if SDM systems are used, there may be a problem in that the amount of signal processing during the MIMO process is increased.

On the contrary, since the multicore fiber10according to the first embodiment has the above-described configuration, for example, light (for example, signal light) input to the core portion10asequentially passes through the core portion11aof the unit multicore fiber11, the core portion12fof the unit multicore fiber12, the core portion13eof the unit multicore fiber13, the core portion14dof the unit multicore fiber14, the core portion15cof the unit multicore fiber15, and the core portion16bof the unit multicore fiber16. In this case, for example, unlike a case where the signal light propagates the core portion11aof the unit multicore fiber11by length corresponding to the multicore fiber10, the signal light propagates based on the refractive index profiles of the six core portions which are different core portions in the original multicore fiber. Unlike a case where the signal light propagates a certain core portion of the unit multicore fiber by length corresponding to the multicore fiber10, the signal light input to the other core portions10bto10falso propagates based on the refractive index profiles of the six core portions which are different core portions in the original multicore fiber. Therefore, the differential group delay between the core portions10ato10fbecomes smaller than the value of a case where the signal light propagates the unit multicore fiber by length corresponding to the multicore fiber10. As a result, the maximum value of the differential group delay between the core portions10ato10fis smaller than a reduced value of the maximum value of the differential group delay between the core portions each of the unit multicore fibers11to16as the value in terms of the length of the multicore fiber10. In addition, more preferably, the maximum value of the differential group delay between the core portions10ato10fis smaller than a reduced value of the minimum value of the differential group delay between the core portions each of the unit multicore fibers11to16as the value in terms of the length of the multicore fiber10. Furthermore, preferably, the differential group delay between the core portions10ato10fis decreased to approach zero.

In addition, from the point of view of the burden of the MIMO process, the maximum value of the differential group delay of the multicore fiber10is preferably smaller than 5 ns from the total length, more preferably equal to or smaller than 3 ns, further more preferably equal to or smaller than 2 ns.

In addition in the multicore fiber10according to the first embodiment, the core portions10ato10fare arranged to have 6-fold rotational symmetry. If the core portion are arranged to have n-fold rotational symmetry (n is an integer of 2 or more) in this manner, since the differential group delay between the core portions may be decreased by rotating the unit multicore fibers around the axis and fusion-splicing the unit multicore fibers, the arrangement is preferred.

Herein, as described above, in a case where the multicore fiber10is manufactured by rotating the unit multicore fibers11,12,13,14,15, and16around the axis and fusion-splicing the unit multicore fibers, the number of splicing positions is increased. In the case of using a typical single-core fiber, splice loss caused by axial misalignment between the core portions may be suppressed. However, in the multicore fiber, since a plurality of the core portions are cyclically spliced, the position accuracy of the core portions greatly affects the splice loss.

As parameters of determining position misalignment of the core portions, a distance r between the center of the clad portion and the center of the core portion and an angle (namely, angle deviation) θ between a straight line connecting the core portion center and the clad portion center and a straight line connecting the core portion center connected to the associated core portion and the clad portion center are checked. A fusion splicing device capable of rotating the optical fiber around the central axis and fusion-splicing the optical fiber rotates the optical fiber by using a typical step motor or the like. Although the rotation adjustment accuracy is about 0.2°, an actual amount of angle misalignment including manufacturing errors becomes about 2°. On the other hand, it is well known that, in a single-mode optical fiber having a zero-dispersion wavelength in a typical band of 1.3 μm in accordance with ITU-T G. 652 a mode field diameter (MFD) at a wavelength of 1310 nm is about 9.2 μm, and splice loss caused by fusion-splicing is defined by the MFD like Mathematical Formula (1) and an amount of axial misalignment between the fusion-spliced core portions. Herein, d is the amount of axial misalignment between the fusion-spliced core portions, w1and w2are diameters of spot sizes of the respective fusion-spliced core portions as amounts of half values of the MFDs of the respective core portions.

Herein, in the embodiment, since cascade connection of a plurality of the core portions is performed, it is preferable that the splice loss at each connection portion is small, it is preferable that the splice loss is 0.5 dB or less, it is more preferable that the splice loss is 0.1 dB or less. Herein, if only the amount of axial misalignment (position alignment between the core portions) between the core portions caused by the angle misalignment is considered, the relationship among r, θ, and d described above may be expressed by the following Mathematical Formula (2).
[Mathematical Formula 2]
d=r*[(1−cos θ)2+sin θ2]0.5(2)

As understood from Mathematical Formula (2), it may be understood that the distance r of the core portion center from clad portion center is the only parameters of determining the position misalignment of the core portion caused by the angle misalignment. In addition,FIG. 3is a diagram illustrating the relationship between the distance r and the splice loss in a case where Mathematical Formula (2) is inserted into Mathematical Formula (1), the MFD is set to 9.2 μm, and the amount of angle misalignment θ is set to 2°. Therefore, in order to set the splice loss to be 0.5 dB or less, it is preferable that the distance r is set to be 45 μm or less, and in order to set the splice loss to be 0.1 dB or less, it is preferable that the distance r is set to be 20 μm or less.

Example and Comparative Example

The present disclosure will be described in detail by using Examples and Comparative Example. First, an original multicore fiber having the same configuration and refractive index profile as those of the first embodiment and having a length of 10.1 km is manufactured with lines being continuously drawn. In order to propagate light having a wavelength in a band of 1.55 μm in two modes of LP01mode and LP11mode and with sufficiently low crosstalk in each core portion, the original multicore fiber is manufactured so that, as design parameters, Δ1is 0.82%, α is 2, Δ2is 0%, Δ3is −0.46%, a1is 7.3 μm, a2is 9.1 μm, a3is 13.4 μm, and core pitch Λ is 30 μm, and a clad diameter is 125 μm.

FIG. 4is a schematic diagram illustrating a cross section of the manufactured original multicore fiber. Herein, as illustrated inFIG. 4, each core portion is denoted by an ID number.FIG. 5is a diagram illustrating a refractive index profile of the manufactured original multicore fiber. InFIG. 5, “Core1” indicates the refractive index profile of the core portion denoted by the ID number “1” inFIG. 4.

Table 1 is a table listing the parameters of the manufactured core portions. As illustrated inFIG. 5and Table 1, due to the manufacturing errors and the like, with respect to each core portion, Δ1varies in a range of 0.025%, α varies in a range of 0.13, Δ2varies in a range of 0.005%, Δ3varies in a range of −0.025%, a1varies in a range of 0.40 μm, a2varies in a range of 0.35 μm, and a3varies in a range of 0.45 μm. However, the core portions have substantially the same refractive index profiles according to the design parameters.

Table 2 is a table listing optical characteristics of the core portions obtained by simulation based on the refractive index profiles of the manufactured core portions. Table 2 lists effective areas (Aeff) and effective refractive index (neff) of the core portions at a wavelength of 1550 nm.

Next, group delays of light having a wavelength of 1530 nm and light having a wavelength of 1570 nm in the LP01and LP11modes of each of the core portions of the original multicore fiber are measured.FIG. 6is a diagram illustrating a measurement system of the group delay of the core portion of the manufactured original multicore fiber. A measurement system100generates test light by modulating continuous laser light output from a light source101by using an LN modulator102, inputs the test light to each core portion of the original multicore fiber10A though a typical single-mode optical fiber104and a highly-nonlinear optical fiber105having a mode field diameter smaller than that of the single-mode optical fiber104, and inputs the test light which propagates the core portion and is output from the core portion to a sampling oscilloscope107through a single-mode optical fiber106. Herein, the LN modulator102modulates a signal output from a pulse pattern generator (PPG)103to a modulation signal, and a trigger signal from the pulse pattern generator103is input to the sampling oscilloscope107. For example, by setting a modulation frequency of the test light input to each core portion of the original multicore fiber10A to be 100 MHz and setting a pulse width of the test light to be 1.5 ns, the LP01mode and the LP11mode of each core portion may be identified. Therefore, the group delay of each core portion of the original multicore fiber10A may be measured.

Table 3 lists differential group delay and DMD (Differential Mode Delay) of the original multicore fiber measured on the basis of the measured group delays. Herein, the differential group delay is defined as a difference of the group delay in the LP11mode at a wavelength of 1530 nm of Core1that is the condition of the smallest group delay. Herein, for example, if the wavelength is 1530 nm, the largest differential group delay is the value in the LP01mode of Core6. In addition, the DMD is defined as a differential group delay per unit length between different modes of the same core portion at the same wavelength. For example, the DMD of Core1at a wavelength of 1530 nm is (0.00−4.97)/10.1=−0.492 ns/km=−492 ps/km. It may be understood from Table 2 that the absolute value of the DMD is largest in Core1and smallest in Core3, and the absolute value of the DMD of each core portion is within 1000 ps/km. In addition, the maximum DMD is defined as a maximum value of a differential group delay per unit length between different modes between different core portions at the same wavelength. For example, the DMD at a wavelength of 1530 nm is (0.00-10.41)/10.1=−1.031 ns/km=−1031 ps/km. In this manner, in the manufactured original multicore fiber, since the maximum DMD per 1 km exceeds 1 ns, the maximum DMD exceeds 2 ns at a length of 2 km, and the maximum DMD exceeds 10 ns at a length of 10 km.

Next, six unit multicore fibers are manufactured by cutting the original multicore fiber by 1 km. Next, first, as Comparative Example, a multicore fiber configured so that the unit multicore fibers are connected in cascade is manufactured by fusion-splicing the unit multicore fibers without rotation around an axis. Next, group delays of the manufactured multicore fiber are measured.

FIG. 7is a diagram illustrating a cumulative group delay in a multicore fiber according to Comparative Example. Herein, a cumulative group delay denotes a group delay cumulated in the longitudinal direction. In addition, sections in the horizontal axis correspond to the unit multicore fibers. The test light from the unit multicore fiber corresponding to Section1is input. The cumulative group delay of Section6corresponds to the group delay over the entire length. In addition, “Core1LP01” is the cumulative group delay in the LP01mode of Core1. “LP11-LP01” is the maximum value of the cumulative differential group delay between different core portions. In addition, the wavelength is 1550 nm. As illustrated inFIG. 7, in the multicore fiber of Comparative Example, the absolute value of the cumulative value (the maximum value of the differential group delay between the core portions over the entire length) of the DMD between the LP11and LP01modes is larger than 6000 ps (6 ns) and becomes a greatly large value.

Next, as Example 1, a multicore fiber configured so that the unit multicore fibers are connected in cascade is manufactured by cutting an original multicore fiber by 1 km to manufacture six unit multicore fibers and by rotating the first to sixth unit multicore fibers by 60° around the axis and clockwise and fusion-splicing the six unit multicore fibers. In the multicore fiber, the core portions of the first to sixth unit multicore fibers are connected from Section1to Section6, for example, like Core1→Core2→Core3→Core4→Core5→Core6. Next, the group delay of the manufactured multicore fiber is measured.

FIG. 8is a diagram illustrating a cumulative group delay in a multicore fiber according to Example 1. The wavelength is 1550 nm. As illustrated inFIG. 8, in the multicore fiber of Example 1, the absolute value of the cumulative value of the DMD between the LP11and LP01modes becomes 2500 ps (2.5 ns) or less, and thus, the value is greatly decreased in comparison with Comparative Example. Namely, in Example 1, the maximum value of the differential group delay between the core portions of the multicore fiber over the entire length becomes smaller than the maximum value (corresponding to a reduced value of the maximum value of the differential group delay between the core portions of each unit multicore fiber as the value in terms of the length of the multicore fiber of Example 1) of the differential group delay between the core portions of the multicore fiber of Comparative Example.

Next, as Example 2, a multicore fiber configured so that the unit multicore fibers are connected in cascade is manufactured by cutting an original multicore fiber by 1 km to manufacture six unit multicore fibers and by rotating the second unit multicore fiber by 180° around the axis and clockwise and fusion-splicing the second unit multicore fiber to the first unit multicore fiber; rotating the third unit multicore fiber by 240° around the axis and clockwise and fusion-splicing the third unit multicore fiber to the second unit multicore fiber; rotating the fourth unit multicore fiber by 180° around the axis and clockwise and fusion-splicing the fourth unit multicore fiber to the third unit multicore fiber; rotating the fifth unit multicore fiber by 240° around the axis and clockwise and fusion-splicing the fifth unit multicore fiber to the fourth unit multicore fiber; and rotating the sixth unit multicore fiber by 180° around the axis and clockwise and fusion-splicing the sixth unit multicore fiber to the fifth unit multicore fiber. In the multicore fiber, the core portions of the first to sixth unit multicore fibers are connected from Section1Section6, for example, like Core1→Core4→Core2→Core5→Core3→Core6. Next, the group delay of the manufactured multicore fiber is measured.

FIG. 9is a diagram illustrating a cumulative group delay in a multicore fiber according to Example 2. The wavelength is 1550 nm. As illustrated inFIG. 9, in the multicore fiber of Example 2, the absolute value of the cumulative value of the DMD between the LP11and LP01modes becomes 2500 ps (2.5 ns) or less, and thus, the value is greatly decreased in comparison with Comparative Example.

Next, as Example 3, a multicore fiber configured so that the unit multicore fibers are connected in cascade is manufactured by cutting an original multicore fiber by 1 km to manufacture six unit multicore fibers and by reversing the second unit multicore fiber in the longitudinal direction and fusion-splicing the second unit multicore fiber to the first unit multicore fiber; rotating the third unit multicore fiber by 240° around the axis and clockwise with reference to the first unit multicore fiber and fusion-splicing the third unit multicore fiber to the second unit multicore fiber; reversing the fourth unit multicore fiber in the longitudinal direction, rotating the fourth unit multicore fiber by 120° with reference to the first unit multicore fiber, and fusion-splicing the fourth unit multicore fiber to the third unit multicore fiber; rotating the fifth unit multicore fiber by 120° with reference to the first unit multicore fiber, and fusion-splicing the fifth unit multicore fiber to the fourth unit multicore fiber; and reversing the sixth unit multicore fiber in the longitudinal direction, rotating the sixth unit multicore fiber by 240° with reference to the first unit multicore fiber, and fusion-splicing the sixth unit multicore fiber to the fifth unit multicore fiber. In the multicore fiber, the core portions of the first to sixth unit multicore fibers are connected, for example, like Core1→Core6(reversed)→Core5→Core4(reversed)→Core3→Core2(reversed). In this manner, in the case of connecting the unit multicore fibers, reversing in the longitudinal direction as well as rotating around the axis may be performed. Therefore, a combination of connection of the core portions which may not be implemented by only the rotating around the axis may be implemented. Next, the group delay of the manufactured multicore fiber is measured.

FIG. 10is a diagram illustrating a cumulative group delay in a multicore fiber according to Example 3. The wavelength is 1550 nm. As illustrated inFIG. 10, in the multicore fiber of Example 3, the absolute value of the cumulative value of the DMD between the LP11and LP01modes becomes 2500 ps (2.5 ns) or less, and thus, the value is greatly decreased in comparison with Comparative Example.

Next, as Example 4, a multicore fiber configured so that the unit multicore fibers are connected in cascade is manufactured by cutting an original multicore fiber by 2 km to manufacture three unit multicore fibers and by reversing the second unit multicore fiber in the longitudinal direction and fusion-splicing the second unit multicore fiber to the first unit multicore fiber; and rotating the third unit multicore fiber by 240° around the axis and clockwise with reference to the first unit multicore fiber and fusion-splicing the third unit multicore fiber to the second unit multicore fiber. In the multicore fiber, the core portions of the first to third unit multicore fibers are connected, for example, like Core1→Core6(reversed)→Core5. Next, the group delay of the manufactured multicore fiber is measured.

FIG. 11is a diagram illustrating a cumulative group delay in a multicore fiber according to Example 4. The wavelength is 1550 nm. As illustrated inFIG. 11, in the multicore fiber of Example 4, the absolute value of the cumulative value of the DMD between the LP11and LP01modes becomes 4000 ps (4 ns) or less, and thus, the value is greatly decreased in comparison with Comparative Example.

Next, as Example 5, a multicore fiber configured so that the unit multicore fibers are connected in cascade is manufactured by cutting an original multicore fiber by 3 km to manufacture two unit multicore fibers and by rotating the second unit multicore fiber by 60° around the axis and clockwise and fusion-splicing the second unit multicore fiber to the first unit multicore fiber. In the multicore fiber, the core portions of the first to second unit multicore fibers are connected, for example, like Core1→Core2. Next, the group delay of the manufactured multicore fiber is measured.

FIG. 12is a diagram illustrating a cumulative group delay in a multicore fiber according to Example 5. The wavelength is 1550 nm. As illustrated inFIG. 12, in the multicore fiber of Example 5, the absolute value of the cumulative value of the DMD between the LP11and LP01modes becomes 4000 ps (4 ns) or less, and thus, the value is greatly decreased in comparison with Comparative Example.

Next, as Example 6, a multicore fiber configured so that the unit multicore fibers are connected in cascade is manufactured by cutting an original multicore fiber by 3 km to manufacture two unit multicore fibers and by reversing the second unit multicore fiber in the longitudinal direction and fusion-splicing the second unit multicore fiber to the first unit multicore fiber. In the multicore fiber, the core portions of the first to second unit multicore fibers are connected, for example, like Core1→Core6(reversed). Next, the group delay of the manufactured multicore fiber is measured.

FIG. 13is a diagram illustrating a cumulative group delay in a multicore fiber according to Example 6. The wavelength is 1550 nm. As illustrated inFIG. 13, in the multicore fiber of Example 6, the absolute value of the cumulative value of the DMD between the LP11and LP01modes becomes 4000 ps (4 ns) or less, and thus, the value is greatly decreased in comparison with Comparative Example.

As illustrated in the above-described Examples, even if the connection after the rotating or reversing is performed on at least one site, the examples are more effective than Comparative Example. However, it is preferable that the connection after the rotating or reversing is performed on two or more sites. In addition, like Examples 1 to 3, it is preferable that the core portions of the multicore fiber are configured to include all the core portions of Core1to Core6because the cumulative value of the DMD is further reduced.

In the multicore fiber10according to the first embodiment, the six core portions are arranged at positions in an equal distance from the central axis of the clad portion at an equal angle to form a circle centered on the central axis. However, the number and arrangement of core portions are not limited thereto. In addition, after cutting the original multicore fiber into a plurality of the unit multicore fibers, it is not necessary to rotate or reverse all the unit multicore fibers to connect the unit multicore fibers. Namely, after the cutting, only the unit multicore fibers which are to be rotated or reversed or had better be rotated or reversed in order to achieve a desired differential group delay are rotated or reversed and, after that, these are connected, so that the unit multicore fibers which may not be particularly rotated or reversed may be connected without rotating or reversing. For example, in Example 4, the multicore fiber is manufactured by the cutting the original multicore fiber by 2 km to manufacture three unit multicore fibers and by rotating or reversing and connecting these unit multicore fibers.

However, the multicore fiber is substantially equivalent to a multicore fiber manufactured by cutting the original multicore fiber by 1 km to manufacture the six unit multicore fibers, connecting two-consecutive unit multicore fibers without rotating or reversing to manufacture three unit multicore fibers, and rotating or reversing these unit multicore fibers like Example 4.

In addition, from the point of view of practical convenience, it is allowable that all the unit multicore fibers are not necessarily rotated or reversed to be connected. For example, a configuration where, after cutting the original multicore fiber into a plurality of the unit multicore fibers, the two end unit multicore fibers are not relatively rotated or reversed to be connected is preferred from the point of view of practical convenience. The configuration where the two end unit multicore fibers are not relatively rotated or reversed to be connected is a configuration where a plurality of the unit multicore fibers are connected like Core1of one-end unit multicore fiber→{rotated or reversed core}→Core1of multi-end multicore fiber. In such a configuration, for example, the relationship between the ID numbers (Core1to Core6) of the core portions and the channels (ch) of the signal light allocated to the core portions is maintained at two ends (incident side and emitting side) so that the signal light incident on Core1is emitted from Core1of the other end. Specifically, if the signal light of the ch 1 is incident from Core1of one end, the signal light of the ch 1 is emitted from Core1of the other end. Therefore, in the configuration where the two end unit multicore fibers are not relatively rotated or reversed to be connected, there is an advantage in that there is little confusion in a user, and the unit multicore fibers other than the two end unit multicore fibers are rotated or reversed to be connected, so that the effect in that the cumulative value of the DMD may be decreased is maintained. In addition, the configuration where the two end unit multicore fibers are not relatively rotated or reversed to be connected may be implemented, for example, by connecting the unit multicore fiber16to the unit multicore fiber15without rotating and reversing the unit multicore fibers with reference to the unit multicore fiber11in the multicore fiber10illustrated inFIG. 2B. In addition, by performing similar modification on the configurations of the other Examples 2 to 4 as well as the configuration of Example 1, it is possible to easily implement the configuration where the two end unit multicore fibers are not relatively rotated or reversed to be connected.

Second Embodiment

FIG. 14is a diagram illustrating a configuration of a multicore fiber according to a second embodiment. As illustrated inFIG. 14, a multicore fiber20according to the second embodiment is configured to include eighteen core portions and a clad portion20dwhich is formed in the outer circumference of the core portions and has a refractive index lower than the maximum refractive index of the core portions. The core portions are arranged so as to form a triangular lattice. In addition, the six core portions20a1,20a2,20a3,20a4,20a5, and20a6are arranged so as to form a regular hexagon centered on a central axis O2of the clad portion20d. In addition, the twelve core portions20b1,20b2,20b3,20b4,20b5,20b6,20c1,20c2,20c3,20c4,20c5, and20c6are formed so as to form a concentric regular hexagon centered at the central axis O2and larger than the regular hexagon formed by the six core portions20a1to20a6. In addition, the configurations and materials of the core portions may be the same as those of the core portions10ato10fof the multicore fiber10according to the first embodiment.

The multicore fiber20is configured so that a plurality of unit multicore fibers are connected in cascade. The unit multicore fibers are manufactured by cutting an original multicore fiber which is manufactured with lines being continuously drawn and has a length substantially the same as or larger than that of the multicore fiber20by equal length and have configurations similar to that of the multicore fiber20. The multicore fiber20is configured by rotating the unit multicore fibers which are manufactured by cutting the original multicore fiber by 60° around the axis and fusion-splicing these unit multicore fibers.

Herein, all the core portions of each of the unit multicore fibers have substantially the same refractive index profile and substantially the same optical characteristics, but since these are not completely the same, the core portions of the same unit multicore fiber have different group delays at the same wavelength in the same propagation mode. In addition, the differential group delay between the different propagation modes at the same wavelength is also different among the core portions.

On the contrary, the multicore fiber20according to the second embodiment is configured in the above-described connection, so that the differential group delay between the core portions becomes smaller than the value of a case where signal light propagates the unit multicore fiber by length corresponding to the multicore fiber20. As a result, the maximum value of the differential group delay between the core portions is smaller than a reduced value of the maximum value of the differential group delay between the core portions of each of the unit multicore fibers as the value in terms of the length of the multicore fiber20.

In addition, as illustrated inFIG. 14, for example, in a case where the core portion20a1is rotated by 60° around the central axis O2to be placed at the position of the core portion20a2like the arrow A11, accordingly, the core portion20b1is rotated by 60° around the central axis O2to be placed at the position of the core portion20b2like the arrow A12, and the core portion20c1is rotated by 60° around the central axis O2to be placed at the position of the core portion20c2like the arrow A13. Namely, in the arrangement of the core portions of the multicore fiber20, the core portions are configured as three groups of a group configured by the core portions20a1to20a6, a group configured by the core portions20b1to20b6, and a group configured by the core portions20c1to20c6. In each unit multicore fiber, the core portions are configured as three groups of a group configured by the core portions existing at the positions corresponding to the core portions20a1to20a6, a group configured by the core portions existing at the positions corresponding to the core portions20b1to20b6, and a group configured by the core portions existing at the positions corresponding to the core portions20c1to20c6. Since the core portions in each group of the unit multicore fiber may be connected to each other through the rotation by 60°, the maximum value of the differential group delay between the core portions of the multicore fiber20may be decreased by connection of the core portions in the three groups between the unit multicore fibers.

Third Embodiment

FIG. 15is a diagram illustrating a configuration of a multicore fiber according to a third embodiment. As illustrated inFIG. 15, a multicore fiber30according to the third embodiment is configured to include eighteen core portions and a clad portion30dwhich is formed in the outer circumference of the core portions and has a refractive index lower than the maximum refractive index of the core portions. Among the core portions, the six core portions30a1,30a2,30a3,30a4,30a5, and30a6are arranged at positions in an equal distance from the central axis O3of the clad portion30dat an equal angle to form a circle centered on the central axis O3. In addition, among the core portions, the twelve core portions30b1,30b2,30b3,30b4,30b5,30b6,30c1,30c2,30c3,30c4,30c5, and30c6are arranged at positions in an equal distance from the central axis O3at an equal angle to form a concentric circle which is centered on the central axis O3and of which radius is larger than that of the circle configured by the six core portions30a1to30a6. In addition, the configurations and materials of the core portions may be the same as those of the core portions10ato10fof the multicore fiber10according to the first embodiment.

The multicore fiber30is configured so that a plurality of unit multicore fibers are connected in cascade. The unit multicore fibers are manufactured by cutting an original multicore fiber which is manufactured with lines being continuously drawn and has a length substantially the same as or larger than that of the multicore fiber30by equal length and have configurations similar to that of the multicore fiber30. The multicore fiber30is configured by rotating the unit multicore fibers which are manufactured by cutting the original multicore fiber by 60° around the axis and fusion-splicing these unit multicore fibers.

Herein, all the core portions of each of the unit multicore fibers have substantially the same refractive index profile and substantially the same optical characteristics, but since these are not completely the same, the core portions of the same unit multicore fiber have different group delays at the same wavelength in the same propagation mode. In addition, the differential group delay between the different propagation modes at the same wavelength is also different among the core portions.

On the contrary, the multicore fiber30according to the third embodiment is configured in the above-described connection, so that the differential group delay between the core portions becomes smaller than the value of a case where signal light propagates the unit multicore fiber by length corresponding to the multicore fiber30. As a result, the maximum value of the differential group delay between the core portions is smaller than a reduced value of the maximum value of the differential group delay between the core portions of each of the unit multicore fibers as the value in terms of the length of the multicore fiber30.

In addition, as illustrated inFIG. 15, for example, in a case where the core portion30a1is rotated by 60° around the central axis O3to be placed at the position of the core portion30a2like the arrow A21, accordingly, the core portion30b1is rotated by 60° around the central axis O3to be placed at the position of the core portion30b2like the arrow A22, and the core portion30c1is rotated by 60° around the central axis O3to be placed at the position of the core portion30c2like the arrow A23. Namely, in the arrangement of the core portions of the multicore fiber30, the core portions are configured as three groups of a group configured by the core portions30a1to30a6, a group configured by the core portions30b1to30b6, and a group configured by the core portions30c1to30c6. In each unit multicore fiber, the core portions are configured as three groups of a group configured by the core portions existing at the positions corresponding to the core portions30a1to30a6, a group configured by the core portions existing at the positions corresponding to the core portions30b1to30b6, and a group configured by the core portions existing at the positions corresponding to the core portions30c1to30c6. Since the core portions in each group of the unit multicore fiber may be connected to each other through the rotation by 60°, the maximum value of the differential group delay between the core portions of the multicore fiber30may be decreased by connection of the core portions in the three groups between the unit multicore fibers.

Fourth Embodiment

FIG. 16is a diagram illustrating a configuration of a multicore fiber according to a fourth embodiment. As illustrated inFIG. 16, a multicore fiber40according to the fourth embodiment is configured to include eighteen core portions and a clad portion40dwhich is formed in the outer circumference of the core portions and has a refractive index lower than the maximum refractive index of the core portions. Among the core portions, the six core portions40a1,40a2,40a3,40a4,40a5, and40a6are arranged at positions in an equal distance from the central axis O4of the clad portion40dat an equal angle to form a circle centered on the central axis O4. In addition, among the core portions, the twelve core portions40b1,40b2,40b3,40b4,40b5,40b6,40c1,40c2,40c3,40c4,40c5, and40c6are arranged at positions in an equal distance from the central axis O4at an equal angle to form a circle which is centered on the central axis O4and of which radius is larger than that of the circle configured by the six core portions40a1to40a6. In addition, the configurations and materials of the core portions may be the same as those of the core portions10ato10fof the multicore fiber10according to the first embodiment.

The multicore fiber40is configured so that a plurality of unit multicore fibers are connected in cascade. The unit multicore fibers are manufactured by cutting an original multicore fiber which is manufactured with lines being continuously drawn and has a length substantially the same as or larger than that of the multicore fiber40by equal length and have configurations similar to that of the multicore fiber40. The multicore fiber40is configured by rotating the unit multicore fibers which are manufactured by cutting the original multicore fiber by 60° around the axis and fusion-splicing these unit multicore fibers.

The multicore fiber40and the multicore fiber30according to the third embodiment have the same configurations except for the difference in that, in the multicore fiber30, for example, the core portion30a1exists on the line connecting the central axis O3and the core portion30b1, but in the multicore fiber40, for example, the core portion40a1exists so as to be shifted from the line connecting the central axis O4and the core portion40b1. Therefore, in the multicore fiber40according to the fourth embodiment, the maximum value of the differential group delay between the core portions is smaller than a reduced value of the maximum value of the differential group delay between the core portions of each of the unit multicore fibers as the value in terms of the length of the multicore fiber40.

In addition, as illustrated inFIG. 16, for example, in a case where the core portion40a1is rotated by 60° around the central axis O4to be placed at the position of the core portion40a2like the arrow A31, accordingly, the core portion40b1is rotated by 60° around the central axis O4to be placed at the position of the core portion40b2like the arrow A32, and the core portion40c1is rotated by 60° around the central axis O4to be placed at the position of the core portion40c2like the arrow A33. Namely, in the arrangement of the core portions of the multicore fiber40, the core portions are configured as three groups of a group configured by the core portions40a1to40a6, a group configured by the core portions40b1to40b6, and a group configured by the core portions40c1to40c6. In each unit multicore fiber, the core portions are configured as three groups of a group configured by the core portions existing at the positions corresponding to the core portions40a1to40a6, a group configured by the core portions existing at the positions corresponding to the core portions40b1to40b6, and a group configured by the core portions existing at the positions corresponding to the core portions40c1to40c6. Since the core portions in each group of the unit multicore fiber may be connected to each other through the rotation by 60°, the maximum value of the differential group delay between the core portions of the multicore fiber40may be decreased by connection of the core portions in the three groups between the unit multicore fibers.

Fifth Embodiment

FIG. 17is a diagram illustrating a configuration of a multicore fiber according to a fifth embodiment. As illustrated inFIG. 17, a multicore fiber50according to the fifth embodiment is configured to include twelve core portions50a,50b,50c,50d,50e,50f,50g,50h,50i,50j,50k, and50land a clad portion50mwhich is formed in the outer circumference of the core portions and has a refractive index lower than the maximum refractive index of the core portions. The core portions50ato50lare arranged at positions in an equal distance from the central axis O5of the clad portion50mat an equal angle to form a circle centered on the central axis O5. In addition, the configurations and materials of the core portions may be the same as those of the core portions10ato10fof the multicore fiber10according to the first embodiment.

The multicore fiber50is configured so that a plurality of unit multicore fibers are connected in cascade. The unit multicore fibers are manufactured by cutting an original multicore fiber which is manufactured with lines being continuously drawn and has a length substantially the same as or larger than that of the multicore fiber50by equal length and have configurations similar to that of the multicore fiber50. All the core portions of each of the unit multicore fibers have substantially the same refractive index profile and substantially the same optical characteristics, but these are not completely the same. The multicore fiber50is configured by rotating the unit multicore fibers which are manufactured by cutting the original multicore fiber by 30° around the axis and fusion-splicing these unit multicore fibers.

The multicore fiber50according to the fifth embodiment is configured in the above-described connection, so that the differential group delay between the core portions becomes smaller than the value of a case where signal light propagates the unit multicore fiber by length corresponding to the multicore fiber50. As a result, the maximum value of the differential group delay between the core portions is smaller than a reduced value of the maximum value of the differential group delay between the core portions of each of the unit multicore fibers as the value in terms of the length of the multicore fiber50.

In addition, as illustrated inFIG. 17, the core portions50ato50lare arranged to have 12-fold rotational symmetry. The core portions of each of the unit multicore fibers are also arranged to have 12-fold rotational symmetry. Therefore, since the core portions of the unit multicore fiber may be connected to each other through the rotation by 30° around the central axis O5like arrow A5, the maximum value of the differential group delay between the core portions of the multicore fiber50may be decreased by connection of the twelve core portions between the unit multicore fibers.

Sixth Embodiment

FIG. 18is a diagram illustrating a configuration of a multicore fiber according to a sixth embodiment. As illustrated inFIG. 18, a multicore fiber60according to the sixth embodiment is configured to include four core portions60a,60b,60c, and60dand a clad portion60ewhich is formed in the outer circumference of the core portions and has a refractive index lower than the maximum refractive index of the core portions. The core portions60ato60dare arranged at positions in an equal distance from the central axis O6of the clad portion60eat an equal angle to form a circle or a square centered on the central axis O6. In addition, the configurations and materials of the core portions may be the same as those of the core portions10ato10fof the multicore fiber10according to the first embodiment.

The multicore fiber60is configured so that a plurality of unit multicore fibers are connected in cascade. The unit multicore fibers are manufactured by cutting an original multicore fiber which is manufactured with lines being continuously drawn and has a length substantially the same as or larger than that of the multicore fiber60by equal length and have configurations similar to that of the multicore fiber60. All the core portions of each of the unit multicore fibers have substantially the same refractive index profile and substantially the same optical characteristics, but these are not completely the same. The multicore fiber60is configured by rotating the unit multicore fibers which are manufactured by cutting the original multicore fiber by 90° around the axis and fusion-splicing these unit multicore fibers.

On the contrary, the multicore fiber60according to the sixth embodiment is configured in the above-described connection, so that the differential group delay between the core portions becomes smaller than the value of a case where signal light propagates the unit multicore fiber by length corresponding to the multicore fiber60. As a result, the maximum value of the differential group delay between the core portions is smaller than a reduced value of the maximum value of the differential group delay between the core portions of each of the unit multicore fibers as the value in terms of the length of the multicore fiber60.

In addition, as illustrated inFIG. 18, the core portions60ato60dare arranged to have 4-fold rotational symmetry. The core portions of each of the unit multicore fibers are also arranged to have 4-fold rotational symmetry. Therefore, since the core portions of the unit multicore fiber may be connected to each other through the rotation by 90° around the central axis O6like arrow A6, the maximum value of the differential group delay between the core portions of the multicore fiber60may be decreased by connection of the four core portions between the unit multicore fibers.

Seventh Embodiment

FIG. 19is a diagram illustrating a configuration of a multicore fiber according to a seventh embodiment. As illustrated inFIG. 19, a multicore fiber70according to the seventh embodiment is configured to include eight core portions and a clad portion70ewhich is formed in the outer circumference of the core portions and has a refractive index lower than the maximum refractive index of the core portions. Among the core portions, the core portions70a1and70a2are arranged at positions in an equal distance from the central axis O7of the clad portion70eto interpose the central axis O7. Similarly, the core portions70b1and70b2, the core portions70c1and70c2, and the core portions70d1and70d2are arranged at positions in respective equal distance from the central axis O7of the clad portion70eto interpose the central axis O7. In addition, the core portion70a1,70b1,70c1, and70d1, and the respective core portions70a2,70b2,70c2, and70d2are arranged in respective shapes of straight lines interposing the central axis O7. In addition, the configurations and materials of the core portions may be the same as those of the core portions10ato10fof the multicore fiber10according to the first embodiment.

The multicore fiber70is configured so that a plurality of unit multicore fibers are connected in cascade. The unit multicore fibers are manufactured by cutting an original multicore fiber which is manufactured with lines being continuously drawn and has a length substantially the same as or larger than that of the multicore fiber70by equal length and have configurations similar to that of the multicore fiber70. All the core portions of each of the unit multicore fibers have substantially the same refractive index profile and substantially the same optical characteristics, but these are not completely the same. The multicore fiber70is configured by rotating the unit multicore fibers which are manufactured by cutting the original multicore fiber by 180° around the axis and fusion-splicing these unit multicore fibers.

The multicore fiber70according to the seventh embodiment is configured in the above-described connection, so that the differential group delay between the core portions becomes smaller than the value of a case where signal light propagates the unit multicore fiber by length corresponding to the multicore fiber70. As a result, the maximum value of the differential group delay between the core portions is smaller than a reduced value of the maximum value of the differential group delay between the core portions of each of the unit multicore fibers as the value in terms of the length of the multicore fiber70.

In addition, as illustrated inFIG. 19, for example, in a case where the core portion70a1is rotated by 180° around the central axis O7to be placed at the position of the core portion70a2like the arrow A71, accordingly, the core portion70b1is rotated by 180° around the central axis O7to be placed at the position of the core portion70b2like the arrow A72, the core portion70c1is rotated by 180° around the central axis O7to be placed at the position of the core portion70c2like the arrow A73, and the core portion70d1is rotated by 180° around the central axis O7to be placed at the position of the core portion70d2like the arrow A74. Namely, in the arrangement of the core portions of the multicore fiber70, the core portions are configured as four groups of a group configured by the core portions70a1and70a2, a group configured by the core portions70b1and70b2, a group configured by the core portions70c1and70c2, and a group configured by the core portions70d1and70d2. In each unit multicore fiber, the core portions are configured as groups of core portions corresponding to the respective groups. Since the core portions in each group of the unit multicore fiber may be connected to each other through the rotation by 180°, the maximum value of the differential group delay between the core portions of the multicore fiber70may be decreased by connection of the core portions in the four groups between the unit multicore fibers.

Eighth Embodiment

FIG. 20is a diagram illustrating a configuration of a multicore fiber according to an eighth embodiment. As illustrated inFIG. 20, a multicore fiber80according to the eighth embodiment is configured to include sixteen core portions and a clad portion80ewhich is formed in the outer circumference of the core portions and has a refractive index lower than the maximum refractive index of the core portions. The core portions may be arranged so as to be a tetragonal lattice. In addition, the four core portions80a1,80a2,80a3, and80a4are arranged to form a square centered on the central axis O8of the clad portion80e. In addition, the twelve core portions80b1,80b2,80b3,80b4,80c1,80c2,80c3,80c4,80d1,80d2,80d3, and80d4are arranged to form a concentric square which is centered on the central axis O8and is larger than the square configured by the core portions80a1to80a4. In addition, the configurations and materials of the core portions may be the same as those of the core portions10ato10fof the multicore fiber10according to the first embodiment.

The multicore fiber80is configured so that a plurality of unit multicore fibers are connected in cascade. The unit multicore fibers are manufactured by cutting an original multicore fiber which is manufactured with lines being continuously drawn and has a length substantially the same as or larger than that of the multicore fiber80by equal length and have configurations similar to that of the multicore fiber80. All the core portions of each of the unit multicore fibers have substantially the same refractive index profile and substantially the same optical characteristics, but these are not completely the same. The multicore fiber80is configured by rotating the unit multicore fibers which are manufactured by cutting the original multicore fiber by 90° around the axis and fusion-splicing these unit multicore fibers.

The multicore fiber80according to the eighth embodiment is configured in the above-described connection, so that the differential group delay between the core portions becomes smaller than the value of a case where signal light propagates the unit multicore fiber by length corresponding to the multicore fiber80. As a result, the maximum value of the differential group delay between the core portions is smaller than a reduced value of the maximum value of the differential group delay between the core portions of each of the unit multicore fibers as the value in terms of the length of the multicore fiber80.

In addition, as illustrated inFIG. 20, for example, in a case where the core portion80a1is rotated by 90° around the central axis O8to be placed at the position of the core portion80a2like the arrow A81, accordingly, the core portion80b1is rotated by 90° around the central axis O8to be placed at the position of the core portion80b2like the arrow A82, the core portion80c1is rotated by 90° around the central axis O8to be placed at the position of the core portion80c2like the arrow A83, and the core portion80d1is rotated by 90° around the central axis O8to be placed at the position of the core portion80d2like the arrow A84. Namely, in the arrangement of the core portions of the multicore fiber80, the core portions are configured as four groups of a group configured by the core portions80a1to80a4, a group configured by the core portions80b1to80b4, a group configured by the core portions80c1to80c4, and a group configured by the core portions80d1to80d4. In each unit multicore fiber, the core portions are configured as groups of core portions corresponding to the respective groups. Since the core portions in each group of the unit multicore fiber may be connected to each other through the rotation by 90°, the maximum value of the differential group delay between the core portions of the multicore fiber80may be decreased by connection of the core portions in the four groups between the unit multicore fibers.

In addition, in the above-described second to eighth embodiments, all the core portions of each of the unit multicore fibers have substantially the same refractive index profiles and substantially the same optical characteristics. However, for example, the core portions of each of the unit multicore fibers may be configured so that the core portions in each group have substantially the same refractive index profiles and substantially the same optical characteristics and the refractive index profiles between the groups are not substantially the same.

In addition, in the above-described embodiments, the unit multicore fibers are manufactured by cutting the original multicore fiber which is manufactured with lines being continuously drawn by equal length. However, the unit multicore fibers may be manufactured by cutting the original multicore fiber which is manufactured with lines being separately drawn, or the unit multicore fibers may be manufactured to have different lengths. In addition, with respect to the connection of the core portions of the unit multicore fiber, it is preferable that, the group delays of the core portions of each unit multicore fiber may be measured in advance, and the connecting is performed through a combination of rotating or reversing of the unit multicore fibers so that the differential group delay between the core portions becomes small on the basis of the measured values.

In addition, in the above-described embodiments, in the multicore fiber, the refractive index profile is set so that two propagation modes of the LP01mode and the LP11mode at a wavelength of the input light exist. In the trench-type refractive index profile set so that the two propagation modes of the LP01mode and the LP11mode exist at a wavelength in a band of 1.55 μm, the design parameters are not limited to the design parameter of the above-described Example (Δ1: 0.82%, α: 2, Δ2: 0%, Δ3: −0.46%, a1: 7.3 μm, a2: 9.1 μm, a3: 13.4 μm, Λ: 30 μm, and clad diameter: 125 μm). For example, the design parameters may be combined from the ranges where Δ1is in a range of 0.2% to 1.6%, Δ2is in the vicinity of 0%, for example, in a range of −0.03% to 0.03%, Δ3is in a range of −0.2% to −0.7%, a1is in a range of 4 μm to 12.5 μm, a2is in a range of 1 to 3 as a ratio to a1(a2/a1=Ra2), and a3is in a range of 2 to 4 as a ratio to a1(a3/a1=Ra3) so that the above-described two propagation modes exist. In addition, α and the core pitch are not particularly limited. In addition, the refractive index profile in a case where there is no region having Δ2and Ra2is 1 is referred to as a w-type refractive index profile.

Table 4 is a table listing the design parameters in the above-described ranges in the trench-type refractive index profile and Aeff, neff, group delay, and DMD in the LP01and LP11modes at a wavelength of 1550 nm in the case of predetermined α and core pitch. In addition, in Table 4, α is “step” denotes that α is 20 or more and the center core portion is considered to have a step-index-type refractive index profile. In the case of the original multicore fiber having the parameters illustrated in Table 4, like the above-described Examples in a case where the refractive index profiles of the core portions are substantially the same, the differential group delay between the core portions may be decreased by appropriately performing cutting, rotating or reversing, and connecting.

In addition, as the refractive index profile where the above-described two propagation modes exist at a wavelength of the input light, there is a single-peak-type refractive index profile where, in the trench-type refractive index profile, there is no region having Δ2or Δ3and Ra2=Ra3=1. In the single-peak-type refractive index profile set so that the above-described two propagation modes exist at a wavelength in a band of 1.55 μm, the design parameters may be combined from the ranges where Δ1is a range of 0.2% to 1.6% and a1is in a range of 3.5 μm to 10.0 μm so that the above-described two propagation modes exist. In addition, α and the core pitch are not particularly limited.

Table 5 is a table listing the design parameters in the above-described ranges in the single-peak-type refractive index profile and Aeff, neff, group delay, and DMD in the LP01and LP11modes at a wavelength of 1550 nm in the case of predetermined α and core pitch. In the case of the original multicore fiber having the parameters illustrated in Table 5, like the above-described Examples in a case where the refractive index profiles of the core portions are substantially the same, the differential group delay between the core portions may be decreased by appropriately performing cutting, rotating or reversing, and connecting.

In addition, in the above-described embodiments, although the refractive index profile of the multicore fiber is set so that the above-described two propagation modes exist, the number of propagation modes is not particularly limited, and a larger number of propagation modes may be used. In addition, the present disclosure may be applied to, for example, a single-mode multicore fiber having core portions of which refractive index profile is set so that a single propagation mode exists at a wavelength of input light. In this case, the differential group delay is a differential group delay between the core portions in the single propagation mode. For example, in the case of a single-mode multicore fiber, in a case where crosstalk may easily occur due to a small core pitch, when the crosstalk is compensated for in a MIMO process, the multicore fiber according to the present disclosure is applied, so that it is possible to reduce load of the MIMO process.

In the trench-type refractive index profile set so that a single propagation mode exists at a wavelength in a band of 1.55 μm, the design parameters may be combined from the ranges where Δ1is in a range of 0.2% to 1.2%, Δ2is in the vicinity of 0%, for example, in a range of −0.05% to 0.05%, Δ3 is in a range of −0.2% to −0.7%, a1is in a range of 2.5 μm to 7.0 μm, a2is in a range of 1 to 3 as a ratio to a1(a2/a1=Ra2), and a3is in a range of 2 to 5 as a ratio to a1(a3/a1=Ra3) so that the single propagation mode exists. In addition, α and the core pitch are not particularly limited. In addition, the refractive index profile in a case where there is no region having Δ2and Ra2is 1 is referred to as a w-type refractive index profile.

Table 6 is a table listing the design parameters in the above-described ranges in the trench-type refractive index profile and Aeff and group delay at a wavelength of 1550 nm and cut-off wavelength in the case of predetermined α and core pitch. In the case of the original multicore fiber having the parameters illustrated in Table 6, like the above-described Examples in a case where the refractive index profiles of the core portions are substantially the same, the differential group delay between the core portions may be decreased by appropriately performing cutting, rotating or reversing, and connecting.

In the single-peak-type refractive index profile set so that a single propagation mode exists at a wavelength in a band of 1.55 μm, the design parameters may be combined from the ranges where Δ1is in a range of 0.2% to 1.5% and a1is in a range of 1.5 μm to 6.0 μm so that the single propagation mode exists. In addition, α and the core pitch are not particularly limited.

Table 7 is a table listing the design parameters in the above-described ranges in the single-peak-type refractive index profile and Aeff and group delay at a wavelength of 1550 nm and cut-off wavelength in the case of predetermined α and core pitch. In the case of the original multicore fiber having the parameters illustrated in Table 7, like the above-described Examples in a case where the refractive index profiles of the core portions are substantially the same, the differential group delay between the core portions may be decreased by appropriately performing cutting, rotating or reversing, and connecting.

The present disclosure is not limited to the above-described embodiments. An appropriate combinational configuration of the components described above is also included in the present disclosure. In addition, new effects and modified examples may be easily derived from the ordinarily skilled in the art. Therefore, aspects wider than those of the present disclosure are not limited to the above-described embodiments, but various changes are available.

As described heretofore, a multicore fiber and a method of manufacturing the multicore fiber according to the present disclosure are useful for information transmission to which SDM systems are applied.

According to the present disclosure, it is possible to obtain an effect that a multicore fiber and a method of manufacturing the multicore fiber where a differential group delay between core portions is reduced may be implemented.