MULTICORE OPTICAL FIBER, METHOD OF DESIGNING MULTICORE OPTICAL FIBER, AND OPTICAL TRANSMISSION METHOD

Abstract

It is an object of the present invention to provide a multicore optical fiber, a design method for the multicore optical fiber and an optical transmission method using the multicore optical fiber including four cores having a standard cladding diameter of 125±1 μm for an existing single mode optical fiber covering several thousands of kilometers of transmission. The multicore optical fiber according to the present invention disposes two-stage claddings with different refractive indices around each core, and designates as a predetermined range, a core radius a1, a radius a2 of a first cladding region surrounding each core, specific refractive index Δ1 relative to the core of the first cladding region and a specific refractive index Δ2 relative to the core of a second cladding region including four cores and the first cladding region.

Claims

1. A multicore optical fiber comprising: four cores having a radius a1 disposed in a longitudinal direction in square lattice shape; first cladding regions disposed around each of the cores and having a radius a2 with a refractive index lower than a refractive index of the cores, a specific relative refractive index difference to the cores being Δ1; and a second cladding region disposed on an outer periphery of the first cladding regions and having a refractive index higher than the refractive index of the first cladding regions and lower than the refractive index of the cores, a specific relative refractive index difference to the core being Δ2, wherein an outer diameter of the second cladding region is 125±1 an effective cross-sectional area at a wavelength of 1550 nm is 80 μm2 or more, bending loss at a wavelength of 1625 nm and a bending radius of 30 mm is 0.1 dB/100 turns or less, and a cutoff wavelength is 1530 nm or less.

2. The multicore optical fiber according to claim 1, further comprising a region having a width w and having a same refractive index as the refractive index of the second cladding region between each of the cores and the first cladding region.

3. The multicore optical fiber according to claim 2, wherein the width w is 0 μm<w≤1.3 μm.

4. The multicore optical fiber according to claim 1, wherein a relationship between the Δ1 and a total value XT (dB/km) of inter-core crosstalk per km satisfies Formula C1,
[Formula C1]
Δ.sub.1≤4.93×10.sup.−5XT−0.00127  (C1).

5. The multicore optical fiber according to claim 1, wherein a relationship between the a1 and the Δ1 satisfies Formula C2,
[Formula C2]
a.sub.1≥−28041Δ.sub.1.sup.2−560.65Δ.sub.1+3.1369  (C2).

6. The multicore optical fiber according to claim 1, wherein a relationship between the Δ2 and a total value XT (dB/km) of inter-core crosstalk per km satisfies Formula C3,
[Formula C3]
Δ.sub.2≥−1.14×10.sup.−5XT−0.00509  (C3).

7. The multicore optical fiber according to claim 1, wherein a ratio a2/a1 between the a1 and the a2 is 1.0 or more and 3.0 or less, a relationship between the Δ1, the a2/a1, and an effective cross-sectional area Aeff at a wavelength of 1550 nm satisfies Formula C4 and a relationship between the Δ2, the a2/a1, and the Aeff satisfies Formula C5,
[Formula C4]
Δ.sub.1≤(1.9×10.sup.−6A.sub.eff.sup.2−0.00033A.sub.eff+0.0163)a.sub.2/a.sub.1+(−8.0×10.sup.−6A.sub.eff.sup.2+0.00139A.sub.eff−0.0705)  (C4)
[Formula C5]
Δ.sub.2≥(5.0×10.sup.−8A.sub.eff.sup.2−6.5×10.sup.−6A.sub.eff−0.00108)a.sub.2/a.sub.1+(−2.9×10.sup.−7A.sub.eff.sup.2+5.58×10.sup.−5A.sub.eff−0.00594)
and
Δ.sub.2≤(5.0×10.sup.−7A.sub.eff.sup.2−9.5×10.sup.−5A.sub.eff+0.0056)a.sub.2/a.sub.1+(−1.6×10.sup.−6A.sub.eff.sup.2+0.00031A.sub.eff−0.0208)  (C5).

8. The multicore optical fiber according to claim 5, wherein the Δ1 and the Δ2 satisfy Formula C6,
[Formula C6]
−0.7%≤Δ.sub.1≤−0.49%
−0.4%≤Δ.sub.2≤−0.27%  (C6).

9. A multicore optical fiber comprising: four cores having a radius a1 disposed in a longitudinal direction in square lattice shape; a first cladding regions disposed around each of the cores and having a radius a2 with a refractive index lower than a refractive index of the cores, a specific relative refractive index difference to the cores being Δ1; and a second cladding region disposed on an outer periphery of the first cladding regions and having a refractive index higher than the refractive index of the first cladding regions and lower than the refractive index of the cores, a specific relative refractive index difference to the core being Δ2, wherein a2/a1=2.0 and an effective cross-sectional area at a wavelength of 1550 nm is 80 μm.sup.2, and when the specific relative refractive index difference of the first cladding regions is represented by an axis of abscissas and the specific relative refractive index difference of the second cladding region is represented by an axis of ordinates, the Δ1 and the Δ2 are within a range enclosed by: A0 (−0.800, −0.396) A1a(−0.800, −0.340) A1b(−0.750, −0.351) A1 (−0.682, −0.378).

10. A multicore optical fiber comprising: four cores having a radius a1 disposed in a longitudinal direction in square lattice shape; a first cladding region disposed around each of the cores and having a radius a2 with a refractive index lower than a refractive index of the cores, a specific relative refractive index difference to the cores being Δ1; and a second cladding region disposed on an outer periphery of the first cladding regions and having a refractive index higher than the refractive index of the first cladding regions and lower than the refractive index of the cores, a specific relative refractive index difference to the core being Δ2, wherein a2/a1=3.0 and an effective cross-sectional area at a wavelength of 1550 nm is 80 μm.sup.2, and when the specific relative refractive index difference of the first cladding region is represented by an axis of abscissas and the specific relative refractive index difference of the second cladding region is represented by an axis of ordinates, the Δ1 and the Δ2 are within a range enclosed by: B0a(−0.700, −0.376) B1a(−0.700, −0.266) B1b(−0.671, −0.286) B1c(−0.618, −0.313) B1d(−0.586, −0.338) B1(−0.490, −0.396) B0d(−0.586, −0.400) B0c(−0.618, −0.391) B0b(−0.671, −0.371).

11. A multicore optical fiber comprising: four cores having a radius a1 disposed in a longitudinal direction in square lattice shape; a first cladding regions disposed around each of the cores and having a radius a2 with a refractive index lower than a refractive index of the cores, a specific relative refractive index difference to the cores being Δ1; and a second cladding region disposed on an outer periphery of the first cladding regions and having a refractive index higher than the refractive index of the first cladding regions and lower than the refractive index of the cores, a specific relative refractive index difference to the core being Δ2, wherein a2/a1=3.0 and an effective cross-sectional area at a wavelength of 1550 nm is 100 μm.sup.2, and when the specific relative refractive index difference of the first cladding region is represented by an axis of abscissas and the specific relative refractive index difference of the second cladding region is represented by an axis of ordinates, the Δ1 and the Δ2 are within a range enclosed by: C0a(−0.600, −0.274) C1a(−0.600, −0.255) C1b(−0.590, −0.262) C1c(−0.570, −0.277) C1(−0.528, −0.305) C0c(−0.570, −0.297) C0b(−0.590, −0.289).

12. A design method for a multicore optical fiber, the multicore optical fiber comprising: four cores having a radius a1 disposed in a longitudinal direction in square lattice shape; a first cladding region disposed around each of the cores and having a radius a2 with a refractive index lower than a refractive index of the cores, a specific relative refractive index difference to the cores being Δ1; and a second cladding region disposed on an outer periphery of the first cladding regions and having a refractive index higher than the refractive index of the first cladding regions and lower than the refractive index of the cores, a specific relative refractive index difference to the core being Δ2, the design method comprising: selecting a combination of the a1 and the Δ1 from requirements for effective cross-sectional area A.sub.eff (μm.sup.2) and a total value XT (dB/km) of inter-core crosstalk per km; selecting the a2 and the Δ2 that satisfy requirements for a cutoff wavelength and bending loss in the combination; acquiring a core interval relationship between the Δ1 with respect to the XT and a core interval Λ, and an OCT relationship between the Δ1 and a shortest distance OCT from the outer periphery of the second cladding region to a center of the core with respect to requirements for an excessive loss α.sub.c; calculating an outer diameter ϕ of the second cladding region at the Δ1 using the core interval relationship and the OCT relationship according to:
φ=2×(Λ/√2+OCT); and determining whether the outer diameter ϕ becomes 125 μm or less.

13. The design method according to claim 12, wherein when A.sub.eff is 80 μm.sup.2 or more, the combination that satisfies Formula C1 and Formula C2 is selected at the selecting the combination,
[Formula C1]
Δ.sub.1≤4.93×10.sup.−5XT−0.00127  (C1)
[Formula C2]
a.sub.1≥−28041Δ.sub.1.sup.2−560.65Δ.sub.1+3.1369  (C2).

14. The design method according to claim 13, wherein when a2/a1 is 3.0 or less, the Δ2 that satisfies Formula C3 is selected at the selecting the Δ2,
[Formula C3]
Δ.sub.2≥−1.14×10.sup.−5XT−0.00509  (C3).

15. An optical transmission method using the multicore optical fiber of 1000 km or more according to any one claim 1 or 9-11 as an optical transmission path, the method comprising: transmitting signal light having a wavelength of 1530 nm or more from four transmitters for each core of the multicore optical fiber; and receiving the signal light for each core of the multicore optical fiber using four receivers.

16. An optical transmission method comprising: replacing a single mode optical fiber of a submarine communication system having a communication distance of 1000 km or more with the multicore optical fiber according to any one claim 1 or 9-11; transmitting signal light having a wavelength of 1530 nm or more from four transmitters for each core of the multicore optical fiber; and receiving the signal light for each core of the multicore optical fiber using four receivers.

17. The multicore optical fiber according to claim 7, wherein the Δ1 and the Δ2 satisfy Formula C6,
[Formula C6]
−0.7%≤Δ.sub.1≤−0.49%
−0.4%≤Δ.sub.2≤−0.27%  (C6).

Description

BRIEF DESCRIPTION OF DRAWINGS

[0070] FIG. 1 is a diagram illustrating an example of a structure of a multicore optical fiber according to the present invention.

[0071] FIG. 2 is a diagram illustrating influences of a width w of the multicore optical fiber according to the present invention.

[0072] FIG. 3 is a characteristic diagram expressing a relationship between an effective cross-sectional area and structural parameters of the multicore optical fiber according to the present invention.

[0073] FIG. 4 is a characteristic diagram expressing a relationship between an effective cross-sectional area and structural parameters of the multicore optical fiber according to the present invention.

[0074] FIG. 5 is a characteristic diagram expressing a relationship between an effective cross-sectional area and structural parameters of the multicore optical fiber according to the present invention.

[0075] FIG. 6 is a characteristic diagram expressing an example of a relationship between core arrangement, XT and excessive loss of the multicore optical fiber according to the present invention.

[0076] FIG. 7 is a characteristic diagram expressing a relationship of structural parameters of the multicore optical fiber according to the present invention in which the cladding diameter is 125±1 μm or less under a predetermined cutoff wavelength, bending loss and an effective cross-sectional area.

[0077] FIG. 8 is a characteristic diagram expressing a relationship of structural parameters of the multicore optical fiber according to the present invention in which the cladding diameter is 125±1 μm or less under a predetermined cutoff wavelength, bending loss and an effective cross-sectional area.

[0078] FIG. 9 is a characteristic diagram expressing a relationship of structural parameters of the multicore optical fiber according to the present invention in which the cladding diameter is 125±1 μm or less under a predetermined cutoff wavelength, bending loss and an effective cross-sectional area.

[0079] FIG. 10 is a characteristic diagram expressing a relationship between XT and a range of necessary structural parameters of the multicore optical fiber according to the present invention.

[0080] FIG. 11 is a characteristic diagram expressing a relationship between XT and a range of necessary structural parameters of the multicore optical fiber according to the present invention.

[0081] FIG. 12 is a characteristic diagram expressing a relationship of structural parameters of the multicore optical fiber according to the present invention in which the cladding diameter is 125 μm or less under a predetermined cutoff wavelength, bending loss and an effective cross-sectional area.

[0082] FIG. 13 is a characteristic diagram expressing a relationship of structural parameters of the multicore optical fiber according to the present invention in which the cladding diameter is 125 μm or less under a predetermined cutoff wavelength, bending loss and an effective cross-sectional area.

[0083] FIG. 14 is a characteristic diagram expressing a relationship of structural parameters of the multicore optical fiber according to the present invention in which the cladding diameter is 125 μm or less under between a predetermined cutoff wavelength, bending loss and an effective cross-sectional area.

[0084] FIG. 15 is a table illustrating a design example of the multicore optical fiber according to the present invention.

[0085] FIG. 16 is a characteristic diagram illustrating a relationship between a core interval and XT in a design example of the multicore optical fiber according to the present invention.

[0086] FIG. 17 is a characteristic diagram illustrating a relationship between OCT and excessive loss in a design example of the multicore optical fiber according to the present invention.

[0087] FIG. 18 is a cross-sectional view of the multicore optical fiber according to the present invention.

[0088] FIG. 19 is a table describing evaluation results of the multicore optical fiber according to the present invention.

[0089] FIG. 20 is a flowchart describing a design method according to the present invention.

[0090] FIG. 21 is a diagram illustrating an optical transmission method according to the present invention.

DESCRIPTION OF EMBODIMENTS

[0091] Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described hereinafter are examples of the present invention, and the present invention is not limited to the following embodiments. Note that components assigned the same reference numerals in the present Description and drawings denote the same components.

[0092] FIG. 1 is a diagram illustrating an example of a structure of an optical fiber 15 according to the present embodiment. FIG. 1(a) illustrates a cross-sectional structure, FIG. 1(b) and FIG. 1(c) illustrate a refractive index distribution of each core. The optical fiber 15 is a multicore optical fiber including:

[0093] four cores 10 having a radius a1 disposed in a longitudinal direction in square lattice shape;

[0094] a first cladding region 11 disposed around the core 10 and having a radius a2 with a refractive index lower than the refractive index of the core, a specific relative refractive index difference between the cladding region and the core being Δ1; and a second cladding region 12 disposed on an outer

[0095] periphery of the first cladding region 11 and having a refractive index higher than the refractive index of the first cladding region and lower than the refractive index of the core, a specific relative refractive index difference between the cladding region and the core being Δ2.

[0096] In the optical fiber 15,

[0097] an outer diameter of the second cladding region 12 is 125±1 μm,

[0098] an effective cross-sectional area at a wavelength of 1550 nm is 80 μm.sup.2 or more,

[0099] bending loss at a wavelength of 1625 nm and a bending radius of 30 mm is 0.1 dB/100 turns or less, and

[0100] a cutoff wavelength is 1530 nm or less.

[0101] The optical fiber 15 includes four cores in one optical fiber having a standard cladding diameter (outer diameter of the second cladding region 12) of 125±1 μm.

[0102] As shown in FIG. 1(b), a refractive index distribution of each core includes the first cladding region 11 around the core 10 and having a refractive index lower than the refractive index of the core and includes the second cladding region 12 therearound and having a refractive index lower than the refractive index of the core and higher than the refractive index of the first cladding region. As shown in FIG. 1(c), a region 13 having a refractive index equivalent to the refractive index of the second cladding region 12 may also be included between the core 10 and the first cladding region 11.

[0103] Regarding the refractive index distribution in FIG. 1(c), FIG. 2 illustrates an amount of change in a cutoff wavelength with respect to a width w of the region 13. As shown in FIG. 2, even when the region 13 having a refractive index equivalent to the refractive index of the second cladding region 12 exists between the core 10 and the first cladding region 11, the amount of change in the cutoff wavelength is small. It can be confirmed from the diagram that when the width w is 1.3 μm or less, the amount of change in the cutoff wavelength is ±10 nm, which is equivalent to a measurement error, and the refractive index distributions in FIG. 1(b) and FIG. 1(c) can be regarded as equivalent. Adopting the structure in FIG. 1(c) can reduce fluctuations in the core structure during manufacturing, and in the case of pure quartz, for example, the structure is more stable regarding the refractive index of the second cladding region, and it is possible to improve manufacturing deviation or yield, which is therefore preferable.

[0104] Note that although the specific relative refractive index difference is defined using the refractive index of the core as a reference here, the materials of the core and the cladding can be selected from among combinations of glass materials that can constitute the refractive index distribution shown in FIG. 1 such as a combination of pure quartz and fluorine added glass or a combination of GeO2 added glass, pure quartz glass and fluorine added glass.

[0105] The “cutoff wavelength” of an optical fiber refers to a condition under which single mode propagation is possible, and light propagates in a single mode when the wavelength of the light is longer than the cutoff wavelength or light propagates in a multimode when the wavelength of the light is shorter than the cutoff wavelength.

[0106] FIG. 3 to FIG. 5 are diagrams illustrating a design range of core parameters to obtain a predetermined effective cross-sectional area A.sub.eff of the optical fiber 15 at a wavelength of 1550 nm. The axis of abscissas represents a specific relative refractive index difference Δ.sub.1 with respect to the core of the first cladding region 11 and the axis of ordinates represents a radius a1 of the core 10. FIG. 3 to FIG. 5 illustrate cases where the ratio a.sub.2/a.sub.1 of the core radius a.sub.1 to the radius a.sub.2 of the first cladding region 11 region is changed from 2.0 to 2.5 to 3.0 respectively. The specific relative refractive index difference Δ of the cladding with respect to the core is set so that the cutoff wavelength becomes 1530 nm.

[0107] Comparing FIG. 3 to FIG. 5, the relationship between a.sub.1 and Δ.sub.1 to obtain constant A.sub.eff is substantially the same. It is considered that this is because the electric field distribution is confined in a region surrounded by the first cladding region 11, and so dependency on the parameters a.sub.2 and Δ.sub.2 (specific relative refractive index difference of the second cladding region 12 with respect to the core) relating to the second cladding region 12 is small.

[0108] Here, a conventional SMF has an A.sub.eff of approximately 80 μm.sup.2 at a wavelength of 1550 nm and the A.sub.eff is preferably 80 μm.sup.2 or more to secure connectivity and low non-linearity equal to or better than conventional. From FIG. 3 to FIG. 5, in order to set A.sub.eff to 80 μm.sup.2,

[Formula 1]

a.sub.1≥−28041Δ.sub.1.sup.2−560.65Δ.sub.1+3.1369  (1)

needs to be satisfied according to an approximate curve shown by the solid line.

[0109] It is known that the core interval needs to be increased by a certain amount in MCF to reduce XT. Deterioration of transmission characteristics caused by XT depends on a transmission scheme and Non-Patent Literature 7 shows that crosstalk should be reduced to −18 dB, −24 dB and −32 dB or less for QPSK, 16QAM and 64QAM respectively. In the case of a multicore optical fiber, inter-core crosstalk is a sum total of crosstalk components from other cores to a given core and crosstalk at a given distance with respect to crosstalk (XT, unit dB/km) per unit distance and a distance L (km) is given by XT+log(L).

[0110] Therefore, in order to perform transmission in 1000 to 10000 km using, for example, a 16QAM signal format, crosstalk of a multicore optical fiber needs to be −54 to −64 dB/km or less. Reducing crosstalk requires the core interval to be increased by a certain amount, but it is known that when the cladding diameter is constant, a thickness (OCT) between a core and a cladding end decreases as the core interval increases, an excessive loss α.sub.c is generated due to light wave leakage.

[0111] FIG. 6 is a diagram illustrating a design example of the optical fiber 15 to reduce inter-core crosstalk (XT) and excessive loss α.sub.c. The axis of abscissas represents a specific relative refractive index difference Δ.sub.1 of the first cladding region 11, a first axis of ordinates represents a core interval Λ or OCT, and a second axis of ordinates represents an outer diameter of the second cladding region 12. Since XT and the excessive loss increase as the wavelength increases, the wavelength is assumed to be 1625 nm in consideration of utilization of an entire C+L bandwidth. The solid line shows a core interval Λ (μm) where XT becomes −64 dB/km and the broken line shows OCT where the excessive loss α.sub.c becomes 0.01 dB/km. That is, XT becomes less than −64 dB/km in a region above the solid line and α.sub.cbecomes less than 0.01 dB/km in a region above the broken line.

[0112] The dotted line shows a “necessary cladding diameter” for XT to satisfy −64 dB/km and for a.sub.c to satisfy 0.01 dB/km, and in the case of a 4-core arrangement, the necessary cladding diameter=2×(Λ/√2+OCT).

[0113] Here a.sub.2/a.sub.1=3.0 and a.sub.1 and Δ.sub.2 are set so that A.sub.eff becomes 110 μm.sup.2 and the cutoff wavelength λ.sub.c becomes 1530 nm.

[0114] It is seen from FIG. 6 that light wave confinement becomes weak as Δ.sub.1 increases, and the core interval Λ and OCT necessary to obtain a desired XT and α.sub.c increase. In the case of FIG. 6, Δ.sub.1 needs to be −0.52% in order to set the cladding diameter to 125±1 μm (see single-dot dashed line). The necessary core interval Λ and OCT at this time are 42.5 μm and 30 μm respectively. Note that the core interval and the OCT shown in FIG. 6 are minimum values that satisfy the predetermined XT and the excessive loss, and setting larger values will cause the XT and the excessive loss to have smaller values. That is, if Δ.sub.1 is set to less than −0.52% in order to set the cladding diameter to 125±1 μm, the necessary cladding diameter becomes 125 μm or less, providing room for the design of Λ and OCT.

[0115] FIG. 7 to FIG. 9 are diagrams illustrating examples of structural parameters of the optical fiber 15 having a cutoff wavelength of 1530 nm or less, bending loss of 0.1 dB/100 turns or less at a wavelength of 1625 nm and a bending radius of 30 mm and a necessary cladding diameter of 125±1 μm. In all the drawings, the aforementioned conditions (XT is −64 dB/km or less and α.sub.c is 0.01 dB/km or less) can be satisfied simultaneously in the shaded regions.

[0116] FIG. 7 shows a case where A.sub.eff is 80 μm.sup.2 at a wavelength of 1550 nm, and a.sub.2/a.sub.1=2.0,

[0117] FIG. 8 shows a case where A.sub.eff is 80 μm.sup.2 at a wavelength of 1550 nm, and a.sub.2/a.sub.1=3.0, and

[0118] FIG. 9 shows a case where A.sub.eff is 110 μm.sup.2 at a wavelength of 1550 nm, and a.sub.2/a.sub.1=3.0.

[0119] In FIG. 7 to FIG. 9, the broken line shows a condition under which the cutoff wavelength becomes 1530 nm (the cutoff wavelength is 1530 nm or less in a region above the broken line) and the dotted line shows a condition under which the bending loss α.sub.b becomes 0.1 dB/100 turns (the bending loss α.sub.b is 0.1 dB/100 turns or less in a region below the dotted line).

[0120] In FIG. 7 to FIG. 9, the solid line shows a condition of XT at a wavelength of 1625 nm and XT becomes −54 dB/km or less, −58 dB/km or less, −61 dB/km or less and −64 dB/km or less respectively in a region below the solid line.

[0121] The region that satisfies all the conditions is a region surrounded by the solid line, the broken line and the dotted line, and when, for example, XT is set to −64 dB/km or less, the region corresponds to a shaded region in the diagram. From FIG. 7 to FIG. 9, an upper limit of Δ.sub.1 and a lower limit of Δ.sub.2 are determined by requirements for XT and cutoff wavelength, and the lower limit of Δ.sub.2 has a relatively smaller change than Δ.sub.1. Therefore, the upper limit of Δ.sub.1 and the lower limit of Δ.sub.2 can be given by an intersection of the solid line and the broken line. It is also seen from FIG. 7 to FIG. 9 that the designable region becomes alleviated as the a.sub.2/a.sub.1 is larger and the required A.sub.eff is smaller.

[0122] Therefore, when A.sub.eff is 80 μm.sup.2 or more, the cutoff wavelength is 1530 nm or less and XT is −64 dB/km or less, FIG. 8 includes the largest region, and in this case,

[Formula 2]

−0.7%≤Δ.sub.1≤−0.49%

−0.4%≤Δ.sub.2≤−0.27%  (2)

[0123] In FIG. 7, if X coordinate is assumed to be Δ1 and Y coordinate is assumed to be Δ2, the shaded region, that is, the region that satisfies:

a.sub.2/a.sub.1=2.0,

A.sub.eff=80 μm.sup.2 at a wavelength of 1550 nm,
cladding diameter D≤125 μm,
excessive loss α.sub.c≤0.01 dB/km,
cutoff wavelength λc≤1530 nm,
bending loss α.sub.b≤0.1 dB/100 turns, and
XT≤−64 dB/km is a range surrounded by:

A0 (−0.800, −0.396)

A1a(−0.800, −0.340)

A1b(−0.750, −0.351)

A1 (−0.682, −0.378).

[0124] Note that if the region satisfies XT≤−61 dB/km, it is a range surrounded by:

A0 (−0.800, −0.396)

A2a(−0.800, −0.335)

A2b(−0.750, −0.346)

A2c(−0.700, −0.363)

A2 (−0.668, −0.380).

[0125] If the region satisfies XT≤−58 dB/km, it is a range surrounded by:

A0 (−0.800, −0.396)

A3a(−0.800, −0.331)

A3b(−0.750, −0.339)

A3c(−0.700, −0.355)

A3d(−0.650, −0.380)

A3 (−0.639, −0.387).

[0126] If the region satisfies XT≤−54 dB/km, it is a range surrounded by:

A0 (−0.800, −0.396)

A4a(−0.800, −0.328)

A4b(−0.750, −0.323)

A4c(−0.700, −0.349)

A4d(−0.650, −0.370)

A4(−0.600, −0.400).

[0127] In FIG. 8, if X coordinate is assumed to be Δ1 and Y coordinate is assumed to be Δ2, the shaded region, that is, the region that satisfies:

a.sub.2/a.sub.1=3.0,

A.sub.eff=80 μm.sup.2 at a wavelength of 1550 nm,
cladding diameter D≤125 μm,
excessive loss α.sub.c≤0.01 dB/km,
cutoff wavelength λc≤1530 nm,
bending loss α.sub.b0.1 dB/100 turns, and
XT≤−64 dB/km is a range surrounded by:

B0a(−0.700, −0.376)

B1a(−0.700, −0.266)

B1b(−0.671, −0.286)

B1c(−0.618, −0.313)

B1d(−0.586, −0.338)

B1(−0.490, −0.396)

B0d(−0.586, −0.400)

B0c(−0.618, −0.391)

B0b(−0.671, −0.371).

[0128] Note that if the region satisfies XT≤−61 dB/km, it is a range surrounded by:

B0a(−0.700, −0.376)

B2a(−0.700, −0.256)

B2b(−0.671, −0.278)

B2c(−0.618, −0.306)

B2d(−0.586, −0.329)

B2 (−0.484, −0.396)

B0d(−0.586, −0.400)

B0c(−0.618, −0.391)

B0b(−0.671, −0.371).

[0129] If the region that satisfied XT≤−58 dB/km, it is a range surrounded by:

B0a(−0.700, −0.376)

B3a(−0.700, −0.242)

B3b(−0.671, −0.267)

B3c(−0.618, −0.299)

B3d(−0.586, −0.319)

B3 (−0.470, −0.396)

B0d(−0.586, −0.400)

B0c(−0.618, −0.391)

B0b(−0.671, −0.371).

[0130] If the region that satisfies XT≤−54 dB/km, it is a range surrounded by:

B0a(−0.700, −0.376)

B4a(−0.700, −0.234)

B4b(−0.671, −0.256)

B4c(−0.618, −0.278)

B4d(−0.586, −0.312)

B4 (−0.458, −0.396)

B0d(−0.586, −0.400)

B0c(−0.618, −0.391)

B0b(−0.671, −0.371).

[0131] In FIG. 9, if X coordinate is assumed to be Δ1 and Y coordinate is assumed to be Δ2, the shaded region, that is, the region that satisfies:

a.sub.2/a.sub.1=3.0,

A.sub.eff=110 μm.sup.2 at a wavelength of 1550 nm
cladding diameter D≤125 μm,
excessive loss α.sub.c≤0.01 dB/km,
cutoff wavelength λc≤1530 nm,
bending loss α.sub.b≤0.1 dB/100 turns, and
XT≤−64 dB/km
is a region surrounded by:

C0a(−0.600, −0.274)

C1a(−0.600, −0.255)

C1b(−0.590, −0.262)

C1c(−0.570, −0.277)

C1(−0.528, −0.305)

C0c(−0.570, −0.297)

C0b(−0.590, −0.289).

[0132] Note that If the region that satisfies XT≤−61 dB/km, it is a region surrounded by:

C0a(−0.600, −0.274)

C2a(−0.600, −0.247)

C2b(−0.590, −0.254)

C2c(−0.570, −0.269)

C2 (−0.513, −0.308)

C0c(−0.570, −0.297)

C0b(−0.590, −0.289).

[0133] If the region that satisfies XT≤−58 dB/km, it is a region surrounded by:

C0a(−0.600, −0.274)

C3a(−0.600, −0.234)

C3b(−0.590, −0.242)

C3c(−0.570, −0.257)

C3 (−0.495, −0.311)

C0c(−0.570, −0.297)

C0b(−0.590, −0.289).

[0134] If the region that satisfies XT≤−54 dB/km, it is a region surrounded by:

C0a(−0.600, −0.274)

C4a(−0.600, −0.220)

C4b(−0.590, −0.233)

C4c(−0.570, −0.249)

C4 (−0.479, −0.313)

C0c(−0.570, −0.297)

C0b(−0.590, −0.289).

[0135] FIG. 10 and FIG. 11 are diagrams illustrating dependency on XT of the upper limit of Δ.sub.1 (FIG. 10) and the lower limit of Δ.sub.2 (FIG. 11) given by intersections of the cutoff wavelength λc (broken line) and the condition of XT (solid line) on the graphs created as shown in FIG. 7 to FIG. 9. Here, a.sub.2/a.sub.1 is assumed to be 3.0. It is seen from FIG. 10 and FIG. 11 that the upper limit of Δ.sub.1 and the lower limit of Δ.sub.2 linearly change with respect to the requirements for XT. If A.sub.eff is 80 μm.sup.2 or more, it is seen from FIG. 10 and FIG. 11 that:

[Formula 3]

Δ.sup.1≤4.93×10.sup.−5XT−0.00127

Δ.sub.2≥−1.14×10.sup.−5XT−0.00509  (3)

Here, comparing FIG. 7 and FIG. 8, the upper limit of Δ.sub.1 decreases and the lower limit of Δ.sub.2 increases as a.sub.2/a.sub.1 decreases, and so when a.sub.2/a.sub.1 is smaller than 3.0, a.sub.2/a.sub.1 is included within ranges of the upper limit of Δ1 and the lower limit of Δ2 shown in Formula 3.

[0136] FIG. 12 to FIG. 14 are diagrams illustrating dependency on a.sub.2/a.sub.1 of the upper limit of Δ.sub.1 (FIG. 12), the lower limit of Δ.sub.2 (FIG. 13) and the upper limit of Δ.sub.2 (FIG. 14) given by intersections of the cutoff wavelength λc (broken line) and the condition of XT (solid line) on the graphs created as shown in FIG. 7 to FIG. 9. Straight lines in the respective drawings are the results of linear approximation.

[0137] Here, the requirements for XT are assumed to be −64 dB/km. It is seen from FIG. 12 to FIG. 14 that the upper limit of Δ.sub.1, the lower limit of Δ.sub.2 (Δ.sub.2_min) and the upper limit of Δ.sub.2 (Δ.sub.2_max) can be approximated relatively better with respect to a.sub.2/a.sub.1 through linear approximation. It is seen from FIG. 12 to FIG. 14 that: With respect to:

A.sub.eff=80 μm.sup.2,

[Formula 4]

Δ.sub.1=0.0020a.sub.2/a.sub.1−0.01069

Δ.sub.2 min=−0.00024a.sub.2/a.sub.1−0.00335

Δ.sub.2 max=0.0012a.sub.2/a.sub.1−0.005833  (4)

With respect to:
A.sub.eff=90 μm.sup.2,

[Formula 5]

Δ.sub.1=0.0019a.sub.2/a.sub.1−0.01045

Δ.sub.2 min=9×10.sup.−5a.sub.2/a.sub.1−0.00329

Δ.sub.2 max=0.0011a.sub.2/a.sub.1−0.005417  (5)

[0138] With respect to:

A.sub.eff=100 μm.sup.2,

[Formula 6]

Δ.sub.1=0.0022a.sub.2/a.sub.1−0.01182

Δ.sub.2 min=−7×10.sup.−5a.sub.2/a.sub.1−0.00328

Δ.sub.2 max=0.0011a.sub.2/a.sub.1−0.00525.  (4)

[0139] When approximate curves of coefficients of Formula 4 to Formula 6 are taken,

[Formula 7]

Δ.sub.1≤(1.9×10.sup.−6A.sub.eff.sup.2−0.00033A.sub.eff+0.0163)a.sub.2/a.sub.1+(−8.0×10.sup.−6A.sub.eff.sup.2+0.00139A.sub.eff−0.0705)

Δ.sub.2≥(5.0×10.sup.−8A.sub.eff.sup.2+6.5×10.sup.−6A.sub.eff−0.00108)a.sub.2/a.sub.1+(−2.9×10.sup.−7A.sub.eff.sup.2+5.58×10.sup.−5A.sub.eff−0.00594)

Δ.sub.2≤(5.0×10.sup.−7A.sub.eff.sup.2−9.5×10.sup.−5A.sub.eff−0.0056)a.sub.2/a.sub.1+(−1.6×10.sup.−6A.sub.eff.sup.2+0.00031A.sub.eff−0.0208)  (7)

[0140] Within the parameter range that satisfies Formula 7, it is possible to realize an optical fiber having a standard cladding diameter of 125 μm, with four cores and having characteristics equivalent to existing optical fibers.

[0141] FIG. 15 shows structural parameters and optical characteristics designed within the design range of Formula 7. The 4-core optical fibers in design examples (1) and (2) have A.sub.eff of 85 μm.sup.2 and 102 μm.sup.2, acquire single mode operation in a C band and an L band, and have bending loss equivalent to or less than conventional SMF.

[0142] FIG. 16 and FIG. 17 show XT characteristics and excessive loss corresponding to the design example shown in FIG. 15. The wavelength is 1625 nm.

[0143] FIG. 16 is a diagram illustrating a relationship between a core interval and XT. In order to obtain XT of −64 dB/km or less, core intervals of 41.6 μm or less and 44 μm or less are necessary with respect to design example (1) and design example (2) respectively.

[0144] FIG. 17 illustrates a relationship between OCT and excessive loss α.sub.c. In order to obtain excessive loss of 0.01 dB/km or less, OCTs of 28.7 μm or less and 30.9 μm or less are necessary with respect to design example (1) and design example (2) respectively.

[0145] It is seen from these results that in the 4-core optical fibers in design example (1) and design example (2), the minimum necessary cladding diameters are 116.2 μm and 124.0 μm respectively. It is seen in both design examples that four cores having a standard cladding diameter of 125 μm can be disposed.

EXAMPLES

[0146] FIG. 18 is a cross-sectional photo of the multicore optical fiber according to the present example. FIG. 19 shows the measurement results.

[0147] It is seen from the cross-sectional photo in FIG. 18 that the multicore optical fiber has a cladding diameter of 125 μm and that four cores are disposed.

[0148] It can also be confirmed from FIG. 19 that in a refractive index distribution of each core, a core radius is 6 μm, a2/a1 is 3, Δ1 and A2 are −0.6% and −0.4% respectively, thus satisfying the formula expressing the aforementioned structural conditions.

[0149] Regarding the optical characteristics, A.sub.eff was 80 μm.sup.2 or more and the cutoff wavelength was 1480 nm or less.

[0150] The transmission losses were 0.18 dB/km or less and 0.25 dB/km or less at wavelengths of 1550 nm and 1625 nm respectively and since no significant increase of loss was observed on the long wavelength side, it is considered that sufficient OCT is secured.

[0151] XT is −66 dB/km or less at a wavelength of 1625 nm and XT characteristics for transmission distances of over 10000 km were obtained in all communication wavelength bands.

[0152] (Design Method)

[0153] FIG. 20 is a flowchart describing a design method for a multicore optical fiber.

[0154] The multicore optical fiber includes:

[0155] four cores having a radius a.sub.1, disposed in a longitudinal direction in square lattice shape;

[0156] a first cladding region disposed around the core and having a radius a.sub.2 with a refractive index lower than the refractive index of the core, a specific relative refractive index difference between the cladding region and the core being Δ.sub.1; and

[0157] a second cladding region disposed on an outer periphery of the first cladding region and having a refractive index higher than the refractive index of the first cladding region and lower than the refractive index of the core, a specific relative refractive index difference between the cladding region and the core being Δ.sub.2, in which

[0158] the design method executes:

[0159] a first step S01 of selecting a combination of the a.sub.1 and the Δ.sub.1 from requirements for effective cross-sectional area A.sub.eff (μm.sup.2) and a total value XT (dB/km) of inter-core crosstalk per km;

[0160] a second step S02 of selecting the a.sub.2 and the Δ.sub.2 that satisfy requirements for a cutoff wavelength and bending loss in the combination selected in the first step S01 and acquiring a core interval relationship between the Δ.sub.1 with respect to the XT and a core interval Λ, and an OCT relationship between the Δ.sub.1 and a shortest distance OCT from the outer periphery of the second cladding region to a center of the core with respect to requirements for an excessive loss α.sub.c; and

[0161] a third step S03 of calculating an outer diameter ϕ of the second cladding region at the Δ.sub.1 acquired in the first step using the core interval relationship and the OCT relationship acquired in the second step S02 according to:

ϕ=2×(Λ/√2+OCT)

and determining whether the outer diameter ϕ becomes 125 μm or less.

[0162] When a 4-core optical fiber having a standard cladding diameter of 125 μm is designed, requirements for the A.sub.eff, the XT, the bending loss and the cutoff wavelength are set first (step S00).

[0163] Next, in step S01, a combination of a.sub.1 and Δ.sub.1 that satisfies the requirements for A.sub.eff is selected as shown in FIG. 3 to FIG. 5. As shown in FIG. 10 and FIG. 11, a condition for Δ.sub.1 corresponding to the desired XT is set. For example, when A.sub.eff is 80 μm.sup.2 or more, a combination of a.sub.1 and Δ.sub.1 that satisfies Formula 1 and Formula 3 is selected in first step S01.

[0164] Next, in step S02, as shown, for example, in FIG. 6, FIG. 7 to FIG. 9, a.sub.2 and Δ.sub.2 that satisfy the requirements for the bending loss and XT are selected for a.sub.1 and Δ.sub.1 selected in advance. More specifically, a.sub.2 is selected from the graph in FIG. 12 obtained from FIG. 7 to FIG. 9 and Δ.sub.2 is selected from the graphs in FIG. 13 and FIG. 14. At this time, parameters of the core 10 and the first cladding region 11 are obtained. For example, when a.sub.2/a.sub.1 is 3.0 or less, Δ.sub.2 that satisfies Formula 3 is selected in second step S02. Since all a.sub.1, a.sub.2, Δ.sub.1 and Δ.sub.2 are available at this time, using FIG. 6, the requirements for XT, and a core interval and an OCT that can sufficiently reduce the excessive loss are calculated and a necessary cladding diameter is calculated.

[0165] Finally, in step S03, if the necessary cladding diameter is 125±1 μm or less, the design is completed. If the cladding diameter is larger than 125±1 μm, parameters of the core and the first cladding region are re-selected so as to obtain 125±1 μm or less (repeat from step S01).

[0166] (Optical Transmission Method)

[0167] FIG. 21 is a diagram illustrating an optical transmission method using the aforementioned multicore optical fiber. The optical transmission method according to the present invention is characterized in that the multicore optical fiber 15 of 1000 km or more is used as an optical transmission path, signal light having a wavelength of 1530 nm or more is transmitted from four transmitters 21 for each core 10 of the multicore optical fiber 15, and four receivers 22 receive the signal light for each core 10 of the multicore optical fiber 15.

[0168] The optical fiber cable 100 is provided with the 4-core optical fiber 15, the transmitter 21 and the receiver 22 are connected to the transmitting side and the receiving side of each core 10 respectively. As described above, the optical fiber cable 100 has a maximum crosstalk of −54 dB/km or less and is suitable for long-distance transmission of several thousands of kilometers. Here, it is assumed that cable installation includes connection points between fibers and light amplifiers, but crosstalk in the fibers becomes dominant in transmission paths of several thousands of kilometers and influences of crosstalk at other connection points or optical components are considered to be sufficiently small.

[0169] The present optical transmission method can replace the single mode optical fiber of the submarine communication system having a communication distance of 1000 km or more with the multicore optical fiber 15, transmit signal light having a wavelength of 1530 nm or more from the four transmitters 21 for each core 10 of the multicore optical fiber 15 and receive the signal light by the four receivers 22 from each core 10 of the multicore optical fiber 15.

[0170] Since the multicore optical fiber 15 adopting a standard cladding diameter is used for the optical fiber cable, the existing optical cable structure can be reused, which is preferable.

INDUSTRIAL APPLICABILITY

[0171] The present invention can be used for optical fibers in optical communication systems.

REFERENCE SIGNS LIST

[0172] 10 core [0173] 11 first cladding region [0174] 12 second cladding region [0175] 21 transmitter [0176] 22 receiver [0177] 100 optical cable