Multi-core fiber
The present invention relates to a multi-core fiber having a structure for effectively suppressing crosstalk increase between cores caused by bending within an allowable range. The multi-core fiber comprises a plurality of types of cores respectively extending along an optical axis and a cladding region, and the effective refractive index of each core is set so that, in all pairs of cores of different types, a relative refractive index difference between an effective refractive index of a core of a certain type and an effective refractive index of a core of another type satisfies a condition regulated according to a core spacing between cores and a bending radius.
1. A multi-core fiber comprising: a plurality of types of cores respectively having different effective refractive indexes in design and extending along a predetermined axis; and a cladding region integrally surrounding the plurality of types of cores, the cores being arranged such that a minimum core spacing D min between the same type of cores, servicing as a center-to-center spacing, becomes not shorter than the minimum spacing that causes no problem with crosstalk,
wherein the multi-core fiber has at least a pair of cores of different types with a core spacing shorter than D min , and
wherein when, in all pairs of cores of different types with the core spacings shorter than D min , the core spacing and a bending radius in a cross-section of the multi-core fiber orthogonal to the predetermined axis are respectively defined as D and R, a relative refractive index difference Δ eff between effective refractive indexes of the core of a certain type and the core of the other type satisfies the following condition:
Δ
eff
(
%
)
>
2
D
R
+
(
D
R
)
2
2
(
1
+
D
R
)
2
·
100.
2. A multi-core fiber according to claim 1 , wherein, when a difference of equivalent refractive index of effective refractive index between cores of the different types falls within a range that causes a problem with crosstalk, the relative refractive index difference Δ eff between effective refractive indexes of the core of the certain type and the core of the other type satisfies the following condition
Δ
eff
(
%
)
≥
2
D
R
+
(
D
R
)
2
2
(
1
+
D
R
)
2
·
100
+
0.005
.
3. A multi-core fiber according to claim 1 , wherein the multi-core fiber is wound around a bobbin with a body radius of R.
4. A multi-core fiber according to claim 3 , wherein the body radius R of the bobbin falls within the range of 200 mm to 30 mm.
5. A multi-core fiber according to claim 3 , wherein the body radius R of the bobbin falls within the range of 150 mm to 30 mm.
6. A multi-core fiber according to claim 3 , wherein the body radius R of the bobbin falls within the range of 100 mm to 30 mm.
7. A multi-core fiber according to claim 3 , wherein the body radius R of the bobbin is one of 76 mm, 85 mm, 115 mm, and 140 mm.
8. A multi-core fiber comprising: a plurality of types of cores respectively having different effective refractive indexes in design and extending along a predetermined axis; and a cladding region integrally surrounding the plurality of types of cores, the cores being arranged such that a minimum core spacing D min between the same type of cores, servicing as a center-to-center spacing, becomes not shorter than the minimum spacing that causes no problem with crosstalk,
wherein the multi-core fiber has at least a pair of cores of different types with a core spacing shorter than D min , and
wherein at least a part of the multi-core fiber is bent at a radius R of not greater than 200 mm, and when, in all pairs of cores of different types with the core spacings of shorter than D min , the core spacing in a cross-section of the multi-core fiber orthogonal to the predetermined axis is defined as D, a relative refractive index difference Δ eff between effective refractive indexes of the core of a certain type and the core of the other type satisfies the following condition
Δ
eff
(
%
)
>
50
(
400
+
D
)
D
(
200
+
D
)
2
.
9. A multi-core fiber according to claim 1 , wherein when, in all pairs of cores of different types with the core spacings of shorter than D min , the bending radius R is 150 mm, the relative refractive index difference Δ eff between effective refractive indexes of the core of the certain type and the core of the other type satisfies the following condition
Δ
eff
(
%
)
>
50
(
300
+
D
)
D
(
150
+
D
)
2
.
10. A multi-core fiber according to claim 1 , wherein when, in all pairs of cores of different types with the core spacings shorter than D min , the bending radius R is 100 mm, the relative refractive index difference Δ eff between effective refractive indexes of the core of the certain type and the core of the other type satisfies the following condition
Δ
eff
(
%
)
>
50
(
200
+
D
)
D
(
100
+
D
)
2
.
11. A multi-core fiber comprising: a plurality of types of cores respectively having different effective refractive indexes in design and extending along a predetermined axis; and a cladding region integrally surrounding the plurality of types of cores, the cores being arranged such that a minimum core spacing D min between the same type of cores, servicing as a center-to-center spacing, becomes not shorter than the minimum spacing that causes no problem with crosstalk,
wherein the multi-core fiber has at least a pair of cores of different types with a core spacing shorter than D min , and
wherein at least a part of the multi-core fiber is bent at a radius R of not greater than 200 mm, and when, under the condition that a difference of equivalent refractive index of effective refractive index between cores of the different types falls within the range that causes a problem with crosstalk, the core spacing in a cross-section of the multi-core fiber orthogonal to the predetermined axis is defined as D, a relative refractive index difference Δ eff between effective refractive indexes of the core of a certain type and the core of the other type, which have the core spacing shorter than D min , satisfies the following condition
Δ
eff
(
%
)
≥
50
(
400
+
D
)
D
(
200
+
D
)
2
+
0.005
.
12. A multi-core fiber according to claim 11 , wherein when, in all pairs of cores of different types with the core spacings shorter than that causes no problem with crosstalk, the bending radius R is 150 mm, the relative refractive index difference Δ eff between effective refractive indexes of the core of the certain type and the core of the other type satisfies the following condition
Δ
eff
(
%
)
≥
50
(
300
+
D
)
D
(
150
+
D
)
2
+
0.005
.
13. A multi-core fiber according to claim 11 , wherein when, in all pairs of cores of different types with the core spacings shorter than that causes no problem with crosstalk, the bending radius R is 100 mm, the relative refractive index difference Δ eff between effective refractive indexes of the core of the certain type and the core of the other type satisfies the following condition
Δ
eff
(
%
)
≥
50
(
200
+
D
)
D
(
100
+
D
)
2
+
0.005
.
14. A multi-core fiber according to claim 1 , wherein, in the cladding region, the plurality of cores are added with a twist.
15. A multi-core fiber according to claim 1 , wherein, a relative refractive index difference Δ between the effective refractive indexes of cores of different types is given by giving a difference between the refractive index of a peripheral region around the core of the certain type and the refractive index of a peripheral region around the core of the other type.
16. A multi-core fiber according to claim 1 , wherein a relative refractive index difference Δ eff between the effective refractive indexes of cores of different types is given by configuring the plurality of types of cores of two or three types among a step index core, a W-shaped core, and a duplex core.
17. A multi-core fiber according to claim 15 , wherein a trench is added around each of the cores surrounded integrally by the cladding region.