FIBER OPTIC CABLE
An object of the present invention is to ensure low crosstalk for all multi-core optical fibers in an optical fiber cable having a plurality of identical multi-core optical fibers of non-coupling type in different spiral shapes. An optical fiber cable according to the present disclosure is an optical fiber cable formed by twisting a plurality of units each of which is formed by bundling a plurality of identical multi-core optical fibers of non-coupling type, which includes: that all the units have the same diameter; that the units are arranged in a plurality of layers from a center in a cross section; that the units are arranged in a spiral shape for each of the layers in a longitudinal direction; that all the units included in one of the layers have the same twist pitch of the spiral shape; and that the twist pitch is different for each layer, and is set based on a curvature Rpk of the multi-core optical fiber where crosstalk is the largest.
1 . An optical fiber cable formed by twisting a plurality of units each of which is formed by bundling a plurality of identical multi-core optical fibers of non-coupling type, comprising:
that all the units have the same diameter;
that the units are arranged in a plurality of layers from a center in a cross section;
that the units are arranged in a spiral shape for each of the layers in a longitudinal direction;
that all the units included in one of the layers have the same twist pitch of the spiral shape; and
that the twist pitch is different for each layer, and is set based on a curvature Rpk of the multi-core optical fiber where crosstalk is the largest.
2 . The optical fiber cable according to claim 1 , wherein comprises:
a twist radius r 1 of the spiral shape of the unit arranged in an innermost layer being set by a Math. C1;
a curvature ρ1 of the spiral shape of the unit arranged in the innermost layer being set by a Math. C2;
a curvature ρm of the spiral shape of the unit arranged in a specific layer different from the innermost layer being set by a Math. C3; and
a twist pitch Pm of the spiral shape of the units arranged in the specific layer being set by a Math. C4,
wherein the curvature Rpk does not coincide with the curvature ρ1.
[
Math
.
C1
]
r
1
=
d
/
{
2
×
sin
(
π
/
n
)
}
(
C
1
)
[
Math
.
C2
]
ρ
1
=
r
1
/
(
r
1
2
+
P
1
/
2
π
)
2
)
(
C2
)
[
Math
.
C3
]
ρ
m
=
{
r
1
+
d
(
m
-
1
)
}
/
[
{
r
1
+
d
(
m
-
1
)
}
2
+
(
Pm
/
2
π
)
2
]
(
C3
)
[
Math
.
C4
]
Pm
=
2
π
r
1
+
d
(
m
-
1
)
·
r
1
2
+
(
P
1
2
π
)
2
-
[
r
1
+
d
(
m
-
1
)
]
2
r
1
(
C4
)
Where d represents the diameter, n represents the number of units in the innermost layer, P 1 represents a twist pitch of the spiral shape of the units arranged in the innermost layer, and m represents the number of layers from the center of the specific layer when the innermost layer assumes a first layer.
3 . The optical fiber cable according to claim 1 , wherein comprises:
a twist radius r 1 of the spiral shape of the unit arranged in an innermost layer being set by a Math. C5;
a curvature ρ1 of the spiral shape of the unit arranged in the innermost layer being set by a Math. C6;
a curvature ρm of the spiral shape of the unit arranged in a specific layer different from the innermost layer being set by a Math. C7; and
a twist pitch Pm of the unit arranged in the specific layer being set by a Math. C8, wherein the curvature Rpk is smaller than the curvature ρ1.
[
Math
.
C5
]
r
1
=
d
/
{
2
×
sin
(
π
/
n
)
}
(
C
5
)
[
Math
.
C
6
]
ρ
1
=
r
1
/
(
r
1
2
+
(
P
1
/
2
π
)
2
)
(
C
6
)
[
Math
.
C
7
]
ρ
m
=
{
r
1
+
d
(
m
-
1
)
}
/
[
{
r
1
+
d
(
m
-
1
)
}
2
+
(
Pm
/
2
π
)
2
]
(
C
7
)
[
Math
.
C
8
]
Pm
≧
2
π
r
1
+
d
(
m
-
1
)
·
r
1
2
+
(
P
1
2
π
)
2
-
[
r
1
+
d
(
m
-
1
)
]
2
r
1
(
C
8
)
Where d represents the diameter, n represents the number of units in the innermost layer, P 1 represents a twist pitch of the spiral shape of the units arranged in the innermost layer, and m represents the number of layers from the center of the specific layer when the innermost layer assumes a first layer.
4 . 4 The optical fiber cable according to claim 1 , wherein comprises:
a twist radius r 1 of the spiral shape of the unit arranged in an innermost layer being set by a Math. C9;
a curvature ρ1 of thpiral shape of the unit arranged in the innermost layer being set by a Math. C10;
a curvature ρm of the spiral shape of the unit arranged in a specific layer different from the innermost layer being set by a Math. C11; and
a twist pitch Pm of the unit arranged in the specific layer being set by a Math. C12, wherein the curvature Rpk is larger than the curvature ρ1.
[
Math
.
C9
]
r
1
=
d
/
{
2
×
sin
(
π
/
n
)
}
(
C
9
)
[
Math
.
C
10
]
ρ
1
=
r
1
/
(
r
1
2
+
(
P
1
/
2
π
)
2
)
(
C
10
)
[
Math
.
C
11
]
ρ
m
=
{
r
1
+
d
(
m
-
1
)
}
/
[
{
r
1
+
d
(
m
-
1
)
}
2
+
(
Pm
/
2
π
)
2
]
(
C
11
)
[
Math
.
C
12
]
Pm
≦
2
π
r
1
+
d
(
m
-
1
)
·
r
1
2
+
(
P
1
2
π
)
2
-
[
r
1
+
d
(
m
-
1
)
]
2
r
1
(
C
12
)
Where d represents the diameter, n represents the number of units in the innermost layer, P 1 represents a twist pitch of the spiral shape of the units arranged in the innermost layer, and m represents the number of layers from the center of the specific layer when the innermost layer assumes a first layer.
5 . The optical fiber cable according to claim 1 , wherein the twisting directions of the spiral shapes of the units arranged in the layers adjacent to each other are different from each other.