Multicore optical fiber and method for manufacturing multicore optical fiber
A multicore optical fiber ( 1 ) includes a plurality of cores ( 11 to 16 ) and a cladding ( 20 ) surrounding the outer circumferential surfaces of the cores ( 11 to 16 ). In the plurality of cores of the multicore optical fiber ( 1 ), a skew value (S) between a pair of cores is expressed by a predetermined expression. The multicore optical fiber ( 1 ) is bent in a specific bending direction, in which in all of the combinations of the pairs of cores in the plurality of cores, the pair of cores has the maximum absolute value of the skew value found by the expression and the skew value of the pair of cores is a minimum value.
1. A multicore optical fiber comprising:
a plurality of cores; and
a cladding surrounding outer circumferential surfaces of the cores, wherein
the plurality of cores is straight when the cladding is straight,
in the plurality of cores, a skew value S for each pair of cores is expressed by an expression below,
the multicore optical fiber includes a specific pair of cores having a maximum absolute value of the skew value S among the skew values of all of combinations of the pairs of cores in the plurality of cores, the absolute value of the skew value S of the specific pair of the cores decreasing according to a bending direction of the multicore fiber, and
the multicore optical fiber is bent in a specific bending direction such that the absolute value of the skew value S of the specific pair of cores decreases to a minimum value,
S
=
L
c
[
(
N
1
m
-
N
1
n
)
+
F
R
b
(
G
m
-
G
n
)
+
1
R
b
(
G
m
N
1
m
-
G
n
N
1
n
)
+
F
R
b
2
(
G
m
2
-
G
n
2
)
]
F
=
-
B
2
C
12
-
B
1
(
C
12
+
C
11
)
G
i
=
x
i
cos
θ
-
y
i
sin
θ
C
11
=
E
(
1
-
v
)
/
[
(
1
+
v
)
(
1
-
2
v
)
]
C
12
=
Ev
/
[
(
1
+
v
)
(
1
-
2
v
)
]
where, i is m or n, and when an axis from a center of the cladding in a predetermined radial direction is defined as an x-axis and a radial direction orthogonal to the x-axis is defined as a y-axis, θ is an angle formed by a bending direction and the x-axis, (x m , y m ) are coordinates of a first core of the pair of cores, (x n , y n ) are coordinates of a second core of the pair of cores, L is a length of the multicore optical fiber, c is a velocity of light in a vacuum, N 1m , is a group refractive index of the first core of the pair of cores, N 1n is a group refractive index of the second core of the pair of cores, R b is a bending radius, B 1 is a photoelastic coefficient of the core to an ordinary ray, B 2 is a photoelastic coefficient of the core to an extraordinary ray, E is Young's modulus of core, and ν is Poisson's ratio of core.
2. The multicore optical fiber according to claim 1 , wherein
the multicore optical fiber is easily bent in the specific bending direction.
3. The multicore optical fiber according to claim 1 , further comprises a mark indicating the specific bending direction in which the multicore optical fiber is bent, the mark being provided in the specific bending direction.
4. The multicore optical fiber according to claim 3 , wherein
the mark is located in the cladding, and is a marker having a refractive index different from a refractive index of the cladding.