Evaluation method and evaluation device for multi-core fiber
An object of the present invention is to provide an evaluation method and an evaluation device for easily determining whether or not a structural parameter of a multi-core fiber satisfies a desired connection loss value (a specification). The evaluation method according to the present invention includes a step of measuring center coordinates of each core with the center coordinates when a clad is approximated by a circle as an origin in an observed cross-sectional structure of the multi-core fiber to be objected, obtaining a length of a line segment connecting the origin and the center of each core and an angle formed by two line segments connecting the origin and two adjacent cores, and judging whether or not a desired connection loss characteristic is satisfied on the basis of whether or not the values satisfy a predetermined determination formula.
1. An evaluation method of a multi-core fiber in which N-pieces of cores (N is an integer of 3 or more) are arranged in an N-angular shape at a distance ∧ in a cross section, comprising:
obtaining a cross-sectional image of the multi-core fiber with a camera;
from the cross-sectional image, approximating, by a computer processor, a clad of the multi-core fiber with a circle;
from the cross-sectional image, measuring, by the computer processor, center coordinates of each of the cores in the clad with center coordinate of the circle as an origin; and
judging, by the computer processor, that a desired connection is obtained, when a length R i of each line segment connecting the origin and the center coordinate of the core (i is a number of the core and is a natural number equal to or less than N) satisfies a range of r s −r d ≤Ri≤r s +r d ; and an angle θ i-j formed by line segments of the adjacent cores (j is the number of the core adjacent to the core of number i, and is a natural number equal to or less than N) satisfies a range of θ s −θ d ≤θ i-j ≤θ s +θ d ;
where
[
Math
.
1
]
θ
s
[
deg
]
=
360
N
,
(
1
)
[
Math
.
2
]
r
s
=
Λ
2
cos
(
90
-
θ
s
2
)
,
(
2
)
[
Math
.
3
]
η
=
{
2
W
1
W
2
W
1
2
+
W
2
2
}
2
exp
[
-
2
d
2
W
1
2
+
W
2
2
]
,
(
3
)
[
Math
.
4
]
r
d
=
-
w
i
2
+
w
j
2
2
ln
[
η
{
2
w
i
w
j
w
i
2
+
w
j
2
}
2
]
,
(
4
)
[
Math
.
5
]
θ
d
=
2
*
tan
-
1
(
r
d
r
s
)
,
(
5
)
d is an axial deviation amount with the multi-core fiber of a connection partner; and W i is a mode field diameter (MFD) of the core of number i at a desired wavelength.
2. An evaluation method of a multi-core fiber in which N-pieces of cores (N is an integer of 3 or more) are arranged in an N-angular shape at a distance ∧ in a cross section, comprising:
obtaining a cross-sectional image of the multi-core fiber with a camera;
from the cross-sectional image, approximating, by a computer processor, a clad of the multi-core fiber with a circle;
from the cross-sectional image, measuring, by the computer processor, center coordinates of each of the cores in the clad with center coordinate of the circle as an origin; and
judging, by the computer processor, that a desired connection is obtained, when a deviation amount δ∧ x between a design center coordinate which is a design center coordinate of the core calculated by Math. 6 and the measured center coordinate satisfies δ∧ x ≤r d ;
[
Math
.
6
]
δΛ
x
=
2
r
s
{
r
s
+
r
-
(
r
s
+
r
)
cos
θ
}
+
r
2
(
6
)
where,
r=R i −r s ,
θ=θ i-j −θ s
R i represents a length of each line segment connecting the origin and the center coordinate of the core (i is a number of the core, and is a natural number equal to or less than N),
θ i-j represents an angle formed by the line segments of the adjacent cores (j is the number of the core adjacent to the core of number i, and is a natural number equal to or less than N)
where
[
Math
.
1
]
θ
s
[
deg
]
=
360
N
,
(
1
)
[
Math
.
2
]
r
s
=
Λ
2
cos
(
90
-
θ
s
2
)
,
(
2
)
[
Math
.
3
]
η
=
{
2
W
1
W
2
W
1
2
+
W
2
2
}
2
exp
[
-
2
d
2
W
1
2
+
W
2
2
]
,
(
3
)
[
Math
.
4
]
r
d
=
-
w
i
2
+
w
j
2
2
ln
[
n
{
2
w
1
w
j
w
1
2
+
w
j
2
}
2
]
,
(
4
)
d is an axial deviation amount with the multi-core fiber of a connection partner; and W i is a mode field diameter (MFD) of the core of number i at a desired wavelength.
3. An evaluation device that evaluates whether or not the multi-core fiber has a desired connection loss characteristic, comprising:
a camera configured to perform a cross-sectional observation of the multi-core fiber; and
a processor configured to perform the evaluation method of the multi-core fiber according to claim 1 .
4. The evaluation method of claim 1 further comprises cutting the multi-core fiber and polishing an exposed surface of the multi-core fiber prior to obtaining the cross-sectional image.