# US 12,181,706 B2 · Multi-core optical fiber and design method

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- Number: US 12,181,706 B2
- Document ID: US12181706B2
- Status: Granted
- Kind: B2
- Application number: 17/605,758
- Filed: 2020-04-06
- Published: 2022-07-07
- Granted: 2024-12-31
- Earliest claimed: 2019-04-25
- Assignees: Nippon Telegraph and Telephone Corporation
- Applicants: Nippon Telegraph and Telephone Corporation
- Inventors: Taiji Sakamoto; Kazuhide Nakajima; Masaki Wada; Shinichi Aozasa; Takashi Yamamoto
- Web page: https://patentgrep.com/patent/US12181706B2

## CPC

- G02B 6/02042: {Multicore optical fibres}
- G02B 27/0012: {Optical design, e.g. procedures, algorithms, optimisation routines}
- G02B 6/02009: {Large effective area or mode field radius, e.g. to reduce nonlinear effects in single mode fibres}
- G02B: OPTICAL ELEMENTS, SYSTEMS OR APPARATUS

## Abstract

An object is to provide a multi-core configuration for acquiring a random mode coupling in a case of an arbitrary core refractive index. A multi-core optical fiber according to the present invention is an optical fiber in which two or more core regions are arranged in a clad region having a refractive index at a minimum core interval Λ smaller than a refractive index of the cores, a configuration of the cores is that including one propagation mode, and the core configuration and the core interval are adjusted so that an inter-mode coupling coefficient between adjacent cores is within a range from 0.73 to 120 m−1.

## Claims

### Claim 1

A multi-core optical fiber, comprising
two or more cores,
wherein each of the cores has one propagation mode in a wavelength λ of a desired communication wavelength band,
an inter-mode coupling coefficient κ between adjacent cores which is a minimum interval among the cores is κmin or more and κmax or less,
where κmin=0.73 m−1 and κmax=120 m−1, and
the inter-mode coupling coefficient κ is defined under the following conditions:
a bend radius of the multi-core optical fiber is 140 mm;
a twisting rate of the multi-core optical fiber is 4 πrad/m or more;
the wavelength λ is 1550 nm; and
a normalized frequency V is varied in a range of 2 to 2.3.

### Claim 2 (depends on 1)

The multi-core optical fiber according to claim 1,
wherein κmin=2.2 m−1 and κmax=98 m−1.

### Claim 3 (depends on 1)

The multi-core optical fiber according to claim 1,
wherein κmin=7.3 m−1 and κmax=77.1 m−1.

### Claim 4 (depends on 1)

The multi-core optical fiber according to claim 1,
wherein a minimum core interval A that is an interval between the adjacent cores is in a range satisfying Math. C1,
[
Math
.
C
⁢
1
]
⁢
a
B
⁢
ln
⁡
(
κ
min
⁢
a
A
⁢
Δ
)
>
Λ
>
a
B
⁢
ln
⁡
(
κ
max
⁢
a
A
⁢
Δ
)
⁢
A
=
-
8.7812
+
5.51
V
⁢
B
=
1.0027
-
1.188
V
⁢
V
=
a
⁢
2
⁢
n
λ
⁢
n
1
⁢
2
⁢
Δ
(
C
⁢
1
)
where a represents a radius of the cores, Δ represents a relative index difference of the cores and, n1 represents a refractive index of the cores.

### Claim 5 (depends on 1)

The multi-core optical fiber according to claim 1,
wherein a cut-off wavelength is 1.26 um or less, a bending loss is 0.1 dB/100 turn or less at a wavelength of 1625 nm and a bend radius of 30 mm, and a mode field diameter is 8.2 um or more and 9.6 um or less at a wavelength of 1310 nm.

### Claim 6 (depends on 1)

The multi-core optical fiber according to claim 1,
wherein a value of the normalized frequency V is varied by changing parameters in the following equations:
V2=u2+w2;
u=a√(K2n12−β2); and
w=a√(β2−K2n02),
where u is a normalized lateral propagation constant, w is a normalized lateral attenuation constant, a is a radius of the core, K is 2π/λ, n1 is a refractive index of the cores, β is a propagation constant of a propagation mode, and n0 is a refractive index of a clad.