# US 11,906,387 B2 · Test method and test device for mode field diameter

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- Number: US 11,906,387 B2
- Document ID: US11906387B2
- Status: Granted
- Kind: B2
- Application number: 17/636,930
- Filed: 2019-09-10
- Published: 2022-09-01
- Granted: 2024-02-20
- Earliest claimed: 2019-09-10
- Assignees: Nippon Telegraph and Telephone Corporation
- Applicants: Nippon Telegraph and Telephone Corporation
- Inventors: Atsushi Nakamura; Daisuke Iida; Hiroyuki Oshida
- Web page: https://patentgrep.com/patent/US11906387B2

## CPC

- G01M 11/02: Testing optical properties
- G01M 11/332: {using discrete input signals (G01M11/333 takes precedence)}
- G01M 11/33: {with a light emitter being disposed at one fibre or waveguide end-face, and a light receiver at the other end-face}
- G01M: TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR

## Abstract

The purpose of the present disclosure is to provide a mode field diameter test method and test device that enable acquisition of a mode field diameter for an arbitrary higher-order mode. The present disclosure is a mode field diameter test method including: a test light incidence procedure for selectively causing test light to be incident in a mode subject to measurement, on one end of an optical fiber 10 under test; a far-field pattern measurement procedure for measuring a far-field pattern of the mode subject to measurement, with respect to a divergence angle θ at the other end of the optical fiber under test, by a far-field scanning technique; and a mode field diameter calculation procedure for calculating, using an equation, a mode field diameter from information about incident mode orders in the test light incidence procedure and the far-field pattern measured in the far-field pattern measurement procedure.

## Claims

### Claim 1

A mode field diameter test device comprising:
test light incidence means for selectively causing test light to be incident in a mode subject to measurement, on one end of an optical fiber under test;
far-field pattern measurement means for measuring a far-field pattern of the mode subject to measurement, with respect to a divergence angle θ at the other end of the optical fiber under test, by a far-field scanning technique; and
mode field diameter calculation means for calculating, using a first equation, a mode field diameter from information about incident mode orders in the optical fiber under test by the test light incidence means and the far-field pattern measured by the far-field pattern measurement procedure, or mode field diameter calculation means for calculating an electric field distribution in a near field from the far-field pattern and further calculating a mode field diameter using a second or third equation;
wherein the first equation is Equation (C4),
[
Math
.
C
⁢
4
]
MFD
=
v
+
2
⁢
μ
-
1
⁢
λ
π
⁢
2
⁢
∫
0
π
/
2
❘
"\[LeftBracketingBar]"
F
v
⁢
μ
(
θ
)
❘
"\[RightBracketingBar]"
2
⁢
sin
⁢
θ
⁢
cos
⁢
θ
⁢
d
⁢
θ
∫
0
π
/
2
❘
"\[LeftBracketingBar]"
F
v
⁢
μ
(
θ
)
❘
"\[RightBracketingBar]"
2
⁢
sin
3
⁢
θcos
⁢
θ
⁢
d
⁢
θ
(
C4
)
where ν is the order in a circumferential direction, μ is the order in a radial direction, λ represents a wavelength of the test light, θ is the divergence angle, Fνμ(θ) is an electric field distribution in a far field, and |Fνμ(θ)|2 represents the far-field pattern.

### Claim 2

A mode field diameter test device comprising:
test light incidence means for selectively causing test light to be incident in a mode subject to measurement, on one end of an optical fiber under test;
far-field pattern measurement means for measuring a far-field pattern of the mode subject to measurement, with respect to a divergence angle θ at the other end of the optical fiber under test, by a far-field scanning technique; and
mode field diameter calculation means for calculating, using a first equation, a mode field diameter from information about incident mode orders in the optical fiber under test by the test light incidence means and the far-field pattern measured by the far-field pattern measurement procedure, or mode field diameter calculation means for calculating an electric field distribution in a near field from the far-field pattern and further calculating a mode field diameter using a second or third equation;
wherein the second equation is Equation (C5),
[
Math
.
C
⁢
5
]
MFD
=
2
⁢
2
v
+
2
⁢
μ
-
1
[
∫
0
∞
E
v
⁢
μ
2
(
r
)
⁢
r
3
⁢
dr
∫
0
∞
E
v
⁢
μ
2
(
r
)
⁢
rdr
]
1
/
2
(
C5
)
where ν is the order in a circumferential direction, μ is the order in a radial direction, r is a radial coordinate in a cross section of the optical fiber, and Eνμ(r) is the electric field distribution of an LPνμ mode in the near field.

### Claim 3

A mode field diameter test device comprising:
test light incidence means for selectively causing test light to be incident in a mode subject to measurement, on one end of an optical fiber under test;
far-field pattern measurement means for measuring a far-field pattern of the mode subject to measurement, with respect to a divergence angle θ at the other end of the optical fiber under test, by a far-field scanning technique; and
mode field diameter calculation means for calculating, using a first equation, a mode field diameter from information about incident mode orders in the optical fiber under test by the test light incidence means and the far-field pattern measured by the far-field pattern measurement procedure, or mode field diameter calculation means for calculating an electric field distribution in a near field from the far-field pattern and further calculating a mode field diameter using a second or third equation;
wherein the second equation is Equation (C6),
[
Math
.
C
⁢
6
]
MFD
=
2
⁢
2
⁢
(
2
⁢
μ
-
1
)
[
∫
0
∞
E
v
⁢
μ
2
(
r
)
⁢
rdr
∫
0
∞
{
dE
v
⁢
μ
(
r
)
dr
}
2
⁢
rdr
]
1
/
2
(
C6
)
where ν is the order in a circumferential direction, μ is the order in a radial direction, r is a radial coordinate in a cross section of the optical fiber, and Eνμ(r) is the electric field distribution of an LPνμ mode in the near field.