MODE CONVERSION DEVICE AND DESIGN METHOD
An objective of the present invention is to provide a mode conversion device capable of designing any coupling efficiency and full width at half maximum, and a design method therefor. According to the present invention, a mode conversion device includes a long period grating at a core of an optical fiber through which light is able to propagate in at least two propagation modes. The long period grating satisfies a relationship of Expression C1, where a full width at half maximum FWHM is a wavelength band in which the coupling efficiency is a half of coupling efficiency of mode conversion at a center wavelength, C is coupling efficiency, L c is a complete coupling length, L g is a grating length, Λ is a grating pitch, and Δβ is a propagation constant difference between the two propagation modes at the center wavelength of a mode conversion target. [ Math . C 1 ] Λ = 2 π Δβ C = sin 2 ( π 2 L g L c ) FWHM · L c = b ❘ "\[LeftBracketingBar]" d Δ β d λ ❘ "\[RightBracketingBar]" - 1 b = - 1169.3 C + 1705.6 ( C 1 )
1 . A mode conversion device comprising a long period grating at a core of an optical fiber and converting a mode of light propagating through the optical fiber from one mode to another mode,
wherein the long period grating satisfies a relationship of Expression C1,
[
Math
.
C
1
]
Λ
=
2
π
Δβ
C
=
sin
2
(
π
2
L
g
L
c
)
FWHM
·
L
c
=
b
❘
"\[LeftBracketingBar]"
d
Δ
β
d
λ
❘
"\[RightBracketingBar]"
-
1
b
=
-
1169.3
C
+
1705.6
(
C
1
)
where
a full width at half maximum FWHM is a wavelength band in which the coupling efficiency is a half of coupling efficiency of mode conversion at a center wavelength, C is coupling efficiency, L c is a complete coupling length, L g is a grating length, Λ is a grating pitch, and Δβ is a propagation constant difference between the two modes at the center wavelength of a mode conversion target.
2 . The mode conversion device according to claim 1 , further comprising a tap waveguide that is at a rear stage of the long period grating in a propagation direction of light and outputs, from a side surface of the optical fiber, light with a desired wavelength converted from one mode to another mode by the long period grating in light propagating through the core of the optical fiber.
3 . The mode conversion device according to claim 2 , wherein a set of the long period grating and the tap waveguide is vertically aligned in the optical fiber.
4 . The mode conversion device according to claim 3 , wherein the long period grating has different design parameters described in Expression C1 so that wavelengths of light converted from the one mode to the other mode are different from each other.
5 . The mode conversion device according to claim 3 , wherein the grating length L g of the long period grating is different so that a wavelength of light to be converted from the one mode to the other mode is identical and the coupling efficiency is different.
6 . A design method of determining, for converting a mode of light propagating through an optical fiber from one mode to another mode, a design parameter of a long period grating installed at a core of the optical fiber, the method comprising:
granting a core radius a (μm) of the optical fiber, a relative refractive index difference Δ (%), a center wavelength λ 0 (nm) of the light subjected to mode conversion, coupling efficiency C, and a full width at half maximum FWHM (nm);
acquiring a propagation constant difference Δβ between the two modes at the center wavelength λ 0 (nm) of a mode conversion target and a wavelength differential dΔβ/dλ through mode analysis from the core radius a (μm) and the relative refractive index difference Λ (%) of the optical fiber;
calculating a grating pitch Λ of the long period grating in Expression C2;
calculating a coefficient b in Expression C3;
calculating a complete coupling length L c with Expression C4; and
calculating a grating length L g in Expression C5,
[
Math
.
C
2
]
Λ
=
2
π
Δβ
(
C
2
)
[
Math
.
C
3
]
b
=
-
1169.3
C
+
1705.6
(
C
3
)
[
Math
.
C
4
]
L
c
=
b
FWHM
·
❘
"\[LeftBracketingBar]"
d
Δ
β
d
λ
❘
"\[RightBracketingBar]"
(
C
4
)
where, the full width at half maximum FWHM is a wavelength band in which the coupling efficiency is a half of coupling efficiency of mode conversion at the center wavelength λ 0 ,
[
Math
.
C
5
]
L
g
=
π
2
L
c
arc
sin
(
C
)
(
C
5
)