Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2001-03-09
2004-08-31
Font, Frank G. (Department: 2877)
Optical waveguides
Optical fiber waveguide with cladding
C385S126000, C385S127000
Reexamination Certificate
active
06785453
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a dispersion shifted optical fiber designed so that the chromatic dispersion value in a 1.55 &mgr;m wavelength band where the loss of quartz based optical fiber is the smallest is substantially zero, and particularly to a dispersion shifted optical fiber having a large effective core area and a small dispersion slope
BACKGROUND ART
Conventionally, in a long distance system such as an optical amplification relay transmission system etc. that uses an optical fiber amplifier, in order to suppress deterioration in the transmission characteristics, it is vital that non-linear optical effects be reduced.
The size of the non-linear effect is represented by n
2
/Aeff. Here, n
2
is the non-linear refractive index of an optical fiber and Aeff is the effective core area of the optical fiber. Accordingly, in order to reduce the non-linear effect, it is necessary to either reduce n
2
or to increase the effective core area. However, because the value of n
2
does not change greatly once the material thereof has been decided, the more effective method is to expand the effective core area for reducing the non-linear optical effect.
In a wavelength multiplex transmission system that is capable of large volume transmission, however, in order to transmit a plurality of optical signals having different wavelengths using a single dispersion shifted optical fiber (i.e. as a transmission path), there are increased demands for a reduction in the dispersion slope. The dispersion slope shows the wavelength dependency of the chromatic dispersion values and is a gradient of the curve when the chromatic dispersion values are plotted when the horizontal axis is set as the wavelength (nm) and the vertical axis is set as the chromatic dispersion values (ps/km·nm).
The larger the dispersion slope of the dispersion shifted optical fiber, the larger the difference in the chromatic dispersion value between a plurality of wavelengths transmitted along one dispersion shifted optical fiber, the more irregular the transmission state, and the more the transmission characteristics are deteriorated. A large dispersion slope is therefore undesirable.
Further, the minimum conditions required for a transmission path for an optical communication system are that it is single mode essentially and that bending loss be held to 100 dB/m or less.
Therefore, recently, proposals intended to achieve a reduction in the dispersion slope and an enlargement in the effective core area using various refractive index distribution configurations (refractive index profiles) have been made in, for example, Japanese Patent Unexamined Application (JP-A) Nos. 10-293225, 10-239550, 11-119046, and the like.
FIGS.
5
(
a
) to
5
(
d
) show examples of the refractive index profile of such dispersion shifted optical fibers.
FIG.
5
(
a
) shows an example of a step type (dual shape core type) of refractive index profile. A core
14
is formed provided with a central core portion
11
and a step core portion
12
provided at the outer periphery of the central core portion
11
and having a lower refractive index than the central core portion
11
. In addition, cladding
16
having a lower refractive index than the step core portion
12
is provided at the outer periphery of the core
14
.
FIG.
5
(
b
) shows an example of a segment core type of refractive index profile. A core
24
is formed provided with a central core portion
21
having a high refractive index and an intermediate portion
22
having a low refractive index at the outer periphery of the central core portion
21
. A ring core portion
23
having a lower refractive index than the central core portion
21
and a higher refractive index than the intermediate portion
22
is further provided at the outer periphery of the intermediate portion
22
. In addition, cladding
26
having a refractive index equal to the intermediate portion
22
is provided at the outer periphery of the ring core portion
23
.
FIG.
5
(
c
) is a variant example of the step type refractive index profile shown in FIG.
5
(
a
). A core
14
is formed with a low refractive index portion
15
, which has a lower refractive index than the cladding
16
, provided between the step core portion
12
and the cladding
16
.
FIG.
5
(
d
) is a variant example of the segment core type refractive index profile shown in FIG.
5
(
b
). A core
24
is formed with a low refractive index portion
25
, which has a lower refractive index than the cladding
26
, provided between the ring core portion
23
and the cladding
26
.
However, in conventionally proposed dispersion shifted optical fibers under the preconditions that essentially the transmission path be single mode and that bending loss be held to 100 dB/m or less, it has been difficult to achieve both an enlargement in the effective core area and a reduction in the dispersion slope.
For example, in the examples having the refractive index profiles shown in FIGS.
5
(
a
) to
5
(
d
), if an attempt is made to increase the effective core area by 90 &mgr;m
2
or more it is not possible to be dispersion slope enough small.
The present invention was conceived in view of the above and it is an object thereof to provide a dispersion shifted optical fiber that fulfills the conditions of being essentially single mode and having bending loss of less than 100 dB/m, and also enables sufficient enlargement of the effective core area and sufficient reduction of the dispersion slope to be achieved.
DISCLOSURE OF INVENTION
In order to achieve the above objects, the first aspect of the present invention is a dispersion shifted optical fiber comprising a core and cladding provided at an outer periphery of the core, and having a refractive index profile in which the core is provided with a central core portion and a peripheral core portion having a higher refractive index than the central core portion provided at an outer periphery of the central core portion, and the cladding has a lower refractive index than the peripheral core portion, wherein, in a used wavelength band selected from 1490 to 1625 nm, the dispersion shifted optical fiber has an effective core area of 45 to 130 &mgr;m
2
, a chromatic dispersion value of −8 to +8 ps/km
m, a dispersion slope of 0.12 ps/km
m
2
or less, a bending loss of 100 dB/m or less, and a cutoff wavelength that provides essentially single mode propagation.
The second aspect of the present invention is the dispersion shifted optical fiber according to the first aspect, wherein a core comprises a central core portion and a peripheral core portion, and if &Dgr;
1
is a relative refractive index difference of the central core when the refractive index of the cladding is taken as a reference, and &Dgr;
2
is a relative refractive index difference of the peripheral core portion when the refractive index of the cladding is taken as the reference, and r
1
is a radius of the central core portion and r
2
is a radius of the peripheral core portion, then the following conditions a to d are fulfilled:
a: 1.35%≦(&Dgr;
2
−&Dgr;
1
)
b: 1.2≦r
2
/r
1
≦2.0
c: &Dgr;
2
≧0.8% and 1.8≦&Dgr;
2
×(r
2
/r
1
)
2
≦3.5
d: When the larger one from out of 0.8 or 1.8/(r
2
/r
1
)
2
is set as &Dgr;
2
min, and a ring inner volume is set as −&Dgr;
1
×r
1
2
, and a ring outer volume is set as &Dgr;
2
min×(r
2
2
−r
1
2
), then the ring outer volume/ring inner volume is either 2 or more, or 0 or less.
The third aspect of the present invention is the dispersion shifted optical fiber according to the second aspect, wherein, in a used wavelength band selected from 1490 to 1625 nm, the dispersion shifted optical fiber has an effective core area of 75 to 130 &mgr;m
2
, a chromatic dispersion value of −6 to +6 ps/km
m, a dispersion slope of 0.12 ps/km
m
2
or less, a bending loss of 100 dB/m or less, and a cutoff wavelength that provides essentially single mode propagation.
The fourth aspect of the present invention is the dispersion shifted optical fiber according to
Matsuo Shoichiro
Tanigawa Shoji
Bell Boyd & Lloyd LLC
Font Frank G.
Fujikura Ltd.
Mooney Michael P.
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