Optical waveguides – Optical fiber waveguide with cladding – Utilizing multiple core or cladding
Reexamination Certificate
2000-05-30
2002-12-31
Sanghavi, Hemang (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
Utilizing multiple core or cladding
C385S123000
Reexamination Certificate
active
06501892
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a dispersion-compensating fiber which is employed to an optical transmission line including a dispersion-shifted fiber capable of long-haul, large-capacity optical communications utilizing wavelength division multiplexing (WDM) signals in a 1.5-&mgr;m wavelength band or 1.6-&mgr;m wavelength band and compensates for the dispersion of the dispersion-shifted fiber.
2. Related Background Art
In optical fiber transmission line networks used for high-speed, large-capacity communications over a long haul, the dispersion (chromatic dispersion) expressed by the sum of the material dispersion (dispersion caused by the wavelength dependence of refractive index inherent in the material of the optical fiber) and structural dispersion (dispersion caused by the wavelength dependence of the group velocity in the propagation mode) in the single-mode optical fiber employed as their transmission medium is a limiting factor for the transmission capacity. Namely, even when light outputted from a light source is assumed to have a single wavelength, it has an uniform spectrum width in the strict sense. When such an optical pulse propagates through a single-mode optical fiber having a predetermined chromatic dispersion characteristic, the pulse form may collapse since the propagation velocity varies among definite spectral components. This dispersion is expressed by a unit (ps/km
m) of propagation delay time difference per unit spectrum width (nm) and unit optical fiber length (km). Also, it has been known that the material dispersion and structural dispersion cancel each other in the single-mode optical fiber, so that the dispersion becomes zero in the vicinity of 1.31 &mgr;m.
A dispersion-shifted fiber is an optical fiber whose zero-dispersion wavelength has been shifted from a 1.3-&mgr;m wavelength band to a 1.55-&mgr;m wavelength band since the transmission loss of optical fibers becomes the lowest in the 1.55-&mgr;m wavelength band; and a dispersion-compensating fiber is used as means for compensating for the dispersion of the dispersion-shifted fiber in general. As a technique for compensating for such a dispersion-shifted fiber, Japanese Patent Application Laid-Open No. HEI 10-39155 discloses a dispersion-compensating fiber and an optical transmission system using the same, for example.
Though being designed such that its dispersion becomes zero at a predetermined wavelength near the wavelength of 1.55 &mgr;m, the dispersion-shifted fiber has a positive dispersion slope, thus being hard to suppress the occurrence of chromatic dispersion over the whole wavelength band in use. As a consequence, in communications based on the wavelength division multiplexing (WDM) system, which multiplexes light signals having wavelengths different from each other and thereby enables the transmission capacity to further increase, and the like, various transmission characteristics may deviate among wavelengths. For this matter, the above-mentioned dispersion-compensating fiber disclosed in Japanese Patent Application Laid-Open No. HEI 10-39155 is configured so as to improve the dispersion slope of the optical transmission line including the dispersion-shifted fiber as well.
Here, the dispersion slope is given by the gradient of a graph which indicates chromatic dispersion, and is expressed by a unit (ps
m
2
/km).
SUMMARY OF THE INVENTION
The inventors have studied the conventional dispersion-compensating fiber and, as a result, have found problems as follows. Namely, the conventional dispersion-compensating fiber aimed at compensating for the dispersion of the dispersion-shifted fiber has a small chromatic dispersion, thereby necessitating a longer fiber length for compensating for the dispersion of the dispersion-shifted fiber. As a consequence, the efficiency of dispersion compensation has been low, and the apparatus and the like have inevitably increased their size. In addition, various characteristics of the conventional dispersion-compensating fiber applied to the dispersion-shifted fiber to be compensated for have not fully been optimized, and the dispersion slope in the optical transmission line to which the dispersion-shifted fiber is applied has not been improved sufficiently.
In order to overcome the foregoing problems, it is an object of the present invention to provide a dispersion-compensating fiber comprising a structure which compensates for the dispersion of a dispersion-shifted fiber, improves the total dispersion slope of an optical transmission system including the dispersion-shifted fiber over a wider wavelength band when applied thereto, and enables the dispersion to be compensated for efficiently and the apparatus to become smaller.
The dispersion-compensating fiber according to the present invention guides light signals of a 1.5-&mgr;m wavelength band or 1.6-&mgr;m wavelength band. It is an optical fiber for compensating for the dispersion of a dispersion-shifted fiber having a zero-dispersion wavelength in the 1.5-&mgr;m wavelength band, and comprises a core region extending along a predetermined reference axis and a cladding region provided on the outer periphery of the core region. The core region is constituted by a core having an outside diameter
2
a
. The cladding region is constituted by a first cladding provided on the outer periphery of the core and having an outside diameter
2
b
; a second cladding provided on the outer periphery of the first cladding and having an outside diameter
2
c
; and a third cladding provided on the outer periphery of the second cladding.
In this dispersion-compensating fiber, the refractive index n
1
of the core, the refractive index n
2
of the first cladding, the refractive index n
3
of the second cladding, and the refractive index n
4
of the third cladding satisfy the condition of n
1
>n
3
>n
4
>n
2
. Also, the respective relative refractive index differences &Dgr;
1
and &Dgr;
2
of the core and first cladding with respect to the third cladding satisfy at least the conditions of 1%≦&Dgr;
1
≦3%, and &Dgr;
2
≦−0.4%. The outside diameter
2
a
of the core and the outside diameter
2
c
of the second cladding preferably satisfy the condition of
2
a
/
2
c
≦0.3.
In this dispersion-compensating fiber, it is preferably that the relative refractive index difference &Dgr;
3
of the second cladding with respect to the third cladding satisfies the condition of &Dgr;
3
≦0.25%, further preferably &Dgr;
3
≦0.15%. Additionally, the outside diameter
2
b
of the first cladding and the outside diameter
2
c
of the second cladding preferably satisfy the condition of
2
b
/
2
c
≦0.3.
The dispersion-compensating fiber according to the present invention is characterized in that, when constituting an optical transmission system together with a dispersion-shifted fiber through which light signals of the 1.5-&mgr;m wavelength band or 1.6-&mgr;m wavelength band propagate, it has a length sufficient for the optical transmission system to yield a total dispersion slope of −0.024 ps
m
2
/km or more but 0.024 ps
m
2
/km or less with respect to respective light signals having a shortest wavelength &lgr;
S
and a longest wavelength &lgr;
L
in signal wavelengths within the wavelength band in use.
Specifically, the dispersion-compensating fiber has a length L
DCF
which is set so as to satisfy the following condition with respect to light having a wavelength &lgr;
m
in signal wavelengths within the wavelength band in use:
|
D
DSF
(&lgr;
m
)·
L
DSF
+D
DCF
(&lgr;
m
)·
L
DCF
|≦200 ps
m
where
D
DSF
(&lgr;m) is the dispersion of the dispersion-shifted fiber at the wavelength &lgr;m;
L
DSF
is the length of the dispersion-shifted fiber;
D
DCF
(&lgr;m) is the dispersion of the dispersion-compensating fiber at the wavelength &lgr;m; and
L
DCF
is the length of the dispersion-compensating fiber.
More preferably, the length L
DCF
of the dispersion-compensating fiber is set so as to satisfy the following condition with respect to
Ishikawa Shinji
Kashiwada Tomonori
Okuno Toshiaki
Knauss Scott A
Sanghavi Hemang
Sumitomo Electric Industries Ltd.
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