Optical waveguides – Optical fiber waveguide with cladding – Utilizing multiple core or cladding
Reexamination Certificate
2000-05-08
2003-08-05
Mack, Ricky (Department: 2873)
Optical waveguides
Optical fiber waveguide with cladding
Utilizing multiple core or cladding
Reexamination Certificate
active
06603913
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a dispersion-compensating fiber which is applied to an optical transmission line including a 1.3-&mgr;m zero-dispersion single-mode optical fiber and improves transmission characteristics of the whole optical transmission line with respect to light in a 1.55-&mgr;m wavelength band.
2. Related Background Art
In optical fiber transmission line networks used for high-speed, large-capacity communications over a long distance, 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 a 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.
As means for compensating for the dispersion of the single-mode optical fiber, a dispersion-compensating fiber is used in general. In particular, since the transmission loss of optical fibers becomes the lowest in the 1.55-&mgr;m wavelength band, it has been desired that optical communications be carried out by use of light in the 1.55-&mgr;m wavelength band. On the other hand, a number of single-mode optical fibers having a zero-dispersion wavelength in the vicinity of 1.3 &mgr;m have been laid so far, and hence there are needs for carrying out optical communications in the 1.55-&mgr;m wavelength band by utilizing such existing single-mode optical fiber transmission line networks. In such a case, when a dispersion-compensating fiber having a negative dispersion in the 1.55-&mgr;m wavelength band is connected to a single-mode optical fiber having a positive dispersion in the 1.55-&mgr;m wavelength band, the chromatic dispersion of the whole optical transmission line can be compensated for. For example, Japanese Patent Application Laid-Open No. HEI 8-136758 and Japanese Patent Application Laid-Open No. HEI 8-313750 disclose dispersion-compensating fibers comprising a double cladding structure. On the other hand, Japanese Patent Application Laid-Open No. HEI 6-11620 discloses a dispersion-compensating fiber comprising a triple cladding structure.
SUMMARY OF THE INVENTION
The inventors have studied the above-mentioned prior art and, as a result, have found problems as follows. Namely, in an optical transmission line constituted by a single-mode optical fiber and a dispersion-compensating fiber for compensating for the chromatic dispersion of the single-mode optical fiber, chromatic dispersion cannot be all prevented from occurring in wavelengths of the wavelength band in use, but it occurs at least in a wavelength band deviated from the vicinity of the zero-dispersion wavelength. Consequently, if various characteristics of the conventional dispersion-compensating fiber are not sufficiently optimized for compensating for the chromatic dispersion of a single-mode optical fiber having a zero-dispersion wavelength near 1.3 &mgr;m (hereinafter referred to as 1.3SMF), various transmission characteristics may be caused to fluctuate among wavelengths in optical communications of wavelength division multiplexing (WDM) system, in which different wavelengths of signal light are multiplexed so as to enable the transmission capacity to further enhance, and the like.
Here, dispersion slope is given by the gradient of a graph which indicates chromatic dispersion, and is expressed by a unit of ps
m
2
/km.
In order to overcome such problems as those mentioned above, it is an object of the present invention to provide a dispersion-compensating fiber comprising a structure which compensates for the chromatic dispersion of a 1.3SMF, improves, when applied to an optical transmission system including the 1.3SMF, the total dispersion slope in the optical transmission system as a whole in a wider wavelength band, and enables dispersion to be compensated for efficiently and its size to become smaller.
The dispersion-compensating fiber according to the present invention is an optical fiber for compensating for the chromatic dispersion of a 1.3SMF, 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 outer diameter
2
a.
The cladding region is constituted by a first cladding, provided on the outer periphery of the core, having an outer diameter
2
b;
a second cladding, provided on the outer periphery of the first cladding, having an outer 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 relative refractive index difference &Dgr;
1
of the core with respect to the third cladding and the relative refractive index difference &Dgr;
2
of the first cladding with respect to the third cladding satisfy at least the conditions of 1%≦&Dgr;
1
≦3%, and &Dgr;
2
≦−0.3%. Further, in this dispersion-compensating fiber, the outer diameter
2
a
of the core and the outer diameter
2
c
of the second cladding satisfy the condition of
2
a
/
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 1.3SMF, 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 components 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
SMF
(&lgr;
m
)·
L
SMF
+D
DCF
(&lgr;
m
)·
L
DCF
|≦200 ps
m
where
D
SMF
(&lgr;m) is the dispersion of the 1.3SMF at the wavelength &lgr;m;
L
SMF
is the length of the 1.3SMF;
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 all signal wavelengths &lgr;
all
of light within the wavelength band in use:
|
D
SMF
(&lgr;
all
)·
L
SMF
+D
DCF
(&lgr;
all
)·
L
DCF
|≦200 ps/ nm
where
D
SMF
(&lgr;
all
) is the dispersion of the 1.3SMF at all the wavelengths &lgr;
all
in use;
L
SMF
is the length of the 1.3SMF;
D
DCF
(&lgr;
all
) is the dispersion of the dispersion-compensating fiber at all the wavelengths &lgr;
all
in use; and
L
DCF
is the length of the dispersion-compensating fiber.
Thus, the dispersion-compensating fiber comprising a core region having a single core and a cladding region, provided on the outer periphery of the core
Harrington Alicia M.
Mack Ricky
Sumitomo Electric Industries Ltd.
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