Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2001-06-13
2002-06-04
Kim, Robert H. (Department: 2882)
Optical waveguides
Optical fiber waveguide with cladding
Reexamination Certificate
active
06400877
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a negative-dispersion optical fiber for compensating for chromatic dispersion of a positive-dispersion optical fiber having positive chromatic dispersion in a signal wavelength band, and to an optical transmission line incorporating the same.
2. Related Background Art
An optical transmission system transmits signals of multiple channels through an optical transmission line consisting of optical fibers to enable long-haul and large-capacity communication. Silica based optical fibers commonly applied to the optical transmission lines exhibit their minimum transmission loss near the wavelength of 1.55 &mgr;m. On the other hand, an Er-doped optical fiber amplifier (EDFA: Erbium-Doped Fiber Amplifier) capable of amplifying signals near the wavelength of 1.55 &mgr;m is available for practical use as optical amplification means. For this reason, C-band (1530 nm to 1560 nm) is mainly utilized as a signal wavelength band.
Since EDFA capable of amplifying signals near the wavelength of 1.58 &mgr;m was also developed recently, L-band (1570 nm to 1610 nm) is also now being utilized as a signal wavelength band. For realizing much larger capacity transmission, use of S-band (1450 nm to 1530 nm) as a signal wavelength band is also under research.
In addition, wavelength division multiplexing (WDM) optical transmission systems are systems that transmit multiplexed signals of multiple channels included in the foregoing S-band, C-band, or L-band and that enable large-capacity information transmission. Concerning such WDM optical transmission systems, there are needs for further increase of information content and this demands to maintain absolute values of chromatic dispersion small across a wider wavelength band throughout the entire optical transmission line.
However, the optical fibers normally applied to the optical transmission lines have positive chromatic dispersion and the positive sign of dispersion slope in either of the S-band, C-band, and L-band. For example, a standard single-mode optical fiber having the zero dispersion wavelength near the wavelength of 1.3 &mgr;m has the chromatic dispersion of about +16 ps
m/km to +21 ps
m/km at the wavelength of 1.55 &mgr;m. A non-zero dispersion-shifted optical fiber (NZ-DSF) having the zero dispersion wavelength near the wavelength of 1.55 &mgr;m has the chromatic dispersion of about +2 ps
m/km to +12 ps
m/km at the wavelength of 1.55 &mgr;m. These single-mode optical fiber and non-zero dispersion-shifted optical fiber both have positive dispersion slopes in the S-band, C-band, and L-band.
When an optical transmission line is constructed by applying only optical fibers with positive chromatic dispersion (hereinafter referred to as positive-dispersion optical fibers) as described above, the optical transmission line has large cumulative chromatic dispersion. This leads to degradation of waveforms of signals and it thus becomes hard to implement long-haul and large-capacity optical transmission. Therefore, application of optical fibers with negative chromatic dispersion (hereinafter referred to as negative-dispersion optical fibers) is under study in order to compensate for the chromatic dispersion of the positive-dispersion optical fibers (e.g., Japanese Patent Applications Laid-Open No. H6-11620, H8-136758, H8-313750, and so on).
SUMMARY OF THE INVENTION
The inventors investigated the above-mentioned prior arts and found the following problem. Namely, it is generally known that the negative-dispersion optical fibers have larger transmission losses than the positive-dispersion optical fibers. Therefore, in the case of using long negative-dispersion optical fibers, there is a problem such that the transmission loss becomes large. According to the knowledge of the inventors, the optical transmission line composed of the positive-dispersion and negative-dispersion optical fibers has such a tendency that average chromatic dispersion on the whole of the optical transmission line is 0 near the zero dispersion wavelength but absolute values of chromatic dispersion increase with deviation from the zero dispersion wavelength. Since the conventional transmission lines had the large deviation of chromatic dispersion in the signal wavelength band as described, there was a limit to implementation of long-haul and large-capacity WDM optical transmission.
The present invention has been accomplished in order to solve the foregoing problem and an object of the invention is to provide a negative-dispersion optical fiber that can compensate in a short length for the chromatic dispersion of the positive-dispersion optical fiber in the signal wavelength band, and an optical transmission line incorporating it and enabling long-haul and large-capacity WDM optical transmission.
In order to accomplish the above object, a negative-dispersion optical fiber according to the present invention has the following properties at the wavelength of 1550 nm: chromatic dispersion D of not more than −150 ps
m/km and more preferably not more than −180 ps
m/km; a dispersion slope S satisfying such a condition that a ratio thereof to the chromatic dispersion D, (S/D), is not less than 2.0×10
−3
m nor more than 4.7×10
−3
m; and an effective area of not less than 12 &mgr;m
2
but less than 25 &mgr;m
2
and more preferably less than 20 &mgr;m
2
. Another negative-dispersion optical fiber according to the present invention may have the following properties at the wavelength of 1550 nm: chromatic dispersion D of not more than −200 ps
m/km; and a dispersion slope S satisfying such a condition that a ratio thereof to the chromatic dispersion D, (S/D), is not less than 2.0×10
−3
m nor more than 4.7×10
−3
m.
Since the negative-dispersion optical fiber has the small chromatic dispersion D (the sign of which is negative and the absolute value of which is large) as described above, an optical transmission line composed of a positive-dispersion optical fiber and the negative-dispersion optical fiber can be constructed at a small ratio of the length of the negative-dispersion optical fiber. This suppresses increase of transmission loss due to insertion of the negative-dispersion optical fiber in the optical transmission line and enables construction of the optical transmission line at low cost. Since the foregoing ratio (S/D) is not less than 2.0×10
−3
m nor more than 4.7×10
−3
m, a dispersion slope compensation rate becomes approximately 60% to 140%, whereby it is feasible to make small both respective absolute values of average chromatic dispersion and average dispersion slope on the whole of the optical transmission line and make small the deviation (maximum−minimum) of average chromatic dispersion among wavelengths on the whole of the optical transmission line in the signal wavelength band. The effective area of not less than 12 &mgr;m
2
is equivalent to or larger than those of the conventional negative-dispersion optical fibers and can effectively restrain the nonlinear optical phenomena. The effective area of less than 25 &mgr;m
2
and more preferably less than 20 &mgr;m
2
can effectively restrain increase of loss in the negative-dispersion optical fiber even in a cabled form as a bundle of optical fibers or in a modularized form as wound in coil shape.
The effective area A
eff
is given by the following equation, as described in Japanese Patent Application Laid-Open No. H8-248251 (EP 0 724171A2).
A
eff
=
2
π
(
∫
0
∞
E
2
r
ⅆ
r
)
2
/
(
∫
0
∞
E
4
r
ⅆ
r
)
In this equation, E represents an electric field caused by propagating light and r a radial distance from the center of the core.
In the negative-dispersion optical fiber according to the present invention, the ratio (S/D) of the dispersion slope S to the chromatic dispersion D is not less than 2.7×10
−3
m n
Hirano Masaaki
Kato Takatoshi
Gemmell Elizabeth
Kim Robert H.
McDermott & Will & Emery
Sumitomo Electric Industries Ltd.
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