Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2002-02-20
2004-02-03
Lee, John D. (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S024000, C385S127000, C398S037000, C398S147000, C398S148000
Reexamination Certificate
active
06687443
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical fiber transmission line suitable for high-speed transmission with a large capacity over a long distance, an optical cable including the same, and an optical transmission system including the same.
2. Related Background Art
Optical transmission systems transmit signal light including a large capacity of information over a long distance at a high speed by way of optical fiber transmission lines. Various proposals have been made in order to realize further larger capacity and longer distance in such an optical transmission system. For example, the optical transmission system disclosed in literature 1—J.-P. Blondel, et al., “Network Application and System Demonstration of WDM Systems with Very Large Spans”, OFC'2000, PD
31
(2000)—is a wavelength division multiplexing (WDM) transmission system which optically transmits a plurality of channels of signals in a wavelength division multiplexing manner, while comprising an optical fiber amplifier (EDFA: Erbium-Doped Fiber Amplifier) employing an optical fiber having an optical waveguide region doped with Er element as an optical amplifying medium, and a Raman amplifier utilizing a Raman scattering phenomenon. Also, in the optical transmission system disclosed in the above-mentioned literature 1, an optical fiber transmission line comprising an optical fiber having a low loss is laid in a repeating section. At least one of transmitting, repeating, and receiving stations is provided with an EDFA and means for supplying Raman amplification pumping light to the optical fiber transmission line. Such a configuration makes repeating sections longer in the optical transmission system disclosed in literature 1.
The optical transmission systems disclosed in literature 2—T. Naito, et al., “1 Terabit/s WDM Transmission over 10,000 km”, ECOC'98, pp. 24-25 (1988)”—and literature 3—K. Takashina, et al., “1 Tbit/s (100 ch·10 Gbit/s) WDM Repeaterless Transmission over 200 km with Raman Amplifier”, OFC'2000, FC8 (2000)—are also WDM transmission systems each comprising an EDFA and a Raman amplifier. In the optical transmission systems disclosed in literatures 2 and 3, the optical fiber transmission line laid in a repeating section is constituted by a positive dispersion optical fiber having a low transmission loss, a large effective area, and a positive chromatic dispersion, and a negative dispersion optical fiber, disposed downstream the positive dispersion optical fiber so as to compensate for the chromatic dispersion in the positive dispersion optical fiber, having a negative chromatic dispersion. At least one of transmitting, repeating, and receiving stations is provided with an EDFA and means for supplying Raman amplification pumping light to the negative dispersion optical fiber. Such a configuration restrains the waveform of signal light from deteriorating due to nonlinear optical phenomena and cumulative chromatic dispersion, whereby a larger capacity in optical transmissions and a longer distance in repeating sections are achieved.
SUMMARY OF THE INVENTION
The inventor studied the conventional optical transmission systems and, as a result, has found the following problems. Namely, as compared with long-distance optical transmission systems (e.g., a system connecting continents to each other with a submarine optical cable), medium-range optical transmission systems (e.g., a system connecting the mainland and an island to each other with a submarine optical cable) are required to further elongate their repeating sections. This is due to the fact that, in a medium-range optical transmission system connecting the mainland and an island to each other, a transmitting station, a repeating station, or a receiving station is provided only in the mainland or island, whereas there has been an increasing demand for making a non-repeating section between the mainland and island. However, there is a limit to elongation of repeating sections in each of the optical transmission systems disclosed in the above-mentioned literatures 1 to 3.
In the optical transmission system disclosed in the above-mentioned literature 1, the optical fiber transmission line is constituted by one kind of optical fiber alone, whereby it is preferred that the optical fiber have a high dopant concentration in its core region or a small effective area from the viewpoint of Raman amplification efficiency with respect to signal light in this optical fiber. However, when the dopant concentration is higher, transmission loss becomes greater due to Rayleigh scattering caused by the dopant. Also, when the effective area is smaller, nonlinear optical phenomena are more likely to occur, thereby deteriorating the waveform of signal light, thus failing to transmit signal light having a high power. Hence, there is a limit to elongation of repeating sections in the optical transmission system disclosed in literature 1.
In the optical transmission system disclosed in the above-mentioned literature 2 or 3, the optical fiber transmission line is constituted by a positive dispersion optical fiber and a negative dispersion optical fiber. In general, the negative dispersion optical fiber has a high dopant concentration in its core region, whereby its transmission loss is large due to the Rayleigh scattering caused by the dopant. Hence, there is a limit to elongation of repeating sections in the optical transmission systems disclosed in literatures 2 and 3 as well.
In order to overcome the problems mentioned above, it is an object of the present invention to provide an optical fiber transmission line comprising a structure which enables repeating sections to become further longer and can yield stable transmission characteristics even when pumping light having a higher power is supplied thereto, an optical cable including the same, and an optical transmission system including the same.
The optical fiber transmission line according to the present invention comprises first and second optical fibers successively disposed along an advancing direction of signal light, and an optical multiplexer for supplying Raman amplification pumping light to one of the first and second optical fibers. The first optical fiber comprises an entrance end for receiving signal light and an exit end for emitting the signal light, whereas the second optical fiber comprises an entrance end fusion-spliced to the exit end of the first optical fiber and an exit end for emitting the signal light, at least one of the first and second optical fibers having a core region substantially made of pure silica glass.
In particular, in this optical fiber transmission line, the first optical fiber has, as characteristics at a wavelength of 1550 nm, a first effective area A
eff1
and a first chromatic dispersion D
1
, and has a first length L1. The second optical fiber has, as characteristics at the wavelength of 1550 nm, a second effective area A
eff2
smaller than the first effective area A
eff1
and a second chromatic dispersion D
2
different from the first chromatic dispersion D
1
, and has a second length L2 different from the first length L1. The optical multiplexer is optically coupled to the entrance end of the first optical fiber so as to supply the Raman amplification pumping light to the first optical fiber together with the signal light, or optically coupled to the exit end of the second optical fiber so as to supply the Raman amplification pumping light to the second optical fiber while transmitting therethrough the signal light emitted from the second optical fiber.
Here, as shown in Japanese Patent application Laid-Open No. HEI 8-248251 (EP 0 724 171 A2), the above-mentioned effective area A
eff
is given by the following expression:
A
eff
=
2
π
(
∫
0
∞
E
2
r
ⅆ
r
)
2
/
(
∫
0
∞
E
4
r
ⅆ
r
)
where E is the electric field accompanying the propagating light, and r is the radial distance from the center of the core region.
Re
Kubo Yuji
Miyamoto Toshiyuki
Shimizu Makoto
Doan Jennifer
Lee John D.
McDermott & Will & Emery
Sumitomo Electric Industries Ltd.
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