Optical: systems and elements – Optical amplifier – Raman or brillouin process
Reexamination Certificate
2002-09-26
2004-09-28
Hellner, Mark (Department: 3663)
Optical: systems and elements
Optical amplifier
Raman or brillouin process
C359S341300
Reexamination Certificate
active
06798564
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a Raman amplification method, a Raman amplifier, and an optical transmission system which Raman-amplify a plurality of channels of signal light having wavelengths different from each other.
2. Related Background Art
Optical fiber amplifiers are optical devices which amplify signal light propagating through optical fiber transmission lines in optical transmission systems, in order to compensate for the transmission loss in the optical transmission lines. An optical fiber amplifier placed on an optical transmission line comprises an amplification optical fiber also functioning as an optical transmission line, and pumping light supply means for supplying pumping light to the amplification optical fiber. When fed into an amplification optical fiber supplied with pumping light, signal light is amplified in the amplification optical fiber.
Known as such an optical fiber amplifier are rare-earth-doped fiber amplifiers doped with rare earth elements such as Er (erbium), and Raman amplifiers utilizing Raman amplification phenomena caused by stimulated Raman scattering.
Here, the rare-earth-doped fiber amplifiers (e.g., EDFA: Erbium-Doped Fiber Amplifier) utilize an optical fiber doped with a rare earth element (e.g., EDF: Erbium-Doped Fiber) as an amplification optical fiber. On the other hand, the Raman amplifiers utilize a silica-type optical fiber constituting optical fiber transmission lines and the like as a Raman amplification optical fiber.
In the above-mentioned optical amplifiers, the Raman amplifiers are advantageous in that they can use a given wavelength band as an amplification wavelength band by choosing an appropriate wavelength of pumping light. If a plurality of pumping light sources respectively supplying different wavelengths of pumping light to a Raman amplification optical fiber are prepared, signal light can be amplified while using respective wavelength bands determined by the individual channel wavelengths included in the pumping light as amplification wavelength bands.
SUMMARY OF THE INVENTION
The inventors studied conventional techniques and, as a result, have found the following problems. Recently, due to social needs at the advent of highly information-oriented society, studies and developments concerning large-capacity, high-speed communications and long-haul communications have vigorously been under way. Here, a wavelength division multiplexing (WDM) transmission system carries out high-speed/large-capacity optical communications by transmitting a plurality of channels of signal light having wavelengths different from each other through an optical fiber transmission line. Also, for further increasing the capacity, the signal wavelength band of WDM signal light has been in the process of widening in the WDM transmission system.
When amplifying WDM signal light in such a WDM transmission system, it is necessary for its amplification wavelength band to become wider. In the above-mentioned Raman amplifiers, on the other hand, the amplification wavelength band obtained by a predetermined wavelength of pumping light is limited. Also, as the amplification wavelength band widens, it has been becoming difficult for an amplification wavelength band enhancing method such as one increasing the number of pumping light sources to sufficiently respond to the widening of the amplification wavelength band.
Examples of other methods proposed therefor include one comprising the steps of dividing the signal wavelength band of WDM signal light into a plurality of wavelength bands and amplifying WDM signal light in each of thus divided wavelength bands, and one amplifying WDM signal light by combining a Raman amplifier and a rare-earth-doped fiber amplifier (see, for example, Japanese Patent Application Laid-Open No. 2001-085773, Japanese Patent Application Laid-Open No. HEI 11-084440, and OFC2001, PD-24). In these methods, however, the configuration of the optical transmission system becomes complicated as a whole, thereby increasing the cost. Also, they have not fully realized signal amplification throughout the signal wavelength band.
In order to overcome the problems mentioned above, it is an object of the present invention to provide a Raman amplification method enabling Raman amplification of WDM signal light in a simpler configuration over a wider amplification wavelength band, a Raman amplifier realizing the Raman amplification method, and an optical transmission system including the Raman amplifier.
The Raman amplification method according to the present invention Raman-amplifies a plurality of channels of signal light (WDM signal light) having wavelengths different from each other. In particular, for achieving the above-mentioned object, the Raman amplification method according to the present invention Raman-amplifies a part of the WDM signal light by supplying pumping light having a wavelength &lgr;
p
to an amplification optical fiber through which the WDM signal light propagates, and utilizes at least a part of the Raman-amplified light (part of the WDM signal light) as pumping light, so as to Raman-amplify a part of WDM signal light including a channel wavelength with a wavelength of (&lgr;
p
+&Dgr;&lgr;+20 nm) or longer, where &Dgr;&lgr; is the Raman shift amount of wavelength caused by the pumping light at the wavelength &lgr;
p
.
The Raman amplifier according to the present invention Raman-amplifies a plurality of channels of signal light (WDM signal light) having wavelengths different from each other. The Raman amplifier comprises a Raman amplification optical fiber, pumping light supply means, and multiplexing means. The pumping light supply means supplies pumping light having a wavelength &lgr;
p
to the Raman amplification optical fiber. The multiplexing means guides to the Raman amplification optical fiber the pumping light from the pumping light supply means. The Raman amplification optical fiber transmits the WDM signal light therethrough, whereas pumping light having a wavelength &lgr;
p
for Raman-amplifying at least a part of the WDM signal light is supplied there to by way of the multiplexing means. In particular, the Raman amplifier utilizes at least a part of the light Raman-amplified by the pumping light having the wavelength &lgr;
p
as pumping light, thereby Raman-amplifying a part of WDM signal light including a channel wavelength with a wavelength of (&lgr;
p
+&Dgr;&lgr;+20 nm) or longer, where &Dgr;&lgr; is the Raman shift amount of wavelength caused by the pumping light at the wavelength &lgr;
p
.
For WDM signal light in a wider signal wavelength band, the above-mentioned Raman amplification method and Raman amplifier carry out Raman amplification utilizing pumping light having a wavelength &lgr;
p
, and Raman amplification utilizing a part of the WDM signal light as pumping light. This Raman-amplifies a part of WDM signal light including a channel wavelength with a wavelength of (&lgr;
p
+&Dgr;&lgr;+20 nm) or longer, thus being able to Raman-amplify WDM signal light over a wider amplification wavelength band. Also, since it is not necessary for the WDM signal light to be divided into a plurality of wavelength bands and amplified in each wavelength band, the configuration and the like as an optical amplifier are simplified.
Specifically, it is preferred that the wavelength &lgr;
p
of pumping light in the Raman amplification method (Raman amplifier) be set so as to Raman-amplify at least first signal light including a signal channel within a first wavelength band located on the shorter wavelength side of an amplification wavelength band. On the other hand, it is preferred that the Raman-amplified first signal light be utilized as pumping light so as to Raman-amplify second signal light including a signal channel within a second wavelength band located on the longer wavelength side than is the first wavelength band. As a consequence, the whole signal wavelength band is included in a wavelength band which can be amplified (ampl
Inoue Masayuki
Nishimura Masayuki
Hellner Mark
Sumitomo Electric Industries Ltd.
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