1. Field of the Invention
The present invention relates to a Raman amplifier for wideband Raman amplification of wavelength division multiplexed (VDM) signal lights.
2. Description of the Related Art
In a wavelength division multiplexing (WDM) optical communication system, a plurality of signal lights at different wavelengths are multiplexed together, and then transmitted through a single optical fiber operating as an optical transmission line. Such a WDM optical communication system has a very high capacity for information transmission, and is currently being used as a basic technology for backbone networks supporting Internet services.
To transmit signal lights over long distances, optical amplifiers are positioned along the transmission line, to amplify the multiplexed signal lights. Such optical amplifiers include rare-earth element dope optical fiber amplifiers, such as erbium doped optical fiber amplifiers (EDFA), and Raman amplifiers.
In a WDM optical communication system using an optical amplifier, the maximum limit of optical transmission power is restricted by the degradation of light waveform and cross-talk caused by nonlinear optical effects in the transmission line.
Moreover, the minimum limit of optical transmission power is restricted by the deterioration of optical signal-to-noise ratio (SNR) caused by optical noise, e.g. amplitude stimulated emission light, generated by the optical amplifier.
For a high-capacity WDM optical communication system which multiplexes tens to hundreds of signal lights together, the use of Raman amplifiers together with EDFAs installed at intervals of several tens of kilometers is regarded as a promising technology to compensate for loss in the transmission line by reducing nonlinear optical effects and deterioration of optical SNR.
Various combinations of Raman amplifiers with EDFAs have been proposed, but such proposals do not consider the situation where the number of multiplexed signal lights at different wavelengths is further increased to thereby increase capacity of the system.
Accordingly, it is an object of the present invention to provide a Raman amplifier which provides a wide gain band for use in a WDM optical communication system.
It is also an object of the present invention to provide a Raman amplifier which provides a wide gain band and is employed in combination with EDFAs in a WDM optical communication system.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.
The foregoing objects of the present invention are achieved by providing an apparatus including (a) a first optical amplifier amplifying a signal light in accordance with a plurality of excitation lights at different wavelengths provided to the first optical amplifier; (b) a band divider dividing the amplified signal light into a plurality of divided signals in a plurality of different wavelength bands, respectively; and (c) a second optical amplifier having a gain in a respective wavelength band of the plurality of wavelength bands, and amplifying the divided signal in the respective wavelength band.
Objects of the present invention are also achieved by providing an apparatus including (a) a first optical amplifier Raman amplifying a signal light in both a first wavelength band and a second wavelength band in accordance with a plurality of excitation lights provided to the first optical amplifier, the first and second wavelength bands being different from each other; (b) a divider dividing the amplified signal light into first and second lights in the first and second wavelength bands, respectively; (c) a second optical amplifier amplifying the first light and having a gain band in the first wavelength band; and (d) a third optical amplifier amplifying the second light and having a gain in the second wavelength band.
Moreover, objects of the present invention are achieved by providing an apparatus including (a) an optical fiber; (b) an excitation light causing a Raman gain to occur in the optical fiber; and (c) a gain equalizer compensating a gain characteristic of the Raman gain.
Objects of the present invention are achieved by providing an apparatus including (a) an optical fiber; (b) a first coupler coupling excitation lights from a plurality of excitation light sources together; (c) a second coupler providing the coupled excitation lights to the optical fiber, the coupled excitation lights provided to the optical fiber causing a Raman gain to occur in the optical fiber; and (d) an optical device optically connected between the first and second couplers and preventing light from the optical fiber from traveling to the excitation light sources.
In addition, objects of the present invention are achieved by providing a method of optical amplifying including (a) Raman amplifying a light with a first optical amplifier in accordance with excitation light provided to the first optical amplifier, the first optical amplifier having a corresponding Raman wavelength band of gain; (b) amplifying amplified light of the first optical amplifier with a second optical amplifier having a corresponding wavelength band of gain; and (c) expanding the Raman wavelength band of gain of the first optical amplifier when adding a third optical amplifier having a corresponding wavelength band of gain different than the wavelength band of gain of the second optical amplifier, the third optical amplifier being added in parallel with the second optical amplifier to amplify amplified light of the first optical amplifier.
Objects of the present invention are also achieved by providing an apparatus including (a) a first optical amplifier Raman amplifying a light in accordance with a plurality of excitation lights provided to the first optical amplifier; and (b) a second optical amplifier the receiving the amplified light and amplifying the received light, wherein the first optical amplifier controls a power level of the amplified light as received by the second optical amplifier to be constant.
Further, objects of the present invention are achieved by providing an apparatus including (a) an optical amplification medium through which light travels, the optical amplification medium including, or being optically connected to, a transmission line through which the light travels; (b) excitation light sources providing excitation lights; (c) a coupler coupling the excitation lights to the optical amplification medium, the coupled f excitation lights causing the light traveling through the optical amplification medium to be Raman amplified; and (d) a controller controlling a gain characteristic of the optical amplification medium in accordance with a condition of the transmission line.
Objects of the present invention are also achieved by providing an apparatus including (a) an optical fiber transmission line having first and second sections, light traveling through the first and second sections; (b) a first excitation light source providing excitation light to the first section so that the light traveling through the first section is Raman amplified in the first section; (c) a second excitation light source providing excitation light to the second section so that the light traveling through the second section is Raman amplified in second section, the excitation light provided by the first excitation light source being at a different wavelength than the excitation light provided by the second excitation light source.