1. Field of the Invention
The present invention relates to a Raman amplifier that amplifies a wavelength division multiplexed (WDM) signal by utilizing a Raman effect, a WDM optical communication system, and a method for controlling Raman amplification, and in particular, relates to technology for controlling pumping light in order to realize the Raman amplification coping with variations in system operating conditions.
2. Description of the Related Art
A WDM optical communication method that multiplexes optical signals having different wavelengths to transmit them in a single optical fiber is an effective means for realizing a more economical optical communication system of larger capacity as well as a flexible optical network. Particularly, as technology with regard to optical amplifiers has been developed in recent years, the WDM optical communication method becomes in practical use rapidly as backbone technology supporting the Internet service.
The WDM optical communication system enables long-distance transmission mainly by repeating signal light while amplifying it in a repeater stage using optical amplification repeating technology. As an optical amplifying means in such a WDM optical communication system, there are used a rare-earth element doped optical fiber amplifier, a Raman amplifier, and so on.
With regard to the Raman amplifier mentioned above, two amplification types are known: that is, a distributed parameter type and a concentrated type. The distributed parameter type is an amplification type in that pumping light is introduced into a transmission path (for example, a silica-based optical fiber and the like) of an optical communication system, to Raman amplify in a distributing manner signal light being propagated through the transmission path, so that a part of loss that occurs when the signal light passes through the transmission path can be compensated. On the other hand, the concentrated type is an amplification type in that the pumping light is introduced in a concentrated manner into an optical fiber having a small effective cross-sectional area, for example, and high non-linearity for Raman amplification.
The Raman amplification of both of the distributed parameter type and the concentrated type described above shows amplification characteristics having a gain peak at a frequency lower than the frequency of the pumping light by a Raman shift amount (for example, 13.2 THz in the case of silica-based medium). Therefore, it is possible to amplify signal light of an arbitrary wavelength by preparing a pumping light source of an appropriate pumping light wavelength in consideration of the shift frequency of the Raman gain, and it is also possible to Raman amplify WDM signal light having wider bandwidth arbitrarily by combining a plurality of pumping light of different wavelengths. Further, by changing a distribution ratio of pumping light power of each of the wavelengths, a profile of the Raman gain corresponding to each pumping light wavelength may also be changed so that wavelength dependence of the Raman amplified WDM signal light can be adjusted arbitrarily.
By the way, in the optical network system of next generation, for example, it is anticipated that operating conditions of the system such as a wavelength band of the WDM signal light and the number of signal light, a signal light level input to an optical transmission path, a type of the optical transmission path and the like may be changed dynamically. However, in the previously proposed technology regarding the Raman amplification, there is a problem in that it is difficult to flexibly cope with the dynamic change of the system operating conditions described above.
More specifically, for example, if the operating conditions are changed such that so-called S-band (a wavelength band of 1480 nm–1520 nm) is added to WDM signal light using C-band (a wavelength band of 1525 nm–1565 nm) and L-band (a wavelength band of 1570 nm–1620 nm), since most of the conventional Raman amplifiers have been designed individually corresponding to each band, it is possible to cope with a change of operating wavelength in one band by controlling the power, wavelength and the like of the pumping light, but it is difficult to cope with such a dynamic change across a plurality of bands using a single type of Raman amplifier.
Further, when the signal light of S-band is added as described above in order to achieve the wider bandwidth of WDM signal light, there is a possibility that a wavelength of pumping light used before the change of the operating conditions may coincide with or considerably approximate to a wavelength of the added S-band signal light, resulting in a problem in degradation of transmission quality of the WDM signal light.
Still further, it is anticipated that the dynamic change of the operating conditions described above is practically carried out in stepwise along with the lapse of required time. In such a case, it is important to cope with the change of the operating conditions without affecting the service in operation. Therefore, it is necessary to establish controlling technology for optimizing a gain and gain wavelength characteristics of the Raman amplification in response to the change with time of the operating conditions.