1.  Field of the Invention
The present invention relates to a Raman amplifier for compensating for, by Raman amplification, a transmission loss generated in an optical communication system for executing communication using signal light when signal light is transmitted through the optical transmission path. 
2.  Related Background Art
In an optical communication system for executing communication using signal light, signal light output from the transmitter and transmitted through the optical transmission path suffers a transmission loss.  Hence, the signal light that has reached the receiver has a small power.  If the power of the signal light that has reached the receiver has a predetermined value or less, normal optical communication may be impossible because of a reception error.  To prevent this problem, an optical amplifier is inserted between the transmitter and the receiver to optically amplify signal light, thereby compensating for the transmission loss generated when the signal light is transmitted through the optical transmission path. 
Examples of such an optical amplifier are a rare-earth-element-doped optical fiber amplifier (e.g., Er-doped optical fiber amplifier) that uses an amplification optical fiber doped with a rare earth element and a Raman amplifier that uses a Raman amplification phenomenon in a Raman amplification optical fiber.  Unlike a rare-earth-element-doped optical fiber amplifier, a Raman amplifier can obtain a desired wavelength band having a gain by appropriately setting the wavelength of pumping light for Raman amplification. 
For a WDM (Wavelength Division Multiplexing) optical communication system for executing optical communication by multiplexing signal light components with multiple wavelengths in a predetermined signal light wavelength band, it is important that the gain spectrum of an optical amplifier in that signal light wavelength band is flat.  Otherwise, even when a signal light component having a certain wavelength in the signal light wavelength band can be normally received by the receiver, another signal light component having a different wavelength with a small gain may cause a reception error.  A technique for flattening the gain spectrum of a Raman amplifier has been studied. 
For example, in a Raman amplifier gain flattening technique described in reference 1 xe2x80x9cY. Emori, et al.,  xe2x80x9c100 nm bandwidth flat gain Raman amplifiers pumped and gain-equalized by 12-wavelength-channel WDM high power laser diodesxe2x80x9d, OFC""99, PD19 (1999)xe2x80x9d, light components output from N (Nxe2x89xa72) pumping light sources are multiplexed and supplied to a Raman amplification optical fiber as Raman amplification pumping light.  The gain spectrum of the Raman amplifier is flattened by appropriately setting the output central wavelengths and output powers of the N pumping light sources.  In reference 1, the number N of pumping light sources is 12. 
In a Raman amplifier gain flattening technique described in reference 2 xe2x80x9cF. Koch, et al.,  xe2x80x9cBroadband gain flattened Raman amplifier to extend operation in the third telecommunication windowxe2x80x9d, OFC""2000, ThD, FF3 (2000)xe2x80x9d, the gain spectrum of the Raman amplifier is flattened using a gain equalizer which has a loss spectrum with almost the same shape as that of the gain spectrum of the Raman amplification optical fiber. 
However, the above conventional Raman amplifier gain flattening techniques have the following problems.  In an optical communication system for executing long-distance optical communication, M (Mxe2x89xa72) Raman amplifiers may be required between the transmitter and the receiver.  In this case, if the gain flattening technique described in reference 1 is employed, the total number of pumping light sources required in the entire optical communication system is Mxc3x97N. Since the output powers of these pumping light sources must be individually controlled, the gain spectrum flattening for the Raman amplifier is hard to control. 
In a Raman amplifier which employs the gain flattening technique described in reference 2, signal light is optically amplified by the Raman amplification optical fiber but attenuated by the gain equalizer.  Hence, the loss spectrum of the gain equalizer must be controlled, and it is also difficult to control the gain spectrum flattening for the Raman amplifier. 
The present invention has been made to solve the above problems, and has as its object to provide a Raman amplifier capable of easily controlling the gain spectrum flattening in a signal light wavelength band. 
According to an aspect of the present invention, there is provided a Raman amplifier characterized by comprising (1) a Raman amplification optical fiber for transmitting signal light and Raman-amplifying the signal light by being provided Raman amplification pumping light, and (2) Raman amplification pumping light supply means, having N (Nxe2x89xa71) pumping light sources for outputting light having a non-unimodal spectrum, for supplying light components output from the N pumping light sources to the Raman amplification optical fiber as the Raman amplification pumping light. 
According to this Raman amplifier, Raman amplification pumping light is supplied from the Raman amplification pumping light supply means having the N pumping light sources to the Raman amplification optical fiber.  The signal light is transmitted through the Raman amplification optical fiber while being Raman-amplified.  That is, a transmission loss generated when the signal light is transmitted through the Raman amplification optical fiber is compensated for by Raman amplification. 
Especially, in this Raman amplifier, since the spectrum of a light component output from each of the N pumping light sources included in the Raman amplification pumping light supply means is non-unimodal, the number of pumping light sources can be made smaller than that in the prior art described in reference 1.  Hence, the gain spectrum flattening can easily be controlled.  Additionally, in this Raman amplifier, since the gain spectrum can be flattened without using any gain equalizer, the gain spectrum flattening can easily be controlled, as compared to the prior art described in reference 2. 
xe2x80x9cThe spectrum is non-unimodalxe2x80x9d means not only that a wavelength at which the output power is maximal is present independently of a wavelength at which the output power is maximum but also that the wavelengths for the maximum and maximal output powers are separated from each other by 5 nm or more.  From the viewpoint of output power, xe2x80x9cunimodalxe2x80x9d means that in an optical spectrum obtained when the wavelength resolving power is set at 0.5 nm or more, there is no maximal peak at which the peak power difference is 5 dB or less or 10 dB or less and the wavelength difference is 5 nm or more, with respect to the peak of the maximum output power.  A spectrum which does not satisfy the above conditions is xe2x80x9cnon-unimodalxe2x80x9d. 
According to another aspect of the present invention, there is provided a Raman amplifier characterized by comprising (1) a Raman amplification optical fiber for transmitting signal light and Raman-amplifying the signal light by being provided Raman amplification pumping light, (2) Raman amplification pumping light supply means, having N (Nxe2x89xa71) pumping light sources for outputting light having a controllable (changeable) spectrum, for supplying light components output from the N pumping light sources to the Raman amplification optical fiber as the Raman amplification pumping light, and (3) control means for controlling the spectrum of the Raman amplification pumping light output from each of the N pumping light sources. 
According to this Raman amplifier, Raman amplification pumping light is supplied from the Raman amplification pumping light supply means having the N pumping light sources to the Raman amplification optical fiber.  The signal light is transmitted through the Raman amplification optical fiber while being Raman-amplified.  That is, a transmission loss generated when the signal light is transmitted through the Raman amplification optical fiber is compensated for by Raman amplification. 
Especially, in this Raman amplifier, since the spectrum of a light component output from each of the N pumping light sources included in the Raman amplification pumping light supply means is changeable and is controlled by the control means, the number of pumping light sources can be made smaller than that in the prior art described in reference 1.  Hence, the gain spectrum flattening can easily be controlled.  Additionally, in this Raman amplifier, since the gain spectrum can be flattened without using any gain equalizer, the gain spectrum flattening can easily be controlled, as compared to the prior art described in reference 2. 
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention. 
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter.  However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.