Patent Publication Number: US-2023142798-A1

Title: Raman amplifier, raman amplification method, and raman amplification system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2022-069820, filed on Apr. 21, 2022, the entire contents of which are incorporated herein by reference. 
     FIELD 
     A certain aspect of embodiments described herein relates to a Raman amplifier, a Raman amplification method, and a Raman amplification system. 
     BACKGROUND 
     A technique for Raman amplification of signal light using pumping light is known as disclosed in, for example, Japanese Patent Application Publication No. 2005-309250. In addition, counter-propagating Raman amplification, in which pumping light is caused to enter a transmission line (more specifically, an optical fiber) for Raman amplification so as to propagate in a direction opposite to the propagation direction of the signal light, is known. Further, co-propagating Raman amplification, in which pumping light is caused to enter the transmission line for Raman amplification so as to propagate in the same direction as the propagation direction of the signal light, is also known. In addition, bidirectional-propagating Raman amplification, in which co-propagating Raman amplification is used simultaneously with counter-propagating Raman amplification, is also known as disclosed in, for example, Japanese Patent Application Publication No. 2020-129143. 
     In co-propagating Raman amplification, also known is a technique in which primary pumping light capable of optically amplifying the wavelength band of the signal light is optically amplified by secondary pumping light, and the signal light is optically amplified using the primary pumping light that has been optically amplified. In this technique, the primary pumping light and the secondary pumping light propagate through the transmission line in the same direction as the signal light as disclosed in, for example, Japanese Patent Application Publication No. 2021-063857. 
     SUMMARY 
     According to an aspect of the embodiments, there is provided a Raman amplifier including: a first light source that outputs a primary pumping light, which propagates in a same direction as a propagation direction of a signal light, to an optical transmission line for Raman amplification; a second light source that outputs a secondary pumping light, which pumps and amplifies the primary pumping light and propagates in the same direction as the propagation direction, to the optical transmission line; and a control unit that controls a gain for the signal light by adjusting power of the secondary pumping light. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    illustrates a transmission system. 
         FIG.  2    illustrates a Raman amplification system in accordance with an embodiment. 
         FIG.  3    illustrates a Raman amplification system in accordance with a comparative example. 
         FIG.  4    is a flowchart illustrating the operation of a forward control unit. 
         FIG.  5    is a flowchart of an example of an initial state transition process. 
         FIG.  6 A  is a diagram for describing an example of the initial state in accordance with the embodiment,  FIG.  6 B  is a diagram for describing tilt control in accordance with the embodiment, and  FIG.  6 C  is a diagram for describing gain control in accordance with the embodiment. 
         FIG.  7 A  is a flowchart of an example of a tilt control process, and  FIG.  7 B  is a flowchart of an example of a gain control process. 
         FIG.  8 A  is a diagram for describing an example of the initial state in accordance with the comparative example,  FIG.  8 B  is a diagram for describing an example of tilt control in accordance with the comparative example, and  FIG.  8 C  is a diagram for describing gain control in accordance with the comparative example. 
         FIG.  9 A  is an example of a graph presenting a relationship between a lump loss and power of pumping light required for compensation of the lump loss, and  FIG.  9 B  is an example of a graph presenting noise characteristics of an i-pump and a c-pump. 
         FIG.  10 A  is a diagram for describing an example of the amplification of the signal light, and  FIG.  10 B  is a diagram for describing an example of a variable range of power of secondary pumping light. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In Raman amplification, the transmission line itself is used as the amplification medium, and therefore, the target gain is required to be changed according to the type of the transmission line. When the target gain is changed, a gain tilt occurs. When the tilt occurs, the quality of the signal light deteriorates. For example, the S/N ratio of the signal light in the wavelength band with a low gain deteriorates compared with the S/N ratio of the signal light in the wavelength band with a high gain. Thus, the quality of the signal light may vary depending on the wavelength band. The deterioration in the quality of the signal light causes deterioration in transmission performance (e.g., the transmission distance or the like) of the transmission device. Therefore, in Raman amplification, tilt control is performed to reduce the tilt. 
     In addition, in Raman amplification, the power of the pumping light introduced into the transmission line, which is the amplification medium, is decreased by a connection loss (so called, a lump loss) in the vicinity of the part where the pumping light is transmitted to the transmission line. When the power of the pumping light decreases, the gain for the signal light decreases, and the output of the signal light decreases. For this reason, in Raman amplification, gain control is performed to reduce a decrease in gain. As seen from the above, in Raman amplification, both the tilt control and the gain control are required. 
     In co-propagating Raman amplification, unlike counter-propagating Raman amplification, as the signal light and the pumping light propagate through the transmission line together over a long distance, a phenomenon in which the noise of the pumping light is gradually transferred as noise to the signal light occurs. This phenomenon is called relative intensity noise (RIN) transfer. 
     As described above, in co-propagating Raman amplification using primary pumping light and secondary pumping light, the primary pumping light is optically amplified by the secondary pumping light, and the signal light is optically amplified using the primary pumping light that has been optically amplified. However, when the primary pumping light is optically amplified by the secondary pumping light, the noise of the secondary pumping light is transferred to the primary pumping light. In addition, since the signal light is optically amplified using the primary pumping light, the noise transferred from the secondary pumping light to the primary pumping light is further transferred to the signal light. When the noise is transferred to the signal light, transmission errors increase, and the transmission performance deteriorates. Therefore, it is required to effectively control the gain for the signal light in co-propagating Raman amplification using the primary pumping light and the secondary pumping light. 
     Hereinafter, a description will be given of embodiments of the present disclosure with reference to the accompanying drawings. 
     As illustrated in  FIG.  1   , a transmission system ST includes two transmission devices  10  and  20 . The transmission devices  10  and  20  are connected via two optical transmission lines  31  and  32 . The optical transmission lines  31  and  32  are examples of a transmission line. The optical transmission lines  31  and  32  include, for example, optical fibers. Note that the transmission line types of the optical transmission lines  31  and  32  are not particularly limited. For example, the optical transmission lines  31  and  32  may include single mode fibers (SMFs), or dispersion shifted fibers (DSFs). The optical transmission lines  31  and  32  may be included in the transmission system ST or are not necessarily included in the transmission system ST. 
     The transmission device  10  includes transceivers  11  and  12 , a multiplexer  13 , a demultiplexer  14 , optical amplifiers  15  and  16 , a co-propagating Raman amplifier  100 , and a counter-propagating Raman amplifier  150 . The co-propagating Raman amplifier  100  is an example of a Raman amplifier (more specifically, a first Raman amplifier). The transmission device  20  includes transceivers  21  and  22 , a multiplexer  23 , a demultiplexer  24 , optical amplifiers  25  and  26 , a counter-propagating Raman amplifier  200 , and a co-propagating Raman amplifier  250 . The counter-propagating Raman amplifier  200  is an example of a second Raman amplifier. The co-propagating Raman amplifier  100  is coupled to the counter-propagating Raman amplifier  200  via the optical transmission line  31 . The co-propagating Raman amplifier  250  is coupled to the counter-propagating Raman amplifier  150  via the optical transmission line  32 . 
     For example, a Raman amplification system STa can be constructed by the co-propagating Raman amplifier  100  and the counter-propagating Raman amplifier  200 . The Raman amplification system STa may include or may not necessarily include the optical transmission line  31 . The Raman amplification system STa can be also constructed by the co-propagating Raman amplifier  250  and the counter-propagating Raman amplifier  150 . Such a Raman amplification system STa may include or may not necessarily include the optical transmission line  32 . 
     The transceivers  11  transmit wavelength lights having different wavelengths, respectively. The transceivers  12  receive wavelength lights having different wavelengths, respectively. The multiplexer  13  multiplexes the wavelength lights having different wavelengths to produce a wavelength division multiplexing (WDM) signal light (hereinafter, simply referred to as a signal light). The demultiplexer  14  demultiplexes wavelength lights having the center wavelengths of a fixed constant wavelength interval from the signal light. The multiplexer  13  and the demultiplexer  14  include, for example, optical couplers. 
     Each of the optical amplifiers  15  and  16  amplifies the signal light. The optical amplifiers  15  and  16  include, for example, erbium doped fiber amplifiers (EDFAs). The optical amplifier  15  may be called, for example, a post-amplifier. The optical amplifier  16  may be called, for example, a pre-amplifier. The co-propagating Raman amplifier  100  outputs a pumping light to the optical transmission line  31  in the same direction as the propagation direction of the signal light. The pumping light enters the optical transmission line  31 , which causes induced Raman scattering, and thereby, the signal light is Raman-amplified. The counter-propagating Raman amplifier  150  outputs a pumping light to the optical transmission line  32  in a direction opposite to the propagation direction of the signal light. The pumping light enters the optical transmission line  32 , which causes induced Raman scattering, and thereby, the signal light is Raman-amplified. 
     The transceivers  21  and  22  basically have the same functions as the transceivers  11  and  12  described above, respectively, and the detailed description thereof is thus omitted. Similarly, the multiplexer  23  and the demultiplexer  24  basically have the same functions as the multiplexer  13  and the demultiplexer  14  described above, respectively, and the detailed description thereof is thus omitted. The optical amplifiers  25  and  26  basically have the same functions as the optical amplifiers  15  and  16  described above, respectively, and the detailed description thereof is thus omitted. The counter-propagating Raman amplifier  200  and the co-propagating Raman amplifier  250  basically have the same functions as the counter-propagating Raman amplifier  150  and the co-propagating Raman amplifier  100  described above, respectively, and the detailed description thereof is thus omitted. 
     With reference to  FIG.  2   , a description will be given of the details of the co-propagating Raman amplifier  100  and the counter-propagating Raman amplifier  200  of the Raman amplification system STa. 
     First, the co-propagating Raman amplifier  100  will be described. The co-propagating Raman amplifier  100  includes a plurality of i-pumps (abbreviated as i-p in  FIG.  2   )  101  and a plurality of c-pumps (abbreviated as c-p in  FIG.  2   )  102 . The i-pump  101  is an example of a first light source. The c-pump  102  is an example of a second light source. Examples of the i-pump  101  and the c-pump  102  are disclosed in the following references. 
     REFERENCES 
     Japanese Patent No. 6774753 (Japanese Patent Application Publication No. 2016-212370) 
     “Co-Propagating Dual-Order Distributed Raman Amplifier Utilizing Incoherent Pumping”, Masahito Morimoto et al., IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 29, NO. 7, Apr. 1, 2017 
     The co-propagating Raman amplifier  100  also includes forward first drivers (abbreviated as Dry in  FIG.  2   )  103 , forward second drivers  104 , and a forward control unit  105 . The forward control unit  105  is an example of the control unit. The forward control unit  105  includes a tilt control unit  105 T and a gain control unit  105 G. Further, the co-propagating Raman amplifier  100  includes a plurality of optical filters  106 ,  107 ,  108 ,  109 , and  110 , and an opto-isolator (ISO in  FIG.  2   )  111 . Further, the co-propagating Raman amplifier  100  includes an OSC communication device  112 , a C-mPD  113 , and an L-mPD  114 . 
     The i-pumps  101  output primary pumping lights Li having different wavelengths, respectively. The primary pumping lights Li are incoherent pumping lights belonging to a first wavelength band (for example, the 1450 nm (nanometers) band). Thus, each of the i-pumps  101  may be called an incoherent light source. 
     The c-pumps  102  output secondary pumping lights Lc having different wavelengths, respectively. The secondary pumping lights Lc are coherent pumping lights belonging to a second wavelength band (for example, the 1350 nm band). Therefore, each of the c-pumps  102  may be called a coherent light source. As seen from the above, the second wavelength band is a wavelength band that is different from the first wavelength band and is narrower than the first wavelength band. The c-pumps  102  are only required to include a fiber bragg grating-laser diode (FBG-LD), a distributed feed-back (DFB)-LD, a distributed bragg reflector (DBR)-LD, a fabry-perot (FP)-LD, or any combination thereof. 
     The primary pumping lights Li amplify a signal light Ls in a third wavelength band that is different from both the first wavelength band and the second wavelength band. The third wavelength band is, for example, the C band, which is the 1550 nm band, or the L band, which is the 1600 nm band. The secondary pumping lights Lc amplify the primary pumping lights Li. The primary pumping lights Li and the secondary pumping lights Lc propagate in the same direction as the propagation direction of the signal light Ls. 
     Although the details will be described later, the primary pumping light Li has noise characteristics that the primary pumping light Li contains a relatively small RIN compared with that of the secondary pumping light Lc. The RIN is a parameter indicating the temporal fluctuation (intensity noise) in the intensity of laser light, and is obtained by dividing the fluctuation (noise) in light intensity per unit frequency by average optical power. When the signal light Ls, the primary pumping lights Li, and the secondary pumping lights Lc propagate together through the optical transmission line  31  over a long distance, the RINs of the primary pumping lights Li and the secondary pumping lights Lc are gradually transferred as noise to the signal light Ls. That is, the RIN transfer occurs. When the RIN transfer occurs, transmission errors due to the RIN increases, and thereby, the transmission performance of the transmission device  10  deteriorates. More specifically, the transmission distance of the transmission device  10  decreases. The present embodiment effectively controls the gain for the signal light Ls to reduce the RIN transferred to the signal light Ls and reduce the deterioration in transmission performance. 
     The forward first driver  103  is a drive circuit that controls the drive of the i-pump  101 . The forward first driver  103  controls the power of the primary pumping light Li output by the i-pump  101  on the basis of a first control signal output from the tilt control unit  105 T. The forward second driver  104  is a drive circuit that controls the drive of the c-pump  102 . The forward second driver  104  controls the power of the secondary pumping light Lc output by the c-pump  102  on the basis of a second control signal output from the gain control unit  105 G. 
     The forward control unit  105  controls the operations of the forward first drivers  103  and the forward second drivers  104 . In particular, the tilt control unit  105 T controls the operations of the forward first drivers  103  by outputting the first control signal. The tilt control unit  105 T adjusts the ratio of power between the wavelengths of the primary pumping lights Li output from the i-pumps  101  by controlling the operations of the forward first drivers  103 . In addition, the gain control unit  105 G controls the operations of the forward second drivers  104  by outputting the second control signal. The gain control unit  105 G adjusts the average power of the secondary pumping lights Lc output from the c-pumps  102  by controlling the operations of the forward second drivers  104 . The forward control unit  105  includes a hardware circuit such as, for example, a memory and a central processing unit (CPU). The forward control unit  105  may be a hardware circuit such as an application specified integrated circuit (ASIC) or a field programmable gate array (FPGA) instead of the CPU. 
     The primary pumping lights Li output from the respective i-pumps  101  are input to the optical filter  106 . The optical filter  106  guides the input primary pumping lights Li to the optical filter  108 . The secondary pumping lights Lc output from the respective c-pump  102  are input to the optical filter  107 . The optical filter  107  guides the input secondary pumping lights Lc to the optical filter  108 . The primary pumping lights Li output from the optical filter  106  and the secondary pumping lights Lc output from the optical filter  107  are input to the optical filter  108 . The optical filter  108  guides the input primary pumping lights Li and the input secondary pumping lights Lc to the optical filter  109  via the opto-isolator  111 . 
     The optical filter  109  guides the primary pumping lights Li and the secondary pumping lights Lc that have been input from the optical filter  108 , to the optical transmission line  31 . When the primary pumping lights Li and the secondary pumping lights Lc are guided to the optical transmission line  31 , the lump loss of the optical transmission line  31  occurs. The gain control unit  105 G can determine the lump loss by calculating, for example, the ratio between the power of the secondary pumping light Lc immediately before the input to the optical filter  109  and the power of the secondary pumping light Lc immediately after the output from the optical filter  109 . The optical filter  109  removes the signal light Ls input from the optical filter  110  without allowing it to be input from the optical transmission line  31  to the optical filter  108 . The optical filter  109  guides the signal light Ls together with the primary pumping lights Li and the secondary pumping lights Lc to the counter-propagating Raman amplifier  200  via the optical transmission line  31 . 
     The OSC communication device  112  includes a small form-factor pluggable (SFP) transceiver. The OSC communication device  112  uses optical supervisory channel (OSC) light Lx to transmit the request of the forward control unit  105  and information to the OSC communication device  215  of the counter-propagating Raman amplifier  200 . The OSC light Lx transmitted by the OSC communication device  112  is input to the optical filter  110 . The optical filter  110  guides the input OSC light Lx to the optical filter  109 . The optical filter  110  removes the signal light Ls without allowing the signal light Ls to be input from the optical transmission line  31  to the OSC communication device  112 . 
     In addition, the OSC communication device  112  uses OSC light Ly to receive the request of the forward control unit (not illustrated) of the co-propagating Raman amplifier  250  and information. The OSC light Ly is output from the OSC communication device (not illustrated) of the co-propagating Raman amplifier  250 . The OSC light Ly is input from an optical filter  155  of the counter-propagating Raman amplifier  150  to the OSC communication device  112 . In the present embodiment, although the optical filter  155  is provided in the counter-propagating Raman amplifier  150 , the optical filter  155  may be provided in the optical amplifier  16  (see  FIG.  1   ). Similarly, in the present embodiment, although optical filters  156  and  157  are provided in the counter-propagating Raman amplifier  150 , the optical filters  156  and  157  may be provided in the optical amplifier  16  (see  FIG.  1   ). The optical filter  158  of the counter-propagating Raman amplifier  150  is coupled to the OSC communication device of the counter-propagating Raman amplifier  150 , but the illustration of the OSC communication device is omitted in  FIG.  2    and  FIG.  3    described later because of space limitations. The optical filter  158  guides the OSC light Ly to the OSC communication device of the counter-propagating Raman amplifier  150 . 
     The C-mPD  113  includes a photo diode (PD) that monitors (measures) the power of the signal light Lt in the C band. The C-mPD  113  detects the power of the signal light Lt in the C band that is output from the co-propagating Raman amplifier  250  and is input to the counter-propagating Raman amplifier  150 . The signal light Lt in the C band is input from the optical filter  156  to the C-mPD  113 . The tilt control unit  105 T of the forward control unit  105  obtains the power of the signal light Lt in the C band from the output signal of the C-mPD  113 . 
     The L-mPD  114  includes a PD that monitors the power of the signal light Lt in the L band. The L-mPD  114  detects the power of the signal light Lt in the L band that is output from the co-propagating Raman amplifier  250  and is input to the counter-propagating Raman amplifier  150 . The signal light Lt in the L band is input to the L-mPD  114  from the optical filter  157 . The tilt control unit  105 T of the forward control unit  105  obtains the power of the signal light Lt in the L band from the output signal of the L-mPD  114 . 
     The tilt control unit  105 T performs tilt control for reducing the gain tilt that occurs between the C and L bands of the signal lights Ls, on the basis of the power of the signal light Lt in the C band, the power of the signal light Lt in the L band, and information contained in the OSC light Ly received by the OSC communication device  112 . The gain tilt means a variation in wavelength characteristics of the gain when the gain provided to the signal light Ls varies. For example, the tilt control unit  105 T performs the tilt control for reducing the tilt by adjusting the ratio of power between the wavelengths of the primary pumping lights Li. To adjust the ratio of power between the wavelengths of the primary pumping lights Li, the tilt control unit  105 T performs the tilt control for the forward first drivers  103 . The details of the tilt control will be described later. 
     The gain control unit  105 G performs gain control for reducing a decrease in gain on the basis of the lump loss of the optical transmission line  31 . For example, the gain control unit  105 G performs the tilt control for reducing the average gain by adjusting the average power of the secondary pumping lights Lc. To adjust the average power of the secondary pumping lights Lc, the gain control unit  105 G performs the gain control for the forward second drivers  104 . The details of the gain control will be described later. 
     Next, a description will be given of the counter-propagating Raman amplifier  200 . The counter-propagating Raman amplifier  200  includes a plurality of FBG-LDs (abbreviated as LD in  FIG.  2   )  201 . The FBG-LD  201  is an example of a third light source. The counter-propagating Raman amplifier  200  also includes backward drivers  203  and a backward control unit  205 . The backward control unit  205  includes a tilt control unit  205 T and a gain control unit  205 G. The counter-propagating Raman amplifier  200  further includes optical filters  206 ,  207 ,  208 ,  209 ,  210 , and  212  and an opto-isolator  211 . In addition, the counter-propagating Raman amplifier  200  includes an OSC communication device  215 , a C-mPD  213 , an L-mPD  214 , and optical filters  255 ,  256 ,  257 , and  258 . The backward control unit  205  basically has the same hardware configuration as the forward control unit  105 . The optical filters  255 ,  256 ,  257 , and  258  correspond to the optical filters  155 ,  156 ,  157 , and  158  described above, respectively. Therefore, for example, the optical filter  255  guides the OSC light Lx to the OSC communication device (not illustrated) of the co-propagating Raman amplifier  250 . For example, the optical filter  258  guides the OSC light Lx to the OSC communication device  215  of the counter-propagating Raman amplifier  200 . The illustration of the OSC communication device of the co-propagating Raman amplifier  250  is omitted because of space limitations. 
     The FBG-LDs  201  output primary pumping lights Lp having different wavelengths, respectively. The primary pumping lights Lp are coherent pumping lights belonging to the first wavelength band described above. Since the primary pumping lights Lp are coherent pumping lights, the primary pumping lights Lp differ from the primary pumping lights Li that are incoherent. As seen from the above, the primary pumping light Lp is another pumping light different from the primary pumping light Li. The primary pumping lights Lp amplify the signal light Ls. The primary pumping lights Lp propagate in a direction opposite to the propagation direction of the signal light Ls. 
     The backward driver  203  is a drive circuit that controls the drive of the FBG-LD  201 . The backward driver  203  controls the power of the primary pumping light Lp output by the FBG-LD  201  on the basis of a third control signal output from the tilt control unit  205 T. The backward driver  203  also controls the power of the primary pumping light Lp output by the FBG-LD  201  on the basis of a fourth control signal output from the gain control unit  205 G. 
     The backward control unit  205  controls the operations of the backward drivers  203 . In particular, the tilt control unit  205 T controls the operations of the backward drivers  203  by outputting the third control signal. The gain control unit  205 G controls the operations of the backward drivers  203  by outputting the fourth control signal. The tilt control unit  205 T and the gain control unit  205 G adjust the power of the primary pumping lights Lp output by the FBG-LDs  201  by controlling the operations of the backward drivers  203 . 
     The primary pumping lights Lp output from some of the FBG-LDs  201  are input to the optical filter  206 . The primary pumping lights Lp output from the rest of the FBG-LDs  201  are input to the optical filter  207 . Both the optical filters  206  and  207  guide the input primary pumping lights Lp to the optical filter  208 . The primary pumping lights Lp output from the optical filters  206  and  207  are input to the optical filter  208 . The optical filter  208  guides the input primary pumping lights Lp to the optical filter  209  via the opto-isolator  211 . 
     The optical filter  209  guides the primary pumping lights Lp input from the optical filter  208  to the optical transmission line  31 . When the primary pumping lights Lp are guided to the optical transmission line  31 , a lump loss occurs. The gain control unit  205 G can determine the lump loss of the optical transmission line  31  by calculating the ratio between the power of the primary pumping light Lp immediately before the input to the optical filter  209  and the power of the primary pumping light Lp immediately after the output from the optical filter  209 , for example. The optical filter  209  removes the signal light Ls input from the co-propagating Raman amplifier  100  to the counter-propagating Raman amplifier  200  without allowing it to be input from the optical transmission line  31  to the optical filter  208 . The optical filter  209  guides the signal light Ls to the optical filter  210  via the optical transmission line  31 . 
     The C-mPD  213  includes a PD that monitors the power of the signal light Ls in the C band. The C-mPD  213  detects the power of the signal light Ls in the C band that is output from the co-propagating Raman amplifier  100  and is input to the counter-propagating Raman amplifier  200 . The signal light Ls in the C band is input from the optical filter  210  to the C-mPD  213 . The tilt control unit  205 T of the backward control unit  205  obtains the power of the signal light Ls in the C band from the output signal of the C-mPD  213 . 
     The L-mPD  214  includes a PD that monitors the power of the signal light Ls in the L band. The L-mPD  214  detects the power of the signal light Ls in the L band that is output from the co-propagating Raman amplifier  100  and is input to the counter-propagating Raman amplifier  200 . The signal light Ls in the L band is input from the optical filter  212  to the L-mPD  214 . The tilt control unit  205 T of the backward control unit  205  obtains the power of the signal light Ls in the L band from the output signal of the L-mPD  214 . 
     The tilt control unit  205 T performs tilt control for reducing the gain tilt that occurs between the C and L bands of the signal lights Ls, on the basis of the power of the signal light Ls in the C band, the power of the signal light Ls in the L band, information contained in the OSC light Lx received by the OSC communication device  215 , and the like. For example, the tilt control unit  205 T performs the tilt control for reducing the tilt by adjusting the ratio of power between the wavelengths of the primary pumping lights Lp. Therefore, the tilt control unit  205 T performs the tilt control for the backward drivers  203 . 
     The gain control unit  205 G performs gain control for reducing a decrease in gain, on the basis of the lump loss of the optical transmission line  31 . For example, the gain control unit  205 G performs the gain control for reducing the average gain by adjusting the average power of the primary pumping lights Lp. Therefore, the gain control unit  205 G performs the gain control for the backward drivers  203  in the same manner as the tilt control unit  205 T. 
     With reference to  FIG.  3   , a Raman amplification system STb in accordance with a comparative example will be described in comparison with the Raman amplification system STa in accordance with the embodiment. In  FIG.  3   , the same components as those of the Raman amplification system STa described with reference to  FIG.  2    are provided with the same reference numerals as those in  FIG.  2   , and the detailed description thereof is omitted. 
     As illustrated in  FIG.  3   , in the Raman amplification system STb in accordance with the comparative example, as in the embodiment, the tilt control unit  105 T controls the operations of the forward first drivers  103  by outputting the first control signal. The tilt control unit  105 T adjusts the ratio of power between the wavelengths of the primary pumping lights Li output from the i-pumps  101  by controlling the operations of the forward first drivers  103 . On the other hand, in the Raman amplification system STb in accordance with the comparative example, unlike the embodiment, the gain control unit  105 G controls the operations of the forward first drivers  103  by outputting the second control signal. The gain control unit  105 G adjusts the average power of the primary pumping lights Li output by the i-pumps  101  by controlling the operations of the forward first drivers  103 . 
     As described above, the control target of the tilt control unit  105 T is the same between the embodiment and the comparative example, but the control target of the gain control unit  105 G differs between the embodiment and the comparative example. That is, the gain control unit  105 G in accordance with the embodiment controls the operations of the forward second drivers  104  by outputting the second control signal. The gain control unit  105 G in accordance with the comparative example controls the operations of the forward first drivers  103  by outputting the second control signal. The difference in effects and advantages based on the difference between the control targets will be described later. 
     Next, a description will be given of the operation of the forward control unit  105  with reference to  FIG.  4    to  FIG.  7   . 
     As illustrated in  FIG.  4   , the forward control unit  105  first executes an initial state transition process (step S 10 ). The initial state transition process is a process of raising the power of the primary pumping lights Li and the power of the secondary pumping lights Lc from the minimum power to the power of the initial state. More specifically, when the forward control unit  105  starts executing the initial state transition process, as illustrated in  FIG.  5   , the forward control unit  105  sets fiber input power, a predetermined ratio, and a target gain for the co-propagating Raman amplifier  100  (step S 11 ). The fiber input power is the power of the signal light Ls that is output from the transmission device  10  to the optical transmission line  31  and is input to the optical transmission line  31 . The predetermined ratio is the ratio by which the power of the secondary pumping light Lc is raised. 
     The fiber input power and the target gain are set on the basis of the transmission line types of the optical transmission lines  31  and  32 . For example, the forward control unit  105  includes a table that stores a correspondence relationship between the transmission line type and the fiber input power in advance, and when the transmission line type is specified, the forward control unit  105  refers to this table to set the fiber input power corresponding to the specified transmission line type. In addition, the forward control unit  105  includes a table that stores a correspondence relationship between the transmission line type and the target gain in advance, and when the transmission line type is specified, the forward control unit  105  refers to this table to set the target gain corresponding to the specified transmission line type. For the power of the secondary pumping light Lc, the predetermined ratio specified in advance is set. 
     When the process in step S 11  is completed, the forward control unit  105  turns on the primary pumping lights Li and the secondary pumping lights Lc at the minimum power (step S 12 ). That is, the forward control unit  105  performs control for causing the i-pumps  101  to output the primary pumping lights Li with the minimum power, for the forward first drivers  103 . The forward control unit  105  also performs control for causing the c-pumps  102  to output the secondary pumping lights Lc with the minimum power, for the forward second drivers  104 . 
     When the process in step S 12  is completed, the forward control unit  105  calculates the pumping ratio of the primary pumping lights Li (step S 13 ). For example, the forward control unit  105  calculates the pumping ratio of the primary pumping lights Li on the basis of the total input power obtained by multiplying the fiber input power by the number of bands (or the number of wavelengths) of the signal light Ls, and the target gain. 
     When the process in step S 13  is completed, the forward control unit  105  maximizes the power of the primary pumping lights Li by the pumping ratio (step S 14 ). Since the primary pumping lights Li are output at the minimum power, by maximizing the primary pumping lights Li by the pumping ratio, the primary pumping lights Li are output at the power according to the pumping ratio (for example, several fold). When the process in step S 14  is completed, the forward control unit  105  raises the power of the secondary pumping lights Lc by the set predetermined ratio (step S 15 ). When the process in step S 15  is completed, the forward control unit  105  finishes the initial state transition process. 
     The completion of the initial state transition process allows the power characteristics of the primary pumping lights Li in the initial state and the power characteristics of the secondary pumping lights Lc in the initial state to be determined, as illustrated in  FIG.  6 A .  FIG.  6 A  illustrates three primary pumping lights Li having different wavelengths and five secondary pumping lights Lc having different wavelengths as examples, but the number of wavelengths is not particularly limited. The number of wavelengths of the primary pumping lights Li may be the same as or different from the number of wavelengths of the secondary pumping lights Lc. 
     The setting of the target gain in the process in step S 11  causes the gain to be changed, causing the gain tilt between the C band and the L band as illustrated in  FIG.  6 A . In  FIG.  6 A , the signal light Ls in the C band including several tens of wavelengths and the signal light Ls in the L band including several tens of wavelengths are illustrated, but the number of wavelengths is not particularly limited, and the number of wavelengths may be the same or may differ between the C band and the L band. As seen from the above, since the gain tilt occurs between the C band and the L band, as illustrated in  FIG.  4   , the forward control unit  105  starts a loop process (step S 20 ), and first, the tilt control unit  105 T performs a tilt control process (step S 30 ). 
     More specifically, as illustrated in  FIG.  7 A , the tilt control unit  105 T calculates the gain difference between the C band and the L band (step S 31 ). For example, the tilt control unit  105 T calculates a first gain of the signal light Lt of each wavelength belonging to the C band by the Raman amplification in the optical transmission line  32 , on the basis of the power of the signal light Lt in the C band and the information indicating the output power of the signal light Lt in the C band contained in the OSC light Ly. Similarly, the tilt control unit  105 T calculates a second gain of the signal light Lt in the L band by the Raman amplification in the optical transmission line  32 , on the basis of the power of the signal light Lt in the L band and information indicating the output power of the signal light Lt belonging to the L band contained in the OSC light Ly. After calculating the first gain and the second gain, the tilt control unit  105 T calculates the gain difference between the C band and the L band, on the basis of the difference between the first gain and the second gain. Therefore, in the gain characteristics illustrated in  FIG.  6 A , the gain difference between the gain of the L band, which is the wavelength band with high gain, and the gain of the C band, which is the wavelength band with a low gain, is calculated. 
     After calculating the gain difference, the tilt control unit  105 T adjusts the pumping power ratios of the primary pumping lights Li, on the basis of the gain difference (step S 32 ). Here, the tilt control unit  105 T includes a pumping ratio table that stores ratio information indicating the pumping power ratios of the primary pumping lights Li for obtaining the specified gain characteristics with respect to the gain by the Raman amplification in the optical transmission line  32 . Therefore, the tilt control unit  105 T adjusts the pumping power ratios of the primary pumping lights Li on the basis of the gain difference and the ratio information of the pumping ratio table, and determines whether the gain difference becomes within an acceptable range. When the gain difference becomes within the acceptable range, the tilt control unit  105 T finishes the tilt control process. 
     For example, as illustrated in  FIG.  6 B , when the ratio of power of the primary pumping light Li between the wavelengths is adjusted on the basis of the gain difference and the ratio information, the gain of the C band increases, and the gain of the L band decreases. This causes the gain characteristics to be substantially uniform or substantially flat. In this case, the tilt control unit  105 T determines that the gain difference becomes within the acceptable range, and finishes the tilt control process. 
     When the tilt control unit  105 T finishes the tilt control process, as illustrated in  FIG.  4   , the gain control unit  105 G performs a gain control process (step S 40 ). More specifically, as illustrated in  FIG.  7 B , the gain control unit  105 G calculates the average gain of the entirety of the C band and the L band (step S 41 ). For example, the gain control unit  105 G sums up the first gain of the signal light Lt in the C band and the second gain of the signal light Lt in the L band, and divides the resulting sum by the number of bands of the C band and the L band to calculate the average gain. The gain control unit  105 G may calculate the average gain by summing up the first gains of the signal lights Lt of respective wavelengths belonging to the C band and the second gains of the signal lights Lt of respective wavelengths belonging to the L band and dividing the resulting sum by the total number of wavelengths included in the C band and the L band. 
     After calculating the average gain, the gain control unit  105 G adjusts the average power of the secondary pumping lights Lc (step S 42 ). For example, the gain control unit  105 G determines the lump loss by calculating the ratio between the power of the secondary pumping light Lc immediately before the input to the optical filter  109  and the power of the secondary pumping light Lc immediately after the output from the optical filter  109 . Then, the gain control unit  105 G adjusts the average power of the secondary pumping lights Lc up and down on the basis of the determined lump loss until the target gain is achieved. 
     As illustrated in  FIG.  6 C , when the average power of the secondary pumping lights Lc is adjusted to be decreased, the gain of the C band and the gain of the L band decrease. As the average power of the secondary pumping lights Lc decreases, the amount of noise of the RINs of the secondary pumping lights Lc decreases. This can reduce the amount of noise of the RINs of the secondary pumping lights Lc transferred to the signal light Ls via the primary pumping lights Li. After adjusting the average power of the secondary pumping lights Lc, the gain control unit  105 G finishes the gain control process. 
     When the gain control process is finished, as illustrated in  FIG.  4   , the forward control unit  105  finishes the loop process (step S 50 ). For example, when both the gain difference and the average gain are within respective acceptable ranges, the forward control unit  105  finishes the loop process, fixes the power of each of the primary pumping lights Li and the power of each of the secondary pumping lights Lc (step S 60 ), and finishes the process. On the other hand, when at least the gain difference or the average gain is not within the acceptable range, the forward control unit  105  returns to the process in step S 20 , and the tilt control unit  105 T and the gain control unit  105 G repeat the same processes. 
     Next, a description will be given of the tilt control and the gain control in accordance with the comparative example with reference to  FIG.  8    and  FIG.  9   . The initial state illustrated in  FIG.  8 A  is the same as the initial state illustrated in  FIG.  6 A , and therefore, the detailed description thereof is omitted. The tilt control illustrated in  FIG.  8 B  is also the same as the tilt control illustrated in  FIG.  6 B , and therefore, the detailed description thereof is omitted. 
     As illustrated in  FIG.  8 C , the gain control unit  105 G in accordance with the comparative example performs the gain control process for the primary pumping lights Li. As described, even when the gain control unit  105 G in accordance with the comparative example performs the gain control process for the primary pumping lights Li, the average gain can be controlled as in the embodiment. However, since the gain control unit  105 G in accordance with the comparative example does not perform the gain control process for the secondary pumping lights Lc, the power of the secondary pumping light Lc remains fixed from the initial state. Therefore, the amount of noise of the RINs of the secondary pumping lights Lc does not vary. Therefore, compared with the case of the embodiment, the amount of noise of the RINs of the secondary pumping lights Lc transferred to the signal light Ls via the primary pumping lights Li cannot be reduced. In other words, the amount of noise of the RINs of the secondary pumping lights Lc transferred to the signal light Ls relatively increases as compared with that in the embodiment. 
     When the lump loss is large as illustrated in  FIG.  9 A , to compensate the lump loss, it is required to increase the power of the pumping light of the Raman amplification. When the power of the pumping light of the Raman amplification is increased, it is conceivable to increase the power of the primary pumping lights Li, the power of the secondary pumping lights Lc, or both of them. 
     As illustrated in  FIG.  9 B , even when the power of the primary pumping light Li output by the i-pump  101  increases, the amount of noise of the RIN of the primary pumping light Li is not proportional to the increase in power, and is substantially constant. That is, the amount of noise of the RIN of the primary pumping light is substantially constant regardless of the increase or decrease in power of the primary pumping light. By contrast, as the power of the secondary pumping light Lc output by the c-pump  102  increases, the amount of noise of the RIN increases in proportion to the increase in power. That is, the amount of noise of the RIN of the secondary pumping light varies according to the increase or decrease in the power of the secondary pumping light. In particular, in many cases, the amount of noise of the RIN of the secondary pumping light Lc is larger than that of the primary pumping light Li. 
     Therefore, to achieve the same average gain, as described as the comparative example, it is not desirable that both the tilt control and the gain control do not adjust the power of the secondary pumping lights Lc and adjust the power of the primary pumping lights Li alone. To achieve the same average gain, as described as the embodiment, it is desirable that the tilt control adjusts the power of the primary pumping light Li and the gain control adjusts the power of the secondary pumping light Lc. By adjusting the power of the secondary pumping light Lc, which contains the RIN having the larger amount of noise than that of the primary pumping light Li, the amount of noise of the RIN transferred to the signal light Ls can be reduced. As a result, the embodiment can improve the transmission performance such as the long transmission distance. 
     For example, it may be conceivable to adjust the ratio of power between the wavelengths of the secondary pumping lights Lc in the tilt control. However, as illustrated in  FIG.  10 A , since the secondary pumping lights Lc amplify the signal light Ls via the primary pumping lights Li, the ratio of the gain for the signal light Ls is less likely to vary. As a result, the variable range of the ratio of gain between wavelengths narrows. As seen from the above, it is considered that the gain for the signal light Ls does not vary so much even when the ratio of power between the wavelengths of the secondary pumping lights Lc is adjusted. 
     In addition, as illustrated in the left part of  FIG.  10 B , when the ratio of power between the wavelengths of the secondary pumping lights Lc is adjusted, since there is a power difference between the wavelengths of the secondary pumping lights Lc, the variable range of the power of the secondary pumping light Lc becomes smaller by the power difference. Therefore, as illustrated in the right part of  FIG.  10 B , changing the average power without adjusting the ratio of power between the wavelengths of the secondary pumping lights Lc is advantageous in that the variable range of the power of the secondary pumping light Lc can be increased. Thus, it is extremely desirable that the tilt control adjusts the ratio of power between the wavelengths of the primary pumping lights Li, and the gain control adjusts the average power of the secondary pumping lights Lc. 
     As described above, the co-propagating Raman amplifier  100  in accordance with the present embodiment includes the i-pump  101 , the c-pump  102 , and the forward control unit  105 . The i-pump  101  outputs the primary pumping light Li, which propagates in the same direction as the propagation direction of the signal light Ls, to the optical transmission line  31  for Raman amplification. The c-pump  102  outputs the secondary pumping light Lc, which amplifies the primary pumping light Li and propagates in the same direction as the propagation direction of the signal light Ls, to the optical transmission line  31 . The forward control unit  105  controls the gain for the signal light Ls by adjusting the average power of the secondary pumping light Lc. These configurations allow the gain for the signal light Ls to be effectively controlled, and as a result, the transmission performance can be improved. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various change, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.