Abstract:
The present invention relates to an optical amplifier and, in particular, it relates to multistage optical amplification having a feature for compensating for the tilt that is generated when a signal having a plurality of optical wavelengths and the like according to a wavelength division multiplexing (WDM) method is amplified in a wide bandwidth. The multistage optical amplifier comprises: a plurality of optical amplifiers that are connected in a multistage manner; and a common control section for commonly controlling pumping light powers of said plurality of optical amplifiers, wherein said common control section includes: tilt detection means for detecting amounts of tilt of each of said plurality of optical amplifiers; and tilt cancellation means for controlling the pumping light powers of said plurality of optical amplifiers so that the amounts of tilt of each of the optical amplifiers detected by said tilt detection means cancel each other.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an optical amplifier and, in particular, it relates to a multistage optical amplifier that amplifies a signal having a plurality of optical wavelengths (hereinafter referred to as a “main signal” or a “channel”), and the like, in the wide bandwidth used in a WDM (wavelength Division Multiplexing) system. 
   2. Description of the Related Art 
   In recent years, as the Internet has rapidly become widespread, an optical wavelength division multiplexing transmission technique that enables large-capacity and long-distance transmission has attracted attention. In the optical amplification used for the long-distance WDM transmission system, an erbium doped fiber amplifier (EDFA), in which erbium is doped into optical fiber cores, is used. Further, as the number of wavelengths is increased due to the increase of speed and capacity of communications, a Raman fiber amplifier that has a wider amplification bandwidth than the EDFA is also used. 
   As the wavelength bandwidth to be used has expanded, a difference (tilt) of the level between the shortest wavelength channel and the longest wavelength channel of the WDM signal has tended to increase. The tilt is generated as a result of fiber loss wavelength characteristics of transmission paths, characteristics of the amplification factor of an optical amplifier, a nonlinear optical effect due to a stimulated Raman effect and the like and is accumulated due to the increase of transmission distances, the number of spans and the like. 
   In an optical transmission system, in order to maintain its transmission quality, it is necessary to ensure that the optical signal-to-noise ratio (OSNR) is higher than a predetermined value. However, as the OSNR of the channels, the level of which is reduced due to the tilt described above, is reduced significantly, the transmission distance, the number of transmission spans or the number of multiplexed wavelengths (or, in other words, the transmission bandwidth) will be restricted. 
   Therefore, conventionally, in order to compensate for the tilt that is generated and accumulated in the transmission path or the optical amplifier, every time an optical wavelength multiplexed signal is transmitted along several spans, the signal is once demultiplexed into separate channel signals by an optical demultiplexer (optical DEMUX), then, the power level of each channel signal is adjusted by an optical variable attenuator (optical ATT) so that the power level is equalized between the channels and, then, the channel signals are converted into a WDM signal again by an optical multiplexer (optical MUX). 
   In another conventional technique, in order to compensate for the tilt of a WDM signal collectively in a one-stage optical amplifier, the tilt generated in the amplifier is detected by using predetermined false signals and, then, based on the detected value of the tilt, a gain is controlled by adjusting pumping light power and an amount of attenuation is controlled by an optical variable attenuator in a combined manner so that the optical amplifier achieves a flat gain (see Documents 1 and 2). 
   Document 1:
         Japanese Unexamined Patent Publication 2003-51791       

   Document 2:
         Japanese Unexamined Patent Publication 2000-91683       

   However, in the device configuration using optical demultiplexers and optical multiplexers every several spans for level adjustment of separate channels as in the conventional example, there is a problem in that the size and cost of the device is increased. In this case, as optical amplifiers are typically required to compensate for insertion losses generated when the channel signals pass through the optical demultiplexers and optical multiplexers, there is also a problem in that amplified spontaneous emission light (ASE light) is generated in these optical amplifiers and superimposed on the main signal to degrade the OSNR and, thus, the transmission quality is degrated. 
   Further, when the pumping light power is controlled based on the detected value of the tilt in the one-stage optical amplifier, there is a problem in that flat gain characteristics can be achieved at a particular amplification gain value corresponding to a specific pumping light power and the maximum gain of the optical amplifier is limited, in a pinpoint manner, to such a particular amplification gain value. 
   In this case, as a reference operating point is set to a level lower than said maximum gain, by some dB, in consideration of fluctuations of the input signal level, the gain of the optical amplification is further restricted. As a result, when this optical amplifier is used as a first-stage amplifier in a multistage configuration, there is a problem in that the OSNR is not improved much and, therefore, good transmission quality cannot be provided. 
   SUMMARY OF THE INVENTION 
   In view of the above problems, it is an object of the present invention to provide a multistage optical amplifier having a tilt compensation feature that can compensate for tilt of a main signal generated in the optical amplifier collectively and automatically and ensure a remarkably improved OSNR and good transmission quality in an appropriate device size and at a reasonable cost. 
   According to the present invention, there is provided a multistage optical amplifier comprising: a plurality of optical amplifiers that are connected in a multistage manner; and a common control section for commonly controlling pumping light powers of said plurality of optical amplifiers, wherein said common control section includes: tilt detection means for detecting amounts of tilt of each of said plurality of optical amplifiers; and tilt cancellation means for controlling the pumping light powers of said plurality of optical amplifiers so that the amounts of tilt of each of the optical amplifiers detected by said tilt detection means cancel each other. 
   Said tilt detection means detects the amounts of tilt of each of said optical amplifiers from the difference of the level between probe lights disposed at both ends of a main signal, and said tilt cancellation means achieves flat gain-wavelength characteristics by canceling the amounts of tilt of each of said optical amplifiers. 
   Further, said tilt detection means includes: common light detection means for commonly detecting probe lights from said plurality of optical amplifiers; and switch means for sequentially switching the probe lights from said plurality of optical amplifiers and supplying the probe lights to said common light detection means. 
   Said multistage amplifier is constituted by a two-stage optical amplifier in which each of a former stage and a latter stage consists of an erbium doped fiber or a two-stage optical amplifier in which a former stage consists of an optical amplifier having an erbium doped fiber and a latter stage consists of an optical amplifier having a Raman fiber. 
   Further, according to the present invention, there is provided a method for controlling the tilt of a multistage optical amplifier in which two stages of optical amplifiers are interconnected, comprising the steps of: activating a former-stage optical amplifier and a latter-stage optical amplifier by respective pumping light powers; controlling the pumping light power of the former-stage optical amplifier so that the optical output of the former-stage optical amplifier reaches a desired level; detecting amounts of tilt of each of the former-stage optical amplifier and the latter-stage optical amplifier; and controlling the pumping light power of the latter-stage optical amplifier so that the amount of tilt of the former-stage optical amplifier is canceled by the amount of tilt of the latter-stage optical amplifier and, as a result, the combined amount of tilt, of the former-stage optical amplifier and the latter-stage optical amplifier, falls within a desired range. 
   According to the present invention, degradation of transmission quality due to accumulated tilt can be inhibited collectively without using expensive modules such as optical demultiplexers and optical multiplexers and, therefore, the transmission quality in a wideband WDM transmission system can be improved in an appropriate device size and at a reasonable cost. 
   Further, according to the present invention, the tilt cancellation control and gain control of the former-stage amplifier and the latter-stage amplifier is performed by the common control section. Therefore, by combining said tilt cancellation control with flexibility in gain allocation between the former-stage amplifier and the latter-stage amplifier, the gain restriction of the former-stage amplifier in the conventional example can be eliminated and, further, a larger gain can be allocated to the former stage on the assumption that the tilt occurs so that an OSNR and transmission quality can be improved significantly. 
   Still further, according to the present invention, as said common control section can control a gain of one or both of the amplifiers while monitoring the condition of the tilt cancellation, the conventional optical variable attenuator may be eliminated. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more clearly understood from the description as set forth below with reference to the accompanying drawings: 
       FIG. 1  is a diagram showing a first embodiment of the present invention; 
       FIG. 2  is a diagram showing a relationship (1) between a pumping light power and gain characteristics in an EDFA; 
       FIG. 3  is a diagram showing a relationship (2) between a pumping light power and gain characteristics in an EDFA; 
       FIG. 4  is a diagram showing an exemplary control flow of tilt compensation in  FIG. 1 ; 
       FIG. 5  is a diagram showing another exemplary aspect of  FIG. 1 ; 
       FIG. 6  is a diagram showing a second embodiment of the present invention; 
       FIG. 7  is a diagram showing a relationship between a pumping light power and gain characteristics in a Raman fiber; 
       FIG. 8  is a diagram showing an exemplary control flow of tilt compensation in  FIG. 6 ; 
       FIG. 9  is a diagram showing another exemplary aspect (1) of  FIG. 6 ; 
       FIG. 10  is a diagram showing an exemplary control flow of tilt compensation in  FIG. 9 ; 
       FIG. 11  is a diagram showing another exemplary aspect (2) of  FIG. 6 ; 
       FIG. 12  is a diagram showing a third embodiment of the present invention; 
       FIG. 13  is a diagram showing an example of a four-wave mixed light; 
       FIG. 14  is a diagram showing an exemplary control flow of tilt compensation in  FIG. 12 ; 
       FIG. 15  is a diagram showing another exemplary aspect (1) of  FIG. 12 ; 
       FIG. 16  is a diagram showing another exemplary aspect (2) of  FIG. 12 ; 
       FIG. 17  is a diagram showing an exemplary control flow of tilt compensation in  FIG. 16 ; and 
       FIG. 18  is a diagram showing another exemplary aspect (3) of  FIG. 12 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows a first embodiment of a multistage optical amplifier having a tilt compensation feature according to the present invention. Though a two-stage optical amplifier is exemplified in this example, the present invention is not limited to such a configuration. 
   In  FIG. 1 , a main signal is input from an input port  11  and, in an optical coupler (CPL)  12 , coupled with a pumping light from an EDFA pumping section (EDFA Pump)  16  for the former stage. The signal is further coupled with two probe lights Pλ 1  and Pλ 2  from probe light sources  17  and  18 , respectively, for the former stage via optical couplers (CPL)  15  and  13  and, then, input to an erbium doped fiber (EDF)  14  for the former stage. 
   The EDF  14  amplifies the input main signal and probe lights by the pumping light and a portion of the amplified signal is separated by an optical coupler (CPL)  19 . The portion of the signal separated by the optical coupler  19  is further divided into two waves in a subsequent optical coupler (CPL)  23  and input into respective optical filters  24  and  25 . 
   The optical filter  24  passes only the probe light Pλ 1  and the optical filter  25  passes only the probe light Pλ 2 . Signal levels of the probe lights Pλ 1 , and Pλ 2  are detected by subsequent photodiodes  29  and  30 , respectively. 
   On the other hand, only the main signal portion of the main signal and the probe lights that have passed through the optical coupler  19  is now passed through a subsequent optical filter  20  and input to an optical variable attenuator (ATT)  21 . The optical variable attenuator  21  changes the amount of attenuation of the input main signal according to the directions of a feedback control section  33  so that the output level of the main signal eventually output from the output port  37  can be kept constant. 
   The main signal, which is attenuated by the optical variable attenuator  21 , is passed through the circulator  22  without change and input to an erbium doped fiber (EDF)  34  in the latter stage. Also in the latter stage, the main signal input to the EDF  34  is amplified by using a pumping light from an EDFA pumping section (EDFA Pump)  41  for the latter stage. The latter stage is configured similarly to the former stage except that the pumping light and probe lights are traveling in the inverse direction to that in the former stage. 
   The pumping light from the EDFA pumping section  41  for the latter stage is coupled to a main signal path via an optical coupler (CPL)  36  and the two probe lights Pλ 1  and Pλ 2  from probe light sources  39  and  40 , respectively, for the latter stage are coupled to the same main signal path via optical couplers  38  and  35 . 
   The pumping light and the two probe lights Pλ 1  and Pλ 2  input in the latter stage are isolated from the former stage by a circulator  22  and flow into an optical coupler (CPL)  26 . The optical coupler  26  divides them into two waves and inputs the two waves to respective optical filters  27  and  28 . The optical filter  27  passes only the probe light Pλ 1  and the optical filter  28  passes only the probe light Pλ 2 . Signal levels of the probe lights Pλ 1  and Pλ 2  are detected by subsequent photodiodes  31  and  32 , respectively. 
   The feedback control section  33 , which is shared by the former and latter stages, compares the signal level between the probe lights Pλ 1  and Pλ 2  from the photodiode  29  and  30 , respectively, for the former stage to detect the amount of tilt of the former-stage optical amplifier. Similarly, the feedback control section  33  compares the signal level between the probe lights Pλ 1  and Pλ 2  from the photodiode  31  and  32 , respectively, for the latter stage to detect the amount of tilt of the latter-stage optical amplifier. 
   Based on these detection results, the feedback control section  33  controls the respective pumping light powers of the EDFA pumping section  16  for the former stage and the EDFA pumping section  41  for the latter stage so that the amounts of tilt of the former-stage and latter-stage optical amplifiers cancel each other. 
   Therefore, according to this embodiment, each optical amplifier in the former and latter stages does not need to have flat gain characteristics and it is not necessary that the maximum gain of the optical amplification is limited, in a pinpoint manner, to a particular amplification gain value at which the flat gain characteristics are exhibited by a specific pumping light power, as in the conventional art. As a result, an OSNR and transmission quality can be improved significantly by allocating a larger portion of the gain to the former-stage amplifier while allowing the occurrence of tilt to some extent. This also contributes to the increase of the transmission distance and the number of spans. 
     FIGS. 2 and 3  show relationships between a pumping light power and EDFA gain characteristics. 
   Item (a) in  FIG. 2  shows an example in which the probe lights Pλ 1  and Pλ 2  for the former stage are disposed at both ends of an input C band (1520-1550 nm) main signal and, along with them, a pumping light (980 nm or 1480 nm) having a larger power than a pumping light power (standard value) that exhibits flat gain characteristics is input to the former-stage EDF  14 . As the pumping light power exceeds the standard value as in this example, the EDF  14  exhibits steadily declining tilt characteristics wherein the gain is reduced in the higher band than in the lower band. 
   On the other hand, Item (b) in  FIG. 2  shows an example in which said main signal and probe lights are amplified by inputting a pumping light having a smaller power than the standard value to the EDF  14 . As the pumping light power is reduced as in this example, the EDF  14  exhibits steadily increasing tilt characteristics wherein the gain is increased in the higher band than in the lower band. 
   Further, Item (a) in  FIG. 3  shows an example in which the main signal from the former stage is input to the EDF  34  in the latter stage and Item (b) in  FIG. 3  shows an example in which the probe lights Pλ 1  and Pλ 2  for the latter stage that are disposed at both ends of the main signal and a pumping light for the latter stage that has a larger power than the standard value are input to the EDF  34  in the latter stage from the opposite direction to the propagating direction of the main signal. 
   It can be seen from the above examples that the tilt characteristics given to the main signal shown in Item (a) of  FIG. 3  agree with the tilt characteristics of the probe lights Pλ 1  and Pλ 2  in Item (b) of  FIG. 3  that are input from the opposite direction to the former. Therefore, the tilt characteristics of the latter-stage optical amplifier can be detected from the difference of the level between the probe lights Pλ 1  and Pλ 2  for the latter stage. 
     FIG. 4  shows an exemplary control flow of tilt compensation in the first embodiment. 
   In  FIG. 4 , the feedback control section  33  activates the former-stage EDF  14  and the latter-stage EDF  34  by using the pumping light powers of respective initial values. At this time, the amount of attenuation of the optical variable attenuator  21  is set to a maximum value (S 001 ). 
   Next, at point B ( FIG. 1 ) where the amplified main signal is output from the former stage EDF  14 , it is determined whether the level of the main signal reaches a predetermined value that is expected in relation to said initial value (S 002 ). If it does not reach said predetermined value, the EDFA pumping section  16  for the former stage is controlled to increase the pumping light power till the level of the main signal reaches said predetermined value without consideration of the occurrence of the tilt in the former-stage EDF  14  (S 003 ). 
   If the level of the main signal reaches said predetermined value, the inclination of the level of the main signal (the amount of tilt) in the former-stage EDF  14  is detected from the difference of the level between the probe lights Pλ 1  and Pλ 2  for the former stage (S 004 ) and, at the same time, the inclination of the level of the main signal (the amount of tilt) in the latter-stage EDF  34  at this time is detected from the difference of the level between the probe lights Pλ 1  and Pλ 2  for the latter stage (S 005 ). 
   Next, it is determined whether the inclinations in the former-stage EDF  14  and the latter-stage EDF  34  cancel each other to achieve a flat gain or not (S 006 ). Based on the result of this determination, if the inclination obtained by combining the two inclinations is steadily increasing (Item (a) in  FIG. 2 ), the EDFA pumping section  41  for the latter stage is controlled to increase the pumping light power until the increasing inclination of the tilt characteristics is eliminated and a flat gain is achieved (S 007 ). 
   On the contrary, if the inclination obtained by combining the two inclinations is steadily declining (Item (b) in  FIG. 2 ), the EDFA pumping section  41  for the latter stage is controlled to reduce the pumping light power till the declining inclination of the tilt characteristics is eliminated and a flat gain is achieved (S 008 ). 
   After the flat gain is achieved, at point C ( FIG. 1 ) where the main signal is output from the latter-stage EDF  34 , in order to keep the optical output of the main signal constant, it is determined whether the level of the main signal reaches a predetermined value or not (S 009 ). If it does not reach said predetermined value, the optical variable attenuator  21  is controlled to reduce the amount of attenuation till the level of the main signal reaches said predetermined value (S 010 ) so that the output level of the main signal is kept constant. 
     FIG. 5  shows another exemplary aspect of the first embodiment shown in  FIG. 1 . 
   In  FIG. 1 , the probe lights Pλ 1  and Pλ 2  from the former stage and the probe lights Pλ 1  and Pλ 2  from the latter stage are detected individually by using the four photodiodes  29 - 32 . In contrast to this, the multistage optical amplifier in this example uses one optical switch  51  and one photodiode  52  so that the common photodiode  52  can detect each probe light Pλ 1  and Pλ 2  by sequentially switching the optical switch  51 . The rest of the configuration is similar to that in  FIG. 1 . 
   According to this configuration, the components shared by the former and latter stages can be configured simply and, further, it is not necessary to consider the unevenness of light receiving characteristics among the photodiodes  29 - 32  in  FIG. 1 . Therefore, it is also not necessary to adjust the light receiving level among the light detection circuits using the photodiodes  29 - 32 . As a result, the feedback control section  33  that is shared by the former and latter stages can compare the respective probe lights Pλ 1  and Pλ 2  accurately without adjustment. 
     FIG. 6  shows a second embodiment of a multistage optical amplifier having a tilt compensation feature according to the present invention. Though a two-stage optical amplifier is exemplified also here, the present invention is not limited to such configuration. 
   In  FIG. 6 , a Raman fiber amplifier (RAMAN)  61  is adopted as an optical amplifier in the latter stage and, therefore, a Raman fiber pumping section (RAMAN Pump)  62  is provided. Other configuration is similar to that in  FIG. 1 . 
   In general, the EDFA can achieve flat gain characteristics only in a relatively narrow bandwidth and the range of the available pumping light power in such bandwidth is restricted. On the other hand, in the Raman fiber amplifier that amplifies signals via stimulated Raman scattering in optical fibers, gain bandwidth is not restricted unlike in the EDFA and, further, arbitrary gain characteristics can be achieved to some extent by using a plurality of pumping light powers. However, in the Raman fiber amplifier, the efficiency of utilization of the pumping light power is reduced and, therefore, its output power is restricted. 
     FIG. 7  shows an example of a pumping light power and gain bandwidth characteristics in a Raman fiber. 
   Item (a) in  FIG. 7  shows an example in which the probe lights Pλ 1  and Pλ 2  for the latter stage are disposed at both ends of the main signal from the former stage and, further, two pumping lights λ 1 ′ and λ 2 ′ that are shifted from the main signal band to be amplified by about 100 nm are disposed. In this case, λ 1 ′ is 1420 nm and λ 2 ′ is 1450 nm. In this connection, it is to be noted that there is no limit on the number of Raman pumping lights, in general. 
   Item (b) in  FIG. 7  shows an example in which the power of the pumping light λ 1 ′ of the two pumping lights λ 1 ′ and λ 2 ′ is larger than that of the pumping light λ 2 ′. In this case, the main signal in a specific bandwidth corresponding to the pumping light λ 1 ′ and the main signal in a specific bandwidth corresponding to the pumping light λ 2 ′ are amplified in a staggered manner according to the powers of the pumping lights λ 1 ′ and λ 2 ′. As a result, the main signal exhibits steadily declining tilt characteristics. 
   On the other hand, Item (c) in  FIG. 7  shows an example in which the power of the pumping light λ 1 ′ of the two pumping lights λ 1 ′ and λ 2 ′ is smaller than that of the pumping light λ 2 ′. In this case, contrary to Item (b) in  FIG. 7 , the main signal is amplified to exhibit steadily increasing tilt characteristics. 
   Thus, though the efficiency of utilization of the pumping light power is reduced, the Raman fiber amplifier has an advantage in that the gain characteristics and the bandwidth of the main signal can be controlled independently of each other by combining a plurality of pumping lights and selecting the power of these pumping lights appropriately. The multistage optical amplifier according to the second embodiment of the present invention is configured so that the advantages of the former-stage EDF  14  and the latter stage Raman fiber  61  can be utilized actively and the both can compensate for each other&#39;s disadvantages. 
   More specifically, in the EDF  14  in the former stage having higher pumping efficiency, in order to improve the OSNR, a higher gain is allocated on the assumption that the tilt occurs. Then, in the Raman fiber  61  in the latter stage, the consumption of its pumping power is restrained by compensating for the tilt that is generated mainly in the former stage. Further, as described below, the Raman fiber  61  in the latter stage can keep the output constant in place of the optical variable attenuator  21  (see  FIG. 9 ). By controlling these components through the common feedback control  33 , it is possible to provide a high-efficiency optical amplifier having an improved OSNR and flat gain characteristics in an appropriate device size and at a reasonable cost. 
     FIG. 8  shows an exemplary control flow of tilt compensation in the second embodiment. 
     FIG. 8  is basically identical to the control flow of tilt compensation in the first embodiment shown in  FIG. 4 . Here, only the distinction between them will be described. 
   First, in Step S 105  of  FIG. 8 , the inclination of the level of the main signal (the amount of tilt) in the latter-stage Raman fiber  61  at this time is detected from the difference of the level between the probe lights Pλ 1  and Pλ 2  for the latter stage. Next, it is determined whether the inclinations in the former-stage EDF  14  and the latter-stage Raman fiber  61  cancel each other to achieve a flat gain or not (S 106 ). 
   Based on the result of this determination, if the inclination obtained by combining the two inclinations is steadily increasing (Item (b) in  FIG. 7 ), the Raman fiber pumping section  62  for the latter stage is controlled to reduce the power of the pumping light λ 2 ′ having a higher frequency till such tilt characteristics are compensated and a flat gain is achieved (S 107 ). 
   On the contrary, if the inclination obtained by combining the two inclinations is steadily declining (Item (c) in  FIG. 7 ), the Raman fiber pumping section  62  for the latter stage is controlled to reduce the power of the pumping light λ 1 ′ having a lower frequency till such tilt characteristics are compensated for and a flat gain is achieved (S 108 ). The steps other than those mentioned above are identical to those in  FIG. 4 . 
     FIG. 9  shows another exemplary aspect of the second embodiment shown in  FIG. 6 . 
   The multistage optical amplifier in  FIG. 9  is configured similarly to that in  FIG. 6  except that the optical variable attenuator (ATT)  21  in  FIG. 6  is omitted. In this example, the Raman fiber  61  and the Raman fiber pumping section  62 , which are controlled by the feedback control section  33 , act in place of the optical variable attenuator (ATT)  21 . 
     FIG. 10  shows an exemplary control flow of tilt compensation in  FIG. 9 . 
     FIG. 10  is also basically identical to the control flow of tilt compensation in the second embodiment shown in  FIG. 8  and, therefore, only the distinction between them will be described here. 
   First, in step S 201 , only the former-stage EDF  14  and the latter-stage Raman fiber  61  are activated by using the pumping light powers of respective initial values but the activation of the optical variable attenuator  21  is naturally not included in this example. Then, this example is characterized by step S 210  in which, after a flat gain is achieved, the Raman fiber amplifier controls the optical output of the main signal to be constant at point C ( FIG. 6 ) where the main signal is output from the Raman fiber  61  in the latter stage. 
   More specifically, if the output level of the main signal does not reach a predetermined value (S 209 ), the feedback control section  33  controls the Raman fiber pumping section  62  to increase the total power of the pumping lights λ 1 ′ and λ 2 ′ (S 210 ) while maintaining the power ratio between the pumping lights λ 1 ′ and λ 2 ′ when the flat gain is achieved (S 206 ). In this example, the output level of the main signal can be kept constant without using the optical variable attenuator  21 . 
     FIG. 11  shows yet another exemplary aspect of the second embodiment shown in  FIG. 6 . 
   In a similar manner to that in  FIG. 5  described above, the multistage optical amplifier in this example uses one optical switch  51  and one photodiode  52  so that the common photodiode  52  can detect each probe light Pλ 1  and Pλ 2  by sequentially switching the optical switch  51 . The rest of the configuration is similar to that in  FIG. 6 . 
   According to this configuration, as described above with reference to  FIG. 5 , the common control section in  FIG. 6  can be configured simply and the feedback control section  33  can compare the respective probe lights Pλ 1  and Pλ 2  accurately without adjustment. 
     FIG. 12  shows a third embodiment of a multistage optical amplifier having a tilt compensation feature according to the present invention. Though a two-stage optical amplifier is exemplified also in this example, the present invention is not limited to such configuration. 
   In  FIG. 12 , the multistage optical amplifier is provided with two latter-stage Raman fiber pumping sections  63  and  64  that can be controlled independently by the feedback control section  33  and utilizes a well-known four-wave mixed light generated by pumping lights λ 1 ′ and λ 2 ′ from the Raman fiber pumping sections  63  and  64  so that the pumping light for the former-stage EDF  14  can also be generated. This eliminates the need for the former-stage EDFA pumping section  16  and the optical coupler  12  for coupling the pumping light from the EDFA pumping section  16  to the transmission path of the main signal and, therefore, these elements are omitted in  FIG. 12 . On the other hand, an optical coupler  65  is provided for multiplexing the pumping lights λ 1 ′ and λ 2 ′ from the Raman fiber pumping sections  63  and  64 , respectively, and supplying the multiplexed light to the latter-stage Raman fiber  61  and, further, optical couplers  66  and  67  are provided for forming an optical path to supply the pumping light for the former stage obtained by the four-wave mixed light to the EDF  14 . The rest of the configuration is similar to that in  FIG. 6 . 
     FIG. 13  shows an example of the four-wave mixed light in  FIG. 12 . 
   Item (a) in  FIG. 13  shows the pumping lights λ 1 ′ and λ 2 ′ in  FIG. 12  by way of example. Here, λ 1 ′ is 1420 nm and λ 2 ′ is 1450 nm (see Item (a) in  FIG. 7 ). Therefore, Δλ=|λ 1 ′−λ 2 ′|=30 nm. 
   Item (b) in  FIG. 13  shows the four-wave mixed light obtained by mixing them. At both ends of the pumping lights λ 1 ′ and λ 2 ′, two optical signals λ 0 ′(=λ 1 ′−Δλ) and λ 3 ′(=λ 2 ′+Δλ) are newly generated as a result of the mixing. Among them, λ 3 ′=λ 2 ′+Δλ=1450 nm+30 nm=1480 nm is used as the pumping light for the former-stage EDF  14  in this example (see  FIG. 2 ). This pumping light is supplied to the EDF  14  in the former stage via the optical couplers  66  and  67 . 
     FIG. 14  shows an exemplary control flow of tilt compensation in  FIG. 12 . 
   In  FIG. 14 , the feedback control section  33  first activates the latter-stage Raman fiber  61  by using the pumping light powers of respective initial values. At this time, the amount of attenuation of the optical variable attenuator  21  is set to a maximum value (S 301 ). At the same time, the former-stage EDF  14  is also activated by the pumping light λ 3 ′ for the former stage that is generated accordingly. 
   Next, at point B ( FIG. 12 ) where the amplified main signal is output from the former-stage EDF  14 , it is determined whether the level of the main signal reaches a predetermined value or not (S 302 ). If it does not reach said predetermined value, the Raman fiber pumping sections  63  and  64  are controlled to increase the total power of the pumping lights (λ 1 ′ and λ 2 ′) until the level of the main signal reaches said predetermined value (S 303 ). 
   If the level of the main signal reaches said predetermined value, the inclination of the level of the main signal (the amount of tilt) in the former-stage EDF  14  is detected from the difference of the level between the probe lights Pλ 1  and Pλ 2  for the former stage (S 304 ) and, at the same time, the inclination of the level of the main signal (the amount of tilt) in the latter-stage Raman fiber  61  is also detected from the difference of the level between the probe lights Pλ 1  and Pλ 2  for the latter stage (S 305 ). 
   Next, it is determined whether the inclinations in the former-stage EDF  14  and the latter-stage Raman fiber  61  cancel each other to achieve a flat gain or not (S 306 ). Based on the result of this determination, if the inclination obtained by combining the two inclinations is steadily increasing (Item (a) in  FIG. 7 ), the Raman fiber pumping sections  63  and  64  are controlled to reduce the power of the pumping light of a higher frequency and, at the same time, increase the power of the pumping light of a lower frequency till the increasing inclination of the tilt characteristics is eliminated and a flat gain is achieved (S 307 ). 
   On the contrary, if the inclination obtained by combining the two inclinations is steadily declining (Item (b) in  FIG. 7 ), the power of the pumping light of a lower frequency is reduced and, at the same time, the power of the pumping light of a higher frequency is increased till the declining inclination of the tilt characteristics is eliminated and a flat gain is achieved (S 308 ). 
   After the flat gain is achieved, at point C ( FIG. 12 ) where the main signal is output from the latter-stage Raman fiber  61 , in order to keep the optical output of the main signal constant, it is determined whether the level of the main signal reaches a predetermined value or not (S 309 ). If it does not reach said predetermined value, the optical variable attenuator  21  is controlled to reduce the amount of attenuation till the level of the main signal reaches said predetermined value (S 310 ) so that the output level of the main signal is kept constant. 
     FIG. 15  shows another exemplary aspect of the third embodiment shown in  FIG. 12 . 
   In a similar manner to that in  FIG. 5  described above, the multistage optical amplifier in this example uses one optical switch  51  and one photodiode  52  so that the common photodiode  52  can detect each probe light Pλ 1  and Pλ 2  by sequentially switching the optical switch  51 . Other configuration is similar to that in  FIG. 12 . 
   According to this configuration, the common control section in  FIG. 12  can be configured simply and the feedback control section  33  can compare the respective probe lights Pλ 1  and Pλ 2  accurately without adjustment. 
     FIG. 16  shows yet another exemplary aspect of the third embodiment shown in  FIG. 12 . 
   The multistage optical amplifier in  FIG. 16  is configured similarly to that in  FIG. 12  except that the optical variable attenuator (ATT)  21  in  FIG. 12  is omitted. In this example, the Raman fiber  61  and the Raman fiber pumping sections  63  and  64 , which are controlled by the feedback control section  33 , act in place of the optical variable attenuator (ATT)  21 . 
     FIG. 17  shows an exemplary control flow of tilt compensation in  FIG. 16 . 
   As for  FIG. 17 , only the difference from the control flow of tilt compensation shown in  FIG. 14  will be described. First, in step S 401 , the setting of the optical variable attenuator  21  is not included, naturally. Then, in step S 402 , after a flat gain is achieved in steps S 406 -S 408 , the variation of the level at point C generated by the process to achieve the flat gain is corrected. 
   To that end, in step S 402 , the Raman fiber pumping sections  63  and  64  are controlled repeatedly to increase or decrease the total power of the pumping lights until the level of the main signal is stabilized at a predetermined constant value (S 403 - 1  and S 403 - 2 ) so that the optical output of the main signal is kept constant. Other steps are similar to those in the control flow in  FIG. 14 . 
     FIG. 18  shows another exemplary aspect of  FIG. 16 . 
   In a similar manner to that in  FIG. 5  described above, the multistage optical amplifier in this example uses one optical switch  51  and one photodiode  52  so that the common photodiode  52  can detect each probe light Pλ 1  and Pλ 2  by sequentially switching the optical switch  51 . The rest of the configuration is similar to that in  FIG. 16 . 
   According to this configuration, the configuration of the common control section in  FIG. 12  can be configured very simply and the feedback control section  33  can compare the respective probe lights Pλ 1  and Pλ 2  accurately without adjustment.