Abstract:
Disclosed is a wavelength-division multiplexing optical transmission system in which an optical lossy medium, optical amplifiers and Raman amplifiers for compensating for loss in the optical lossy medium are cascade-connected. The system includes power-level equalizing means for equalizing optical power levels input to an optical amplifier of a succeeding stage by adjusting excitation ratio of a Raman amplifier; optical-SNR equalizing means for adjusting power levels at a transmitting end to equalize optical SNRs at a receiving end; and correction-value acquisition means for acquiring a correction value that represents an amount of change in power of each wavelength before and after optical-SNR equalization control. At control for equalizing power levels by a Raman amplifier, the power-level equalizing means performs equalization control using the correction value that represents the amount of change in power of each wavelength before and after optical-SNR equalization control the previous time, and the optical-SNR equalizing means subsequently performs optical-SNR equalization control.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to a wavelength-division multiplexing optical transmission system and repeater station in this system. More particularly, the invention relates to a wavelength-division multiplexing optical communication system and repeater station in which the capacity and transmission distance of an optical transmission system are increased by suppressing interchannel variations (inter-wavelength variations) in received light power, which are caused by wavelength-dependent gain of optical amplifiers and wavelength-dependent loss in the optical fiber of the transmission line.  
           [0002]    Interchannel variations (inter-wavelength scattering) in the power of received light in a WDM optical amplifying repeater transmission system are caused by the characteristics and wavelength dependence of an optical lossy medium (optical devices, optical amplifiers and the optical transmission line) through which the wavelength-division multiplexed signal is transmitted, and may be classified into the following components depending upon the cause and characteristics:  
           [0003]    1) a slope (primary slope) component caused by the wavelength-dependent loss of the optical transmission line and optical devices and by the Raman effect of the optical transmission line;  
           [0004]    2) a beat component (a comparatively gentle rise and fall in the shape of the spectrum) caused by the wavelength-dependent gain of the optical amplifiers; and  
           [0005]    3) a ripple component (a deviation on the order of 0.1 to 1 nm) caused by a gain equalizer in the optical amplifiers, an optical device used in an OADM (Optical Add/Drop Multiplexer), etc., a deviation in the output level of the transmitter in each channel and an adjustment error following wavelength-division multiplexing.  
           [0006]    Interchannel variations in optical power at the receiving end that are the result of these factors produces variations in optical SNR and, as a result, degrades the transmission characteristic (bit error rate, or BER) and imposes a severe limitation upon the capacity and transmission distance of WDM optical transmission. More specifically, since the wavelength signal of lowest power among the multiplexed wavelength signals is the lower-limit value of receive power after transmission, the maximum transmission distance is limited by the wavelength signal of lowest power. Accordingly, reducing the variation between wavelengths after transmission is critical in terms of enlarging the maximum relay transmission distance.  
           [0007]    To achieve this, flattening control and optical pre-emphasis control are available. Flattening control eliminates slope components of the optical spectrum with respect to wavelength (primary slope of the wavelength characteristic) and beat components by changing the excitation ratio of the excitation light of a Raman amplifier. Optical pre-emphasis control equalizes optical SNR (Signal-to-Noise Ratio) at the receiving end by changing the optical output level at the transmitting end channel by channel.  
           [0008]    Flattening control by a Raman amplifier seeks to equalize, as much as possible, the input levels to optical amplifiers at the system nodes (optical repeater stations), and the object of pre-emphasis control is to calculate or measure optical SNR of each channel, adjust the output levels at the transmitting end and equalize the optical SNRs at the receiving end. By exercising such control, it is possible to improve the optical SNR characteristic of the overall system and achieve transmission of greater capacity and over longer distances.  
           [0009]    [0009]FIG. 14 is a diagram useful in describing optical pre-emphasis control (see the specification of Japanese Patent Application Laid-Open No. 2001-203414). A WDM optical signal generated by an optical transmitter  11   a  in an optical transmitting station  11  is amplified by a plurality of optical repeaters  13   a,    13   b,  . . .  13   n,  which are provided in optical transmission lines  12 , so as to compensate for loss along the optical transmission lines  12  and loss in the optical repeaters  13   a,    13   b,  . . .  13   n,  the amplified signal is transmitted to an optical receiving station  14  and the signal is received and processed by an optical receiver  14   a.  Loss in the optical repeaters  13   a,    13   b,  . . .  13   n  is produced by optical component parts such as a dispersion compensating fiber used in the stations.  
           [0010]    When the WDM optical signal is sent from the optical transmitter  11   a  to the optical transmission lines  12 , pre-emphasis is applied by a pre-emphasis control circuit  11   b  within the optical transmitting station. That is, the pre-emphasis control circuit  11   b  calculates the difference between an average value of optical SNRs of all channels received from the optical receiving station  14  and the optical SNR of each individual channel and adjusts the optical level of each channel by an optical attenuator  11   c  so as to compensate for this difference. The optical transmitter  11   a  wavelength-division multiplexes the adjusted optical signals of all channels and sends the multiplexed signals to the optical transmission lines  12 . The optical SNR of the optical signal of each wavelength is measured by an optical-SNR measurement circuit  14   b  provided in the optical receiving station  14 , the information concerning the SNR is transmitted to the optical transmitting station  11  via a line  15  and then the above-described pre-emphasis control is repeated. As a result of the above operation, control is exercised in the optical receiving station  14  so as to uniformalize SNR.  
           [0011]    [0011]FIG. 15 is a block diagram illustrating flattening control by a Raman amplifier.  
           [0012]    A Raman amplifier produces gain in a signal wavelength that has been shifted from the wavelength of the excitation light by the amount of the Raman shift in the amplifying medium, as shown in FIG. 16. The amount of Raman shift and the Raman band are specific to the amplifying medium. Accordingly, if the excitation wavelength is shifted to the long-wavelength side, then the center wavelength of the gain and the gain band will be shifted toward the long-wavelength side by an amount identical with the amount of shift of the excitation wavelength. Further, optical amplification over a wide band is possible, as shown in FIG. 17, by inputting excitation light sources, which have slightly different excitation wavelengths from one another, to the amplifying medium collectively. Further, since gain varies in such a manner that the higher the power of wavelength of the excitation light, the greater the gain, any gain characteristic can be assigned to a Raman amplifier by controlling the power of each excitation wavelength (see the specification of Japanese Patent Application Laid-Open No. 2002-72262).  
           [0013]    A plurality of optical signals (WDM signal light) are wavelength-division multiplexed and input to a back-excited Raman amplifying medium  21  from the input side of a Raman amplifier  20 . A wavelength-division multiplexer  22  multiplexes excitation light of wavelengths λp 1  to λp 3  from excitation light-source blocks  23   a,    23   b,    23   c,  respectively, having different center wavelengths, and inputs the multiplexed signal to a combining coupler  24 . The latter combines the excitation light of wavelengths λp 1  to λp 3  and the wavelength-multiplexed main-signal light, and supplies the combined signal to the Raman amplifying medium  21 . A spectrum analyzer  25  detects the spectrum at the input section or output section (the input section in FIG. 15) of an optical amplifier  26  and inputs the detected spectrum to an excitation light controller  27 . The latter calculates the slope (tilt) of the wavelength characteristic from the output of the spectrum analyzer  25 , calculates the power of each excitation light signal so as to obtain a characteristic that will be the inverse of this wavelength characteristic and inputs the power to the excitation light-source blocks  23   a,    23   b,    23   c.  As a result, the excitation light-source blocks  23   a,    23   b,    23   c  generate excitation light of the wavelengths λp 1  to λp 3  having an intensity (excitation ratio) conforming to the input power, correct the tilt that is generated in the optical transmission line in the interval that undergoes compensation, flattens the wavelength characteristic and inputs the flattened characteristic to the optical amplifier  26 .  
           [0014]    Interchannel optical power variations and optical SNR deviations at the receiving end are minimized and high-capacity, long-haul transmission is made possible by the compensating scheme described above.  
           [0015]    A further technique is to provide optical attenuators, the degree of attenuation of which can be varied, between a plurality of optical amplifiers disposed in an optical transmission line and flatten the wavelength characteristic by these optical attenuators (see the specification of Japanese Patent Application Laid-Open No. 2002-84024).  
           [0016]    Flattening control by a Raman amplifier is extremely effective as a method of eliminating slope and beat components of a spectrum beforehand so long as the amount of attenuation applied by the optical attenuator of each channel disposed at the transmitting end in order to carry out optical pre-emphasis control has sufficient margin.  
           [0017]    However, whereas the goal of flattening control by a Raman amplifier is to equalize input/output power of optical amplifiers, the goal of optical pre-emphasis is to equalize optical SNR. In view of this fact, it is necessary to consider items {circle over (1)} to {circle over (3)} below when both types of control are used conjointly.  
           [0018]    {circle over (1)} If the input spectrum to the optical amplifier at each node is flat, then, theoretically, the optical SNRs at the receiving end should be uniform. However, since a Raman amplifier performs flattening one to several excitation light signals, there is a limit to flattening, beat components cannot be eliminated and neither can ripple components that produce a deviation in level from channel to channel. Consequently, in order to eventually equalize optical SNRs, it is necessary to carry out optical pre-emphasis control after flattening.  
           [0019]    {circle over (2)} In a case where a wavelength-division multiplexing optical transmission system is introduced, usually such a system is introduced initially starting from a small number of wavelengths even though the system has a large capacity and a function capable of supporting a large number of wavelengths. Flattening control is control that adjusts excitation light so as to flatten the spectrum while observing the results of measurement by a spectrum analyzer at each node. However, it is highly likely that the excitation state which prevails when flattening is performed with a small number of wavelengths at the time of initial system introduction will differ from that which will eventually prevail when flattening is performed after the addition of a large number of wavelengths. In order to exercise optimum flattening control for a number of wavelengths and for every provided wavelength in such a manner that optical pre-emphasis control will be subjected to as little load as possible, it is preferred that flattening control be carried out when wavelengths are added on and when wavelengths are removed.  
           [0020]    {circle over (3)} Thus, in order to eventually bring about the optimum state in terms of optical characteristics by pre-emphasis control, it is necessary to perform control in a certain order, namely flattening control first and then optical pre-emphasis control. It should be noted that after wavelengths are added on or removed, it is necessary that flattening control be performed again in order to lighten the load on pre-emphasis control. At such time there is the possibility that the condition of the spectrum that was optimized by pre-emphasis control with regard to the already existing wavelengths will be upset. This is a cause of degradation of the optical signal after the start of service and can lead to error. In order to avoid such a situation, control must be exercised a particular way when flattening control is carried out after wavelengths are added on or removed. Specifically, it is necessary to perform control in such a manner that wavelengths subsequently added on are flattened as much as possible while the spectrum that prevails following the preceding pre-emphasis is maintained. However, such flattening control is not performed by the prior art described in the examples of the patent specifications cited above.  
         SUMMARY OF THE INVENTION  
         [0021]    Accordingly, an object of the present invention is to so arrange it that flattening control after wavelengths are added on or removed is carried out in such a manner that wavelengths added on are flattened as much as possible while the spectrum that prevails following the preceding pre-emphasis is maintained.  
           [0022]    According to a first aspect of the present invention, the foregoing object is attained by providing a wavelength-division multiplexing optical transmission system in which an optical lossy medium, optical amplifiers and Raman amplifiers for compensating for loss in the optical lossy medium are cascade-connected, the system comprising: {circle over (1)} power-level equalizing means for equalizing optical power levels input to an optical amplifier of a succeeding stage by adjusting excitation ratio of a Raman amplifier; {circle over (2)} optical-SNR equalizing means for adjusting power levels at a transmitting end to equalize optical SNRs at a receiving end; and {circle over (3)} correction-value acquisition means for acquiring a correction value that represents an amount of change in power of each wavelength before and after optical-SNR equalization control. At control for equalizing power levels by a Raman amplifier, the power-level equalizing means performs equalization control using the correction value that represents the amount of change in power of each wavelength before and after optical-SNR equalization control the previous time, and the optical-SNR equalizing means subsequently performs optical-SNR equalization control.  
           [0023]    For example, assume that the correction-value acquisition means is a monitoring control unit provided at a repeater station. The monitoring control unit {circle over (1)} calculates and retains, as the correction value, the difference in optical power of each wavelength, before and after optical-SNR equalization control, detected by a spectrum analyzer provided in a Raman amplifier and {circle over (2)} at optical-power equalization control, subtracts the correction value from the optical power of each wavelength detected by the spectrum analyzer and inputs the result of subtraction to the Raman amplifier; and {circle over (3)} the Raman amplifier performs optical-power equalization control based upon the result of subtraction.  
           [0024]    According to a second aspect of the present invention, the foregoing object is attained by providing a wavelength-division multiplexing optical transmission system in which an optical lossy medium, optical amplifiers and Raman amplifiers for compensating for loss in the optical lossy medium are cascade-connected, the system comprising: {circle over (1)} power-level equalizing means for equalizing optical power levels input to an optical amplifier of a succeeding stage by adjusting excitation ratio of a Raman amplifier; and {circle over (2)} correction-value acquisition means for acquiring a correction value that represents an amount of change in power of each wavelength before and after optical-SNR equalization control, which is for adjusting power levels at a transmitting end to equalize optical SNRs at a receiving end. At control for equalizing power levels by a Raman amplifier, the power-level equalizing means performs equalization using the correction value that represents the amount of change in power of each wavelength before and after optical-SNR equalization control of the previous time.  
           [0025]    For example, the correction-value acquisition means is a monitoring control unit provided at a repeater station. The monitoring control unit {circle over (1)} calculates and retains, as the correction value, the difference in optical power of each wavelength, before and after optical-SNR equalization control, detected by a spectrum analyzer provided in a Raman amplifier and {circle over (2)} at optical-power equalization control, subtracts the correction value from the optical power of each wavelength detected by the spectrum analyzer and inputs the result of subtraction to the Raman amplifier; and {circle over (3)} the Raman amplifier performs optical-power equalization control based upon the result of subtraction.  
           [0026]    In accordance with the wavelength-division multiplexing optical transmission system and repeater according to the present invention, flattening control after wavelengths are added on can be carried out in such a manner that wavelengths added on are flattened as much as possible while the spectrum that prevails following preceding pre-emphasis is maintained.  
           [0027]    Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]    [0028]FIGS. 1A and 1B are flowcharts of overall processing according to the present invention at the time of initial start-up and when wavelengths are added on or removed;  
         [0029]    [0029]FIG. 2 is a diagram illustrating a wavelength-division multiplexing optical transmission system;  
         [0030]    [0030]FIG. 3 is a diagram illustrating minimal node structure useful in describing flattening control according to a first embodiment;  
         [0031]    [0031]FIG. 4 is a flowchart of processing by a Raman amplifier and spectrum analyzer at the time of initial start-up in the first embodiment;  
         [0032]    [0032]FIG. 5 is a flowchart of processing by a Raman amplifier and spectrum analyzer when wavelengths are added on or removed in the first embodiment;  
         [0033]    [0033]FIG. 6 is a diagram illustrating minimal mode structure useful in describing flattening control according to a second embodiment;  
         [0034]    [0034]FIG. 7 is a flowchart of processing by a Raman amplifier and spectrum analyzer at the time of initial start-up in the second embodiment;  
         [0035]    [0035]FIG. 8 is a flowchart of processing by a Raman amplifier and spectrum analyzer when wavelengths are added on or removed in the second embodiment;  
         [0036]    [0036]FIG. 9 is a diagram illustrating minimal mode structure useful in describing flattening control according to a third embodiment;  
         [0037]    [0037]FIG. 10 is a flowchart of processing by a Raman amplifier and spectrum analyzer at the time of initial start-up in the third embodiment;  
         [0038]    [0038]FIG. 11 is a flowchart of processing by a Raman amplifier and spectrum analyzer when wavelengths are added on or removed in the third embodiment;  
         [0039]    [0039]FIG. 12 is a diagram illustrating minimal mode structure useful in describing flattening control according to a fourth embodiment;  
         [0040]    [0040]FIG. 13 is a diagram useful in describing sending and receiving of a monitoring control signal between nodes;  
         [0041]    [0041]FIG. 14 is a diagram useful in describing optical pre-emphasis control according to the prior art;  
         [0042]    [0042]FIG. 15 is a block diagram illustrating flattening control by a Raman amplifier according to the prior art.  
         [0043]    [0043]FIG. 16 is a first diagram showing the relationship between excitation wavelength and gain according to the prior art; and  
         [0044]    [0044]FIG. 17 is a second diagram showing the relationship between excitation wavelength and gain according to the prior art.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0045]    (A) Overview of the Present Invention  
         [0046]    Once the shape of a spectrum has been optimized by performing pre-emphasis control, flattening control (equalization control) is carried out in such a manner that the optimized shape of the spectrum will not be disturbed when wavelengths are added on or removed. In order to achieve this, an effective method is to previously store the amount of change in power level before and after the above-mentioned pre-emphasis control and carry out flattening control using information representing this amount of change when wavelengths are added on or removed.  
         [0047]    More specifically, information regarding spectrum shape that prevails after pre-emphasis (i.e., the amount of change in power level) is calculated or measured and then stored for every channel (wavelength). When flattening control is performed by a Raman amplifier at such time that a wavelength is added on or removed, the stored information regarding spectrum shape is subtracted from information (power level) obtained from a spectrum analyzer. This makes it possible to carry out flattening control upon taking into consideration the change in power level that is due to pre-emphasis control.  
         [0048]    (B) Overall Control According to the Present Invention at the Time of Initial Start-up and when Wavelengths are Added on or Removed  
         [0049]    [0049]FIGS. 1A and 1B are flowcharts of overall processing according to the present invention at the time of initial start-up and when wavelengths are added on or removed. As shown for example in FIG. 2, a wavelength-division multiplexing optical system is provided with a number of repeater stations  53   a  to  53   d,  which are disposed between a transmitting end  51  and a receiving end  52 , for amplifying optical signals and compensating for interchannel variations. If the transmitting end, receiving end and repeater stations are called nodes, then the system will have nodes A to F as illustrated. Node A, which is the transmitting end  51 , nodes B to E, which are the repeater stations  53   a  to  53   d,  and node F, which is the receiving end  52 , are each provided with an optical amplifier PAM. Raman amplifiers RMA and spectrum analyzers SPA are provided where appropriate. Further, the transmitting end  51  is provided with a pre-emphasis control circuit  51   a  and an optical attenuator  51   b  having a construction similar to that described in FIG. 14, and the receiving end  52  is provided with an optical-SNR measuring circuit  54   b.    
         [0050]    At initial start-up control (FIG. 1A), the optical amplifier and Raman amplifier at each node are started up (step  101 ). After start-up, each spectrum analyzer SPA measures the wavelength characteristic (the power of every wavelength) at the input section or output section of the optical amplifier PAM, and each Raman amplifier performs flattening control using the wavelength characteristic detected by the spectrum analyzer SPA (step  102 ). Optical pre-emphasis control is performed next. However, in order to observe to what extent spectrum shape is changed by optical pre-emphasis, the Raman amplifier RMA or a monitoring controller (not shown) collects the information that prevailed prior to pre-emphasis control. For example, the Raman amplifier RMA or monitoring controller retains the power of each wavelength before pre-emphasis measured by the spectrum analyzer SPA (step  103 ).  
         [0051]    The transmitting end  51  thenceforth performs pre-emphasis (step  104 ). After pre-emphasis is executed, the Raman amplifier RMA collects the information that prevails after pre-emphasis. For example, the Raman amplifier RMA retains the wavelength characteristic (the power of each wavelength) measured by the spectrum analyzer SPA (step  105 ). Further, the Raman amplifier calculates the difference between power D2n of each wavelength after pre-emphasis and power D1n of each wavelength before pre-emphasis and retains this difference as a correction value Dn (step  106 ). This value is actually used in a case where flattening control by the Raman amplifier is performed at a later date when wavelengths are added on or removed. This ends processing executed at the time of initial start-up. The excitation ratio of the Raman amplifier is thenceforth left unchanged and a constant excitation light continues to be output until a wavelength is added on or removed, etc.  
         [0052]    If a wavelength is added on or removed under these conditions (step  151 ), the Raman amplifier RMA performs flattening control (step  152 ) using the correction value that was measured or calculated at start-up. More specifically, in regard to the portion of the spectrum that has undergone a shift owing to pre-emphasis, the Raman amplifier RMA applies flattening control to this portion so as to maintain the shape of the spectrum. For example, the Raman amplifier RMA subtracts the retained correction value from the optical power of each wavelength prevailing at the time of flattening control and detected by the spectrum analyzer SPA and performs optical-power equalization control based upon the result of subtraction. As a result, the amount of change in the spectrum due to the adding on or removal of a wavelength is adjusted to achieve a better condition by flattening control.  
         [0053]    Next, the state prevailing prior to pre-emphasis control is measured or calculated and retained in a manner similar to that at start-up (step  153 ). For example, the Raman amplifier RMA retains the power of every wavelength prevailing prior to pre-emphasis and measured by the spectrum analyzer SPA before pre-emphasis control is performed.  
         [0054]    This is followed by execution of pre-emphasis at the transmitting end  51  (step  154 ). After execution of pre-emphasis, the Raman amplifier RMA collects information that prevails following pre-emphasis. For example, the Raman amplifier RMA retains the wavelength characteristic (the power of each wavelength), which prevails after pre-emphasis, measured by the spectrum analyzer SPA (step  155 ). Further, the Raman amplifier RMA calculates the difference between the power of each wavelength after pre-emphasis and the power of each wavelength before pre-emphasis, adds the difference to the preceding correction value Dn and stores the sum as the new correction value (step  156 ). This ends processing executed at the time of wavelength add-on or removal. The excitation ratio of the Raman amplifier is thenceforth left unchanged and a constant excitation light continues to be output until the next time a wavelength is added on or removed, etc.  
         [0055]    By repeating the above-described operation and control whenever a wavelength is added on or removed, the change in the shape of the spectrum due to pre-emphasis is held and it is possible to carry out control so as to compensate for a change in the shape of the spectrum due to the addition or removal of a wavelength.  
         [0056]    (C) Flattening Control According to First Embodiment  
         [0057]    [0057]FIG. 3 is a diagram illustrating minimal node structure useful in describing flattening control according to a first embodiment. As shown in FIG. 3, the node includes an optical transmission line  61  comprising optical fiber; a Raman amplifier (RMA)  62  for performing flattening control; and a spectrum analyzer (SPA)  64  for detecting power of each wavelength at the input section or input section of the optical amplifier  63 .  
         [0058]    The Raman amplifier  62  has a structure similar to that of the prior art shown in FIG. 15. Specifically, WDM signal light is input to a back-excited Raman amplifying medium  62   a  from the input side of the Raman amplifier  62 . A wavelength-division multiplexer  62   b  multiplexes excitation light of wavelengths λp 1  to λp 3  from excitation light-source blocks  62   c   1 ,  62   c   2 ,  62   c   3 , respectively, having different center wavelengths, and inputs the multiplexed signal to a combining coupler  62   d.  The latter combines the excitation light of wavelengths λp 1  to λp 3  and the WDM signal light, and supplies the combined signal to the Raman amplifying medium  62   a.    
         [0059]    A spectrum analyzer  64  detects the power of each wavelength at the input section or output section of the optical amplifier  63  and inputs the detected power to an excitation light controller  62   e.  The latter calculates the slope (tilt) of the wavelength characteristic from the output of the spectrum analyzer  25 , calculates the power of each excitation light signal so as to obtain a characteristic that will be the inverse of this wavelength characteristic and inputs the power to the excitation light-source blocks  23   a,    23   b,    23   c.  As a result, the excitation light-source blocks  62   c   1 ,  62   c   2 ,  62   c   3  generate excitation light of the wavelengths λp 1  to λp 3  having an intensity (excitation ratio) conforming to the input power, correct the tilt that is generated in the optical transmission line  61 , flattens the wavelength characteristic and inputs the flattened characteristic to the optical amplifier  63 .  
         [0060]    [0060]FIGS. 4 and 5 are flowcharts of processing by a Raman amplifier and spectrum analyzer at the time of initial start-up and when wavelengths are added on or removed, respectively, in the first embodiment. According to the first embodiment, information (the wavelength characteristic) from the spectrum analyzer  64  before and after pre-emphasis is held by the Raman amplifier  62 , the latter calculates and stores the difference between the wavelength characteristic before pre-emphasis control and the wavelength characteristic after pre-emphasis as the correction value and performs flattening control using this correction value when a wavelength is added on or removed.  
         [0061]    The processing of FIG. 4 is such that after the optical amplifier and Raman amplifier at each node are started up, the spectrum analyzer SPA measures the wavelength characteristic (the power of each wavelength) at the input section or output section of the optical amplifier  63 , inputs the wavelength characteristic to the Raman amplifier  62  (step  201 ) and waits for end of flattening control by the Raman amplifier (step  202 ).  
         [0062]    When the power of each wavelength is received from the spectrum analyzer  64  (step  251 ), the excitation light controller  62   e  of the Raman amplifier  62  implements flattening control so as to flatten the wavelength characteristic (step  252 ) and thenceforth waits for receipt of the power D1n (n=1, 2, 3, . . . ) of each wavelength before pre-emphasis control (step  253 ). It should be noted that n represents the wavelength number.  
         [0063]    When flattening control by the Raman amplifier ends, the spectrum analyzer  64  measures the power D1n (n=1, 2, 3, . . . ) of each wavelength at the input section or output section of the optical amplifier  63  prior to pre-emphasis control, reports this to the Raman amplifier  62  (step  203 ) and waits for completion of pre-emphasis control (step  204 ). When pre-emphasis control is completed, the spectrum analyzer  64  measures power D2n at the input section or output section of the optical amplifier  63 , reports this to the Raman amplifier  62  (step  205 ) and terminates control for initial system start-up.  
         [0064]    When the power D1n (n=1, 2, 3, . . . ) of each wavelength prior to pre-emphasis is received at step  253 , the excitation light controller  62   e  of the Raman amplifier  62  retains the power (step  254 ) and then waits for receipt of power D2n (n=1, 2, 3, . . . ) of each wavelength after pre-emphasis control (step  255 ). When the power D2n (n=1, 2, 3, . . . ) of each wavelength after pre-emphasis control is received, the excitation light controller  62   e  retains the power (step  256 ) and terminates control for initial system start-up. It should be noted that the excitation light controller  62   e  can also calculate the difference Dn (=D2n−D1n) between D2n and D1n and retain this difference as the correction value.  
         [0065]    In the processing of FIG. 5, the spectrum analyzer  64  monitors whether a wavelength has been added on or removed (step  211 ). The excitation light controller  62   e  of the Raman amplifier monitors whether the power Pn (where n represents the wavelength number) of each wavelength at the input section or output section of the optical amplifier has been received from the spectrum analyzer  64  (step  261 ).  
         [0066]    If the spectrum analyzer  64  detects the addition or removal of a wavelength, it measures the power Pn of each wavelength at the input section or output section of the optical amplifier  63 , reports this to the Raman amplifier  62  (step  212 ) and waits for end of flattening control by the Raman amplifier (step  213 ).  
         [0067]    If the power Pn of each wavelength is received from the spectrum analyzer  64 , the excitation light controller  62   e  of the Raman amplifier calculates power Pn′ of each wavelength (step  262 ) in accordance with the following equation using the correction value Dn: 
           P 1 n′=Pn−Dn=Pn− ( D 2 n−D 1 n )   (1)  
         [0068]    and performs flattening control using Pn′ (step  263 ). Thereafter, the excitation light controller  62   e  waits for receipt of power D1n′ (n=1, 2, 3, . . . ) of each wavelength prior to pre-emphasis control (step  264 ).  
         [0069]    When flattening control by the Raman amplifier ends, the spectrum analyzer  64  measures power D1n′ (n=1, 2, 3, . . . ) of each wavelength at the input section or output section of the optical amplifier  63 , reports this to the Raman amplifier  62  (step  214 ) and waits for completion of pre-emphasis control (step  215 ). If pre-emphasis control is completed, the spectrum analyzer  64  measures power D2n′ (n=1, 2, 3, . . . ) at the input section or output section of the optical amplifier  63 , reports this to the Raman amplifier  62  (step  216 ) and terminates control for when a wavelength is added on or removed.  
         [0070]    If power D1n′ (n=1, 2, 3, . . . ) of each wavelength prior to pre-emphasis control is received at step  264 , the excitation light controller  62   e  of Raman amplifier  62  retains the power D1n′ (step  265 ) and thenceforth waits for receipt of power D2n′ (n=1, 2, 3, . . . ) of each wavelength after pre-emphasis control (step  266 ). If the power D2n′ (n=1, 2, 3, . . . ) of each wavelength after pre-emphasis control is received, the excitation light controller  62   e  of Raman amplifier  62  retains the power D1n′ (step  267 ) and terminates control for when a wavelength is added on or removed. The excitation light controller  62   e  calculates the difference Dn′ (=D2n′−D1n′) between D2n′ and D1n′ and stores the correction value Dn of flattening control anew in place of Dn=Dn(old)+Dn′, where Dn(old) represents the preceding correction value Dn.  
         [0071]    (D) Flattening Control According to First Embodiment  
         [0072]    [0072]FIG. 6 is a diagram illustrating minimal node structure useful in describing flattening control according to a second embodiment, in which components identical with those of the first embodiment are designated by like reference characters. This embodiment differs in that the spectrum analyzer  64  calculates the power Pn′ of each wavelength in accordance with Equation (1) and inputs the power to the Raman amplifier  62  when a wavelength is added on or removed. Further, the spectrum analyzer  64  has a spectrum detector  64   a  and an arithmetic unit  64   b.    
         [0073]    [0073]FIGS. 7 and 8 are flowcharts of processing by a Raman amplifier and spectrum analyzer at the time of initial start-up and when wavelengths are added on or removed, respectively. According to the second embodiment, the wavelength characteristic at the input section or output section of the optical amplifier  63  before pre-emphasis control and the wavelength characteristic after pre-emphasis control are measured and held by the spectrum analyzer  64 , the latter performs the calculation of Equation (1) when a wavelength is added on or removed, and inputs the power Pn′ of each wavelength to the spectrum analyzer  64 .  
         [0074]    The processing of FIG. 7 is such that after the optical amplifier and Raman amplifier at each node are started up, the spectrum analyzer  64  measures the wavelength characteristic (the power of each wavelength) at the input section or output section of the optical amplifier  63 , inputs the wavelength characteristic to the Raman amplifier  62  (step  301 ) and waits for end of flattening control by the Raman amplifier (step  302 ).  
         [0075]    When the power of each wavelength is received from the spectrum analyzer  64  (step  351 ), the excitation light controller  62   e  of the Raman amplifier  62  implements flattening control so as to flatten the wavelength characteristic (step  352 ) and terminates control for initial system start-up.  
         [0076]    When flattening control by the Raman amplifier ends, the spectrum analyzer  64  measures and retains the power D1n (n=1, 2, 3, . . . ) of each wavelength at the input section or output section of the optical amplifier  63  prior to pre-emphasis (step  303 ) and waits for completion of pre-emphasis control (step  304 ), where n represents the wavelength number.  
         [0077]    Next, if pre-emphasis control is completed, the spectrum analyzer  64  measures and retains the power D2n of each wavelength at the input section or output section of the optical amplifier  63  (step  305 ) and terminates control for initial system start-up. In this case, the arithmetic unit  64   b  of the spectrum analyzer  64  can also calculate the difference Dn (=D2n−D1n) between D2n and D1n beforehand and retain this value as the correction value.  
         [0078]    In the processing of FIG. 8, the spectrum analyzer  64  monitors whether a wavelength has been added on or removed (step  311 ). The excitation light controller  62   e  of the Raman amplifier waits for receipt of the power Pn′ of each wavelength from the spectrum analyzer  64  (step  361 ).  
         [0079]    If the spectrum analyzer  64  detects the addition or removal of a wavelength, it measures the power Pn of each wavelength at the input section or output section of the optical amplifier (step  312 ). Next, the arithmetic unit  64   b  of the spectrum analyzer  64  calculates the power Pn′ of each wavelength in accordance with Equation (1) using the correction value Dn (=D2n−D1n) (step  313 ), reports this to the Raman amplifier  62  (step  314 ) and waits for end of flattening control by the Raman amplifier (step  315 ).  
         [0080]    Upon receiving the power Pn′ of each wavelength from the spectrum analyzer  64 , the excitation light controller  62   e  of the Raman amplifier performs flattening control using the value Pn′ (step  362 ) and terminates control for when a wavelength is added on or removed.  
         [0081]    When flattening control performed by the Raman amplifier ends, the spectrum analyzer  64  measures and retains the power D1n′ (n=1, 2, 3, . . . ) of each wavelength at the input section or output section of the optical amplifier  63  (step  316 ) and waits for completion of pre-emphasis control (step  317 ). When pre-emphasis control is completed, the spectrum analyzer  64  measures and retains the power D2n′ of each wavelength at the input section or output section of the optical amplifier  63  (step  318 ) and terminates control for when a wavelength is added on or removed. The arithmetic unit  64   b  of the spectrum analyzer  64  calculates the difference Dn′ (=D2n′−D1n′) between D2n′ and D1n′ and stores the correction value Dn of flattening control anew in place of Dn=Dn(old)+Dn′, where Dn(old) represents the preceding correction value Dn.  
         [0082]    (E) Flattening Control According to Third Embodiment  
         [0083]    [0083]FIG. 9 is a diagram illustrating the configuration of a wavelength-division multiplexing optical transmission system for describing flattening control according to a third embodiment. The wavelength-division multiplexing optical system is provided with the number of repeater stations  53   a  to  53   d,  which are disposed between the transmitting end  51  and the receiving end  52 , for amplifying optical signals and compensating for interchannel variations. Node A, which is the transmitting end  51 , nodes B to E, which are the repeater stations  53   a  to  53   d,  and node F, which is the receiving end  52 , are each provided with an optical amplifier  63  and a monitoring controller  71 . Raman amplifiers  62  and spectrum analyzers  64  are provided where appropriate. Further, the transmitting end  51  is provided with a pre-emphasis control circuit  51   a  and an optical attenuator  51   b  having a construction similar to that described in FIG. 14, and the receiving end  52  is provided with an optical-SNR measuring circuit  54   b.    
         [0084]    [0084]FIGS. 10 and 11 are flowcharts of processing by a Raman amplifier, spectrum analyzer and monitoring controller at the time of initial start-up and when wavelengths are added on or removed, respectively, in the third embodiment. According to the third embodiment, the wavelength characteristic at the input section or output section of the optical amplifier  63  before pre-emphasis control and that after pre-emphasis control are retained by the monitoring controller  71 , the latter calculates the correction value Dn at the time that a wavelength is added on or removed, inputs this value to the spectrum analyzer  64 , calculates Pn′ in accordance with Equation (1) and executes flattening control.  
         [0085]    The processing of FIG. 10 is such that after the optical amplifier and Raman amplifier at each node are started up, the spectrum analyzer  64  measures the wavelength characteristic (the power of each wavelength) at the input section or output section of the optical amplifier  63 , inputs the wavelength characteristic to the Raman amplifier  62  (step  401 ) and waits for end of flattening control by the Raman amplifier (step  402 ).  
         [0086]    When the power of each wavelength is received from the spectrum analyzer  64  (step  451 ), the Raman amplifier  62  implements flattening control so as to flatten the wavelength characteristic (step  452 ) and terminates control for initial system start-up.  
         [0087]    When flattening control by the Raman amplifier ends, the spectrum analyzer  64  measures the power D1n (n=1, 2, 3, . . . ) of each wavelength at the input section or output section of the optical amplifier  63  prior to pre-emphasis control, reports this to the monitoring controller  71  (step  403 ) and waits for completion of pre-emphasis control (step  404 ). It should be noted that n represents the wavelength number. When pre-emphasis control is completed, the spectrum analyzer  64  measures power D2n of each wavelength at the input section or output section of the optical amplifier  63 , reports this to the monitoring controller  71  (step  405 ) and terminates control for initial system start-up.  
         [0088]    The monitoring controller  71  waits for receipt of power D1n (n=1, 2, 3, . . . ) of each wavelength before pre-emphasis control (step  481 ). Upon receiving the power D1n of each wavelength from the spectrum analyzer  64 , the monitoring controller  71  retains the power (step  482 ) and waits for receipt of power D2n (n=1, 2, 3, . . . ) of each wavelength after pre-emphasis control (step  483 ). Upon receiving the power D2n (n=1, 2, 3, . . . ) of each wavelength after pre-emphasis control, the monitoring controller  71  retains the power (step  484 ) and terminates control for initial system start-up. It should be noted that the monitoring controller  71  can also calculate the difference Dn (=D2n−D1n) between D2n and D1n and retain this difference as the correction value.  
         [0089]    In the processing of FIG. 11, the spectrum analyzer  64  and monitoring controller  71  monitor whether a wavelength has been added on or removed (steps  411 ,  491 ). Further, the Raman amplifier monitors whether the power Pn (where n represents the wavelength number) of each wavelength at the input section or output section of the optical amplifier  63  has been received from the spectrum analyzer  64  and monitors the correction value Dn has been received from the monitoring controller  71  (step  461 ).  
         [0090]    If the spectrum analyzer  64  detects the addition or removal of a wavelength, it measures the power Pn of each wavelength at the input section or output section of the optical amplifier  63 , reports this to the Raman amplifier  62  (step  412 ) and waits for end of flattening control by the Raman amplifier (step  413 ). Further, if the monitoring controller  71  detects the addition or removal of a wavelength, it calculates the difference Dn (=D2n−D1n) between the wavelength characteristics D1n, D2n prevailing before and after pre-emphasis control as the correction value, reports the correction value to the Raman amplifier  62  (step  492 ) and waits for receipt of the power D1n (n=1, 2, 3, . . . ) prevailing prior to pre-emphasis control (step  493 ).  
         [0091]    If the Raman amplifier  62  receives the power Pn of each wavelength from the spectrum analyzer  64  and receives the correction value Dn from the monitoring controller  71 , the Raman amplifier  62  calculates power Pn′ of each wavelength (step  262 ) in accordance with Equation (1), performs flattening control using the power Pn′ (step  463 ) and terminates control for when a wavelength is added on or removed.  
         [0092]    When flattening control by the Raman amplifier  62  ends, the spectrum analyzer  64  measures power D1n′ (n=1, 2, 3, . . . ) of each wavelength at the input section or output section of the optical amplifier  63  prevailing prior to pre-emphasis control, reports this to the monitoring controller  71  (step  414 ) and waits for completion of pre-emphasis control (step  415 ). If pre-emphasis control is completed, the spectrum analyzer  64  measures power D2n′ at the input section or output section of the optical amplifier  63 , reports this to the monitoring controller  71  (step  416 ) and terminates control for when a wavelength is added on or removed.  
         [0093]    If the monitoring controller  71  receives power D1n′ (n=1, 2, 3, . . . ) of each wavelength before pre-emphasis control (step  481 ), the monitoring controller  71  retains the power D1n′ (step  494 ) and thenceforth waits for receipt of power D2n′ (n=1, 2, 3, . . . ) of each wavelength after pre-emphasis control (step  495 ). Upon receiving the power D2n′ (n=1, 2, 3, . . . ) of each wavelength after pre-emphasis control, the monitoring controller  71  retains the power (step  496 ) and terminates control for when a wavelength is added on or removed. The monitoring controller  71  calculates the difference Dn′ (=D2n′−D1n′) between D2n′ and D1n′ and stores the correction value Dn of flattening control anew in place of Dn=Dn(old)+Dn′, where Dn(old) represents the preceding correction value Dn.  
         [0094]    (F) Flattening Control According to Fourth Embodiment  
         [0095]    [0095]FIG. 12 is a diagram illustrating the configuration of a wavelength-division multiplexing optical transmission system for describing flattening control according to a fourth embodiment. Components identical with those of the wavelength-division multiplexing optical transmission system of FIG. 9 are designated by like reference characters. This embodiment differs from the system of FIG. 9 in that whereas the third embodiment uses the monitoring controller to calculate the correction value Dn, the fourth embodiment is provided with an external control unit  81 , the latter calculates the correction value Dn and transmits this to the Raman amplifier  62  of each node.  
         [0096]    The external control unit  81 , which is constituted by a personal computer, is connected to the monitoring controller  71  at node F and is capable of sending and receiving a monitoring control signal to and from each node via the monitoring controller and up/down links. FIG. 13 is a diagram useful in describing the sending and receiving of the monitoring control signal between nodes.  
         [0097]    Neighboring first and second nodes  101 ,  102 , respectively, are connected by an EW-side link (uplink)  103  and a WE-side link (downlink)  104 . Each node is provided with uplink and downlink optical amplifiers  111 ,  112 , respectively, branchers  113 ,  114  are provided on the input sides of the optical amplifiers  111 ,  112 , respectively, and combiners  115 ,  116  are provided on the output sides of the optical amplifiers  111 ,  112 , respectively. The branchers  113 ,  114  branch off light of a wavelength assigned to the monitoring control signal and input the light of this wavelength to a monitoring controller (MNT)  119  via O/E (optoelectronic) transducers  117 ,  118 , respectively. The monitoring controller  119  performs control to extract the correction value Dn, which is contained in the received monitoring control signal, and to input Dn to the Raman amplifier  62 , controls implementation/non-implementation of slope correction by the optical amplifiers  111 ,  112 , and carries out other control as well. Further, the monitoring controller  119  receives the power values D1n, D2n of the optical amplifier prevailing before and after pre-emphasis control from the spectrum analyzer  64  and inputs these to E/O (electro-optic) transducers  121 ,  122  as the monitoring control signal. The E/O transducers  121 ,  122  convert the monitoring control signal to light of a prescribed wavelength, the combiners  115 ,  116  combine the WDM main-signal light and the light of the monitoring control signal and transmit the result to the neighboring node. Though Raman amplifiers and spectrum analyzers are not shown in FIG. 13, they are inserted appropriately between the branchers  113 ,  114  and optical amplifiers  111 ,  112 .  
         [0098]    In FIG. 12, the monitoring controller  71  of each node reports the power data D1n, D2n prevailing before and after pre-emphasis control to the external control unit  81  by way of the monitoring control signal. The external control unit  81  uses the power data D1n, D2n, which has been received from the monitoring controller  71  of each node, to calculate the correction value Dn of each and every node, and stores the correction value Dn calculated.  
         [0099]    When a wavelength is added on or removed, the external control unit  81  uses the monitoring control signal to report the correction value Dn of the Raman amplifier  62  of each node to the Raman amplifier via the monitoring controller  71 . In optical-power equalization control at addition or removal of a wavelength, the Raman amplifier  62  control optical power equalization using the value Pn′, which is the result of subtracting the received correction value Dn from the power Pn of the optical amplifier  63 .  
         [0100]    (G) Modification  
         [0101]    In the above embodiments, the correction value Dn is calculated based upon optical power at the input section or output section of the optical amplifier  63 . However, it can be so arranged that the correction value is acquired based upon the difference between amount of optical power attenuation of each wavelength at the transmitting end before optical-SNR equalization control and amount of optical power attenuation of each wavelength at the transmitting end after optical-SNR equalization control. In this case, the amount of optical power attenuation of each wavelength would be reported to the Raman amplifier at each node using the monitoring control signal.  
         [0102]    In accordance with the wavelength-division multiplexing optical transmission system and repeater station according to the present invention, flattening control after wavelengths are added on or removed can be carried out in such a manner that wavelengths added on are flattened as much as possible while the spectrum that prevails following the preceding pre-emphasis is maintained.  
         [0103]    Further, in accordance with the present invention, it can be so arranged that optical pre-emphasis control is performed after flattening, and therefore optical pre-emphasis control is not subjected to excessive load.  
         [0104]    Further, in accordance with the present invention, a monitoring controller or external control unit can be made to exercise control to calculate and retain correction values, thereby alleviating load upon the Raman amplifiers.  
         [0105]    As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.