Abstract:
A WDM transmission device includes a level adjustment unit adjusting levels of optical signals having different wavelengths, a multiplexer multiplexing the optical signals, an amplifier amplifying a multiplexed optical signal, and a monitor unit monitoring the multiplexed optical signal applied to the amplifier and a level of an output signal of the amplifier and controlling the level adjustment unit so that the levels of the optical signals fall within a predetermined level range in which the amplifier can operate normally.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to a level adjustment method and a wavelength division multiplexing device and system. More particularly, the present invention is concerned with a level adjustment method for adjusting the level of an input signal, and a device and a system using such a method. 
   Recently, as a rapid increase in a demand for communications due to advance of intelligent societies, many communication channels have been newly installed. However, installation of optical fiber cables costs a great deal. Thus, it is requirement to efficiently utilize the existing optical fiber cables. Wavelength division multiplexing (WDM) is the main current in the development of efficient utilization of the existing cables and increase in the aggregate number of channels per fiber. 
   2. The description of the Related Art 
     FIG. 1  is a block diagram of a communication system using WDM transmission devices. The communication system shown in  FIG. 1  includes WDM transmission devices  1   a  and  1   b . Transmission devices  2   a - 1  through  2   a - 3  are connected to the WDM transmission device  1   a , and transmission devices  2   b - 1  through  2   b - 3  are connected to the WDM transmission device  1   b . A plurality of terminals  3   a  are connected to the transmission devices  2   a - 1  through  2   a - 3 , and a plurality of terminals  3   b  are connected to the transmission devices  2   b - 1  through  2   b - 3 . Each of the transmission devices  2   a - 1  through  2   a - 3  multiplexes signals from the terminals  3   a  by time-division multiplexing. Similarly, each of the transmission devices  2   b - 1  through  2   b - 3  multiplexes signals from the terminals  3   b  by time-division multiplexing. 
   The transmission devices  2   a - 1  through  2   a - 3  send multiplexed optical signals respectively having different wavelengths λ 1 , λ 2  and λ 3  to the WDM transmission device  1   a . Similarly, the transmission devices  2   b - 1  through  2   b - 3  send multiplexed optical signals respectively having different wavelengths λ 1 , λ 2  and λ 3  to the WDM transmission device  1   b . A repeater device  4  is provided between the WDM transmission devices  1   a  and  1   b . The repeater device  4  may be omitted when the WDM transmission devices  1   a  and  1   b  are close to each other so that there is no need to repeat the optical signals transmitted therebetween. 
   The WDM transmission device  1   a  multiplexes the optical signals of the wavelengths λ 1 , λ 2  and λ 3 , and sends the multiplexed optical signal thus obtained to the opposing the WDM transmission device  1   b  via the repeater  4 . The WDM transmission device  1   b  demultiplexes the received optical signal into optical signals of the wavelengths λ 1 , λ 2  and λ 3 , which are then supplied to the transmission devices  2   b - 1 ,  2   b - 2  and  2   b - 3 , respectively. The transmission devices  2   b - 1  through  2   b - 3  separate the received signals by time-division multiplexing, the individual signals thus obtained being supplied to the terminals  3   b.    
     FIG. 2  is a block diagram of a WDM transmission device  10 , which corresponds to the WDM transmission device  1   a  or  1   b  shown in FIG.  1 . The WDM transmission device  10  is made up of a transmission unit and a reception unit. The optical signals of the wavelengths λ 1 , λ 2  and λ 3  are applied to the transmission unit of the WDM transmission device  10  via optical variable attenuators  20 - 1  through  20 - 3  provided outside of the device  10 . The transmission unit includes a wavelength multiplexer  12  and an amplifier  14  for transmission. The wavelength multiplexer  12  multiplexes the optical signals of the wavelengths λ 1 , λ 2  and λ 3  respectively coming from the attenuators  20 - 1  through  20 - 3 , and outputs the multiplexed optical signal to the amplifier  14 . Then, the amplifier  14  amplifies the optical signal, and outputs the amplified optical signal to an optical transmission path (optical fiber). The wavelength multiplexer  12  includes, for example, a WDM coupler utilizing a grating or the like. 
   An optical signal transmitted over the wavelength-multiplexed optical signal is applied to a reception unit of the WDM transmission device  10 . The reception unit includes an amplifier  18  for reception, and a wavelength demultiplexer  16 . The amplifier  18  amplifies the received optical signal, the amplified signal being applied to the wavelength demultiplexer  16 . Then, the wavelength demultiplexer  16  demultiplexes the received optical signal into optical signals of the wavelengths λ 1 , λ 2  and λ 3 , which are then supplied to, for example, the transmission devices  2   a - 1  through  2   a - 3  shown in FIG.  1 . 
   A factor used to evaluate the quality of transmission in the system including the WDM transmission device  10  is an optical signal-to-noise ratio (OSNR). It is desired that the OSNR is high and uniform to the optical signals of the wavelengths λ 1 , λ 2  and λ 3  on the receive side. 
     FIG. 3  shows spectra of lights that are input and output signals of the WDM transmission device  10 . In  FIG. 3 , only four wavelengths are illustrated for the sake of simplicity. However, the number of wavelengths is not limited to four. 
   Part (A) of  FIG. 3  shows a spectrum of light that is the output signal of the amplifier  14  for transmission shown in FIG.  2 . The four peaks of the wavelengths λ 1 -λ 4  have been subjected to a level adjustment, and have almost equal peak levels. The optical signals exhibiting the spectrum shown in part (A) of  FIG. 3  are transmitted over the optical fiber. In this case, the output signal of the amplifier  18  for reception shown in  FIG. 2  has a spectrum of light as shown in part (B) of  FIG. 3. A  gentle envelope having a mountain shape is a spectrum of an amplified spontaneous emission light (ASE light) accumulated in the optical signal due to amplifiers on the transmission path. Four sharp peaks of light shown in part (B) of  FIG. 3  have the wavelengths λ 1 -λ 4 . It is to be noted that the four peaks shown in part (B) have different levels. It will be noted that an ASE light introduced in the amplifier  14  is neglected in the illustration of part (A) of FIG.  3 . 
   The OSNR corresponds to the difference between the peak level of the optical spectrum and the level of the ASE light. For example, as shown in part (B) of  FIG. 3 , the OSNR of the optical spectrum of the wavelength λ 4  is indicated by an arrow  22 . 
   Thus, as shown in part (A) of  FIG. 3 , even if the amounts of attenuation by the optical variable attenuators  20 - 1  through  20 - 3  are varied so that the lights of the wavelengths λ 1 -λ 4  have a constant level, the OSNRs of the wavelengths λ 1 -λ 4  are not constant. This is due to the following. First, the ASE light is accumulated in the light each time the light passes through the amplifier or the like. Second, the gain of each amplifier has a wavelength dependence. Third, the optical fiber has a loss of wavelength dependence. 
   The levels of the optical variable attenuators  20 - 1  through  20 - 3  shown in  FIG. 2  are adjusted taking into consideration the above factors. This adjustment on the transmission side is called pre-emphasis control. 
     FIG. 4  is a block diagram of a WDM transmission device  30  having a different configuration as that of the WDM transmission device  10 . The WDM transmission device  30  includes, in the transmission unit, variable attenuators  36 - 1  through  36 - 3 , the wavelength multiplexer  12 , the amplifier  14  for transmission, an optical coupler  32 , and a spectrum monitor unit  34 . The reception unit of the WDM transmission device  30  is the same as that of the device  10  shown in FIG.  2 . 
   Lights of the wavelengths λ 1 , λ 2  and λ 3  pass through the variable attenuators  36 - 1  through  36 - 3  of the WDM transmission device  10 , and are applied to the wavelength multiplexer  12 . Then, the multiplexer  12  multiplexes the wavelengths λ 1 , λ 2  and λ 3  of the optical signals, and outputs the multiplexed optical signal to the transmission amplifier  14 . Then, the amplifier  14  amplifies the received signal, the amplified optical signal being output to the optical fiber via the coupler  32 . 
   The coupler  32  outputs a part of the optical signal from the amplifier  14  to the spectrum monitor unit  34 , which may be formed by a spectrum analyzer. The spectrum monitor unit  34  defines target levels with regard to the optical signals of the wavelengths λ 1 , λ 2  and λ 3 . The spectrum monitor unit  34  measures the wavelengths, levels and OSNR of the light components contacted in the branch light coming from the coupler  32 . 
   The spectrum monitor unit  34  supplies the variable attenuators  36 - 1  through  36 - 3  with respective control signals, which control the levels of the respective optical signals on the basis of the results of the measurement in a feedback fashion. For example, the monitored level of the light of the wavelength λ 1  is higher than the corresponding target level, the spectrum monitor unit  34  supplies the variable attenuator  36 - 1  with the control signal which controls the amount of attenuation thereof so that the monitored level reduces. 
   However, the WDM transmission device  10  shown in  FIG. 2  has a disadvantage in that there is no way other than actual measurement of dispersion of the losses of the optical signals having the different wavelengths in the communication system. More particularly, the amounts of attenuation of the variable attenuators  20 - 1  through  20 - 3  are manually changed to adjust the differences among the losses of the optical signals. Thus, it takes a long time to perform initial installation, operation and maintenance of the system. Further, the WDM transmission device  10  does not have any means for coping with age deterioration and seasonal variation in performance. 
   The WDM transmission device  30  shown in  FIG. 4  has the performance that depends on the required transmission distance. Thus, the device  30  is required to have the amplifier  14  of a type suitable for the situation in which the device  30  is used. The dynamic range of the amplifier  14  may vary. In the case where the variable attenuators  36 - 1  through  36 - 3  are feedback-controlled by the results of measurement by the spectrum monitor unit  34 , the level of the input signal applied to the amplifier  14  may go beyond the dynamic range thereof. 
   Further, in order to maintain the OSNR at an appropriate level on the reception side, it is desirable that the optical signals of the different wavelengths have levels as high as possible. In this regard, the manual adjustment of the variable attenuators  20 - 1  through  20 - 3  of the WDM transmission device  10  shown in  FIG. 2  is cumbersome and inefficient. In the WDM transmission device  30 , the level of the input signal applied to the amplifier  14  may exceed the dynamic range thereof although the feedback control of the variable attenuators  36 - 1  through  36 - 3  enables the levels of the optical signals to be maintained as high as possible. 
   SUMMARY OF THE INVENTION 
   It is a general object of the present invention to eliminate the above disadvantages of the related art. 
   A more specific object of the present invention is to provide a level adjustment method and a wavelength division multiplexing device and system capable of efficiently adjusting the levels of the input signals of different wavelengths while taking into account the dynamic range of an amplifier. 
   The above objects of the present invention are achieved by A WDM (Wavelength Division Multiplexing) transmission device comprising: a level adjustment unit adjusting levels of optical signals having different wavelengths; a multiplexer multiplexing the optical signals; an amplifier amplifying a multiplexed optical signal; and a monitor unit monitoring the multiplexed optical signal applied to the amplifier and a level of an output signal of the amplifier and controlling the level adjustment unit so that the levels of the optical signals fall within a predetermined level range in which the amplifier can operate normally. 
   The above objects of the present invention are also achieved by a system comprising a plurality of wavelength division multiplexing (WDM) transmission devices, and an optical fiber cable connecting the WDM transmission devices. Each of the WDM transmission devices is configured as described above. 
   The above objects of the present invention are also achieved by a level adjustment method comprising the steps of: adjusting levels of optical signals having different wavelengths; multiplexing the optical signals; amplifying a multiplexed optical signal: and monitoring the multiplexed optical signal applied to the amplifier and a level of an output signal of the amplifier and controlling the level adjustment unit so that the levels of the optical signals fall within a predetermined level range in which the amplifier can operate normally. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a block diagram of a communication system using WDM transmission devices; 
       FIG. 2  is a block diagram of a configuration of a WDM transmission device; 
       FIG. 3  is a spectrum diagram of input and output signals of the WDM transmission device; 
       FIG. 4  is a block diagram of another configuration of the WDM transmission device; 
       FIG. 5  is a block diagram of a WDM transmission device according to a first embodiment of the present invention; 
       FIG. 6  is a flowchart of a process sequence of the WDM transmission device; 
       FIGS. 7A and 7B  are diagrams of a decision on level convergence; 
       FIG. 8  is a flowchart of another process sequence of the WDM transmission device; 
       FIGS. 9A and 9B  are diagrams of a level decision with respect to an upper limit; 
       FIG. 10  is a block diagram of a WDM transmission device according to a second embodiment of the present invention; 
       FIG. 11  is a block diagram of a system including WDM transmission devices; 
       FIG. 12  is a diagram of a pre-emphasis control; and 
       FIG. 13  is a flowchart of a process sequence of the system shown in FIG.  11 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 5  is a block diagram of a WDM transmission device according to a first embodiment of the present invention. 
   A WDM transmission device  40  shown in  FIG. 5  includes variable attenuators  42 - 1  through  42 - 3 , a wavelength multiplexer  44 , an amplifier  46  for transmission, a photocoupler  48 , a spectrum monitor unit  50 , and a monitor control unit  52 . 
   A process sequence of the WDM transmission device  30  will be described by referring to a flowchart shown in FIG.  6 . After the WDM transmission device  30  is activated, the spectrum monitor unit  50  sets the provisional target levels of the optical signals at step S 10 . At step S 12 , the spectrum monitor unit  50  is supplied with information concerning the dynamic range of the transmission amplifier  46 , this information indicating a range of the input signal level within which the transmission amplifier  46  operates normally. Hereinafter, the above information will be referred to as dynamic range information. 
   At step S 14 , the spectrum monitor unit  50  reads information indicative of the upper and lower limits of the input signal level contained in the dynamic range information, and computes the target input level of the transmission amplifier  46  by using the following formula:
 
(target input level)=[(upper limit level)−(lower limit level)] ×α+(lower limit level)  (1)
 
where 0&lt;α&lt;1.
 
   At step S 16 , the spectrum monitor unit  50  compares the provisional target input levels of the optical signals tentatively selected with the levels of the optical signals included in the branch light from the photocoupler  48 , and determines whether the optical signals converge at the respective target levels. 
   If it is determined that the optical signals do not converge at the respective target levels (NO at step S 16 ), at step S 18  the spectrum monitor unit  50  supplies the variable attenuators  42 - 1  through  42 - 3  with control signals that control the levels of the respective optical signals on the basis of the results of comparison carried out at step S 16 . More particularly, if the level of the optical signal having the wavelength λ 1  is lower than the corresponding target level, the spectrum monitor unit  50  supplies the variable attenuator  42 - 1  with the control signal that acts to reduce the amount of attenuation of the variable attenuator  42 - 1 . By the manner mentioned above, the levels of the optical signals are adjusted so as to become equal to the respective target levels. The target levels for the optical signals of different wavelengths may be identical to or different from each other taking into consideration the setting of pre-emphasis. Then, the process proceeds with step S 20 . 
   Step S 20  is also executed if it is determined that optical signals converge at the respective target levels (YES at step S 16 ). At step S 20 , the spectrum monitor unit  50  compares the input level of the transmission amplifier  46  with the target amplifier input level, and determines whether the input level of the amplifier  46  is settled at the target amplifier input level. 
   If it is determined that the input level of the transmission amplifier  46  is settled at the target amplifier input level (YES at step S 20 ), the spectrum monitor unit  50  executes step S 24 . If not (NO at step S 20 ), the unit  50  executes step S 22 . 
   At step S 22 , the spectrum monitor unit  50  updates the target levels for the respective optical signals on the basis of the results of comparison carried out at step S 20 . More particularly, if the input level of the amplifier  46  is lower than the target amplifier input level, the spectrum monitor unit  50  raises the target levels for the optical signals. In contrast, if the input level of the amplifier  46  is higher than the target amplifier input level, the spectrum monitor unit  50  lowers the target levels. Then, the spectrum monitor unit  50  executes step S 16 . 
   When the input level of the transmission amplifier  46  is settled at the target amplifier input level, the spectrum monitor unit  50  monitors the stability of the levels of the optical signals and the stability of the multiplexed optical signal applied to the transmission amplifier  46  for a given time. More particularly, the spectrum monitor unit  50  checks whether the levels of the optical signals are constantly at the respective target levels. Further, the spectrum monitor unit  50  checks whether the input level of the transmission amplifier  46  is constantly at the target amplifier input level. 
   If the stability of the individual optical signals and the stability of the optical signal applied to the transmission amplifier  46  are confirmed at step S 24 , the spectrum monitor unit  50  shifts to the stationary state. In contrast, if not at step S 24 , the spectrum monitor unit  50  returns to step S 16 . Even in the stationary state, the spectrum monitor unit  50  constantly executes steps S 16 -S 20  and stores information thus obtained. 
   A description will be given, with reference to  FIGS. 7A and 7B , of a decision on convergence made at step S 16 . The horizontal axis of a graph of  FIG. 7A  denotes the wavelength and the vertical axis thereof denotes the level. A tolerable range defined by +a (a is a fine quantity) is defined with respect to the target level Pref. If the level of the optical signal on which a decision should be made falls within the range Pref ±α, the optical signal is acknowledged to be settled at the target level. In the case shown in  FIG. 7A , the optical signals of the wavelengths λ 2  and λ 3  are acknowledged to be settled at the target level, and the optical signals of the wavelengths λ 1  and λ 4  are acknowledged not to be settled at the target level. 
   As described above, the WDM transmission device  40  can control the optical signals to have an appropriate level within the tolerable range and can adjust dispersion of the losses of the optical signals. The principle of the convergence decision shown in  FIGS. 7A and 7B  can be applied to the decision made at step S 20 . 
     FIG. 8  is a flowchart of another process sequence of the WDM transmission device  40 . In  FIG. 8 , steps that are the same as those shown in  FIG. 6  are given the same reference numbers as previously. 
   Referring to  FIG. 8 , after the sequence of steps S 10 -S 20 , the spectrum monitor unit  50  determines whether there is any variable attenuator among the variable attenuators  42 - 1  through  42 - 3  that has reached an adjustment limit by step S 18  or S 22 . The spectrum monitor unit  50  has a management table in which information concerning a variable attenuator which has reached the adjustment limit is registered. The information in the management table is derived from status information concerning the variable attenuators  42 - 1  through  42 - 3  supplied therefrom. 
   Thus, the spectrum monitor unit  50  is capable of discriminating which variable attenuator has reached the adjustment limit by referring to the management table. If it is determined that there is any variable attenuator that has reached the adjustment limit (YES at step S 30 ), the spectrum monitor unit  50  executes step S 24 . If not at step S 30  (NO at step S 30 ), the spectrum monitor unit  50  executes step S 32 . 
   At step S 32 , the spectrum monitor unit  50  determines whether the target amplifier input level has reached the upper limit of the dynamic range of the transmission amplifier  46 . If the answer of step S 32  is YES, the spectrum monitor unit  50  executes step S 24 . If the answer of step S 32  is NO, the spectrum monitor unit  50  executes step S 34 . 
   At step S 34 , the spectrum monitor unit  50  increases the target amplifier input level by a given amount, and executes step S 20 . At a raised target amplifier input level by the sequence of steps S 20 , S 30 , S 32  and S 34 , the optical signal applied to the transmission amplifier  46  does not converge at the target amplifier input level. In this case, the spectrum monitor unit  50  executes step S 22  rather than step S 20 . 
   The spectrum monitor unit  50  executes step S 24  if there is any variable attenuator among the variable attenuators  42 - 1  through  42 - 3  that has reaches the adjustment limit (YES at step S 30 ) or if the target amplifier input level has reached the upper limit of the dynamic range (YES at step S 32 ). 
   A description will be given, with reference to  FIGS. 9A and 9B , of the judgment at step S 32 . As shown in  FIG. 9A , a fine quantity -β is defined with regard to the upper limit Lup. If the level of the optical signal subjected to the judgment of step S 32  falls within the range of Lup-,β, it is judged that the optical signal has reached the upper limit. For example, an optical signal indicated as case A is acknowledged to have reached the upper limit, and an optical signal indicated as case B is acknowledged not to have reached the upper limit. 
   As described above, the WDM transmission device  40  can control the optical signal applied to the transmission amplifier  46  to be appropriately as high as possible within the dynamic range of the amplifier  46  and to adjust dispersion of the losses of the optical signals. 
   A description will be given of pre-emphasis control of the present invention WDM transmission device. The pre-emphasis control is intended to adjust the level of the optical signal on the transmission side taking into consideration accumulative introduction of the ASE light resulting from the amplifiers and the like on the transmission path, the level difference among the wavelengths due to the wavelength-dependence amplifying abilities of the amplifiers, and the level difference among the wavelengths due to the wavelength-dependence losses of the fibers. 
     FIG. 10  is a block diagram of a WDM transmission device according to a second embodiment of the present invention in which the pre-emphasis control is employed. In  FIG. 10 , any part shown therein that is the same as a part shown in  FIG. 5  is denoted by the same reference numeral in both Figures.  FIG. 11  is a block diagram of a system having two WDM transmission devices, each being configured as shown in FIG.  10 . 
   A WDM transmission device  60  shown in  FIG. 10  includes the variable attenuators  42 - 1  through  42 - 3 , a transmission unit, and a reception unit. The transmission unit includes the wavelength multiplexer  44 , the transmission amplifier  46 , the photocoupler  48 , and the spectrum monitor unit  50 . The reception unit includes a wavelength demultiplexer  64 , a reception amplifier  66 , a photocoupler  68 , and a spectrum monitor unit  70 . Further, the WDM transmission device  60  includes the monitor control unit  52  provided in common to the transmission and reception units. 
   The spectrum monitor unit  50  performs the setting of pre-emphasis instructed by the monitor control unit  52  in addition to the aforementioned control operations thereof in the WDM transmission device  40  shown in FIG.  5 . The setting of pre-emphasis will be described with reference to  FIG. 12 , which shows an example of the setting of pre-emphasis. 
   If the setting of pre-emphasis by the monitor control unit  52  is not performed, the spectrum monitor unit  50  adjusts the variable attenuators  42 - 1  through  42 - 3  so that the optical signals of the wavelengths λ 1 -λ 4  output by the transmission amplifier  46  converge at the target level, as shown in part (A) of FIG.  12 . 
   If the setting of pre-emphasis by the monitor control unit  52  is performed, the spectrum monitor unit  50  adjusts the variable attenuators  42 - 1  through  42 - 3  so that the optical signals of the wavelengths λ 1 -λ 4  converge at the respective target levels, as shown in part (B) of FIG.  12 . For example, the spectrum monitor unit  50  adjusts the variable attenuator  42 - 1  so that the optical signal of the wavelengthλ 1  converges at [(target level)+1.0 dB]. 
   The reception amplifier  66  receives the wavelength-multiplexed optical signal from the opposing WDM transmission device, and detects the monitor control signal therefrom. The monitor control signal is sent to the monitor control unit  52 . A part of the optical signal from the photocoupler  68  is supplied to the spectrum monitor unit  70 . The spectrum monitor unit  70  measures the wavelengths, levels and OSNRs of the optical signals contained in the branch light from the photocoupler  68 . Then, the spectrum monitor unit  70  supplies the monitor control unit  52  with the results of measurement as reception monitor information. 
     FIG. 13  is a flowchart of a process sequence of the communication system shown in FIG.  11 . 
   At step S 40 , a reception amplifier  66   b  of a WDM transmission device  60   b  receives a wavelength-multiplexed optical signal transmitted by a WDM transmission device  60   a . The reception amplifier  66   b  amplifies the received optical signal and supplies the amplified optical signal to a spectrum monitor unit  50   b  via a photocoupler (not shown for the sake of simplicity). 
   At step S 42 , the spectrum monitor unit  50   b  measures the wavelengths, levels and OSNRs of the received optical signals, and outputs the reception monitor information to a monitor control unit  52   b . At step S 44 , the monitor control unit  52   b  adds the supplied reception monitor information to the monitor control signal, which is then sent to a transmission amplifier  46   b . At step S 46 , the transmission amplifier  46   b  sends the monitor control signal to the WDM transmission device  60   a  on the transmission side. 
   At step S 48 , a reception amplifier  66   a  receives the monitor control signal sent by the WDM transmission device  60   b  on the reception side. Then, the reception amplifier  66   a  supplies the received monitor control signal to a monitor control unit  52   a . At step S 50 , the monitor control unit  52   a  acquires the reception monitor information concerning the WDM transmission device  60   b  from the supplied monitor control signal. 
   At step S 52 , the monitor control unit  52   a  calculates a setting value of pre-emphasis from the acquired reception monitor information. The pre-emphasis setting value is selected in accordance with the reception monitor information, as shown in part (B) of FIG.  12 . 
   At step S 54 , the monitor control unit  52   a  supplies the pre-emphasis setting value to a spectrum monitor unit  50   a . At step S 56 , the spectrum monitor unit  50   a  adjusts a variable attenuator  42   a  so that the optical signal can converge at the target level based on the pre-emphasis setting value supplied from the monitor control unit  52   a.    
   As described above, the pre-emphasis control can automatically be carried out between the opposing WDM transmission devices  60   a  and  60   b , so that the initial installation and operation/maintenance work can be performed efficiently. 
   The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention. 
   The present application is based on Japanese Priority application no. 11-368470 filed on Dec. 24, 1999, the entire contents of which are hereby incorporated by reference.