Abstract:
A wavelength division multiplexing (WDM) device, an optical leakage prevention method, and a WDM communication system are disclosed. The WDM device includes a level adjusting unit for receiving an optical signal, for adjusting the optical power level of the received optical signal, and for outputting the level-adjusted optical signal; and a controlling unit for analyzing the optical power level of the optical signal output from the level adjusting unit, for controlling the level adjusting unit so that the optical power level of the output optical signal stays at a constant level, and for transitioning to an “off-state” when the optical power level of the output optical signal becomes less than a predetermined disconnection-detection threshold so that the level adjusting unit gives a predetermined attenuation to the input optical signal. The controlling unit transitions from the “off-state” when the optical power level of the output optical signal is greater than a predetermined recovery threshold and an upstream WDM device is not in the “off-state”.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention generally relates to a wavelength division multiplexing device, an optical leakage prevention method, and a wavelength division multiplexing communication system, and especially relates to a wavelength division multiplexing device, an optical leakage prevention method, and a wavelength division multiplexing communication system that can adjust the optical power level.  
         [0003]     2. Description of the Related Art  
         [0004]     In order to maintain transmission quality of a wavelength division multiplexing communication system, it is important to reduce variations in the optical power level due to, for example, attenuation variation of optical parts and transmission lines, and gain variation of optical amplifiers. Especially, where the wavelength division multiplexing communication system is constituted by a great number of relays, tilts (variations of the optical power level by wavelength) are accumulated, and the wavelength variation of OSNR (optical S/N ratio) at a receiving end increases. Since degradation of OSNR causes an adverse influence on the system performance of the wavelength division multiplexing communication system as a reduction in the span length over which an optical signal can be transmitted, variations in the optical power level by wavelength have to be reduced.  
         [0005]     As a method of reducing the variation in optical power level, a method of controlling and reducing the amount of the tilt is conceivable, wherein the tilt of the optical amplifier is compensated for by applying a tilt in an opposite direction of the tilt generated in the transmission line. Further, in the case where the wavelength division multiplexing communication system uses a distributed Raman amplifier (DRA), a method of controlling the optical power level of the excitation light of the DRA and compensating for the tilt is available. Either method is suitable for reduction of the tilt, such as reduction of a primary inclination.  
         [0006]      FIG. 1  is a block diagram of an example of an OADM (Optical Add/Drop Multiplexing) node that constitutes a wavelength division multiplexing communication system. The OADM node is an example of a wavelength division multiplexing device. In the OADM node of  FIG. 1 , the variation in optical power level is reduced by the following methods.  
         [0007]     According to a conventional OADM node (henceforth a node), a received optical signal is divided by wavelengths by a splitter  100 . The divided optical signal is output to an adder  150  through a coupler (CPL)  120 , after the optical power level is adjusted by a variable optical attenuator (VOA)  110 . The coupler  120  divides the output of the variable optical attenuator  110  and sends one of the divided parts to a monitor photo diode (PD 1 )  130 .  
         [0008]     A control unit  140  analyzes the optical power level of the output of the variable optical attenuator  110  using the monitor photo diode (PD 1 )  130 , and controls the magnitude of attenuation of the variable optical attenuator  110  so that the optical power level of the signal output from the variable optical attenuator  110  becomes a predetermined level (constant level control). According to this method, the optical power level of the signal output from the variable optical attenuator  110  is finely adjusted for every channel, and the variation in optical power level is properly reduced.  
         [0009]      FIG. 2  is a control flowchart of the Node of  FIG. 1 . At Step S 100 , the control unit  140  controls the magnitude of attenuation of the variable optical attenuator  110  so that the optical power level of the signal output from the variable optical attenuator  110  becomes equal to a target VOA output level L 1 . The control unit  140  repeats Step S 100  until the optical power level of the signal output from the variable optical attenuator  110  becomes less than a disconnection-detection threshold Th_d (i.e., so long as the condition is NO at S 101 ). If the optical power level of the signal output from the variable optical attenuator  110  becomes less than the disconnection-detection threshold Th_d (i.e., YES at S 101 ), the control unit  140  determines that the optical signal is no longer input (optical signal off), and transitions to an “off-state”.  
         [0010]     During the “off-sate”, the control unit  140  controls the variable optical attenuator  110  so that the magnitude of attenuation of the variable optical attenuator  110  is set to a predetermined “off-state” attenuation A 1  that is set at a magnitude of attenuation that can detect an input optical signal when the optical signal is resumed from the “off-state”. If the optical power level of the signal output from the variable optical attenuator  110  becomes greater than a recovery threshold Th_r (i.e., YES at S 103 ), the control unit  140  transitions from the “off-state”, and resumes processing at Step S 100 . That is, the control unit  140  controls the magnitude of attenuation of the variable optical attenuator  110  so that the optical power level of the signal output from the variable optical attenuator  110  serves as the target VOA output level L 1 .  
         [0011]     Patent Reference 1 discloses a technology wherein a variable optical attenuator of a node is controlled so that it autonomously reduces variations such that an optical surge, and the like, does not occur when a system becomes faulty.  
         [0012]     [Patent Reference 1] JPA 2004-7058  
         [0013]     [Problem(s) to be solved by the Invention] 
         [0014]     According to the Node structured as shown in  FIG. 1 , a coupler is provided in front of the variable optical attenuator  110 , which is an advantage in that a level diagram of the Node can be improved, and that the Node can be made at a lesser cost with reduction of size. However, there are the following problems.  
         [0015]     For example, when the attenuation of the transmission line is great, the Node has to increase the gain of a preamplifier  90 . Accordingly, a spontaneous emission optical signal (Amplified Spontaneous Emission: ASE) generated by the Node increases. In the case of a wavelength division multiplexing communication system having a great number of relays, when the transmission line attenuation is great, the spontaneous-emission optical signal generated by the Node is great, and is accumulated. Consequently, at a down-stream node, it often happens that the optical power level of the signal output from the variable optical attenuator  110  becomes greater than the recovery threshold Th_r due only to the spontaneous-emission optical signal, even if the optical signal is not recovered.  
         [0016]      FIG. 3  is a system diagram that shows a spontaneous emission optical signal being accumulated in the wavelength division multiplexing communication system having a great number of relays. At Section (a) of  FIG. 3 , an optical signal multiplexed by an adder  150 - 1  is represented. At Section (b) of  FIG. 3 , the optical signal amplified by a post amplifier  160 - 1  and a preamplifier  90 - m  is shown, to which a spontaneous-emission optical signal is added.  
         [0017]     When the optical signal with the spontaneous emission optical signal has passed through a splitter  100 - m  and an adder  150 - m , the spontaneous-emission optical signal is filtered except for the wavelengths of the optical signal. Section (c) of  FIG. 3  shows the optical signal after being filtered by the splitter  100 - m  and the adder  150 - m . However, a wen (remnant, trace) of a spontaneous-emission optical signal remains in a channel that has no optical signal as shown at section (c). At section (d), the optical signal amplified by a post amplifier  160 - m  is shown, to which a spontaneous-emission optical signal is added.  
         [0018]     Whenever the wen of a spontaneous-emission optical signal passes a node, the wen is accumulated. In this way, it is often the case that the optical power level of the spontaneous-emission optical signal becomes greater than the recovery threshold Th_r. If the optical power level of the spontaneous-emission optical signal becomes greater than recovery threshold Th_r, the control unit  140  resumes an “on-state” from the “off-state”, and controls the magnitude of attenuation of the variable optical attenuator  110  so that the optical power level of the signal output from the variable optical attenuator  110  serves as the target VOA output level L 1 . That is, the variable optical attenuator  110  is fully released, the spontaneous-emission optical signal superfluously penetrates, and an optical leakage occurs. Section (e) expresses the optical leakage and the optical signal.  
         [0019]     The optical leakage of the Node in  FIG. 1  causes problems as follows. First, the optical leakage is a problem in that it generates a surge when a wavelength is added. For example, when there is an optical leakage, the variable optical attenuator  110  is fully released for applying constant level control of the output of the variable optical attenuator  110 . If an optical signal is added to a channel that has the optical leakage, the optical power level cannot be appropriately adjusted by the variable optical attenuator  110  in the Node until the magnitude of attenuation of the variable optical attenuator  110  is reconfigured, which causes a surge to occur.  
         [0020]     Second, the optical leakage causes a problem of producing a gap in an output set point of constant level control performed by the preamplifier  90  and the post amplifier  160 . For example, the output value of the constant level control of the preamplifier  90  and the post amplifier  160  is varied by an optical leakage being added. When gains of several wavelengths are set up, the optical power level per wavelength is reduced according to the magnitude of the optical leakage that is added. Further, depending on methods of the constant level control, a solution may be attempted by recognizing an optical leakage as one wavelength, and by setting up the gain for all the optical signals and the optical leakage; however, in this case, an error of the output value is generated by the difference between the optical power level for one wavelength of the optical signals and the optical power level of the optical leakage.  
         [0021]      FIG. 4  is a waveform chart showing the optical leakage and the optical signal. With the waveform chart of  FIG. 4 , the optical leakage occurs in a part of the frequency range that contains no optical signals.  FIGS. 5A, 5B  and  5 C give graphs that express an example of level transition of an input optical signal, an output optical signal, and optical attenuation, respectively, of the variable optical attenuator  110  in normal operations.  FIGS. 6A, 6B , and  6 C give graphs that express an example of level transition of the input optical signal, the output optical signal, and the optical attenuation, respectively, of the variable optical attenuator  110  when there is an optical leakage. The example is where a resumption of the “on-state” is carried out from the “off-state” due to a spontaneous-emission optical signal.  
         [0022]     As shown in  FIG. 6A , even if the spontaneous-emission optical signal is added in the frequency range wherein no optical signal channels are present, the incident optical signal to the variable optical attenuator  110  in the “off-state” is greater than the case shown by  FIG. 5A . For this reason, the optical power level of the optical signal output from the variable optical attenuator  110  becomes greater than the recovery threshold Th_r as shown in  FIG. 6B , and the “on-state” is resumed. Accordingly, the problem with the Node shown in  FIG. 1  is in that control of the variable optical attenuator  110  cannot be properly performed due to accumulation of the spontaneous-emission optical signal, and that there is a danger of an excessive surge occurring at the time of resumption.  
       SUMMARY OF THE INVENTION  
       [0023]     The present invention is made in view of the problems, and aims at offering a wavelength division multiplexing device, an optical leakage prevention method, and a wavelength division multiplexing communication system that can prevent an optical leakage from being generated, substantially obviating one or more of the problems caused by the limitations and disadvantages of the related art.  
         [0024]     Features and advantages of the present invention are set forth in the description that follows, and in part will become apparent from the description and the accompanying drawings, or may be learned by practice of the invention according to the teachings provided in the description. Objects as well as other features and advantages of the present invention will be realized and attained by a wavelength division multiplexing device, an optical leakage prevention method, and a wavelength division multiplexing communication system particularly pointed out in the specification in such full, clear, concise, and exact terms as to enable a person having ordinary skill in the art to practice the invention.  
         [0025]     [Means for Solving the Problem] 
         [0026]     In order to achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a wavelength division multiplexing device that includes  
         [0027]     a level adjusting unit for adjusting an output optical power level of an optical signal that is input (input optical signal), and  
         [0028]     a controlling unit for analyzing the optical power level of the optical signal output by the level adjusting unit, controlling the level adjusting unit so that the optical power level of the optical signal output is fixed, transitioning to an “off-state” when the optical power level of the output optical signal becomes less than the disconnection-detection threshold, and controlling the level adjusting unit such that the input optical signal is optically attenuated by a predetermined amount. Here, if the optical power level of the output optical signal is greater than a recovery threshold, and if an upstream wavelength division multiplexing device is in the “on-state”, the controlling unit resumes the “on-state”.  
         [0029]     The present invention further offers an optical leakage prevention method for a wavelength division multiplexing device, the method including  
         [0030]     a step of inputting an optical signal, adjusting the level thereof, and outputting the level-adjusted optical signal,  
         [0031]     a step of analyzing the optical power level of the optical signal that is output,  
         [0032]     a step of controlling the optical power level of the output optical signal to become fixed,  
         [0033]     a step of controlling a transition to the “off-state” when the optical power level of the output optical signal becomes less than the disconnection-detection threshold, so that a predetermined optical attenuation is given to the input optical signal, and  
         [0034]     a step of resuming the “on-state” from the “off-state” if the optical power level of the output optical signal becomes greater than a recovery threshold, and if an upstream wavelength division multiplexing device is in the “on-state”.  
         [0035]     The present invention further offers a wavelength division multiplexing communication system that includes two or more wavelength division multiplexing devices. The wavelength division multiplexing device includes  
         [0036]     a level adjusting unit for adjusting an output optical power level of a optical signal that is input (input optical signal), and  
         [0037]     a controlling unit for analyzing the optical power level of the optical signal output by the level adjusting unit, controlling the level adjusting unit so that the optical power level of the optical signal output is fixed, transitioning to an “off-state” when the optical power level of the output optical signal becomes less than the disconnection-detection threshold, and controlling the level adjusting unit such that the input optical signal is optically attenuated by a predetermined amount. Here, if the optical power level of the output optical signal is greater than a recovery threshold, and if an upstream wavelength division multiplexing device is in the “on-state”, the controlling unit resumes the “on-state”.  
         [0038]     As described above, according to the present invention, resuming “on-state” from “off-state” takes place under two conditions simultaneously fulfilled, i.e., when the optical power level of the output optical signal is greater than the recovery threshold, and when an upstream wavelength division multiplexing device is not in the “off-state”; in this way, resumption from the “off-state” due to a spontaneous-emission optical signal is prevented from occurring, and an optical leakage is prevented from occurring.  
         [0039]     [Effect of the Invention] 
         [0040]     As described above, the present invention provides the wavelength division multiplexing device, the optical leakage prevention method, and the wavelength division multiplexing communication system, wherein generation of the optical leakage is avoided. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0041]      FIG. 1  is a block diagram showing an example of a conventional OADM node that constitutes a wavelength division multiplex communication system;  
         [0042]      FIG. 2  is a control flowchart of the OADM node of  FIG. 1 ;  
         [0043]      FIG. 3  is a block diagram with associated graphs showing the wavelength division multiplexing communication system of  FIG. 1  with multiple relays, wherein a spontaneous-emission optical signal is accumulated;  
         [0044]      FIG. 4  is a waveform chart showing an optical leakage and an optical signal;  
         [0045]      FIGS. 5A, 5B , and  5 C are level diagrams showing an example of level transition of an input optical signal, an output optical signal, and optical attenuation, respectively, in normal operations;  
         [0046]      FIGS. 6A, 6B , and  6 C are level diagrams showing an example of level transition of the input optical signal, the output optical signal, and the optical attenuation, respectively, when there is an optical leakage;  
         [0047]      FIG. 7  is a block diagram of the OADM node that constitutes a wavelength division multiplexing communication system according to an embodiment of the present invention;  
         [0048]      FIG. 8  is a control flowchart showing an example of a control process carried out by the control unit of the OADM node of  FIG. 7 ;  
         [0049]      FIGS. 9A, 9B , and  9 C are level diagrams showing an example of level transition of the input optical signal, the output optical signal, and the optical attenuation, respectively, of a variable optical attenuator according to the present invention;  
         [0050]      FIG. 10  is a control flowchart showing an example of the control process carried out by the control unit of  FIG. 7 ;  
         [0051]      FIG. 11  is a block diagram of the OADM node that constitutes the wavelength division multiplexing communication system according to another embodiment of the present invention; and  
         [0052]      FIGS. 12A and 12B  are block diagrams showing examples of the wavelength division multiplexing communication system that are configured in the shape of a ring. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0053]     In the following, embodiments of the present invention are described with reference to the accompanying drawings. In the following, an OADM node is described as an example of the wavelength division multiplexing device of the present invention.  
       Embodiment 1  
       [0054]      FIG. 7  is a block diagram of the OADM node that constitutes a wavelength division multiplexing communication system according to the embodiment of the present invention.  FIG. 7  shows an OADM node A (henceforth Node A), and an OADM node B (henceforth Node B) that represent a part of the wavelength division multiplexing communication system. Node A includes a preamplifier  1 A, a splitter  2 A, a coupler  3 A, a switch  4 A, the variable optical-attenuator  5 A, a coupler  6 A, an adder  7 A, a post amplifier  8 A, a monitor PD 1   9 A, and the control unit  10 A. Node B includes a preamplifier  1 B, a splitter  2 B, a coupler  3 B, a switch  4 B, a variable optical-attenuator  5 B, a coupler  6 B, an adder  7 B, a post amplifier  8 B, a monitor PD 1   9 B, and the control unit  10 B. Here, the configurations and operations of Node A and Node B are the same.  
         [0055]     Node A receives an optical signal from a transmission line, and outputs the optical signal to a transmission line via the preamplifier  1 A, the splitter  2 A, the coupler  3 A, the switch  4 A, the variable optical-attenuator  5 A, the coupler  6 A, the adder  7 A, and the post amplifier  8 A. Node A also provides the optical signal that is branched off by the coupler  6 A to the monitor PD 1   9 A. The monitor PD 1   9 A outputs a voltage based on the optical power level of the branched off optical signal to the control unit  10 A. The control unit  10 A analyzes the voltage received from the monitor PD 1   9 A, the voltage being based on the optical power level of the optical signal, and controls the magnitude of attenuation of the variable optical-attenuator  5 A so that the optical power level of the signal output from the variable optical-attenuator  5 A becomes equal to a target VOA output level L 1 .  
         [0056]     The preamplifier  1 A amplifies the optical signal input from the transmission line, and outputs the optical signal to the splitter  2 A. The splitter  2 A divides the amplified optical signal by wavelengths. Each wavelength is output to a path provided for the corresponding wavelength. The path is a structure that includes the coupler  3 A, the switch  4 A, the variable optical-attenuator  5 A, the coupler  6 A, the monitor PD 1   9 A, and the control unit  10 A. The optical signal of each wavelength is output to the adder  7 A through the coupler  6 A, after the optical power level is adjusted by the variable optical-attenuator  5 A. The adder  7 A adds optical signals of every wavelength, and outputs the added optical signal to the post amplifier  8 A. The post amplifier  8 A amplifies the received optical signal, and outputs the amplified signal to the transmission line.  
         [0057]     Here, the control process carried out by the control unit  10 A is explained, referring to  FIG. 8 , which is a control flowchart showing an example of the control process of the control unit  10 A.  
         [0058]     At Step S 1 , the control unit  10 A controls the magnitude of attenuation of the variable optical-attenuator  5 A so that the optical power level of the signal output from the variable optical-attenuator  5 A becomes equal to the target VOA output level L 1 . The control unit  10 A carries out Step S 1  until the optical power level of the signal output from the variable optical-attenuator  5 A becomes below the disconnection-detection threshold Th_d (i.e., NO at S 2 ). When the optical power level of the signal output from the variable optical-attenuator  5 A becomes less than the disconnection-detection threshold Th_d (i.e., YES at S 2 ), the control unit  10 A determines that the input signal is disconnected, and transitions to the “off-state”.  
         [0059]     In the “off-state”, the control unit  10 A carries out Step S 3 , wherein the variable optical-attenuator  5 A is controlled so that the magnitude of attenuation of the variable optical-attenuator  5 A becomes equal to a fixed magnitude A 1  defined for the “off-state”. The fixed magnitude of attenuation A 1  is made the magnitude of attenuation at which an input optical signal can be detected when the input signal returns during the “off-state”. Progressing to Step S 4 , the control unit  10 A provides information about the “off-state” to a down-stream node (for example, Node B). The control unit  10 A can provide the information about the “off-state” to the down-stream node via an OSC (Optical Supervisory Channel).  
         [0060]     Then, the process proceeds to Step S 5 , wherein the control unit  10 A determines whether the optical power level of the signal output from the variable optical-attenuator  5 A is greater than a recovery threshold Th_r. If affirmative (YES at S 5 ), the process progresses to Step S 6 , wherein whether the node concerned is “the highest node” is determined. (Here, “the highest node” is a node to which an optical signal is first provided, wherein no spontaneous optical signal is emitted.) If affirmative (YES at S 6 ), the process progresses to Step S 7 , where information about transitioning from the “off-state” is provided to the down-stream node. The control unit  10 A can use the OSC for providing the information to the down-stream node. Then, the control unit  10 A returns to processing Step S 1 . That is, the control unit  10 A controls the magnitude of attenuation of the variable optical-attenuator  5 A so that the optical power level of the signal output from the variable optical-attenuator  5 A becomes equal to the target VOA output level L 1 .  
         [0061]     On the other hand, if the determination is negative, i.e., NO at S 6 , the process proceeds to Step S 8 , wherein it is determined whether an upstream node has transitioned from “off-state” to “on-state”. If the determination is affirmative, i.e., YES at S 8 , the process progresses to Step S 7 . Otherwise, i.e., if NO at S 8 , the process returns to Step S 5 .  
         [0062]     As shown by the control flowchart of  FIG. 8 , Node A by the present invention transitions from the “off-state” when two conditions are met; namely, when the optical power level of the signal output from the variable optical-attenuator  5 A is greater than the recovery threshold Th_r, and when the upstream Node is not in the “off-state”. Therefore, though the optical power level of the signal output from the variable optical-attenuator  5 A becomes greater than the recovery threshold Th_r by accumulation of a spontaneous-emission optical signal, since the “off-state” is maintained, the optical leakage is not generated at Node A according to the present invention.  
         [0063]     Here, “the highest node” transitions from the “off-state” when one condition is met; namely, when the optical power level of the signal output from the variable optical-attenuator  5 A is greater than recovery threshold Th_r. This is because “the highest node”, to which the optical signal is first input, does not generate a spontaneous-emission optical signal, and no optical leakage is generated.  
       Embodiment 2  
       [0064]     As described above, according to Embodiment 1, the phenomenon of the “off-state” being canceled by the optical power level of the spontaneous-emission optical signal can be prevented. However, the case is conceivable, wherein the optical power level of the signal output from the variable optical-attenuator  5 A does not become less than the disconnection-detection threshold Th_d due to the accumulated spontaneous-emission optical signal, as explained below with reference to  FIGS. 9A, 9B , and  9 C, which show an example of level transition of the input optical signal, the output optical signal, and optical attenuation, respectively, of the variable optical-attenuator  5 A.  
         [0065]     As shown in  FIG. 9A , the power level of the optical signal input to the variable optical-attenuator  5 A does not fall as much as shown in  FIG. 5A  by an amount corresponding to the accumulated spontaneous-emission optical signal even if it is in the unused frequency range. For this reason, as shown in  FIG. 9B , the optical power level of the output optical signal from the variable optical-attenuator  5 A becomes greater than the disconnection-detection threshold Th_d, and a transition to the “off-state” cannot be attained. In view of this, each Node of Embodiment 2 is made capable of detecting a rapid fall of the optical power level of own node such that the transition to the “off-state” is attained when the optical power level rapidly decreases.  
         [0066]     The structure of Node A of Embodiment 2 is the same as that of Node A of Embodiment 1; accordingly, the structure shown in  FIG. 7  applies. Descriptions follow about the control process of the control unit  10 A of Node A according to Embodiment 2 with reference to the control flowchart of  FIG. 10 , which shows an example of the control process of the control unit  10 A.  
         [0067]     At Step S 10 , the control unit  10 A controls the magnitude of attenuation of the variable optical-attenuator  5 A so that the optical power level of the signal output from the variable optical-attenuator  5 A becomes equal to the target VOA output level L 1 . Then, at Step S 11 , the control unit  10 A determines whether the optical power level of the signal output from the variable optical-attenuator  5 A is less than the disconnection-detection threshold Th_d.  
         [0068]     If the determination is negative, i.e., NO at S 11 , the process proceeds to Step S 12 , where it is determined whether the optical power level of the signal output from the variable optical-attenuator  5 A has rapidly decreased. If the determination is negative, i.e., NO at S 12 , the process returns to Step S 10 .  
         [0069]     Under a condition that the optical power level of the signal output from the variable optical-attenuator  5 A has become less than the disconnection-detection threshold Th_d, i.e., YES at S 11 , or if the determination at S 12  is affirmative, YES at S 12 , the control unit  10 A determines that the input signal is disconnected, and the transition to the “off-state” takes place.  
         [0070]     Since Steps S 13  through S 18  after transitioning to the “off-state” are the same as Steps S 3  through S 8  of  FIG. 8 , explanation thereof is not repeated. As described above with reference to  FIG. 10 , Node A according to Embodiment 2 is capable of transitioning to the “off-state” even if the optical power level of the signal output from the variable optical-attenuator  5 A does not become less than the disconnection-detection threshold Th_d, as shown in  FIG. 9 , due to the accumulated spontaneous-emission optical signal.  
         [0071]     The rapid fall of the optical power level of the signal output from the variable optical-attenuator  5 A is detected by continuously monitoring a voltage based on the optical power level of the optical signal received by the monitor PD 1   9 A, and by comparing a change of the voltage in a unit of time with a predetermined threshold (e.g., 5 dB/3 ms, and 10 dB/500 μs). When the speed of the change exceeds the threshold, it is determined that the optical power level is rapidly decreasing.  
         [0072]     As described, in Node A of Embodiment 2, the transition to the “off-state” takes place even if the optical power level from the variable optical-attenuator  5 A does not become less than the disconnection-detection threshold Th_d due to the accumulated spontaneous-emission optical signal, as shown in  FIG. 9 . Accordingly, the optical leakage is prevented from occurring.  
       Embodiment 3  
       [0073]     The OADM node as shown in  FIG. 7  can also be configured as shown in  FIG. 11 , which is a block diagram of another OADM node (Node) that constitutes a wavelength division multiplexing communication system according to Embodiment 3 of the present invention. The Node includes a preamplifier  1 , a splitter  2 , a coupler  3 , a switch  4 , a variable optical attenuator  5 , a coupler  6 , an adder  7 , a post amplifier  8 , a control unit  10 , and a spectrum analyzer unit (SAU)  11 .  
         [0074]     Node of  FIG. 11  differs from Node A shown in  FIG. 7  in that the coupler  6  is provided after the adder  7 , and the spectrum analyzer unit  11  is provided between the coupler  6  and the control unit  10 .  
         [0075]     That is, the Node of Embodiment  3  includes the coupler  6  and the spectrum analyzer unit  11  after the adder  7  instead of the monitor PD 1   9 A provided for every channel in  FIG. 7 . The Node of  FIG. 11  outputs the optical signal that is branched off by the coupler  6  to the spectrum analyzer unit  11 . The spectrum analyzer unit  11  outputs a voltage based on the optical power level of the branched off optical signal to the control unit  10 . The control unit  10  analyzes the voltage received from the spectrum analyzer unit  11 , and controls the magnitude of attenuation of the variable optical attenuator  5  as described in Embodiment 1 and Embodiment 2.  
         [0076]     Therefore, in the Node of Embodiment 3, since the “off-state” is maintained even if the optical power level of the signal output from the variable optical attenuator  5  becomes greater than recovery threshold Th_r due to accumulation of spontaneous-emission optical signals, generation of an optical leakage can be prevented. Further, since the transition to the “off-state” takes place even if the optical power level of the signal output from the variable optical attenuator  5  does not become less than the disconnection-detection threshold Th_d due to the accumulated spontaneous-emission optical signals, generation of the optical leakage can be prevented.  
       Embodiment 4  
       [0077]     In Nodes according to Embodiments 1 through 3, information about the “off-state” is provided to a down-stream node via the OSC. However, since the OSC provides optical termination for every node, when the wavelength division multiplexing communication system has many relays, the information about the “off-state”, and information about transition from the “off-state” may not be timely provided, i.e., a delay is a concern. For example, in the Nodes according to Embodiments 1 through 3, the “off-state” may resume before the information is provided by the OSC, and an optical leakage can occur. The optical leakage can occur more often in a downstream node where the spontaneous-emission optical signal is accumulated.  
         [0078]     Then, according to Embodiment 4, the wavelength division multiplexing communication system is configured in the shape of a ring as shown in  FIGS. 12A and 12B .  FIG. 12A  shows the case wherein the information about the “off-state” and resumption of the “on-state” is transmitted by the OSC in one direction (forward direction).  FIG. 12B  shows the case wherein the information about the “off-state” and resumption is transmitted in both directions (the forward and the reverse directions) by the OSC.  
         [0079]      FIGS. 12A and 12B  shows an example wherein Node I transitions to the “off-state”. In  FIG. 12A , the information about of the “off-state” is transmitted only in the forward direction (counterclockwise rotation). In  FIG. 12B , the information is transmitted both in the forward direction and the opposite direction (clockwise rotation). In the Nodes of Embodiment 4, since the information can quickly reach each node by transmitting the information in both directions by the OSC, momentary generation of the optical leakage can be prevented.  
         [0080]     Further, the present invention is not limited to these embodiments, but variations and modifications may be made without departing from the scope of the present invention.  
         [0081]     The present application is based on Japanese Priority Application No. 2005-099882 filed on Mar. 30, 2005 with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.