Patent Publication Number: US-2023155673-A1

Title: Burst optical relaying device and burst optical relaying method

Description:
TECHNICAL FIELD 
     The present invention relates to a burst beam relay device used in a node of a metro network that is applied to a rural area in which areas with a low traffic demand are scattered, for example, and a burst beam relay method. 
     BACKGROUND ART 
     As a network for flexibly concentrating metro networks in a rural area with an accommodation traffic of several tens of Gbps, for example, at a low cost and with low power, an optical TDM (Time Division Multiplexing) network ( FIG.  9   ) to which the optical TDM technique is applied has been studied. In such an optical TDM network, a burst optical signal with controlled timing is used as a main signal. Further, in the optical TDM network, an EDFA (Erbium Doped optical Fiber Amplifier) as an optical amplifier for addressing burst is needed to cover the transmission distance for a metro network in which transmission over a long distance of several hundreds of Km, for example, is performed in multiple stages. 
     In an optical TDM network  10  illustrated in  FIG.  9   , optical fibers are laid in a double-ring shape, forming a double-ring configuration of an outer optical transmission line  11  as an outer ring and an inner optical transmission line  12  as an inner ring. A plurality of optical couplers  14   a ,  14   b ,  14   c , and  14   d  are connected to the outer optical transmission line  11  at predetermined intervals. Likewise, a plurality of optical couplers  15   a ,  15   b ,  15   c , and  15   d  are connected to the inner optical transmission line  12  at predetermined intervals. 
     An optical signal is transmitted through the outer optical transmission line  11  in a clockwise direction as indicated by the arrow Y 1 , and an optical signal is transmitted through the inner optical transmission line  12  in a counterclockwise direction as indicated by the arrow Y 2 . In the outer optical transmission line  11 , EDFAs  16   a  and  16   b  are connected to the respective input sides of the optical couplers  14   c  and  14   d . In the inner optical transmission line  12  also, EDFAs  17   a  and  17   b  are connected to the respective input sides of the optical couplers  15   c  and  15   d . 
     A burst TRX (transceiver)  18   a , which serves as a transceiver for sending and receiving an optical signal with data superposed thereon, is connected to the optical coupler  14   a  connected to the outer optical transmission line  11  and to the optical coupler  15   a  connected to the inner optical transmission line  12  via optical fibers. With the burst TRX  18   a  and the optical couplers  14   a  and  15   a , a node  19   a  as a communication device is formed. 
     Likewise, a burst TRX  18   b  is connected to the optical couplers  14   b  and  15   d  via optical fibers. With the burst TRX  18   b  and the optical couplers  14   b  and  15   d , a node  19   b  is formed. A burst TRX  18   c  is connected to the optical couplers  14   c  and  15   c . With the burst TRX  18   c  and the optical couplers  14   c  and  15   c , a node  19   c  is formed. A burst TRX  18   d  is connected to the optical couplers  14   d  and  15   b . With the burst TRX  18   d  and the optical couplers  14   d  and  15   b , a node  19   d  is formed. 
     Each of the burst TRXs  18   a  to  18   d  has a transmission device  20  connected thereto as represented by the burst TRX  18   a . The transmission device  20  exchanges optical signals with the burst TRX  18   a . 
     In such a configuration, it is assumed that the burst TRX  18   b  has sent a burst optical signal 1B with a wavelength λ1 illustrated in a blowing frame  21   a  to the optical coupler  14   b  within a predetermined time period t1 as indicated by the arrow Y 3 . The sent burst optical signal 1B is transmitted through the outer optical transmission line 1 in a clockwise direction as indicated by the arrow Y 1  via the optical coupler  14   b , and is amplified by the EDFA  16   a  as indicated by the arrow Y 4  and is then output to the optical coupler  14   c . 
     It is also assumed that the burst TRX  18   c  has sent a burst optical signal 2B with a wavelength λ1 illustrated in a blowing frame  21   b  within a predetermined time period t2 after the time period t1 has elapsed as indicated by the arrow Y 5 . The sent burst optical signal 2B is time-division multiplexed with the burst optical signal 1B by the optical coupler  14   c , and the resulting signals are transmitted through the outer optical transmission line  11  in a clockwise direction. The transmitted burst optical signals 1B and 2B are amplified by the EDFA  16   b  and are then input to the optical coupler  14   d . 
     Meanwhile, it is assumed that the burst TRX  18   d  has sent a burst optical signal 3B with a wavelength λ1 illustrated in a blowing frame  21   c  within a predetermined time period t3 after the time period t2 has elapsed as indicated by the arrow Y 6 . The sent burst optical signal 3B is time-division multiplexed with the burst optical signals 1B and 2B by the optical coupler  14   d  as illustrated in a blowing frame  21   d , and the resulting signals are transmitted through the outer optical transmission line  11  in a clockwise direction. The transmitted burst optical signals 1B to 3B are branched by the optical coupler  14   a , for example, and are received by the burst TRX  18   a  and are then sent to the transmission device  20 . 
     However, when the burst optical signals 1B and 2B are amplified by the aforementioned EDFAs  16   a  and  16   b , overshoot occurs that would cause an increase in the degradation of transmission quality. Such occurrence of overshoot will be described using the EDFA  16   a  illustrated in  FIG.  10    as a representative example. It is assumed that the burst optical signal 1B transmitted through the outer optical transmission line  11  is input to the EDFA  16   a . 
     The EDFA  16   a  receives a pump beam  1 P based on a laser beam from a pumping semiconductor laser (not illustrated). With the pump beam  1 P, the burst optical signal 1B with a level L 1  is amplified to a level L 2 . At the beginning of the amplification, a transient response occurs in the EDFA  16   a  so that overshoot with a level L 3  occurs. 
     To suppress such overshoot, there is known a technique using a clamp beam  1 C illustrated in  FIG.  11    (Non-Patent Literature 1). Such a technique can be implemented with a suppressed cost since a general-purpose EDFA that has been already used is used. 
     As illustrated in  FIG.  11   , a CW (Continuous Wave) light source  23  is connected to an optical coupler  14   b   1  that is connected to the outer optical transmission line  11  in an inserted manner. The CW light source  23  sends to the optical coupler  14   b   1  the continuous-wave clamp beam  1 C with a wavelength λ4 different from that of the burst optical signal 1B. The optical coupler  14   b   1  synthesizes the clamp beam  1 C with the burst optical signal 1B so that the non-signal sections of the burst optical signal 1B are eliminated. 
     The EDFA  16   a  amplifies the synthesized burst optical signal 1B and clamp beam  1 C. In the amplification, as the power of the clamp beam  1 C is higher than the power of the burst optical signal 1B, the percentage of the synthesized signals that are detected as a continuous signal by the EDFA  16   a  is increased. This can further suppress overshoot. The burst optical signal 1B and the clamp beam  1 C amplified to a predetermined level L 2  ( FIG.  10   ) with the suppressed overshoot are input to a filter  24 . The filter  24  removes the clamp beam  1 C, and passes only the burst optical signal 1B with the wavelength λ1. 
     Such a technique of suppressing overshoot using a clamp beam has been fully studied for a PON (Passive Optical Network) with a passive star configuration. For a metro network, a configuration is applied in which the clamp beam  1 C sent from the CW light source  23  is synthesized with the burst optical signal 1B immediately before the EDFA  16   a  in the double-ring configuration, and the clamp beam  1 C is removed with the filter  24  immediately after the EDFA  16   a  as described above. In such a configuration, the components, such as the CW light source  23 , the optical coupler  14   b   1 , and the filter  24 , are needed for each of a plurality of EDFAs arranged in each of a plurality of nodes of the double-ring. This results in increased resources of the metro network. 
     Thus,  FIG.  12    illustrates the configuration of a burst beam relay system (also referred to as a system) applied to a metro network with reduced resources. A system  30  is based on the technique of Non-Patent Literature 2, and has a configuration in which a plurality of nodes  31   a ,  31   b ,  31   c , and  31   d  are connected to a representative node  31  via an outer optical transmission line  41  and an inner optical transmission line  42  forming a double-ring, using the aforementioned optical TDM technique. 
     The representative node  31  has an active/auxiliary configuration including a primary node and a secondary node. The primary node includes transponders  33   a  each having a clamp light source  32   a , a MUX (multiplexer)  34   a , and a DEMUX (demultiplexer)  35   a . The secondary node includes transponders  33   b  each having a clamp light source  32   b , a MUX  34   b , and a DEMUX  35   b . 
     The node  31   a  includes a burst beam relay device (also referred to as a relay device)  39   a  and active/auxiliary transponders  37   a  and  37   b . Likewise, the node  31   b  includes a relay device  39   b  and active/auxiliary transponders  37   c  and  37   d , the node  31   c  includes a relay device  39   c  and active/auxiliary transponders  37   e  and  37   f , and the node  31   d  includes a relay device  39   d  and active/auxiliary transponders  37   g  and  37   h . 
     Since each of the nodes  31   a  to  31   d  does not require the components, such as the CW light source  23 , the optical coupler  14   b   1 , and the filter  24 , used for each of the plurality of EDFAs illustrated in  FIG.  11    described above, the amount of resources is reduced. 
     It should be noted that optical transmission lines (not illustrated) are respectively connected to the aforementioned transponders  33   a ,  33   b , and  37   a  to  37   h . 
     The output side of the MUX  34   b  of the secondary node and the input side of the DEMUX  35   a  of the primary node are connected through the outer optical transmission line  41  via each of the relay devices  39   d ,  39   c ,  39   b , and  39   a . An optical signal sent from the MUX  34   b  of the secondary node is transmitted through the outer optical transmission line  41 , and is received by the DEMUX  35   a  of the primary node via each of the relay devices  39   d ,  39   c ,  39   b , and  39   a  as indicated by the arrow Y 11 . 
     The output side of the MUX  34   a  of the primary node and the input side of the DEMUX  35   b  of the secondary node are connected through the inner optical transmission line  42  via each of the relay devices  39   a ,  39   b ,  39   c , and  39   d . An optical signal sent from the MUX  34   a  of the primary node is received by the DEMUX  35   b  of the secondary node via each of the relay devices  39   a ,  39   b ,  39   c , and  39   d  as indicated by the arrow Y 12 . 
     Next, the configurations of the burst beam relay devices  39   a  to  39   d  will be described using the relay device  39   b  illustrated in  FIG.  13    as a representative example. The relay device  39   b  includes optical couplers  44   c  and  44   d  and EDFAs  45   c  and  45   d  connected to the outer optical transmission line  41 , and optical couplers  47   c  and  47   d  and EDFAs  48   c  and  48   d  connected to the inner optical transmission line  42 . 
     The EDFA  45   d , the optical couplers  44   d  and  44   c , and the EDFA  45   c  are connected in this order to the outer optical transmission line  41  in the direction of the arrow Y 11 . The EDFA  48   c , the optical couplers  47   c  and  47   d , and the EDFA  48   d  are connected in this order to the inner optical transmission line  42  in the direction of the arrow Y 12 . 
     Further, the relay device  39   b  includes a buffer  46   c   1  connected between the transponder  37   c  and the optical coupler  44   c , a buffer  46   c   2  connected between the optical coupler  47   c  and the transponder  37   c , a buffer  46   d   1  connected between the transponder  37   d  and the optical coupler  47   d , and a buffer  46   d   2  connected between the optical coupler  44   d  and the transponder  37   d . 
     In the system  30  illustrated in  FIG.  12    with such a configuration, for example, in the node  39   d , a burst optical signal sent from the transponder  37   g  is transmitted to the outer optical transmission line  41 , and is then received by the DEMUX  35   a  of the primary node as the representative node  31  as indicated by the arrow Y 11   a . Likewise, in the relay device  39   b  illustrated in  FIG.  13   , a burst optical signal sent from the transponder  37   c  passes through the buffer  46   c   1  and the optical coupler  44   c  and is amplified by the EDFA  45   c , and is then transmitted through the outer optical transmission line  41  and is received by the DEMUX  35   a  of the primary node as indicated by the arrow Y 11   a . Further, the signal is received by each transponder  33   a  as indicated by the arrow Y 11   b . 
     Meanwhile, in the primary node illustrated in  FIG.  12   , a burst optical signal sent from the MUX  34   a  is transmitted to the inner optical transmission line  42  as indicated by the arrow Y 12 . The burst optical signal is amplified by the EDFA  48   c  illustrated in  FIG.  13   , for example, and is then branched by the optical coupler  47   c . Then, one of the resulting signals is sent to the transponder  37   c  via the buffer  46   c   2 , and the other is amplified by the EDFA  48   d  via the optical coupler  47   d  and is then sent to the DEMUX  35   b  of the secondary node via the inner optical transmission line  42 . Then, the signal is received by each transponder  33   b  as indicated by the arrow Y 12   a . 
     The primary node and the secondary node are redundant nodes with an active/auxiliary configuration, and are configured such that one of them (for example, the primary node) is automatically switched to the other (i.e., the secondary node) to be selected upon occurrence of a failure. That is, each transponder  33   a  of the primary node and each transponder  33   b  of the secondary node are configured such that one of them is automatically switched to the other upon occurrence of a failure. Along with this, the transponder  37   a  and the transponder  37   b  of the node  39   a  are configured such that one of them is switched to the other. The same holds true for the other nodes  39   b  to  39   d . 
     For the clamp light source  32   b  arranged in each transponder  33   b  of the secondary node, the aforementioned CW light source  23  ( FIG.  11   ) is used. The clamp light source  32   b  sets the gain of a continuous-wave optical signal with a wavelength different from that of a burst optical signal to be constant, and sends the signal as a clamp beam as indicated by the arrow Y 11   b . The sent clamp beam is transmitted from the MUX  34   b  to the outer optical transmission line  41  in the direction indicated by the arrow Y 11 , and passes through each of the nodes  31   d  to  31   a , and is then received by the DEMUX  35   a  of the primary node. 
     A clamp beam sent from the clamp light source  32   a  arranged in each transponder  33   a  of the primary node as indicated by the arrow Y 12   a  is transmitted from the MUX  34   a  to the inner optical transmission line  42  in the direction indicated by the arrow Y 12 , and passes through each of the nodes  31   a  to  31   d , and is then received by the DEMUX  35   b  of the secondary node. 
     It should be noted that the clamp beam that passes through the outer optical transmission line  41  or the inner optical transmission line  42  is removed with a filter as appropriate as described above. 
     In this manner, as a clamp beam is transmitted through the outer optical transmission line  41  or the inner optical transmission line  42 , overshoot of a burst optical signal is suppressed. 
     Citation List 
     Non-Patent Literature 
     Non-Patent Literature 1: H.H. Lee, et al., “All-optical gain-clamped EDFA using external saturation signal for burst-mode upstream in TWDM-PONs.”, Optics Express 22.15 (2014). 
     Non-Patent Literature 2: Kana Masumoto, and three others. “Evaluation of usefulness of node configuration capable of suppressing transient response that occurs during burst beam amplification for metro networks,” January, IEICE technical report, 2020. 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     In the aforementioned system  30 , suppose that a failure, such as disconnection of an optical fiber indicated by reference sign  1   k , has occurred in the outer optical transmission line  41  between the node  31   b  and the node  31   c  illustrated in  FIG.  14   . In such a case, it becomes impossible for a burst optical signal and an optical signal of a clamp beam, which travel toward the primary node from the position of the failure  1   k  in the outer optical transmission line  41 , to reach the nodes  31   b  and  31   a . 
     Therefore, as illustrated in  FIG.  15   , since a clamp beam does not reach the EDFA  45   c  when a burst optical signal indicated by the arrow Y 11   a  from the active transponder  37   c  is input to the EDFA  45   c , it would be impossible to appropriately perform amplification while suppressing overshoot as indicated by a cross mark  2   k . 
     When the aforementioned failure  1   k  has occurred, the configuration of the representative node  31  is switched to the auxiliary configuration so that an optical signal travels from the primary node to the secondary node via the inner optical transmission line  42  as indicated by the arrow Y 12  in  FIG.  14   . In such a case, as illustrated in  FIG.  15   , an optical signal transmitted through the inner optical transmission line  42  is amplified by the EDFA  48   c  and is branched by the optical coupler  47   c , and is then received by the active transponder  37   c  via the buffer  46   c   2  as indicated by the arrow Y 12   b . 
     Meanwhile, a burst optical signal sent from the auxiliary transponder  37   d  as indicated by the arrow Y 12   c  is multiplexed with a clamp beam by the optical coupler  47   d  via the buffer  46   d   1 , and is amplified by the EDFA  48   d  and is then transmitted to the secondary node via the inner optical transmission line  42 . 
     However, since overshoot of a burst optical signal on the transmission side from the active transponder  37   c  indicated by the arrow Y 11   a  is not appropriately suppressed as indicated by the cross mark  2   k  on the EDFA  45   c , it would be impossible to use the burst optical signal. Meanwhile, since the auxiliary transponder  37   d  cannot receive an optical signal from the secondary node due to the occurrence of the failure  1   k , it would be impossible to use the optical signal. 
     Therefore, when the configuration of the representative node  31  is switched to the auxiliary configuration in which an optical signal travels from the primary node to the secondary node as described above, communication cannot be performed properly even if the active transponder  37   c  is switched to the auxiliary transponder  37   d  in the node  31   b . This also holds true for the other nodes  31   a ,  31   c , and  31   d . Therefore, even when the configuration of the representative node  31  is switched from the active primary node to the auxiliary secondary node, communication cannot be performed properly. 
     The present invention has been made in view of the foregoing circumstances, and it is an object of the present invention to, when a failure has occurred in one of optical transmission lines with a double-ring configuration, allow a burst optical signal to be sent to the other optical transmission line with suppressed overshoot. 
     Means for Solving the Problem 
     To solve the aforementioned problem, a burst beam relay device of the present invention includes an optical signal return unit connected to a representative node with an active/auxiliary configuration that sends and receives a burst optical signal and sends a continuous-wave clamp beam with a wavelength different from a wavelength of the burst optical signal, via two optical transmission lines with a double-ring configuration that perform optical transmission in mutually opposite directions, in a manner relaying the burst optical signal and an optical signal of the clamp beam, the optical signal return unit being connected across the two optical transmission lines in each of a plurality of nodes that sends and receives the burst optical signal; and a detection unit that, when an optical signal input via one of the two optical transmission lines has not been detected for a predetermined time or longer, outputs a disconnection signal of the optical transmission line to the optical signal return unit, in which only when there is an input of the disconnection signal, the optical signal return unit returns to the one of the optical transmission lines only a clamp beam that has been sent from the representative node via another optical transmission line in a direction opposite to the optical signal input via the one of the optical transmission lines. 
     Effects of the Invention 
     The present invention allows, when a failure has occurred in one of optical transmission lines with a double-ring configuration, a burst optical signal to be sent to the other optical transmission line with suppressed overshoot. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a block diagram illustrating the configuration of a burst beam relay system according to an embodiment of the present invention. 
         FIG.  2    is a block diagram illustrating the configuration of a burst beam relay device according to an embodiment of the present invention. 
         FIG.  3    is a first flowchart for illustrating the operation of the burst beam relay device according to an embodiment of the present invention. 
         FIG.  4    is a second flowchart for illustrating the operation of the burst beam relay device according to an embodiment of the present invention. 
         FIG.  5    is a block diagram illustrating a state in which both double rings have failed in the burst beam relay device according to an embodiment of the present invention. 
         FIG.  6    is a block diagram illustrating the configuration of a burst beam relay device according to Modified Example 1 of the embodiment of the present invention. 
         FIG.  7    is a block diagram illustrating the configuration of a burst beam relay device according to Modified Example 2 of the embodiment of the present invention. 
         FIG.  8    is a block diagram illustrating the configuration of a burst beam relay device according to Modified Example 3 of the embodiment of the present invention. 
         FIG.  9    is a block diagram illustrating the configuration of an optical TDM network. 
         FIG.  10    is a view illustrating overshoot that occurs when a burst optical signal is amplified by an EDFA. 
         FIG.  11    is a view illustrating the suppression of overshoot that occurs in an EDFA. 
         FIG.  12    is a block diagram illustrating the configuration of a conventional burst beam relay system. 
         FIG.  13    is a block diagram illustrating the configuration of a conventional burst beam relay device. 
         FIG.  14    is a block diagram illustrating a failure of an outer optical transmission line in the conventional burst beam relay system. 
         FIG.  15    is a block diagram illustrating a state in which overshoot cannot be suppressed with an EDFA of the conventional burst beam relay device. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an embodiment of the present invention will be described with reference to the drawings. It should be noted that throughout the drawings of this specification, functionally corresponding portions are denoted by identical reference signs, and the description thereof will be omitted as appropriate. 
     Configuration of Embodiment 
       FIG.  1    is a block diagram illustrating the configuration of a burst beam relay system according to an embodiment of the present invention. 
     A burst beam amplification system (i.e., a system)  30 A illustrated in  FIG.  1    differs from the conventional system  30  ( FIG.  12   ) in burst beam relay devices (i.e., relay devices)  39   a A,  39   b A,  39   c A, and  39   d A provided in respective nodes  31   a  to  31   d  that are connected to a representative node  31  through double rings. The double rings include an outer optical transmission line  41  and an inner optical transmission line  42  made of double optical fibers that transmit optical signals in mutually opposite directions as indicated by the arrows Y 11  and Y 12 . 
     The representative node  31  has an active/auxiliary configuration including a primary node and a secondary node. The primary node includes transponders  33   a  each having a clamp light source  32   a , a MUX (multiplexer)  34   a , and a DEMUX (demultiplexer)  35   a . The secondary node includes transponders  33   b  each having a clamp light source  32   b , a MUX  34   b , and a DEMUX  35   b . 
     The nodes  31   a  to  31   d  respectively include pairs of active and auxiliary transponders  37   a  to  37   h  in addition to the relay devices  39   a A to  39   d A. It should be noted that a transmission device (not illustrated) is connected to each of the transponders  37   a  to  37   h  and each of the transponders  33   a  and  33   b  of the representative node  31 . 
     The configurations of the burst beam relay devices  39   a A to  39   d A will be described using the relay device  39   b A illustrated in  FIG.  2    as a representative example. The relay device  39   b A includes optical signal return units  50   a  and  50   b  and detection units  55   a  and  55   b  as the characteristic elements of the present invention. Besides, the relay device  39   b A includes an EDFA  45   d , optical couplers  44   d  and  44   c , and an EDFA  45   c  connected in this order to the outer optical transmission line  41  in the direction of the arrow Y 11 . The relay device  39   b A also includes an EDFA  48   c , optical couplers  47   c  and  47   d , and an EDFA  48   d  connected in this order to the inner optical transmission line  42  in the direction of the arrow Y 12 . Further, the relay device  39   b A includes buffers  46   c   1 ,  46   c   2 ,  46   d   1 , and  46   d   2  connected between the transponders  37   c  and  37   d  and the optical couplers  44   c ,  44   d ,  47   c , and  47   d . 
     The optical signal return units  50   a  and  50   b  are provided across the outer optical transmission line  41  and the inner optical transmission line  42 , and each include isolators  51   a  and  51   b , optical couplers  52   a  and  52   b , a filter  53 , and a gate unit  54 . 
     First, referring to the optical signal return unit  50   a , the isolator  51   a  and the optical coupler  52   a , which are connected to the inner optical transmission line  42 , are connected in this order between the EDFA  48   c  and the optical coupler  47   c  on the side of the inner optical transmission line  42  in the direction toward the secondary node ( FIG.  1   ) indicated by the arrow Y 12   (referred to as a secondary node direction Y 12 ). It should be noted that the isolator  51   a  forms a first isolator recited in the claims, and the optical coupler  52   a  forms a first optical coupler recited in the claims. 
     The isolator  51   b  and the optical coupler  52   b , which are connected to the outer optical transmission line  41 , are connected in this order between the optical coupler  44   c  and the EDFA  45   c  on the side of the outer optical transmission line  41  in the direction toward the primary node ( FIG.  1   ) indicated by the arrow Y 11  (referred to as a primary node direction Y 11 ) . 
     The filter  53  and the gate unit  54  are connected in this order between the optical coupler  52   a  connected to the inner optical transmission line  42  and the optical coupler  52   b  connected to the outer optical transmission line  41  in the direction from the inner optical transmission line  42  toward the outer optical transmission line  41 . 
     The detection unit  55   a  is connected to the input side of the EDFA  48   c  having connected thereto the isolator  51   a  on the side of the inner optical transmission line  42 . That is, the detection unit  55   a  is connected to an input terminal for an optical signal transmitted through the inner optical transmission line  42  in the secondary node direction Y 12  in the relay device  39   b A. 
     Next, referring to the optical signal return unit  50   b , the isolator  51   b  and the optical coupler  52   b  connected to the outer optical transmission line  41  are connected in this order between the EDFA  45   d  and the optical coupler  44   d  on the side of the outer optical transmission line  41  in the primary node direction Y 11 . It should be noted that the isolator  51   b  forms a second isolator recited in the claims, and the optical coupler  52   b  forms a second optical coupler recited in the claims. 
     The isolator  51   a  and the optical coupler  52   a  connected to the inner optical transmission line  42  are connected in this order between the optical coupler  47   d  and the EDFA  48   d  on the side of the inner optical transmission line  42  in the secondary node direction Y 12 . 
     The filter  53  and the gate unit  54  are connected in this order between the optical coupler  52   b  connected to the outer optical transmission line  41  and the optical coupler  52   a  connected to the inner optical transmission line  42  in the direction from the outer optical transmission line  41  toward the inner optical transmission line  42 . 
     The detection unit  55   b  is connected to the input side of the EDFA  45   d  having connected thereto the isolator  51   b  on the side of the outer optical transmission line  41 . That is, the detection unit  55   b  is connected to an input terminal for an optical signal transmitted through the outer optical transmission line  41  in the primary node direction Y 11  in the relay device  39   b A. 
     The process operation of the components of such optical signal return units  50   a  and  50   b , which have the same configuration, will be described with reference to a flowchart of  FIG.  3   . It should be noted that the description will be made using the optical signal return unit  50   a  as a representative example. 
     In step S 1   a  of  FIG.  3   , when the detection unit  55  of the relay device  39   b A has not detected an optical signal input via, of the outer optical transmission line  41  and the inner optical transmission line  42 , the outer optical transmission line  41  for a predetermined time or longer, the detection unit  55  outputs a disconnection signal of the outer optical transmission line  41 . 
     Next, in step S 2   a , only when there is an input of the disconnection signal, the optical signal return unit  50   a  returns to the outer optical transmission line  41  only a clamp beam that has been sent from the representative node  31  via the inner optical transmission line  42  in the direction opposite to the optical signal input via the outer optical transmission line  41 . 
     Next, the detailed operation will be described with reference to a flowchart of  FIG.  4   . 
     In step S 1 , the isolator  51   a  passes optical signals based on a clamp beam with a wavelength λ4 and a burst optical signal with a wavelength λ1, which have been sent from the primary node via the inner optical transmission line  42 , only in the secondary node direction Y 12 . The optical coupler  52   a  branches the optical signals having passed through the isolator  51   a  so that the resulting signals are transmitted in the secondary node direction Y 12  and to the filter  53 . 
     In step S 2 , the filter  53  is adapted to pass only an optical signal with a wavelength λ4, and thus passes only the clamp beam with the wavelength λ4 of the optical signals from the optical coupler  52   a , toward the gate unit  54 . 
     In step S 3 , if the detection unit  55   b  has detected an optical signal input to the relay device  39   b  via the outer optical transmission line  41  in the primary node direction Y 11  within a predetermined time, the detection unit  55   b  sends no signal to the gate unit  54 . Meanwhile, if the detection unit  55  has detected no optical signal within a predetermined time, the detection unit  55  outputs to a control terminal of the gate unit  54  a disconnection signal indicating the disconnected state of the outer optical transmission line  41 . 
     For example, when a failure  1   k  has occurred in the outer optical transmission line  41  on the upstream side of the relay device  39   b A in the primary node direction Y 11 , an optical signal is no longer transmitted to the relay device  39   b . Thus, the detection unit  55   b  detects no optical signal, and if such a state has continued for a predetermined time or longer, the detection unit  55   b  outputs a disconnection signal to the gate unit  54 . 
     In step S 4 , the gate unit  54  is usually OFF and thus blocks the passage of a clamp beam, but is turned ON when its control terminal has received a disconnection signal from the detection unit  55   b , and thus passes a clamp beam. 
     In step S 5 , the clamp beam having passed through the gate unit  54  is input to the optical coupler  52   b . The upstream side of the optical coupler  52   b  in the primary node direction Y 11  has the isolator  51   b  connected thereto such that the isolator  51   b  passes an optical signal only in the primary node direction Y 11 . Therefore, the clamp beam input to the optical coupler  52   b  is not transmitted to the side of isolator  51   b  but is input to the EDFA  45   c . 
     Through such a process, upon occurrence of the failure  1   k  in the outer optical transmission line  41 , a clamp beam that has been sent via the inner optical transmission line  42  in the secondary node direction Y 12  is returned in the primary node direction Y 11  via the outer optical transmission line  41  from the isolator  51   a , the optical coupler  52   a , the filter  53 , the gate unit  54 , and the optical coupler  52   b  as indicated by the dashed arrow Y 12   e . 
     In this manner, returning a clamp beam allows a burst optical signal sent from the transponder  37   c  to be synthesized with the clamp beam by the optical coupler  44   c  via the buffer  46   c   1  and then be output to the EDFA  45   c . In step S 6 , since the synthesized burst optical signal and clamp beam are amplified as a continuous signal by the EDFA  45   c , the aforementioned overshoot is suppressed. After the overshoot is suppressed in this manner, the clamp beam is removed with a filter (not illustrated), and only the burst optical signal may be transmitted in the primary node direction Y 11 . 
     The transponder  37   c  can receive a burst optical signal, which has been transmitted via the inner optical transmission line  42 , via the optical coupler  47   c  and the buffer  46   c   2 . Thus, sending and receiving of the burst optical signal are possible. 
     Such return of a clamp beam is also illustrated in  FIG.  1    as indicated by the dashed arrow Y 12   e . That is, a clamp beam sent from the clamp light source  32   a  of the primary node is transmitted from the MUX  34   a  to the inner optical transmission line  42 , and is then returned by the relay device  39   b A of the node  31   b  via the node  31   a . The returned clamp beam is received by the DEMUX  35   a  of the primary node via the node  31   a . Then, the received clamp beam is removed with a filter, for example. 
     Likewise, in the optical signal return unit  50   b  illustrated in  FIG.  2   , upon occurrence of a failure in the inner optical transmission line  42  on the upstream side of the relay device  39   b A, a clamp beam sent via the outer optical transmission line  41  in the primary node direction Y 11  is returned in the secondary node direction Y 12  via the inner optical transmission line  42  from the isolator  51   b , the optical coupler  52   b , the filter  53 , the gate unit  54 , and the optical coupler  52   a  as indicated by the dashed arrow Y 12   f . 
     As illustrated in  FIG.  5   , even when a failure  3   k  has occurred both in the outer optical transmission line  41  on the upstream side of the relay device  39   b A in the primary node direction Y 11  and in the inner optical transmission line  42 , the aforementioned optical signal return unit  50   a  can return a clamp beam sent from the primary node to the primary node as indicated by the arrow Y 12   e . Such return of a clamp beam allows for the transmission of a burst optical signal from the transponder  37   c  in the primary node direction Y 11  while suppressing overshoot. 
     Effects of Embodiment 
     Next, the effects of the burst beam relay devices  39   a A to  39   d A according to the present embodiment will be described using the relay device  39   b A illustrated in  FIG.  2    as a representative example. 
     (1a) The relay device  39   b A includes the optical signal return unit  50   a  and the detection unit  55   b . 
     The optical signal return unit  50   a  is connected to the representative node  31  with an active/auxiliary configuration, which sends and receives a burst optical signal and sends a continuous-wave clamp beam with a wavelength different from that of the burst optical signal, via the two optical transmission lines  41  and  42  with a double-ring configuration, which perform optical transmission in mutually opposite directions, in a manner relaying the burst optical signal and an optical signal of the clamp beam. The optical signal return unit  50   a  is connected across the two optical transmission lines  41  and  42  in each of the plurality of nodes  31   a  to  31   d  that sends and receives the burst optical signal. 
     The detection unit  55  outputs, when an optical signal input via, of the two optical transmission lines  41  and  42 , the optical transmission line  41  has not been detected for a predetermined time or longer, a disconnection signal of the optical transmission line. 
     The optical signal return unit  50   a  is configured to, only when there is an input of the disconnection signal, return to one of the optical transmission lines  41  only a clamp beam that has been sent from the representative node  31  via the other optical transmission line  42  in the direction opposite to the optical signal input via the one of the optical transmission lines  41 . 
     According to such a configuration, the detection unit  55   b  outputs a disconnection signal to the optical signal return unit  50   a  upon occurrence of a failure in one of the optical transmission lines  41  with a double-ring configuration. The optical signal return unit  50   a  that has received the disconnection signal returns to the one of the optical transmission lines  41  only a clamp beam that has been sent from the other optical transmission line  42  in the direction opposite to the optical signal. The returned clamp beam is synthesized with a burst optical signal sent from the node  31   b , and the synthesized signals are input to the EDFA  45   c  connected to some midpoint of the one of the optical transmission lines  41 . Since the EDFA  45   c  amplifies the synthesized burst optical signal and clamp beam as a continuous signal, it is possible to suppress overshoot that would occur if the burst optical signal is amplified alone. Therefore, the EDFA  45   c  can, upon occurrence of a failure in the one of the optical transmission lines  41  with a double-ring configuration, send a burst optical signal to the other optical transmission line  42  while suppressing overshoot. 
     (2a) The optical signal return unit  50   a  includes the isolator  51   a  for passing an optical signal, which is transmitted through the other optical transmission line  42  in the node  31   b  in the direction opposite to the one of the optical transmission lines  41 , only in the transmission direction of the optical signal, and the optical coupler  52   a  for branching the optical signal having passed through the isolator  51   a . The optical signal return unit  50   a  also includes the isolator  51   b  for passing an optical signal, which is transmitted through the one of the optical transmission lines  41  and  42  in each of the nodes  31   a  to  31   d , only in the transmission direction of the optical signal, the optical coupler  52   b  connected to the output side of the optical signal of the isolator  51   b , and the filter  53  and the gate unit  54  connected in a cascade arrangement between the optical coupler  52   a  and the optical coupler  52   b . 
     The filter  53  passes only the clamp beam branched by the optical coupler  52   a , and the gate unit  54  passes the clamp beam having passed through the filter  53  only when there is an input of the aforementioned disconnection signal, and further, the optical coupler  52   b  synthesizes the clamp beam having passed through the gate unit  54  with a burst optical signal that has passed through the isolator  51   b  and that is to be sent to the representative node  31 , and then sends the synthesized signals to the one of the optical transmission lines  41 . 
     According to such a configuration, the optical coupler  52   b  has connected thereto, on the side opposite to the optical signal transmission direction of the one of the optical transmission lines  41 , the isolator  51   b  that passes an optical signal only in the optical signal transmission direction. Therefore, a clamp beam synthesized with a burst optical signal by the optical coupler  52   b  is not transmitted in the direction of the isolator  51   b , and is reliably transmitted in the original optical signal transmission direction. Accordingly, the burst optical signal and the clamp beam synthesized by the optical coupler  52   b  are amplified as a continuous signal by the EDFA  45   c  connected to some midpoint of the one of the optical transmission lines  41 . Therefore, the EDFA  45   c  can suppress overshoot that would occur if the burst optical signal is amplified alone. 
     Modified Example 1 of Embodiment 
       FIG.  6    is a block diagram illustrating the configuration of a burst beam relay device according to Modified Example 1 of the embodiment of the present invention. 
     The burst beam relay device  39   b A of Modified Example 1 illustrated in  FIG.  6    differs from the relay device  39   b A ( FIG.  2   ) of the aforementioned embodiment in that the isolators  51   a  and  51   b  ( FIG.  2   ) on the optical signal input sides of the optical signal return units  50   a  and  50   b  ( FIG.  2   ) are removed. 
     That is, an optical signal return unit  50   a   1  illustrated in  FIG.  6    includes the optical couplers  52   a  and  52   b , the filter  53 , and the gate unit  54 . An optical signal return unit  50   b   1  includes the optical couplers  52   a  and  52   b , the filter  53 , and the gate unit  54 . 
     Without the isolator  51   a  ( FIG.  2   ) on the input side of the optical signal return unit  50   a   1 , a passage loss of a clamp beam through the isolator is reduced. With a reduced loss, the transmission efficiency of the clamp beam can be improved. In addition, since the optical signal return unit  50   a   1  has no isolator on the inner optical transmission line  42  and has the isolator  51   b  on the outer optical transmission line  41 , a loss of the inner ring and a loss of the outer ring in the double-ring configuration become asymmetrical, which facilitates the design of the optical transmission lines. Further, since the optical signal return unit  50   a   1  has only one isolator  51   b , the amount of resources can be reduced than in the relay device  39   b A ( FIG.  2   ) of the embodiment. 
     Modified Example 2 of Embodiment 
       FIG.  7    is a block diagram illustrating the configuration of a burst beam relay device according to Modified Example 2 of the embodiment of the present invention. 
     The burst beam relay device  39   b A of Modified Example 2 illustrated in  FIG.  7    differs from the relay device  39   b A ( FIG.  2   ) of the aforementioned embodiment in that in each of optical signal return units  50   a   2  and  50   b   2 , a T filter (i.e., a tunable wavelength filter)  57  is connected between the optical coupler  52   a  connected to the inner optical transmission line  42  and the optical coupler  52   b  connected to the outer optical transmission line  41 , instead of the filter  53  and the gate unit  54 . 
     The T filter  57  is a filter with a variable pass band for optical signals, and usually has a band that passes neither a burst optical signal nor a clamp beam, but, when there is an input of a disconnection signal from the detection unit  55   a  or the detection unit  55   b , has a band that passes only a clamp beam. 
     Each of such optical signal return units  50   a   2  and  50   b   2  includes one component that is the T filter  57  instead of the two components that are the filter  53  and the gate unit  54 . Thus, the amount of resources can be reduced than in the relay device  39   b A ( FIG.  2   ) of the embodiment. 
     Modified Example 3 of Embodiment 
       FIG.  8    is a block diagram illustrating the configuration of a burst beam relay device according to Modified Example 3 of the embodiment of the present invention. 
     The burst beam relay device  39   b A of Modified Example 3 illustrated in  FIG.  8    differs from the relay device  39   b A ( FIG.  2   ) of the aforementioned embodiment in that each of optical signal return units  50   a   3  and  50   b   3  does not include the gate unit  54  illustrated in  FIG.  2   , and includes the isolators  51   a  and  51   b , the optical couplers  52   a  and  52   b , and the filter  53  as illustrated in  FIG.  8   . It should be noted that the burst beam relay device  39   b A of Modified Example 3 does not include the detection units  55   a  and  55   b . 
     According to such a configuration, referring to the optical signal return unit  50   a   3  as a representative example, only a clamp beam that has passed through the isolator  51   a  on the inner optical transmission line  42  and has been branched by the optical coupler  52   a  always passes through the filter  53  and is synthesized with a burst optical signal by the optical coupler  52   b , and is then returned to the inner optical transmission line  42 . Such a sending operation is performed even when the failure  1   k  has occurred in the outer optical transmission line  41  on the input side of the relay device  39   b A in the primary node direction Y 11 . 
     Since each of the optical signal return units  50   a   3  and  50   b   3  can be configured with two isolators  51   a  and  51   b , two optical couplers  52   a  and  52   b , and one filter  53 , the amount of resources can be significantly reduced than in the relay device  39   b A ( FIG.  2   ) of the embodiment. 
     Effects 
     (1) There is provided a burst beam relay device including an optical signal return unit connected to a representative node with an active/auxiliary configuration that sends and receives a burst optical signal and sends a continuous-wave clamp beam with a wavelength different from a wavelength of the burst optical signal, via two optical transmission lines with a double-ring configuration that perform optical transmission in mutually opposite directions, in a manner relaying the burst optical signal and an optical signal of the clamp beam, the optical signal return unit being connected across the two optical transmission lines in each of a plurality of nodes that sends and receives the burst optical signal; and a detection unit that, when an optical signal input via one of the two optical transmission lines has not been detected for a predetermined time or longer, outputs a disconnection signal of the optical transmission line to the optical signal return unit, in which only when there is an input of the disconnection signal, the optical signal return unit returns to the one of the optical transmission lines only a clamp beam that has been sent from the representative node via another optical transmission line in a direction opposite to the optical signal input via the one of the optical transmission lines. 
     According to such a configuration, the detection unit outputs a disconnection signal to the optical signal return unit upon occurrence of a failure in one of the optical transmission lines with a double-ring configuration. The optical signal return unit that has received the disconnection signal returns to the one of the optical transmission lines only a clamp beam that has been sent from the other optical transmission line in the direction opposite to the optical signal. The returned clamp beam is synthesized with a burst optical signal sent from the node, and the synthesized signals are input to an EDFA connected to some midpoint of the one of the optical transmission lines. Since the EDFA amplifies the synthesized burst optical signal and clamp beam as a continuous signal, it is possible to suppress overshoot that would occur if the burst optical signal is amplified alone. Therefore, when a failure has occurred in one of the optical transmission lines with a double-ring configuration, it is possible to send a burst optical signal to the other optical transmission line while suppressing overshoot. 
     (2) There is provided the burst beam relay device according to (1) described above, in which the optical signal return unit includes a first isolator that passes an optical signal transmitted through the other optical transmission line in the node in a direction opposite to the one of the optical transmission lines, only in a transmission direction of the optical signal, a first optical coupler that branches the optical signal having passed through the first isolator, a second isolator that passes an optical signal transmitted through the one of the optical transmission lines in the node, only in a transmission direction of the optical signal, a second optical coupler connected to an output side of the optical signal of the second isolator, and a filter and a gate unit connected in a cascade arrangement between the first optical coupler and the second optical coupler, the filter passes only the clamp beam branched by the first optical coupler, the gate unit passes the clamp beam having passed through the filter only when there is an input of the disconnection signal, and the second optical coupler synthesizes the clamp beam having passed through the gate unit with a burst optical signal that has passed through the second isolator and that is to be sent to the representative node, and returns the synthesized signals to the one of the optical transmission lines. 
     According to such a configuration, the second optical coupler has connected thereto, on the side opposite to the optical signal transmission direction of the one of the optical transmission lines, the second isolator that passes an optical signal only in the optical signal transmission direction. Therefore, a clamp beam synthesized with a burst optical signal by the second optical coupler is not transmitted in the direction of the second isolator, and is reliably transmitted in the original optical signal transmission direction. Accordingly, the burst optical signal and the clamp beam synthesized by the second optical coupler are amplified as a continuous signal by the EDFA connected to some midpoint of the one of the optical transmission lines. Therefore, the EDFA can suppress overshoot that would occur if the burst optical signal is amplified alone. 
     (3) There is provided the burst beam relay device according to (1) described above, in which the optical signal return unit includes a first optical coupler that branches an optical signal transmitted through the other optical transmission line in the node in a direction opposite to the one of the optical transmission lines, an isolator that passes an optical signal transmitted through the one of the optical transmission lines in the node, only in a transmission direction of the optical signal, a second optical coupler connected to an output side of the optical signal of the isolator, and a filter and a gate unit connected in a cascade arrangement between the first optical coupler and the second optical coupler, the filter transmits only the clamp beam branched by the first optical coupler, the gate unit passes the clamp beam having passed through the filter only when there is an input of the disconnection signal, and the second optical coupler synthesizes the clamp beam having passed through the gate unit with a burst optical signal that has passed through the isolator and that is to be sent to the representative node, and returns the synthesized signals to the one of the optical transmission lines. 
     According to such a configuration, since there is no isolator on the input side on the other optical transmission line, a passage loss of a clamp beam is reduced. With a reduced loss, the transmission efficiency of the clamp beam can be improved. In addition, since the optical signal return unit has an isolator on one of the optical transmission lines and has no isolator on the other optical transmission line, a loss of the one of the optical transmission lines (i.e., the outer ring) and a loss of the other optical transmission line (i.e., the inner ring) in the double-ring configuration become asymmetrical, which facilitates the design of the optical transmission lines. 
     (4) There is provided the burst beam relay device according to (1) described above, in which the optical signal return unit includes a first isolator that passes an optical signal transmitted through the other optical transmission line in the node in a direction opposite to the one of the optical transmission lines, only in a transmission direction of the optical signal, a first optical coupler that branches the optical signal having passed through the first isolator, a second isolator that passes an optical signal transmitted through the one of the optical transmission lines in the node, only in a transmission direction of the optical signal, a second optical coupler connected to an output side of the optical signal of the second isolator, and a tunable wavelength filter connected between the first optical coupler and the second optical coupler and having a variable pass band for an optical signal, the tunable wavelength filter passes only the clamp beam when there is an input of the disconnection signal, and the second optical coupler synthesizes the clamp beam having passed through the tunable wavelength filter with a burst optical signal that has passed through the second isolator and that is to be sent to the representative node, and returns the synthesized signals to the one of the optical transmission lines. 
     According to such a configuration, the optical signal return unit includes one component that is the tunable wavelength filter instead of the two components that are the filter and the gate unit of the optical signal return unit of claim 1 described above. Thus, the amount of resources in the device configuration can be reduced than in the burst beam relay device described in claim 2. 
     (5) There is provided a burst beam relay device including an optical signal return unit connected to a representative node with an active/auxiliary configuration that sends and receives a burst optical signal and sends a continuous-wave clamp beam with a wavelength different from a wavelength of the burst optical signal, via two optical transmission lines with a double-ring configuration that perform optical transmission in mutually opposite directions, in a manner relaying the burst optical signal and an optical signal of the clamp beam, the optical signal return unit being connected across the two optical transmission lines in each of a plurality of nodes that sends and receives the burst optical signal, in which the optical signal return unit includes a first isolator that passes an optical signal transmitted in a direction opposite to one of the optical transmission lines through another optical transmission line in the node, only in a transmission direction of the optical signal, a first optical coupler that branches the optical signal having passed through the first isolator, a second isolator that passes an optical signal transmitted through the one of the optical transmission lines in the node, only in a transmission direction of the optical signal, a second optical coupler connected to an output side of the optical signal of the second isolator, and a filter connected between the first optical coupler and the second optical coupler, the filter passes only the clamp beam branched by the first optical coupler, and the second optical coupler synthesizes the clamp beam having passed through the filter with a burst optical signal that has passed through the second isolator and that is to be sent to the representative node, and returns the synthesized signals to the one of the optical transmission lines. 
     According to such a configuration, the optical signal return unit can be configured with two isolators including the first and second isolators, two optical couplers including the first and second optical couplers, and one filter. Thus, the amount of resources can be significantly reduced than in the burst beam relay device described in claim 2. 
     Besides, the specific configurations can be changed as appropriate within the spirit and scope of the present invention. 
     
       
         
           
               
               
             
               
                 Reference Signs List 
               
             
            
               
                   
                   31  Representative node 
               
               
                   
                   31   a  to  31   d  Node 
               
               
                   
                   39   a A to  39   d A Burst beam relay device 
               
               
                   
                   41  Outer optical transmission line (One of 
               
               
                 optical transmission lines) 
               
               
                   
                   42  Inner optical transmission line (the 
               
               
                 other optical transmission line) 
               
               
                   
                   50   a ,  50   b ,  50   a   1 ,  50   b   1 ,  50   a   2 ,  50   b   2 ,  50   a   3 ,  50   b   3   
               
               
                 Optical signal return unit 
               
               
                   
                   45   c ,  45   d ,  48   c ,  48   d  EDFA 
               
               
                   
                   51   a ,  51   b  Isolator 
               
               
                   
                   52   a ,  52   b  Optical coupler 
               
               
                   
                   53  Filter 
               
               
                   
                   54  Gate unit 
               
               
                   
                   55   a ,  55   b  Detection unit 
               
               
                   
                   57  T filter (tunable wavelength filter)