Optical transmission device and optical transmission system

Abstract

To suppress the fast transient response caused by the reduction in the number of optical signals in a WDM signal.SOLUTION: An optical transmission system transmits an optical signal through a plurality of nodes. A first optical transmission device implemented in a first node of the plurality of nodes outputs a WDM signal including a wavelength channel that propagates a real signal and a wavelength channel that propagates a pseudo signal. A second optical transmission device implemented in a second node of the plurality of nodes terminates the pseudo signal from the WDM signal and generates a second WDM signal by inserting an add signal and a new pseudo signal into the unused wavelength channel of the WDM signal after the pseudo signal has been terminated.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to an optical transmission device and an optical transmission system for transmitting wavelength division multiplexed signals.

Background Art

[0002] In order to achieve high-capacity communication, wavelength division multiplexing (WDM) has become widespread. WDM multiplexes and transmits a plurality of optical signals using a plurality of different wavelength channels. In addition, in order to further expand the transmission capacity, there is an increasing interest in wideband wavelength multiplexing (multi-band WDM). In multi-band WDM, for example, the C-band and the L-band are used.

[0003] In WDM transmission for transmitting WDM signals, due to stimulated Raman scattering (SRS) occurring in an optical fiber transmission line, part of the power of an optical signal on the short-wavelength side is absorbed by an optical signal on the long-wavelength side. For this reason, if many wavelength channels in the WDM signal stop for some reason and a small number of wavelength channels remain, the influence of stimulated Raman scattering changes, and the optical power of the remaining wavelength channels fluctuates temporarily or transiently. In the following description, this phenomenon may be referred to as a transient response (or Transient).

[0004] In a large-scale optical transmission system, a WDM signal is transmitted through a large number of spans. And the transient response caused by stimulated Raman scattering occurs in each span. For this reason, when many wavelength channels in the WDM signal stop as described above, the transient responses accumulate, and transmission errors may occur. Alternatively, when the optical power of the remaining wavelength channels increases, there is a risk of damaging the optical receiver.

[0005] Note that a configuration using dummy light has been proposed to prevent damage to the receiver (for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0006] Patent Document 1 Japanese Patent Application Laid-Open No. 2012-100103 Summary of the Invention Problems to be Solved by the Invention

[0007] In a WDM transmission system, a technique for stabilizing the optical power of each wavelength channel in a feedback system including a variable optical attenuator is known. However, the transient response caused by stimulated Raman scattering is very fast, and it is difficult to sufficiently suppress the transient response in the feedback system.

[0008] One object of the present invention is to suppress a high-speed transient response caused by a decrease in the number of optical signals in a WDM signal. Means for Solving the Problems

[0009] An optical transmission system according to one aspect of the present invention transmits an optical signal via a plurality of nodes. A first optical transmission device mounted on a first node among the plurality of nodes outputs a WDM signal including a wavelength channel for propagating a real signal and a wavelength channel for propagating a pseudo signal. A second optical transmission device mounted on a second node among the plurality of nodes includes a wavelength processing circuit that terminates the pseudo signal from the WDM signal and inserts an add signal and a new pseudo signal into an unused wavelength channel of the WDM signal after the pseudo signal is terminated to generate a second WDM signal. Effects of the Invention

[0010] According to the above aspect, a high-speed transient response caused by a decrease in the number of optical signals in a WDM signal is suppressed. Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Embodiments for Carrying Out the Invention

[0012] FIG. 1 is a diagram for explaining power shift due to stimulated Raman scattering (SRS). When a WDM signal propagates through an optical fiber, due to stimulated Raman scattering, part of the power of the optical signal on the short wavelength side is absorbed by the optical signal on the long wavelength side. That is, a power shift from the short wavelength side to the long wavelength side occurs. For example, as shown in FIG. 1A, when the power of each wavelength channel of the WDM signal output from the transmitting node is equalized, a WDM signal with a small power on the short wavelength side and a large power on the long wavelength side arrives at the receiving node.

[0013] Here, in order to suppress the variation in the quality of each wavelength channel, it is preferable that the power of each wavelength channel of the WDM signal arriving at the receiving node is equalized. For this reason, for example, as shown in FIG. 1B, at the transmitting node, a WDM signal with a large power on the short-wavelength side and a small power on the long-wavelength side is generated. Then, due to stimulated Raman scattering, a power shift from the short-wavelength side to the long-wavelength side occurs, so that the power of each wavelength channel of the WDM signal arriving at the receiving node becomes almost constant. In other words, the power of each wavelength channel of the WDM signal is adjusted at the transmitting node so that the power of each wavelength channel of the WDM signal arriving at the receiving node becomes almost constant.

[0014] FIG. 2 shows an example of a transient response generated due to the input interruption of a WDM signal. In this embodiment, in a reconfigurable optical add-drop multiplexer (ROADM) 101, an add signal is inserted into the WDM signal. The WDM signal is composed of wavelength channels λ1 to λn. Among the WDM signals input to the ROADM 101, wavelength channels λ2 to λn are used. The add signal is inserted into the wavelength channel λ1. The wavelength λ1 is the shortest among the wavelengths λ1 to λn.

[0015] In Case 1, as described with reference to FIG. 1B, the ROADM 101 generates and outputs a WDM signal with a large power on the short-wavelength side and a small power on the long-wavelength side. Here, the wavelength of the wavelength channel λ1 into which the add signal is inserted is short. Therefore, in the ROADM 101, the power of the add signal (that is, the wavelength channel λ1) is controlled to be larger compared to other wavelength channels, as described with reference to FIG. 1B. Then, the power of each wavelength channel of the WDM signal received by the next node (ROADM 102) becomes almost uniform.

[0016] ROADM102 also outputs a WDM signal with a large power on the short-wavelength side and a small power on the long-wavelength side. Then, the power of each wavelength channel of the WDM signal received by the next node becomes almost uniform. Similarly, each ROADM outputs a WDM signal with a large power on the short-wavelength side and a small power on the long-wavelength side. As a result, the power of each wavelength channel of the WDM signal arriving at the receiving node 103 is almost uniform.

[0017] In Case 2, the optical fiber on the input side of ROADM101 is cut, and no WDM signal is input to ROADM101. In this case, ROADM101 outputs only the add signal inserted into the wavelength channel λ1. However, since no optical signal propagates through the other wavelength channels, no power shift due to stimulated Raman scattering occurs. That is, no power shift occurs from the wavelength channel λ1 to the other wavelength channels.

[0018] Thus, when the optical fiber propagating the WDM signal is cut and many wavelength channels (in this example, λ2 to λn) in the WDM signal stop, the power of the remaining wavelength channel (in this example, λ1) fluctuates. That is, a transient response (or Transient) occurs. And since this transient response accumulates in each span, the power of the remaining signal arriving at the receiving node 103 may become very large. In this case, an error occurs at the receiving node 103 if the power of the remaining signal exceeds the dynamic range of the light-receiving element. Or, if the power of the remaining signal is very large, the light-receiving element may fail.

[0019] Note that, in many cases, the ROADM can individually control the optical power of each wavelength channel. Therefore, when detecting the interruption of the WDM signal input, the ROADM may control the power of the remaining wavelength channels so as to reduce the transient response. However, the transient response caused by stimulated Raman scattering is very fast (for example, 100 microseconds to 1 millisecond). For this reason, it is difficult to suppress the transient response caused by stimulated Raman scattering in the procedure of detecting the input interruption and controlling the power of the wavelength channels.

[0020] Also, in the example shown in FIG. 2, since the residual signal is arranged on the short-wavelength side within the signal band of the WDM signal, the power of the residual signal becomes large at the receiving node. On the other hand, when the residual signal is arranged on the long-wavelength side within the signal band of the WDM signal, the power of the residual signal becomes small at the receiving node. In this case, an error may occur at the receiving node.

[0021] FIG. 3 shows an example of an optical transmission system according to an embodiment of the present invention. The optical transmission system 1 according to the embodiment of the present invention includes a plurality of WDM nodes (nodes A to N) 10 and a network management system (NMS: Network Management System) 20. An ROADM is installed in each of the nodes A to N. In FIG. 3, the ROADM installed in node B is depicted. Also, in FIG. 3, a WDM signal is transmitted from node A toward node N, but the optical transmission system 1 can also transmit a WDM signal from node N toward node A. Hereinafter, the case where a WDM signal is transmitted from node A toward node N will be described.

[0022] The ROADM includes a wavelength selective switch (WSS) circuit 11. The WSS circuit 11 is an example of a wavelength processing circuit and can individually process a plurality of wavelength channels constituting the WDM signal.

[0023] The WSS circuit 11 can terminate one or more desired optical signals from the input WDM signal. The wavelength of the optical signal to be terminated is specified by the control unit 12. The optical signal terminated by the WSS circuit 11 is guided to the demultiplexer (DeMux) 13 via the drop port of the WSS circuit 11. The demultiplexer 13 is realized by, for example, a multicast switch (MCS). Then, the demultiplexer 13 guides the optical signal terminated from the input WDM signal to the corresponding transponder (TRP). Each transponder is connected to, for example, an access line or a client device. Note that "terminating an optical signal from a WDM signal" includes branching an optical signal from the WDM signal and removing an optical signal from the WDM signal.

[0024] The WSS circuit 11 can insert an optical signal into an unused channel of the WDM signal. That is, the optical signal transmitted from the access line or the client device is guided to the add port of the WSS circuit 11 via the multiplexer (Mux) 14. When the WSS circuit 11 terminates an optical signal from the input WDM signal, the ROADM can insert a new optical signal into the wavelength channel to which the terminated optical signal was assigned. The multiplexer 14 is realized by, for example, a multicast switch (MCS).

[0025] The ROADM further includes a pseudo signal generation unit 15 that generates a pseudo signal (PS: Pseudo Signal). The pseudo signal is an optical signal that does not carry data or information. The pseudo signal is guided to a predetermined input port of the WSS circuit 11. Then, the WSS circuit 11 generates a pseudo signal in an unused channel of the WDM signal. Note that the pseudo signal generation unit 15 is not particularly limited, and may be realized using, for example, an ASE (Amplified Spontaneous Emission) light source and a WSS.

[0026] The control unit 12 controls the operation of the ROADM. Also, the control unit 12 sets the state of the WSS circuit 11. Specifically, the control unit 12 sets the WSS circuit 11 to terminate one or more optical signals from the input WDM signal according to an instruction from the network management system 20. Also, the control unit 12 sets the WSS circuit 11 to insert one or more optical signals into the WDM signal according to an instruction from the network management system 20.

[0027] The network management system 20 manages the communication of the optical transmission system 1 and controls the operation of each WDM node 10. For example, the network management system 20 manages the wavelengths to be assigned to each optical path, generates control information for setting the WSS circuit 11 implemented in each WDM node 10, and gives it to each WDM node 10. Also, the network management system 20 determines the wavelength channels into which the dummy signals should be inserted and notifies each WDM node 10.

[0028] In the optical transmission system 1 with the above configuration, the WDM signal transmitted from node A arrives at node B. This WDM signal is composed of a plurality of wavelength channels and transmits real signals and dummy signals. That is, the WDM signal includes a wavelength channel for propagating the real signal and a wavelength channel for propagating the dummy signal. The real signal represents an optical signal carrying data or information. The data or information is generated by a client, for example. The dummy signal is an optical signal that does not carry data or information as described above and is inserted by the WDM node 10. For example, the dummy signal in the WDM signal received by node B is inserted by node A.

[0029] Note that in the transmission between nodes, it is preferable that all wavelength channels of the WDM signal are used. That is, when the WDM signal carries a real signal using one or more wavelength channels, it is preferable that dummy signals are inserted into all the remaining wavelength channels.

[0030] The WSS circuit 11 branches a dummy signal from the input WDM signal. Here, the dummy signal branched from the input WDM signal is discarded without being used. That is, the WSS circuit 11 substantially removes the dummy signal from the input WDM signal. Also, when the input WDM signal includes a drop signal to be led to the access line, the WSS circuit 11 branches the drop signal from the input WDM signal. The drop signal is a real signal carrying data or information. The drop signal branched at node B represents an optical signal to be led to the access line of node B and is specified by the network management system 20.

[0031] In the following description, the WDM signal from which the drop signal and the dummy signal are branched by the WSS circuit 11 may be referred to as the "in-node WDM signal". However, when the input WDM signal does not include a drop signal, the "in-node WDM signal" represents the WDM signal in which the dummy signal is terminated by the WSS circuit 11.

[0032] As described above, the in-node WDM signal is generated by branching the dummy signal (and the drop signal) from the input WDM signal. Therefore, the wavelength channels to which the dummy signal (and the drop signal) were assigned in the input WDM signal are unused channels in the in-node WDM signal.

[0033] The WSS circuit 11 inserts an add signal and a new dummy signal into the unused channels of the in-node WDM signal. The add signal is an optical signal received via the access line. Also, the add signal is a real signal carrying data or information. The new dummy signal is generated by the dummy signal generation unit 15. Then, the WSS circuit 11 generates a transmission WDM signal by inserting the add signal and the new dummy signal into the in-node WDM signal.

[0034] The transmitted WDM signal is amplified by the optical amplifier 16 and then transmitted to an adjacent node. The optical amplifier 16 is, for example, an EDFA (Erbium Doped Fiber Amplifier). When the WDM signal includes an optical signal in the C band and an optical signal in the L band, the optical amplifier 16 may include an optical amplifier that amplifies the C band and an optical amplifier that amplifies the L band.

[0035] Figures 4 to 7 show operation examples of the optical transmission system 1. In this embodiment, the WDM signal is composed of 192 wavelength channels. The 192 wavelength channels C1 to C192 are composed of, for example, 96 wavelength channels arranged in the C band and 96 wavelength channels arranged in the L band. In the C band, the 96 wavelength channels are arranged at equal intervals, and in the L band, the 96 wavelength channels are arranged at equal intervals. Hereinafter, node B provided between node A and node C processes the WDM signal traveling from node A to node C.

[0036] In the case shown in Figures 4 to 5, one of the 192 wavelength channels is used to transmit a dummy signal. As an example, wavelength channel C101 is used to transmit a dummy signal.

[0037] Figure 4 shows a case where the optical transmission system 1 operates normally. That is, as shown in Figure 4A, the WDM signal transmitted from node A is transmitted to node C via node B.

[0038] The WDM signal (WDM_AB) transmitted from node A includes a drop signal and a dummy signal, as shown in Figure 4B. The drop signal is an actual signal branched at node B. Also, the drop signal is arranged in wavelength channel C1.

[0039] As shown in Fig. 4A, node B branches a drop signal and a dummy signal from the WDM signal (WDM_AB). That is, the drop signal and the dummy signal are removed from the WDM signal (WDM_AB). Thereby, the in-node WDM signal shown in Fig. 4C is generated. In the in-node WDM signal, wavelength channel C1 and wavelength channel C101 are unused.

[0040] Also, node B inserts an add signal and a new dummy signal into the unused channels of the in-node WDM signal. The add signal arrives at node B via, for example, an access line. The new dummy signal is generated within node B. At this time, the add signal is inserted into wavelength channel C1, and the new dummy signal is inserted into wavelength channel C101. That is, the add signal is inserted into the wavelength channel where the drop signal was arranged, and the new dummy signal is inserted into the wavelength channel where the dummy signal was arranged. Thereby, the WDM signal (WDM_BC) shown in Fig. 4D is generated. Then, node B transmits this WDM signal (WDM_BC) to node C.

[0041] Fig. 5 shows an example of the operation of the optical transmission system 1 when the optical fiber for transmitting the optical signal from node A to node B is cut. In this case, node A transmits the WDM signal (WDM_AB) shown in Fig. 5B in the same case as shown in Fig. 4. However, as shown in Fig. 5A, the WDM signal (WDM_AB) does not arrive at node B.

[0042] Node B inserts an add signal into the wavelength channel where the drop signal was arranged and a new dummy signal into the wavelength channel where the dummy signal was arranged, in the same case as shown in Fig. 4. Specifically, the add signal is inserted into wavelength channel C1, and the new dummy signal is inserted into wavelength channel C101. Thereby, the WDM signal (WDM_BC) shown in Fig. 5C is generated. Then, node B transmits this WDM signal (WDM_BC) to node C.

[0043] In the case shown in FIGS. 6 to 7, about 20 percent of the 192 wavelength channels are used to transmit dummy signals. As an example, 42 wavelength channels C101 to C142 are used to transmit dummy signals.

[0044] FIG. 6 shows a case where the optical transmission system 1 operates normally. That is, as shown in FIG. 6A, the WDM signal transmitted from node A is transmitted to node C via node B.

[0045] The WDM signal (WDM_AB) transmitted from node A includes a drop signal and a dummy signal, as shown in FIG. 6B. The drop signal is arranged in wavelength channel C1. The dummy signals are arranged in wavelength channels C101 to C142.

[0046] Node B branches the drop signal and the dummy signal from the WDM signal (WDM_AB), as shown in FIG. 6A. That is, the drop signal and the dummy signal are removed from the WDM signal (WDM_AB). Thereby, the in-node WDM signal shown in FIG. 6C is generated. In the in-node WDM signal, wavelength channel C1 and wavelength channels C101 to C142 are unused.

[0047] Also, node B inserts an add signal and new dummy signals into the unused channels of the in-node WDM signal. At this time, the add signal is inserted into wavelength channel C1, and new dummy signals are inserted into wavelength channels C101 to C142, respectively. That is, the add signal is inserted into the wavelength channel where the drop signal was arranged, and new dummy signals are inserted into the wavelength channels where the dummy signals were arranged. Thereby, the WDM signal (WDM_BC) shown in FIG. 6D is generated. Then, node B transmits this WDM signal (WDM_BC) to node C.

[0048] FIG. 7 shows an example of the operation of the optical transmission system 1 when the optical fiber for transmitting the optical signal from node A to node B is cut. In this case, node A transmits the WDM signal (WDM_AB) shown in FIG. 7B, similar to the case shown in FIG. 6. However, as shown in FIG. 7A, the WDM signal (WDM_AB) does not reach node B.

[0049] Similar to the case shown in FIG. 6, node B inserts an add signal into the wavelength channel where the drop signal was arranged and inserts a new dummy signal into the wavelength channel where the dummy signal was arranged. Specifically, an add signal is inserted into wavelength channel C1, and new dummy signals are inserted into wavelength channels C101 to C142, respectively. Thereby, the WDM signal (WDM_BC) shown in FIG. 7C is generated. Then, node B transmits this WDM signal (WDM_BC) to node C.

[0050] Thus, in the optical transmission system 1, even when the input of the WDM signal to the WDM node 10 stops, the WDM node 10 outputs a dummy signal in addition to the add signal. For example, in the case shown in FIG. 5, when the WDM signal transmitted from node A does not reach node B, node B outputs an add signal and one dummy signal. In the case shown in FIG. 7, when the WDM signal transmitted from node A does not reach node B, node B outputs an add signal and 42 dummy signals.

[0051] FIG. 8 is a diagram for explaining the effect of transmitting a dummy signal. In this example, the WDM signal is composed of 192 wavelength channels (C1 to C192). Wavelength channels C1 to C96 are arranged within the C band, and wavelength channels C97 to C192 are arranged within the L band.

[0052] The horizontal axis of the graph shown in FIG. 8 represents the wavelength. The vertical axis represents the amount of variation in the optical power caused by stimulated Raman scattering per span. The dashed line represents the amount of variation in the optical power when the optical signal is transmitted through all the wavelength channels of the WDM signal.

[0053] When a WDM signal is transmitted via an optical fiber, due to stimulated Raman scattering, part of the power of the optical signal on the short-wavelength side is absorbed by the optical signal on the long-wavelength side. For this reason, for example, the optical signal with the shortest wavelength in the WDM signal (wavelength channel C1 in FIG. 8) has its optical power reduced by about 2.7 dB due to stimulated Raman scattering. Also, the optical signal with the longest wavelength (wavelength channel C192 in FIG. 8) has its optical power increased by about 2.1 dB.

[0054] On the other hand, when the WDM signal transmits only one optical signal (for example, wavelength channel C1), the variation in optical power due to stimulated Raman scattering is substantially zero. Therefore, for example, when transitioning from a state where an optical signal is transmitted through all wavelength channels of the WDM signal to a state where the optical signal is transmitted only through wavelength channel C1 due to, for example, a cut in the optical fiber, the optical power of wavelength channel C1 transiently increases by ΔP1 (about 2.7 dB). Here, the variation in optical power (i.e., the transient response) due to stimulated Raman scattering occurs for each span. And when the WDM signal is transmitted through multiple spans, the transient responses due to stimulated Raman scattering accumulate. That is, the transient response when the WDM signal is transmitted through 2 spans is about 5.4 dB, and the transient response when the WDM signal is transmitted through 3 spans is about 8.1 dB.

[0055] Here, assume that when the cumulative transient response exceeds 10 dB, a transmission error exceeding the threshold level occurs at the receiving node. In this case, in the worst-case scenario (i.e., the case of transitioning from a state where an optical signal is transmitted through all wavelength channels of the WDM signal to a state where the optical signal is transmitted only through wavelength channel C1), an error exceeding the threshold level will occur in 4-span transmission.

[0056] In an embodiment of the present invention, each WDM node 10 generates and outputs a dummy signal. In the example shown in FIG. 8, the WDM node 10 outputs an add signal using the wavelength channel C1, and outputs dummy signals via the wavelength channels C95 to C102 and C142 to C192. That is, together with the add signal, 69 dummy signals are output. Therefore, when the input of the WDM signal to the WDM node 10 stops, the state transitions from the state where the optical signal is transmitted through all the wavelength channels of the WDM signal to the state where 70 optical signals (that is, the add signal and 69 dummy signals) are transmitted. At this time, due to stimulated Raman scattering, a part of the optical power of the add signal is absorbed by the dummy signal. As a result, the optical power of the add signal transmitted using the wavelength channel C1 decreases by about 1.4 dB due to stimulated Raman scattering, as indicated by the asterisk.

[0057] Therefore, when the input of the WDM signal to the WDM node 10 stops, the optical power of the wavelength channel C1 transiently increases by ΔP2 (about 1.3 dB). That is, the transient response becomes smaller compared to the case where the dummy signal is not transmitted. As a result, when a transmission error exceeding the threshold level occurs at the receiving node when the cumulative transient response exceeds 10 dB, transmission over 6 spans becomes possible using the wavelength channel C1. Thus, according to the embodiment of the present invention, by inserting a dummy signal into the WDM signal, the transient response is suppressed, so that the number of spans over which the optical signal can be transmitted increases.

[0058] The effect of suppressing the transient response depends on the number of dummy signals inserted into the WDM signal. That is, the larger the number of dummy signals, the more the transient response is suppressed. In the example shown in FIG. 8, by inserting dummy signals into 69 of the 192 channels, the transient response per span is improved by 1.4 dB. Therefore, in order to sufficiently suppress the transient response, as an example, the channel utilization rate may be limited to 70 to 80 percent, and dummy signals may be inserted into 20 to 30 percent of the wavelength channels.

[0059] However, increasing the number of dummy signals reduces the number of wavelength channels for transmitting data or information. That is, the transmission efficiency deteriorates. Therefore, it is preferable to determine the upper limit of the channel utilization rate in consideration of both suppression of the transient response (i.e., the number of spans through which an optical signal can be transmitted) and the transmission efficiency.

[0060] Note that the transient response caused by a change in the number of wavelength channels for transmitting a signal may be suppressed by using a feedback system including a variable optical attenuator. However, the transient response due to stimulated Raman scattering is very fast. Therefore, in order to sufficiently suppress this transient response with a feedback system, it is necessary to increase the speed of the variable optical attenuator, which increases the cost or is difficult to implement. In contrast, the embodiment of the present invention suppresses the transient response by using an inexpensive ASE light source and an existing WSS, so it is advantageous in terms of cost and easy to implement.

[0061] FIG. 9 shows an example of a ROADM device implemented in each WDM node. As shown in FIG. 9, the ROADM device 30 includes a control unit 31, an optical amplifier 32, WSSs 33 and 34, an optical amplifier 35, an OSC (Optical Supervisory Channel) circuit 36, an OTDR (Optical Time Domain Reflectometer) 37, an optical channel monitor (OCM) 38, an optical channel monitor 39, an OTD 40, an OSC circuit 41, and a dummy signal generation unit 42. Note that the ROADM device 30 may further include other circuits or devices not shown in FIG. 9.

[0062] The control unit 31 includes a processor and a memory, and controls the operation of the ROADM device 30 in response to an instruction transmitted from the network management system 20. Further, the control unit 31 may control the operation of the ROADM device 30 based on an OSC signal received by the OSC circuit 36 or the OSC circuit 41. Furthermore, the control unit 31 may control the operation of the ROADM device 30 based on control information stored in the header of a packet received via an arbitrary wavelength channel in the WDM signal.

[0063] The optical amplifier 32 amplifies the input WDM signal. Also, the optical amplifier 35 amplifies the output WDM signal. The gains of the optical amplifier 32 and the optical amplifier 35 are adjusted by the control unit 31.

[0064] The WSSs 33 and 34 correspond to the WSS circuit 11 shown in FIG. 3 and can individually process a plurality of wavelength channels constituting the WDM signal. The WSS 33 branches a drop signal from the input WDM signal according to an instruction from the control unit 31. The drop signal branched from the input WDM signal is guided to the corresponding transponder by the demultiplexer as described with reference to FIG. 3. Also, when the input WDM signal includes a dummy signal, the WSS 33 branches the dummy signal from the input signal. At this time, the WSS 33 does not need to output the dummy signal branched from the input WDM signal. That is, the WSS 33 removes the dummy signal from the input WDM signal. Then, the WSS 33 outputs the in-node WDM signal obtained by branching the drop signal and the dummy signal from the input WDM signal.

[0065] The WSS 34 can insert an add signal into the in-node WDM signal. The add signal is transmitted from the corresponding transponder and guided to the WSS 34 via the multiplexer as described with reference to FIG. 3. Also, the WSS 34 can insert a new dummy signal into the in-node WDM signal. Then, the WSS 34 outputs the WDM signal into which the add signal and the new dummy signal are inserted. The new dummy signal is generated by a dummy signal generation unit 42 described later.

[0066] The OSC circuit 36 receives the OSC signal transmitted from an adjacent node. The OSC signal is an optical signal having a predetermined wavelength set outside the wavelength band of the WDM signal. Also, the OSC signal is used to notify control information between nodes. For example, the OSC signal may notify an adjacent node of information representing a wavelength to be branched from the WDM signal and information representing a wavelength where the dummy signal is arranged. Then, the OSC circuit 36 passes the control information transmitted by the OSC signal to the control unit 31. Also, the OSC circuit 41 generates an OSC signal according to an instruction from the control unit 31 and transmits it to an adjacent node.

[0067] The OTDRs 37 and 40 measure the state of the optical fiber cable by inputting an optical pulse into the optical fiber cable and monitoring the reflected pulse. The measurement results of the OTDRs 37 and 40 are notified to the control unit 31.

[0068] The optical channel monitor 38 detects the power of each wavelength channel of the input WDM signal. Here, the optical channel monitor 38 can detect the optical power before amplification by the optical amplifier 32 and the optical power after amplification by the optical amplifier 32. The optical channel monitor 39 detects the power of each wavelength channel of the output WDM signal. Here, the optical channel monitor 39 can detect the optical power before amplification by the optical amplifier 35 and the optical power after amplification by the optical amplifier 35.

[0069] The dummy signal generation unit 42 outputs light of a wavelength specified by the control unit 31 as a dummy signal. In the examples shown in FIGS. 6 to 7, the dummy signal generation unit 42 generates light of wavelengths corresponding to the wavelength channels C101 to C142.

[0070] The pseudo-signal generation unit 42 is realized by, for example, an ASE light source and a WSS. The ASE light source outputs high-intensity and broadband ASE light. The WSS operates as an optical filter that allows wavelength components to be used as pseudo-signals to pass through the ASE light. In this case, control information representing the wavelength of the pseudo-signal is given to this WSS. Then, the WSS generates a pseudo-signal by filtering (or channelizing) the ASE light according to this control information. Alternatively, the WSS 34 may be used to generate a pseudo-signal. For example, ASE light is input to a predetermined input port of the WSS 34. Then, the WSS 34 is set to operate as an optical filter that allows wavelength components to be used as pseudo-signals to pass through. In this case, the pseudo-signal generation unit 42 is realized by the ASE light source and the WSS 34.

[0071] The network management system 20 manages the communication of the optical transmission system 1 and controls the operation of each WDM node 10. For example, the network management system 20 notifies each WDM node of information representing the wavelength of the optical signal to be inserted into the WDM signal and information representing the wavelength of the optical signal to be branched from the WDM signal. Further, the network management system 20 determines the wavelength channel into which the pseudo-signal is to be inserted and notifies each WDM node 10.

[0072] FIG. 10 is a flowchart showing an example of the processing of the ROADM device 30. However, FIG. 10 shows the procedure related to the control of the WSS circuit. Further, FIG. 10 represents the operation of the ROADM device 30 implemented in node B shown in FIG. 11. In the case shown in FIG. 11, a WDM signal is transmitted from node A to node C via node B.

[0073] In S1, the ROADM device 30 receives wavelength setting information related to the setting of the WSS circuit from the network management system 20. The wavelength setting information represents the wavelength of the through signal T, the wavelength of the drop signal D, the wavelength of the add signal A, the wavelength of the input dummy signal P1, and the wavelength of the output dummy signal P2. The through signal T represents an actual signal among the actual signals in the WDM signal transmitted from node A that is transmitted to node C without being branched at node B. The drop signal D represents an optical signal branched from the WDM signal at node B. The add signal A represents an optical signal inserted into the WDM signal at node B. The input dummy signal P1 represents a dummy signal transmitted from node A to node B. The output dummy signal P2 represents a dummy signal transmitted from node B to node C. Note that the network management system 20 notifies each node of the wavelength setting information. Note that the wavelength setting information is created by the network management system 20 for each node and notified to the corresponding node.

[0074] The wavelength setting information may be notified to each node by other methods. For example, the wavelength setting information may be notified from an adjacent node using the OSC. Alternatively, the wavelength setting information may be stored in the header of a data packet transmitted through an arbitrary wavelength channel in the WDM signal. In this case, the ROADM device 30 implemented in each node can acquire the wavelength setting information by analyzing the header of the received packet from the adjacent node.

[0075] In S2, the control unit 31 sets the WSS 33 and the WSS 34 so that the through signal T is transmitted to node C without being branched at node B. In S3, the control unit 31 sets the WSS 33 so that the drop signal D is branched from the WDM signal at node B. At this time, the WSS 33 is set so that the drop signal D is guided to the corresponding drop port. In S4, the control unit 31 sets the WSS 33 so that the input dummy signal P1 is removed from the WDM signal at node B. At this time, the WSS 33 is set so that the input dummy signal P1 is not output.

[0076] In S5, the control unit 31 sets the WSS 34 so that the add signal A is inserted into the WDM signal at node B. At this time, the WSS 34 is set so that the input optical signal of the add port is guided to the output port. In S6, the control unit 31 sets the WSS 34 so that the output pseudo signal P2 is inserted into the WDM signal at node B. At this time, the WSS 34 is set so that the light given to the input port from the pseudo signal generation unit 42 is guided to the output port.

[0077] In S7, the ROADM device 30 processes the WDM signal according to the settings in S2 to S6. Then, the WDM signal arriving at node B is processed as shown in Fig. 11A. That is, in the WSS 33, the drop signal D is branched from the WDM signal and guided to the corresponding transponder, and the input pseudo signal P1 is removed from the WDM signal. Thereby, an in-node WDM signal including the through signal T is generated. Then, in the WSS 34, the add signal and the output pseudo signal P2 are inserted into the in-node WDM signal. As a result, node B outputs a WDM signal including the through signal T, the add signal A, and the output pseudo signal P2.

[0078] When the WDM signal transmitted from node A does not arrive at node B due to, for example, the breakage of an optical fiber cable, the ROADM device 30 mounted on node B operates as shown in Fig. 11B. That is, node B outputs a WDM signal including the add signal A and the output pseudo signal P2. Note that this operation is equivalent to the operation of inserting the add signal A and the output pseudo signal P2 into an in-node WDM signal in which all wavelength channels are unused channels in the WSS 34.

[0079] As described above, even when the input of the WDM signal to node B stops, the ROADM device 30 mounted on node B outputs a pseudo signal in addition to the add signal. Therefore, since a power shift due to stimulated Raman scattering occurs between the add signal and the pseudo signal, the transient response at the stop of the WDM signal is suppressed.

[0080] <Variation> FIG. 12 is a flowchart showing a variation of the processing of the ROADM device 30. Note that S11 is substantially the same as S1 shown in FIG. 10, and the ROADM device 30 receives wavelength setting information. Also, S12 is substantially the same as S2 to S7 shown in FIG. 10, and the ROADM device 30 sets the WSSs 33 and 34 according to the wavelength setting information and processes the WDM signal.

[0081] In S13, the control unit 31 determines whether a WDM signal transmitted from an adjacent node (here, node A) has arrived at the ROADM device 30. For example, when the OSC circuit 36 cannot detect an OSC signal, the control unit 31 determines that the WDM signal transmitted from the adjacent node has not arrived at the ROADM device 30. Alternatively, the control unit 31 may detect a WDM signal interruption based on the optical power detected by the optical channel monitor 38.

[0082] When the WDM signal transmitted from the adjacent node has arrived at the ROADM device 30, S14 to S15 are skipped. In this case, the ROADM device 30 executes the operation shown in FIG. 11A. That is, the ROADM device 30 outputs a through signal T, an add signal A, and an output dummy signal P2.

[0083] On the other hand, when the WDM signal transmitted from the adjacent node has not arrived at the ROADM device 30, the ROADM device 30 executes the operation shown in FIG. 11B. That is, the ROADM device 30 outputs an add signal A and an output dummy signal P2. In this case, the wavelength channel where the through signal T was arranged becomes an empty channel. Here, when the number of optical signals transmitted by the WDM signal changes, control is performed to adjust the gain of the optical amplifier at each WDM node. Therefore, it is preferable that the number of optical signals transmitted by the WDM signal is constant. Thus, the ROADM device 30 executes the processing of S14 to S15 so that all wavelength channels transmit signals.

[0084] In S14, the ROADM device 30 generates an additional dummy signal P3. The additional dummy signal P3 is generated by the dummy signal generation unit 42. The wavelength of the additional dummy signal P3 is the same as the wavelength of the through signal T. Note that the wavelength of the through signal T is notified in S11.

[0085] In S15, the control unit 31 sets the WSS 34 so that the additional dummy signal P3 is inserted into the WDM signal at node B. As a result, the additional dummy signal P3 is inserted into the WDM signal. That is, as shown in FIG. 13, the ROADM device 30 outputs the add signal A, the output dummy signal P2, and the additional dummy signal P3. As a result, all wavelength channels of the WDM signal output from node B will transmit signals (real signals or dummy signals).

[0086] Note that in the above-described embodiment, one drop signal is branched from the WDM signal at each WDM node, and one add signal is inserted into the WDM signal. However, the embodiment of the present invention is not limited to this configuration. That is, the ROADM device 30 mounted on each WDM node may branch a desired number of drop signals from the WDM signal, and may insert a desired number of add signals into the WDM signal. Further, the WDM signal may include one or more unused channels.

Explanation of Reference Numerals

[0087] 1 Optical transmission system 10 WDM node 11 WSS circuit 12 Control unit 15 Dummy signal generation unit 20 Network management system 30 ROADM device 31 Control unit 33, 34 Wavelength selective switch (WSS) 36, 41 OSC circuit 42 Dummy signal generation unit

Claims

1. An optical transmission system for transmitting an optical signal via a plurality of nodes, wherein a first optical transmission device mounted on a first node among the plurality of nodes outputs a WDM (Wavelength Division Multiplexing) signal including a wavelength channel for propagating an actual signal and a wavelength channel for propagating a pseudo signal; a second optical transmission device mounted on a second node among the plurality of nodes, comprises a wavelength processing circuit that terminates the pseudo signal from the WDM signal and inserts an add signal and a new pseudo signal into an unused wavelength channel of the WDM signal after the pseudo signal is terminated to generate a second WDM signal. An optical transmission system characterized by the above.

2. The wavelength processing circuit, comprises a first wavelength selection switch that terminates the pseudo signal from the WDM signal to generate an in-node WDM signal, and a second wavelength selection switch that inserts the add signal and the new pseudo signal into an unused wavelength channel of the in-node WDM signal to generate the second WDM signal. The optical transmission system according to claim 1, characterized by the above.

3. The actual signal propagated by the WDM signal includes a drop signal to be branched at the second node, the first wavelength selection switch branches the drop signal and the pseudo signal from the WDM signal to generate the in-node WDM signal, and the second wavelength selection switch inserts the add signal into the wavelength channel where the drop signal was arranged and inserts the new pseudo signal into the wavelength channel where the pseudo signal was arranged. The optical transmission system according to claim 2, characterized by the above.

4. The first wavelength selection switch generates the in-node WDM signal by terminating all the pseudo signals included in the WDM signal from the WDM signal. The optical transmission system according to claim 2, characterized by the above.

5. When the WDM signal output from the first node does not reach the second node, the wavelength processing circuit inserts the add signal into one or more wavelength channels among the plurality of wavelength channels constituting the WDM signal, and inserts the new pseudo signal into one or more other wavelength channels among the plurality of wavelength channels to generate the second WDM signal. The optical transmission system according to claim 1, characterized by the above.

6. When the WDM signal transmitted from the first node does not reach the second node, the wavelength processing circuit inserts the add signal into one or more wavelength channels among the plurality of wavelength channels constituting the WDM signal, and inserts the new dummy signal into all the other wavelength channels among the plurality of wavelength channels, thereby generating the second WDM signal. The optical transmission system according to claim 5, characterized in that.

7. The upper limit number of wavelength channels assigned for transmitting the real signal is preset. The optical transmission system according to claim 1, characterized in that.

8. A network management system that controls a plurality of optical transmission devices mounted on the plurality of nodes, or from the first node to the second node, wavelength setting information representing the wavelength channels in which the real signal and the dummy signal are inserted in the WDM signal is notified, Based on the wavelength setting information, the wavelength processing circuit terminates the dummy signal from the WDM signal. The optical transmission system according to claim 1, characterized in that.

9. The second optical transmission device further includes an ASE (Amplified Spontaneous Emission) light source, The new dummy signal is generated using ASE light output from the ASE light source. The optical transmission system according to claim 1, characterized in that.

10. An optical transmission device that processes a WDM (Wavelength Division Multiplexing) signal including a wavelength channel for propagating a real signal and a wavelength channel for propagating a dummy signal, A first wavelength selection switch; A second wavelength selection switch provided on the output side of the first wavelength selection switch; A control unit that controls the first wavelength selection switch and the second wavelength selection switch, The control unit, Sets the first wavelength selection switch to branch the dummy signal from the WDM signal, Sets the second wavelength selection switch to insert a new real signal and a new dummy signal into an unused wavelength channel of the WDM signal. The optical transmission device is characterized in that.

Images