Optical transmission system and optical transmission device

Abstract

To suppress the variation in optical power due to transmission optical power control in an optical transmission system which transmits a WDM signal.SOLUTION: An optical transmission device which transmits a WDM signal includes a control unit, an optical circuit and a convergence determination unit. The control unit controls the optical power of each wavelength channel of the WDM signal on the basis of optical power information expressing the optical power of each wavelength channel of the WDM signal detected in a reception node. The optical circuit adjusts the optical power of each wavelength channel of the WDM signal on the basis of the control signal from the control unit. The convergence determination unit determines whether the optical power of each wavelength channel of the WDM signal converges at a target level on the basis of the optical power information. When the optical power of each wavelength channel does not converge, the control unit controls the optical circuit in a first cycle. When the optical power of each wavelength channel converges, the control unit controls the optical circuit in a second cycle longer than the first cycle.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to an optical transmission system and an optical transmission device for transmitting a WDM signal. [Background technology]

[0002] Wavelength Division Multiplexing (WDM) has been put to practical use to provide high-capacity optical communications. WDM transmits signals using multiple wavelength channels. Therefore, high-capacity optical communications can be achieved by multiplexing a large number of wavelength channels.

[0003] In many optical transmission systems that transmit WDM signals, a reconfigurable optical add-drop multiplexer (ROADM) is implemented at each node. The ROADM includes a wavelength selective switch (WSS) and an optical amplifier circuit, and processes each wavelength channel of the WDM signal. The WSS drops an optical signal of a desired wavelength channel from the WDM signal and adds the optical signal to an empty wavelength channel of the WDM signal. The optical amplifier circuit amplifies the WDM signal output from the WSS. Note that if dropping or adding an optical signal is not required, a dynamic gain equalizer (DGE) may be provided instead of the WSS.

[0004] The optical power of each wavelength channel of a WDM signal is wavelength-dependent. Therefore, pre-emphasis control and slope control may be performed to suppress the effects of this wavelength dependency. For example, the optical power of each wavelength channel is monitored at the receiving node. The transmitting node controls the transmission optical power of each wavelength channel based on the monitoring results obtained at the receiving node. At this time, the transmitting node controls the WSS and optical amplifier circuit so that the output optical power of the optical fiber transmission line (i.e., the received optical power at the receiving node) is flat with respect to wavelength. Alternatively, the receiving node may control the output optical power of the receiving optical amplifier so that it is flat with respect to wavelength. As a result, the received optical power of each wavelength channel at the receiving node is flat with respect to wavelength, improving the quality of the WDM signal.

[0005] In addition, a WDM optical communication system has been proposed that suppresses variations in transmission characteristics for optical signals of each wavelength based on received information such as OSNR (Optical Signal-to-Noise Ratio) measured on the receiving side (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-057624 Summary of the Invention [Problem to be solved by the invention]

[0007] As described above, the transmitting node controls the optical power of each wavelength channel of a WDM signal based on information detected by the receiving node. However, in a large-scale network, many ROADMs are connected. Therefore, optical power control performed by one ROADM node may affect optical power control performed by other ROADM nodes. In other words, the optical power controls of multiple ROADM nodes may interfere with each other. For example, if multiple ROADM nodes determine that "optical power needs to be increased" and increase their optical power simultaneously, the optical power may become excessively high. In other words, unexpected large optical power fluctuations may occur during steady-state operation.

[0008] An object of one aspect of the present invention is to suppress fluctuations in optical power caused by transmission optical power control in an optical transmission system that transmits WDM signals. [Means for solving the problem]

[0009] An optical transmission system according to one aspect of the present invention transmits a WDM signal from a first optical transmission device to a second optical transmission device via an optical fiber transmission line. The optical transmission system includes: an optical channel monitor in the second optical transmission device that detects the optical power of each wavelength channel of the WDM signal; a control unit in the first optical transmission device that controls the optical power of each wavelength channel of the WDM signal based on the detection result by the optical channel monitor; an optical circuit in the first optical transmission device that adjusts the optical power of each wavelength channel of the WDM signal based on a control signal from the control unit; and a convergence determination unit that determines whether the optical power of each wavelength channel of the WDM signal has converged to a target level based on the detection result by the optical channel monitor. When the optical power of each wavelength channel of the WDM signal has not converged to the target level, the control unit controls the optical circuit using the control signal in a first period. When the optical power of each wavelength channel of the WDM signal has converged to the target level, the control unit controls the optical circuit using the control signal in a second period longer than the first period. [Effects of the Invention]

[0010] According to the above-described aspect, in an optical transmission system that transmits a WDM signal, fluctuations in optical power caused by transmission optical power control are suppressed. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a diagram illustrating an example of an optical communication network according to an embodiment of the present invention. [Figure 2] FIG. 1 illustrates an example of an optical transmission system. [Figure 3] FIG. 10 illustrates an example of transmission light power control. [Figure 4] 1 is a diagram illustrating an example of an optical transmission system according to an embodiment of the present invention. [Figure 5] FIG. 10 is a diagram illustrating convergence determination. [Figure 6] 10 is a flowchart illustrating an example of processing by a convergence determination unit. [Figure 7] FIG. 10 is a sequence diagram illustrating an example of processing by a determination result acquisition unit and a cycle selection unit. [Figure 8] 10 is a flowchart showing a variation of the process of the convergence determination unit. [Figure 9] 9 is a sequence diagram showing an example of processing by a determination result acquisition unit and a cycle selection unit when a convergence determination unit executes the procedure shown in FIG. 8. FIG. [Figure 10] 10 is a flowchart showing yet another variation of the process of the convergence determination unit. [Figure 11] FIG. 1 is a diagram illustrating a first variation of an optical transmission system according to an embodiment of the present invention. [Figure 12] FIG. 10 is a diagram illustrating a second variation of the optical transmission system according to the embodiment of the present invention. [Figure 13] FIG. 10 is a diagram illustrating a third variation of the optical transmission system according to the embodiment of the present invention. [Figure 14] FIG. 10 is a diagram illustrating an example of GSNR at a downstream station. [Figure 15]FIG. 1 illustrates an example of an optical transmission system that controls transmission optical power based on GSNR. [Figure 16] FIG. 1 illustrates an example of a method for calculating the linear SNR of a wavelength channel. [Figure 17] 1 is a flowchart illustrating an example of a method for calculating a GSNR. DETAILED DESCRIPTION OF THE INVENTION

[0012] FIG. 1 shows an example of an optical communication network according to an embodiment of the present invention. The optical communication network 100 according to the embodiment of the present invention includes a plurality of ROADMs 1 (1a to 1n). The plurality of ROADMs 1 are connected by an optical fiber transmission line 2. Each ROADM 1 transmits a WDM signal. For example, a WDM signal generated by ROADM 1a is transmitted to ROADM 1n via ROADMs 1b to 1d. However, each ROADM 1 can drop an optical signal of a desired wavelength channel from a received WDM signal. Furthermore, each ROADM 1 can add an optical signal to an empty wavelength channel of the WDM signal. Note that the ROADM is an example of an optical transmission device.

[0013] FIG. 2 shows an example of an optical transmission system. The optical transmission system shown in FIG. 2 is configured by two adjacent ROADMs out of the multiple ROADMs 1 shown in FIG. 1. In the example shown in FIG. 2, the optical transmission system is configured by ROADM 1a and ROADM 1b. A WDM signal is transmitted from ROADM 1a to ROADM 1b via an optical fiber transmission line 2x. Therefore, in the following description, ROADM 1a may be referred to as the "upstream station 10." Furthermore, ROADM 1b may be referred to as the "downstream station 50 (or receiving node)."

[0014] The upstream station 10 includes a WSS 11, an optical amplifier circuit 12, photodetectors (PD) 13 and 14, an AGC control unit 15, pump light sources (LD) 16 and 17, an OSC receiving unit 18, a power control information generating unit 19, a WSS control unit 20, an OCM 21, and a VOA control unit 22. The upstream station 10 may include other elements, circuits, or functions not shown in FIG. 2. The downstream station 50 includes an OCM 51 and an OSC transmitting unit 52. The downstream station 50 may include other elements, circuits, or functions not shown in FIG. 2.

[0015] The WSS 11 adjusts the optical power of each wavelength channel of the WDM signal in response to instructions given by the WSS control unit 20. The WSS 11 is an example of an optical circuit that controls the optical power of each wavelength channel of the WDM signal in response to instructions given by the WSS control unit 20. In other words, the upstream station 10 may be provided with another type of optical circuit, instead of the WSS 11, that adjusts the optical power of each wavelength channel of the WDM signal.

[0016] The optical amplifier circuit 12 amplifies the WDM signal output from the WSS 11. In this embodiment, the optical amplifier circuit 12 includes an optical amplifier 12a, a variable optical attenuator (VOA) 12b, and an optical amplifier 12c. The optical amplifier 12a amplifies the WDM signal output from the WSS 11. The VOA 12b attenuates the WDM signal output from the optical amplifier 12a. The optical amplifier 12c amplifies the WDM signal output from the VOA 12b. The optical amplifiers 12a and 12c are, for example, erbium-doped fiber amplifiers (EDFAs).

[0017] The gains of the optical amplifiers 12a and 12c are determined by AGC (Automatic Gain Control). That is, the optical receiver 13 converts the WDM signal input to the optical amplifier circuit 12 into an electrical signal. The optical receiver 14 converts the WDM signal output from the optical amplifier circuit 12 into an electrical signal. The AGC control unit 15 controls the pumping light sources 16 and 17 based on the output signals of the optical receivers 13 and 14 so that the gain of the optical amplifier circuit 12 for the WDM signal approaches a target value. The pumping light sources 16 and 17 each generate pumping light in response to a signal provided from the AGC control unit 15. The pumping light generated by the pumping light source 16 is provided to the optical amplifier 12a, and the pumping light generated by the pumping light source 17 is provided to the optical amplifier 12c. The attenuation of the WDM signal in the VOA 12b is controlled by the VOA control unit 22.

[0018] The WDM signal output from the upstream station 10 propagates through the optical fiber transmission line 2x, and the downstream station 50 receives this WDM signal through the optical fiber transmission line 2x.

[0019] In the downstream station 50, the OCM 51 detects the optical power of each wavelength channel of the WDM signal. That is, the OCM 51 can detect the spectrum of the WDM signal received by the downstream station 50. The OSC transmitter 52 uses an OSC (Optical Supervisory Channel) to transmit information indicating the optical power detected by the OCM 51 to the upstream station 10. The OSC is realized, for example, by a predetermined wavelength channel provided separately from the wavelength channel for transmitting data. In the following description, the information indicating the optical power detected by the OCM 51 may be referred to as "optical power information." The OSC is also set in the optical fiber transmission line 2y that transmits the optical signal from the downstream station 50 to the upstream station 10.

[0020] In the upstream station 10, the OSC receiver 18 extracts optical power information from the OSC. The optical power information is guided to the power control information generator 19.

[0021] The power control information generator 19 generates power control information based on optical power information sent from the downstream station 50. The power control information includes WSS loss information indicating the amount of loss in each wavelength channel of the WDM signal and tilt information indicating the tilt of the WDM signal with respect to the wavelength. The WSS loss information is provided to the WSS controller 20. The tilt information is also provided to the VOA controller 22.

[0022] The WSS control unit 20 controls the WSS 11 in accordance with the WSS loss information. At this time, the OCM 21 detects the optical power of each wavelength channel of the WDM signal input to the WSS 11 and the optical power of each wavelength channel of the WDM signal output from the WSS 11. The WSS control unit 20 also monitors the loss of each wavelength channel in the WSS 11 based on the measurements by the OCM 21. The WSS control unit 20 then controls the WSS 11 so that the loss of each wavelength channel in the WSS 11 matches the amount of loss indicated by the WSS loss information.

[0023] The VOA control unit 22 controls the VOA 12b in accordance with the tilt information. Here, in a configuration in which the VOA 12b is provided between a pair of optical amplifiers (12a, 12c), if the optical power of the WDM signal is adjusted using the VOA 12b while performing AGC control on the optical amplifiers 12a and 12c, the tilt of the WDM signal received by the downstream station 50 changes. Therefore, by controlling the VOA 12b based on the optical power information generated in the downstream station 50, the optical power of each wavelength channel of the WDM signal received by the downstream station 50 can be flattened.

[0024] Fig. 3 shows an example of transmission optical power control. In this example, a WDM signal is composed of wavelength channels CH1 to CH8. As shown in Fig. 3(a), when the optical power of wavelength channels CH1 to CH8 transmitted from an upstream station 10 is the same, the WDM signal shown in Fig. 3(b) arrives at a downstream station 50.

[0025] 3(b) is obtained by monitoring the WDM signal using the OCM 51 in the downstream station 50. The power control information generator 19 in the upstream station 10 then acquires this optical power information from the downstream station 50.

[0026] The power control information generator 19 generates power control information based on the optical power information received from the downstream station 50. Specifically, the power control information generator 19 generates tilt information for making the tilt (i.e., inclination) of the WDM signal relative to the wavelength closer to flat. Here, when the optical power of the WDM signal is adjusted using the VOA 12b, the tilt of the WDM signal received by the downstream station 50 changes. That is, the power control information generator 19 can make the tilt of the WDM signal received by the downstream station 50 closer to flat, as shown in FIG. 3(c).

[0027] However, simply controlling the tilt of the WDM signal may result in variations in the optical power of wavelength channels CH1 to CH8 of the WDM signal received by the downstream station 50, as shown in Fig. 3(d). For this reason, variations in the optical power of each wavelength channel are compensated for using the WSS 11. That is, the power control information generator 19 generates WSS loss information for controlling the loss of each wavelength channel in the WSS 11 based on the optical power information.

[0028] For example, in the example shown in Figure 3(d), the received optical power of wavelength channels CH1, CH3, and CH6 to CH8 is greater than the average power, and the received optical power of wavelength channels CH2 and CH4 to CH5 is less than the average power. In this case, the power control information generator 19 generates WSS control information that increases the loss of wavelength channels CH1, CH3, and CH6 to CH8 and decreases the loss of wavelength channels CH2 and CH4 to CH5. WSS 11 then controls the optical power of each wavelength channel in accordance with this WSS control information. As a result, the optical power of each wavelength channel of the WDM signal output from WSS 11 is adjusted as shown in Figure 3(e).

[0029] In this way, the upstream station 10 controls the loss of each wavelength channel in the WSS 11 and the attenuation amount of the VOA 12b based on the optical power information received from the downstream station 50. As a result, the optical power of each wavelength channel of the WDM signal received by the downstream station 50 becomes uniform, and the quality of each wavelength channel becomes stable.

[0030] It is also possible to equalize the optical power of each wavelength channel of a received WDM signal using only the WSS 11 without using the VOA 12b. However, in this case, loss in the WSS 11 increases, and the OSNR is likely to decrease. On the other hand, controlling the tilt of the WDM signal relative to the wavelength by adjusting the attenuation of the VOA 12b results in less degradation of the OSNR. Therefore, a configuration that equalizes the optical power of each wavelength channel of a received WDM signal by controlling the WSS 11 and VOA 12b, as shown in Figure 2, is preferable.

[0031] The power control information generator 19 controls the transmission optical power of each wavelength channel of a WDM signal, for example, at a predetermined interval. However, shortening the control interval may result in interference between the optical power controls of multiple ROADM nodes. For example, in the case of transmitting a WDM signal from ROADM 1a to ROADM 1n shown in FIG. 1, the transmission power control of ROADM 1a, which equalizes the received WDM signal of ROADM 1b, the transmission power control of ROADM 1b, which equalizes the received WDM signal of ROADM 1c, and the transmission power control of ROADM 1c, which equalizes the received WDM signal of ROADM 1d, may affect each other, resulting in unexpected large fluctuations in optical power during steady state operation. On the other hand, lengthening the control interval may increase the time required for the received WDM signal to be equalized at each ROADM node when the transmission conditions change. In this case, there is a risk that the quality of some wavelength channels may remain low for a long period of time. The optical transmission device or optical transmission system according to an embodiment of the present invention alleviates this trade-off.

[0032] FIG. 4 shows an example of an optical transmission system according to an embodiment of the present invention. The optical transmission system 200 according to the embodiment of the present invention is configured by two adjacent ROADMs among the multiple ROADMs 1 shown in FIG. 1. In the example shown in FIG. 4, the optical transmission system 200 is configured by ROADM 1a and ROADM 1b. A WDM signal is transmitted from ROADM 1a to ROADM 1b via an optical fiber transmission line 2x. Therefore, in the following description, ROADM 1a may be referred to as the "upstream station 10." Furthermore, ROADM 1b may be referred to as the "downstream station 50 (or receiving node)."

[0033] In the upstream station 10, the WSS 11 and the optical amplifier circuit 12 are an example of an optical circuit that adjusts the optical power of each wavelength channel of the WDM signal based on control signals provided from the WSS control unit 20 and the VOA control unit 22. In this case, the WSS control unit 20 and the VOA control unit 22 are an example of a control unit that generates a control signal to control the optical power of each wavelength channel of the WDM signal based on the detection result by the OCM 51 implemented in the downstream station 50.

[0034] The configuration of the optical transmission system is generally the same in Fig. 2 and Fig. 4. However, in the optical transmission system 200 according to the embodiment of the present invention, the upstream station 10 includes a convergence determination unit 31, a determination result acquisition unit 32, a cycle selection unit 33, a determination result acquisition unit 34, and a cycle selection unit 35 in addition to the configuration shown in Fig. 2.

[0035] The convergence determination unit 31 determines whether a control system that controls the transmission optical power of the WDM signal has converged, based on optical power information indicating the optical power of each wavelength channel of the WDM signal received by the downstream station 50. At this time, the convergence determination unit 31 determines whether the optical power of each wavelength channel has converged to a target level, based on the optical power information. Here, the optical power of each wavelength channel of the WDM signal received by the downstream station 50 is detected by the OCM 51. The optical power information is transmitted from the downstream station 50 to the upstream station 10 using an OSC. The convergence determination unit 31 then extracts the optical power information from the OSC to recognize the optical power of each wavelength channel of the WDM signal received by the downstream station 50.

[0036] FIG. 5 is a diagram illustrating convergence determination. Here, FIG. 5 shows the optical power of each wavelength channel of the WDM signal received by the downstream station 50. In this embodiment, the target level is the average of the optical power of each wavelength channel. The convergence range is set by adding a predetermined error to the target level.

[0037] The convergence determination unit 31 determines whether the optical power of each wavelength channel is within the convergence range. In the example shown in FIG. 5, the optical power of wavelength channel CH2 is greater than the upper limit of the convergence range. Therefore, in this case, the convergence determination unit 31 determines that the optical power of each wavelength channel has not converged to the target level. In other words, the convergence determination unit 31 determines that the control system that controls the transmission optical power of the WDM signal has not yet converged. The convergence determination unit 31 performs the convergence determination at a predetermined interval (for example, approximately 1 to 5 seconds).

[0038] 4 and 5, the convergence determination unit 31 monitors the optical power of each wavelength channel of the received WDM signal, but the present invention is not limited to this configuration. For example, when each node is equipped with a receiving optical amplifier that amplifies the received WDM signal, the convergence determination unit 31 may monitor the optical power of each wavelength channel of the WDM signal output from the receiving optical amplifier.

[0039] The judgment result acquisition unit 32 acquires the judgment result by the convergence judgment unit 31. Then, when the judgment result changes, the judgment result acquisition unit 32 transmits a switching request to the cycle selection unit 33. Specifically, when the state of the control system changes from a converged state to a non-converged state, the judgment result acquisition unit 32 requests the cycle selection unit 33 to switch from the low-speed mode to the high-speed mode. Also, when the state of the control system changes from a non-converged state to a converged state, the judgment result acquisition unit 32 requests the cycle selection unit 33 to switch from the high-speed mode to the low-speed mode.

[0040] The cycle selection unit 33 controls the operating cycle of the WSS control unit 20 in accordance with a request from the determination result acquisition unit 32. For example, when a request to switch from low-speed mode to high-speed mode is received, the cycle selection unit 33 operates the WSS control unit 20 at a first cycle (e.g., 10 seconds). On the other hand, when a request to switch from high-speed mode to low-speed mode is received, the cycle selection unit 33 operates the WSS control unit 20 at a second cycle (e.g., 10 minutes) that is longer than the first cycle. The WSS control unit 20 then controls the WSS 11 at the cycle selected by the cycle selection unit 33.

[0041] The operations of the decision result acquisition unit 34 and the period selection unit 35 are substantially the same as those of the decision result acquisition unit 32 and the period selection unit 33. That is, when the decision result changes, the decision result acquisition unit 34 transmits a switching request to the period selection unit 35. Then, the period selection unit 35 controls the operating period of the VOA control unit 22 in accordance with the request from the decision result acquisition unit 34. The VOA control unit 22 controls the VOA 12b at the period selected by the period selection unit 35.

[0042] As described above, in the optical transmission system 200, when the optical power of each wavelength channel of the WDM signal received by the downstream station 50 has not converged to a target level, the high-speed mode is selected, and the frequency of the process of adjusting the transmission optical power of the WDM signal increases. Therefore, even if the transmission conditions of the optical transmission system 200 change, it takes a short time for the received WDM signal to be equalized at each ROADM node. Furthermore, when the optical power of each wavelength channel of the WDM signal received by the downstream station 50 converges to a target level, the low-speed mode is selected, and the frequency of the process of adjusting the transmission optical power of the WDM signal decreases. Therefore, the optical power controls of multiple ROADM nodes are less likely to interfere with each other, and unexpected optical power fluctuations are suppressed.

[0043] It is preferable that the WSS control unit 20 and the VOA control unit 22 operate in cooperation with each other. For example, the WSS control unit 20 and the VOA control unit 22 may alternately control the corresponding optical circuits.

[0044] In the embodiment shown in Fig. 4, the ROADM provided at each node includes a WSS, but the present invention is not limited to this configuration. That is, an optical transmission device that transmits a WDM signal may include a channel power adjusting device that adjusts the optical power of each wavelength channel of the WDM signal. Here, the WSS is an example of a channel power adjusting device. A DGE may also be used as the channel power adjusting device.

[0045] 6 is a flowchart showing an example of the processing of the convergence determination unit 31. The processing of this flowchart is executed continuously while the ROADM 1 is operating. At the start of this flowchart, the control system that controls the transmission optical power of the WDM signal is assumed to be operating in high-speed mode.

[0046] In S1, the convergence determination unit 31 receives optical power information generated in the downstream station 50. The optical power information represents the optical power of each wavelength channel of the WDM signal received by the downstream station 50, as described above.

[0047] In S2, the convergence determination unit 31 determines whether the optical power of each wavelength channel is within the convergence range. The convergence range has been described with reference to FIG. 5. If the optical power of each wavelength channel has not converged within the convergence range, the process of the convergence determination unit 31 returns to S1. Therefore, the control system that controls the transmission optical power of the WDM signal operates in high-speed mode until the optical power of each wavelength channel converges within the convergence range. Then, when the optical power of all wavelength channels converges within the convergence range, the convergence determination unit 31 transmits a convergence notification to the determination result acquisition units 32 and 34 in S3. As will be described later, when the convergence determination unit 31 issues the convergence notification, the operating mode of the control system that controls the transmission optical power of the WDM signal switches from high-speed mode to low-speed mode.

[0048] In S4, the convergence determination unit 31 receives the optical power information. In S5, the convergence determination unit 31 determines whether the optical power of one or more wavelength channels is outside the convergence range. If the optical power of each wavelength channel has converged within the convergence range, the process of the convergence determination unit 31 returns to S4. That is, while the optical power of each wavelength channel is converging within the convergence range, the control system that controls the transmission optical power of the WDM signal operates in low-speed mode. Then, if the optical power of one or more wavelength channels falls outside the convergence range, the convergence determination unit 31 transmits a non-convergence notification to the determination result acquisition units 32 and 34 in S6. As will be explained later, when the convergence determination unit 31 issues the non-convergence notification, the operating mode of the control system that controls the transmission optical power of the WDM signal switches from low-speed mode to high-speed mode.

[0049] 7 is a sequence diagram showing an example of the processing of the determination result acquisition unit and the cycle selection unit. In this embodiment, when the ROADM 1 starts operating, the cycle selection units 33 and 35 select the high-speed mode. That is, the control system that controls the transmission optical power of the WDM signal starts operating in the high-speed mode.

[0050] The judgment result acquisition units 32 and 34 wait for a notification sent from the convergence judgment unit 31. Then, when the convergence judgment unit 31 issues a convergence notification in S3 shown in Fig. 6, the judgment result acquisition units 32 and 34 send a switching request to the cycle selection units 33 and 35, respectively. This switching request requests switching from the high-speed mode to the low-speed mode.

[0051] Upon receiving this switching request, the cycle selector 33 switches the operation mode of the WSS controller 20 from high-speed mode to low-speed mode. That is, the cycle selector 33 provides the WSS controller 20 with an instruction to control the WSS 11 at the second cycle. Similarly, upon receiving this switching request, the cycle selector 35 switches the operation mode of the VOA controller 22 from high-speed mode to low-speed mode. That is, the cycle selector 35 provides the VOA controller 22 with an instruction to control the VOA 12b at the second cycle.

[0052] Next, the judgment result acquisition units 32 and 34 wait for a notification sent from the convergence judgment unit 31. Then, when the convergence judgment unit 31 issues a non-convergence notification in S6 shown in Fig. 6, the judgment result acquisition units 32 and 34 send a switching request to the cycle selection units 33 and 35, respectively. This switching request requests switching from the low-speed mode to the high-speed mode.

[0053] Upon receiving this switching request, the period selector 33 switches the operation mode of the WSS control unit 20 from the low-speed mode to the high-speed mode. That is, the period selector 33 provides the WSS control unit 20 with an instruction to control the WSS 11 at a first period that is shorter than the second period. Similarly, upon receiving this switching request, the period selector 35 switches the operation mode of the VOA control unit 22 from the low-speed mode to the high-speed mode. That is, the period selector 35 provides the VOA control unit 22 with an instruction to control the VOA 12b at the first period.

[0054] Fig. 8 is a flowchart showing a variation of the processing of the convergence determination unit 31. In the procedure shown in Fig. 6, the control period of the WSS 11 and the control period of the VOA 12b are switched simultaneously. In contrast, in the procedure shown in Fig. 8, the timing at which the control period of the WSS 11 is switched and the timing at which the control period of the VOA 12b is switched are different from each other. In the flowchart shown in Fig. 8, the processing in which the convergence determination unit 31 receives optical power information is omitted.

[0055] In S11, the convergence determination unit 31 determines whether the optical power of each wavelength channel is within the convergence range. If the optical power of each wavelength channel is within the convergence range, the convergence determination unit 31 issues a VOA convergence notification in S12. When the convergence determination unit 31 issues the VOA convergence notification, the operation mode of the VOA control unit 22 switches from high-speed mode to low-speed mode. Thereafter, in S13, the convergence determination unit 31 waits for a predetermined time to elapse.

[0056] In S14, the convergence determination unit 31 determines whether the optical power of each wavelength channel is within the convergence range. That is, it determines whether the optical power of each wavelength channel is maintained within the convergence range when the VOA 12b is controlled in the low-speed mode. If the optical power of each wavelength channel is maintained within the convergence range, the convergence determination unit 31 issues a WSS convergence notification in S15. When the convergence determination unit 31 issues the WSS convergence notification, the operation mode of the WSS control unit 20 switches from the high-speed mode to the low-speed mode.

[0057] In S16, the convergence determination unit 31 determines whether the optical power of one or more wavelength channels is outside the convergence range. If the optical power of one or more wavelength channels is outside the convergence range, the convergence determination unit 31 issues a VOA non-convergence notification in S17. When the convergence determination unit 31 issues the VOA non-convergence notification, the operation mode of the VOA control unit 22 switches from low-speed mode to high-speed mode. Thereafter, in S18, the convergence determination unit 31 waits for a predetermined time to elapse. If it is determined in S14 that the optical power of one or more wavelength channels is outside the convergence range, the convergence determination unit 31 also issues a VOA non-convergence notification in S17.

[0058] In S19, the convergence determination unit 31 determines whether the optical power of one or more wavelength channels is outside the convergence range. That is, in a situation where the VOA 12b is controlled in high-speed mode, it determines whether the optical power of one or more wavelength channels is outside the convergence range. If the optical power of one or more wavelength channels is outside the convergence range, the convergence determination unit 31 issues a WSS non-convergence notification in S20. When the convergence determination unit 31 issues a WSS non-convergence notification, the operation mode of the WSS control unit 20 switches from the low-speed mode to the high-speed mode.

[0059] When the convergence determination unit 31 executes the process shown in Fig. 8, the determination result acquisition unit and the period selection unit execute the process shown in Fig. 9. The procedures shown in Fig. 7 and Fig. 9 are generally the same. However, the determination result acquisition unit 34 and the period selection unit 35 switch the operation mode of the VOA control unit 22 from high-speed mode to low-speed mode when a VOA convergence notification is issued in S12, and switch the operation mode of the VOA control unit 22 from low-speed mode to high-speed mode when a VOA non-convergence notification is issued in S17. Furthermore, the determination result acquisition unit 32 and the period selection unit 33 switch the operation mode of the WSS control unit 20 from high-speed mode to low-speed mode when a WSS convergence notification is issued in S15, and switch the operation mode of the WSS control unit 20 from low-speed mode to high-speed mode when a WSS non-convergence notification is issued in S20.

[0060] Fig. 10 is a flowchart showing yet another variation of the processing of the convergence determination unit 31. In the embodiment shown in Figs. 8 and 9, the control period of the WSS 11 and the control period of the VOA 12b are switched in conjunction with each other. In contrast, in the embodiment shown in Fig. 10, the control period of the WSS 11 and the control period of the VOA 12b are switched independently of each other.

[0061] The control period of the WSS 11 is controlled in accordance with steps S51 to S55 of the flowchart shown in FIG. 10(a). That is, if the optical power of each wavelength channel is within the convergence range, the convergence determination unit 31 issues a WSS convergence notification. Furthermore, if the optical power of one or more wavelength channels is outside the convergence range, the convergence determination unit 31 issues a WSS non-convergence notification. The operations of the determination result acquisition unit 32 and the period selection unit 33 are as shown in FIG. 9. Therefore, when the convergence determination unit 31 issues a WSS convergence notification, the operation mode of the WSS control unit 20 is switched from high-speed mode to low-speed mode. Furthermore, when the convergence determination unit 31 issues a WSS non-convergence notification, the operation mode of the WSS control unit 20 is switched from low-speed mode to high-speed mode.

[0062] The control period of the VOA 12b is controlled in accordance with S61 to S65 of the flowchart shown in FIG. 10(b). That is, if the optical power of each wavelength channel is within the convergence range, the convergence determiner 31 issues a VOA convergence notification. Furthermore, if the optical power of one or more wavelength channels is outside the convergence range, the convergence determiner 31 issues a VOA non-convergence notification. The operations of the determination result acquirer 34 and the period selector 35 are as shown in FIG. 9. Therefore, when the convergence determiner 31 issues a VOA convergence notification, the operation mode of the VOA control unit 22 is switched from high-speed mode to low-speed mode. Furthermore, when the convergence determiner 31 issues a VOA non-convergence notification, the operation mode of the VOA control unit 22 is switched from low-speed mode to high-speed mode. The convergence determiner 31 can independently execute the process shown in FIG. 10(a) and the process shown in FIG. 10(b).

[0063] <Configuration variations> In the configuration shown in FIG. 4, the upstream station 10 has a function of generating power control information (including WSS loss information and tilt information) and a function of determining the convergence of the control system. However, the embodiments of the present invention are not limited to this configuration. For example, the optical transmission system according to the embodiments of the present invention may have the configurations shown in FIGS. 11 to 13.

[0064] In the configuration shown in FIG. 11, the downstream station 50 includes a power control information generation unit 19. In this case, power control information is generated in the downstream station 50. The downstream station 50 transmits the power control information to the upstream station 10 in addition to the optical power information representing the optical power of each wavelength channel of the WDM signal received by the downstream station 50. Then, the upstream station 10 controls the transmission optical power of each wavelength channel of the WDM signal based on the optical power information and the power control information received from the downstream station 50. However, the determination as to whether the control system has converged is made by the convergence determination unit 31 implemented in the upstream station 10.

[0065] In the configuration shown in FIG. 12, the downstream station 50 includes a convergence determination unit 31. In this case, the determination as to whether the control system has converged is made in the downstream station 50. The downstream station 50 transmits the determination result to the upstream station 10 in addition to the optical power information. Then, the upstream station 10 controls the transmission optical power of each wavelength channel of the WDM signal based on the optical power information and the determination result received from the downstream station 50. However, the power control information is generated by the power control information generation unit 19 implemented in the upstream station 10.

[0066] In the configuration shown in FIG. 13, the downstream station 50 includes a power control information generation unit 19 and a convergence determination unit 31. In this case, power control information is generated in the downstream station 50, and the determination as to whether the control system has converged is made. The downstream station 50 transmits the optical power information, the power control information, and the determination result to the upstream station 10. Then, the upstream station 10 controls the transmission optical power of each wavelength channel of the WDM signal based on the optical power information, the power control information, and the determination result received from the downstream station 50. <00002​​​​In the above-described embodiment, the upstream station 10 controls the transmission optical power so that the optical power of each wavelength channel of the WDM signal received by the downstream station 50 is equalized. However, to equalize the quality of each wavelength channel, it may be preferable to control the transmission optical power based on the optical signal-to-noise ratio (OSNR) or generalized signal-to-noise ratio (GSNR) of each wavelength channel. In the following embodiment, the transmission optical power is controlled so that the GSNR of each wavelength channel is equalized.

[0068] The GSNR is expressed by equation (1).

number

[0069] FIG. 14 shows an example of the GSNR at the downstream station 50. In the example shown in FIG. 14(a), the optical power of each wavelength channel of the WDM signal transmitted from the upstream station 10 is uniform. However, when the WDM signal propagates through an optical fiber transmission line, linear noise and nonlinear noise are generated. For this reason, the GSNR at the downstream station 50 is not flat with respect to wavelength. In the example shown in FIG. 14(a), the GSNR is smallest in the short wavelength region of the C band. Here, errors are likely to occur in wavelength channels with small GSNR. Therefore, it is important to increase the minimum GSNR.

[0070] As described above, the upstream station 10 controls the transmission optical power of each wavelength channel of the WDM signal based on the optical power information generated by the downstream station 50. At this time, it is preferable that the upstream station 10 controls the optical power of each wavelength channel of the WDM signal so as to increase the minimum GSNR at the downstream station 50. For example, as shown in FIG. 14(b), the upstream station 10 controls the optical power of each wavelength channel of the WDM signal so that the GSNR is flat with respect to wavelength. This increases the minimum GSNR and improves the quality of the WDM signal.

[0071] Fig. 15 shows an example of an optical transmission system that controls transmission optical power based on GSNR. In this embodiment, the upstream station 10 includes an OCM 41 and a GSNR calculation unit 42 in addition to the configuration shown in Fig. 4. The configuration of the downstream station 50 is substantially the same in Figs. 4 and 15.

[0072] The OCM 41 monitors the optical power of each wavelength channel of the WDM signal output from the upstream station 10 to the optical fiber transmission line 2. The monitoring results of the OCM 41 are notified to the GSNR calculation unit 42. The GSNR calculation unit 42 calculates the GSNR of each wavelength channel of the WDM signal. As described above, the GSNR is calculated from the linear SNR and the nonlinear SNR. Note that, although the linear SNR is not strictly the same as the OSNR, in this embodiment it is assumed to be equivalent to the OSNR.

[0073] The linear SNR is calculated based on the optical power information received from the downstream station 50. As described above, the optical power information represents the optical power of each wavelength channel of the WDM signal received by the downstream station 50. In other words, the optical power information represents the spectrum of the WDM signal received by the downstream station 50.

[0074] FIG. 16 shows an example of a method for calculating the linear SNR of a wavelength channel. In the method shown in FIG. 16(a), the GSNR calculation unit 42 detects the optical power of the center wavelength λ0 of the wavelength channel (measurement 1). As a result, the optical power P_CH(i) of the signal in wavelength channel i is detected. The GSNR calculation unit 42 also detects the optical power of a wavelength shifted by Δλ from the wavelength λ0 (measurement 2). λ0 + Δλ is a wavelength at which the signal component is sufficiently small. Therefore, the optical power detected at λ0 + Δλ corresponds to the power of the ASE noise. In other words, the optical power P_ASE of the ASE noise is detected. The GSNR calculation unit 42 then calculates the linear SNR of wavelength channel i using equation (2).

number

[0075] 16(a), it is possible to calculate the linear SNR while transmitting a data signal. However, when the wavelength channel spacing of a WDM signal is narrow, it is difficult to accurately measure the optical power of ASE noise.

[0076] In the method shown in FIG. 16(b), the optical power of the center wavelength λ0 of the wavelength channel is detected while a signal is being transmitted (measurement 1). This detects the optical power of the signal of wavelength channel i. Furthermore, the optical power of wavelength λ0 is detected while the signal is stopped (measurement 2). This detects the optical power of the ASE noise. The GSNR calculation unit 42 then calculates the linear SNR of wavelength channel i using equation (2). In this way, the GSNR calculation unit 42 calculates the linear SNR of each wavelength channel based on the optical power information received from the downstream station 50. Alternatively, the linear SNR may be calculated from the input power to the amplifier and the NF (Noise Figure).

[0077] The nonlinear SNR of each wavelength channel is calculated based on the optical power detected by the OCM 41 installed in the upstream station 10. Here, the intensity of the nonlinear noise is proportional to the cube of the optical power input to the optical fiber transmission line 2. That is, the nonlinear noise P_NLI is expressed by equation (3).

[0078]

number

[0079] Here, if the bandwidth of the wavelength channel is constant, the nonlinear noise per unit bandwidth (for example, 12.5 GHz) is expressed by equation (4).

number

[0080] Therefore, the nonlinear SNR is expressed by equation (5).

number

[0081] Here, the proportionality coefficient ηd is assumed to be known. Also, the bandwidth of the wavelength channel is known. Therefore, if the optical power P_CH(T) of the wavelength channel is detected using the OCM 41, the nonlinear SNR can be calculated. Note that "P_CH(T) / B_CH" corresponds to the optical fiber input power per unit bandwidth.

[0082] The GSNR calculation unit 42 calculates the GSNR for each wavelength channel based on the linear SNR and the nonlinear SNR. In this embodiment, the GSNR is calculated by equation (6).

number

[0083] 17 is a flowchart showing an example of a method for calculating the GSNR. In this embodiment, the process of this flowchart is executed by the GSNR calculation unit 42 installed in the upstream station 10.

[0084] In S31, the GSNR calculation unit 42 acquires optical power information from the downstream station 50. In S32, the GSNR calculation unit 42 calculates the linear SNR of each wavelength channel based on the optical power information. In S32, the GSNR calculation unit 42 measures the transmission optical power of each wavelength channel using the OCM 41. In S34, the GSNR calculation unit 42 calculates the nonlinear SNR of each wavelength channel based on the measurement result of S33. Then, in S35, the GSNR calculation unit 42 calculates the GSNR for each wavelength channel according to equation (6).

[0085] Returning to the explanation of Fig. 15, the power control information generator 19 controls the transmission optical power of each wavelength channel of the WDM signal based on the GSNR of each wavelength channel calculated by the GSNR calculator 42. Specifically, the power control information generator 19 may control the transmission optical power of each wavelength channel of the WDM signal so as to reduce the variation in GSNR of each wavelength channel. In this case, the power control information generator 19 controls the attenuation amount for each wavelength channel in the WSS 11 and also controls the attenuation amount of the VOA 12b.

[0086] Here, the target value TP_CH(i) of the transmission optical power of wavelength channel i (ie, fiber input power) is updated by equation (7).

number

[0087] The adjustment value of the target value of the fiber input power of wavelength channel i is determined based on the GSNR of wavelength channel i and the average value of the GSNR, for example, as shown in equation (8).

number

[0088] Then, the power control information generator 19 determines the gain of the optical amplifier circuit 12 based on, for example, the average of the target values ​​of the fiber input power of each wavelength channel. Here, the gains of the optical amplifiers 12a and 12c are controlled by the AGC controller 15. Therefore, the power control information generator 19 essentially determines the attenuation amount of the VOA 12b.

[0089] Furthermore, the power control information generator 19 determines the attenuation amount of each wavelength channel in the WSS 11 based on the target value of the fiber input power and the gain of the optical amplifier circuit 12. For example, the attenuation amount ATT_CH(i) of wavelength channel i in the WSS 11 is calculated by equation (9).

[0090]

number

[0091] In the embodiment shown in FIG. 4, the convergence determination unit 31 determines whether the optical power of each wavelength channel has converged to a target level. In contrast, in the embodiment shown in FIG. 15, the convergence determination unit 31 determines whether the GSNR of each wavelength channel has converged to a target level. The target level is, for example, the average of the GSNRs of the wavelength channels. In this case, the convergence determination unit 31 determines that the control system that controls the transmission optical power of the WDM signal has converged if the GSNRs of all wavelength channels are within a convergence range corresponding to the target level. Furthermore, the convergence determination unit 31 determines that the control system has not converged if the GSNRs of one or more wavelength channels are outside the convergence range.

[0092] The operations of the determination result acquisition units 32 and 34 and the cycle selection units 33 and 35 are as described with reference to Fig. 7. Therefore, also in the embodiment shown in Fig. 15, if the control system has not converged, the transmission optical power is adjusted in a first cycle, and if the control system has converged, the transmission optical power is adjusted in a second cycle that is longer than the first cycle.

[0093] <Hardware configuration> The power control information generator 19, WSS controller 20, VOA controller 22, convergence determiner 31, determination result acquirers 32 and 34, period selectors 33 and 35, and GSNR calculator 42 are realized, for example, by a computer including a processor and memory. In this case, the processor executes a program stored in the memory to provide the functions of the power control information generator 19, WSS controller 20, VOA controller 22, convergence determiner 31, determination result acquirers 32 and 34, period selectors 33 and 35, and GSNR calculator 42. However, some or all of these functions may be realized by hardware circuits. [Explanation of symbols]

[0094] 1(1a~1n) ROADM 2(2x, 2y) optical fiber transmission line 10 Upstream Station 11 Wavelength Selective Switch (WSS) 12 Optical amplifier circuit 12a, 12c Optical amplifier 12b Variable Optical Attenuator (VOA) 19 Power control information generator 20 WSS control section 22 VOA control section 31 Convergence judgment unit 32, 34 Judgment result acquisition part 33, 35 Period selection section 41 Optical Channel Monitor (OCM) 42 GSNR calculation section 50 Downstream station 51 Optical Channel Monitor (OCM) 200 Optical Transmission System

Claims

1. An optical transmission system that transmits a WDM signal from a first optical transmission device to a second optical transmission device via an optical fiber transmission line, an optical channel monitor in the second optical transmission device that detects the optical power of each wavelength channel of the WDM signal; a control unit in the first optical transmission device that controls the optical power of each wavelength channel of the WDM signal based on a detection result by the optical channel monitor; an optical circuit in the first optical transmission device that adjusts the optical power of each wavelength channel of the WDM signal based on a control signal from the control unit; a convergence determination unit that determines whether the optical power of each wavelength channel of the WDM signal has converged to a target level based on the detection result by the optical channel monitor, when the optical power of each wavelength channel of the WDM optical signal has not converged to the target level, the control unit controls the optical circuit using the control signal in a first period; When the optical power of each wavelength channel of the WDM optical signal has converged to the target level, the control unit controls the optical circuit using the control signal at a second period longer than the first period. An optical transmission system comprising:

2. The optical circuit comprises: a channel power adjusting device for adjusting the optical power of each wavelength channel of the WDM optical signal; an optical amplifier circuit for adjusting the optical power of the WDM signal output from the channel power adjusting device; The optical amplifier circuit comprises: a first optical amplifier for amplifying the WDM signal output from the channel power adjusting device; a variable optical attenuator for attenuating the WDM signal output from the first optical amplifier; a second optical amplifier that amplifies the WDM signal output from the variable optical attenuator; the control signals generated by the control unit include a first control signal representing an amount of loss of each wavelength channel in the channel power adjusting device, and a second control signal representing an amount of attenuation of the variable optical attenuator; the channel power adjusting device adjusts the optical power of each wavelength channel according to the first control signal; The variable optical attenuator attenuates the WDM signal output from the first optical amplifier in accordance with the second control signal.

2. The optical transmission system according to claim 1.

3. when the optical power of each wavelength channel of the WDM optical signal transitions from a non-converged state to a converged state, the control unit changes a control cycle for adjusting the attenuation amount of the variable optical attenuator using the second control signal from the first cycle to the second cycle; If the optical power of each wavelength channel of the WDM optical signal has converged to the target level after a predetermined time has elapsed since the control cycle for adjusting the amount of attenuation of the variable optical attenuator was changed from the first cycle to the second cycle, the control unit changes the control cycle for adjusting the amount of loss of each wavelength channel in the channel power adjusting device from the first cycle to the second cycle using the first control signal.

3. The optical transmission system according to claim 2.

4. when the optical power of each wavelength channel of the WDM optical signal transitions from a converged state to a non-converged state, the control unit changes a control cycle for adjusting the attenuation amount of the variable optical attenuator using the second control signal from the second cycle to the first cycle; If the optical power of each wavelength channel of the WDM optical signal has not converged to the target level after a predetermined time has elapsed since the control cycle for adjusting the amount of attenuation of the variable optical attenuator was changed from the second cycle to the first cycle, the control unit changes the control cycle for adjusting the amount of loss of each wavelength channel in the channel power adjusting device from the second cycle to the first cycle using the first control signal.

3. The optical transmission system according to claim 2.

5. when the optical power of each wavelength channel of the WDM optical signal transitions from a non-converged state to a converged state, the control unit changes a control cycle for adjusting the loss amount of each wavelength channel in the channel power adjusting device from the first cycle to the second cycle using the first control signal; When the optical power of each wavelength channel of the WDM optical signal transitions from a converged state to a non-converged state, the control unit changes a control cycle for adjusting the loss amount of each wavelength channel in the channel power adjusting device from the second cycle to the first cycle using the first control signal.

3. The optical transmission system according to claim 2.

6. when the optical power of each wavelength channel of the WDM optical signal transitions from a non-converged state to a converged state, the control unit changes a control cycle for adjusting the attenuation amount of the variable optical attenuator using the second control signal from the first cycle to the second cycle; When the optical power of each wavelength channel of the WDM optical signal transitions from a converged state to a non-converged state, the control unit changes a control cycle for adjusting the attenuation amount of the variable optical attenuator using the second control signal from the second cycle to the first cycle.

3. The optical transmission system according to claim 2.

7. An optical transmission system that transmits a WDM signal from a first optical transmission device to a second optical transmission device via an optical fiber transmission line, a GSNR calculation unit that calculates a GSNR of each wavelength channel of the WDM signal; a control unit in the first optical transmission device that controls the optical power of each wavelength channel of the WDM signal based on the GSNR calculated by the GSNR calculation unit; an optical circuit in the first optical transmission device that adjusts the optical power of each wavelength channel of the WDM signal based on a control signal from the control unit; a convergence determination unit that determines whether the GSNR of each wavelength channel of the WDM signal has converged to a target level, When the GSNR of each wavelength channel of the WDM optical signal has not converged to the target level, the control unit controls the optical circuit using the control signal in a first period; When the GSNR of each wavelength channel of the WDM optical signal has converged to the target level, the control unit controls the optical circuit using the control signal in a second period longer than the first period. An optical transmission system comprising:

8. An optical transmission device that transmits a WDM signal to a receiving node via an optical fiber transmission line, a control unit that controls the optical power of each wavelength channel of the WDM signal based on optical power information that indicates the received optical power of each wavelength channel of the WDM signal detected at the receiving node; an optical circuit that adjusts the optical power of each wavelength channel of the WDM optical signal based on a control signal from the control unit; a convergence determination unit that determines whether the optical power of each wavelength channel of the WDM optical signal has converged to a target level based on the optical power information, when the optical power of each wavelength channel of the WDM optical signal has not converged to the target level, the control unit controls the optical circuit using the control signal in a first period; When the optical power of each wavelength channel of the WDM optical signal has converged to the target level, the control unit controls the optical circuit using the control signal at a second period longer than the first period. An optical transmission device characterized by:

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