Optical transmission device, optical transmission system, and transmission optical power control method

Abstract

To suppress the variations in transmission performance of a wavelength channel of a broad-band WDM signal.SOLUTION: An optical transmission system comprises: a first optical channel monitor which detects optical power of each wavelength channel of a WDM signal in a transmission node; a second optical channel monitor which detects optical power of each wavelength channel in a reception node; first to third calculation units; and a power control unit. The first calculation unit calculates the linear SNR of each wavelength channel on the basis of the optical power detected by the second optical channel monitor. The second calculation unit calculates the non-linear SNR of each wavelength channel on the basis of the optical power detected by the first optical channel monitor. The third calculation unit calculates the GSNR of each wavelength channel by utilizing the linear SNR and the non-linear SNR. The power control unit controls the transmission power of each wavelength channel on the basis of the GSNR of each wavelength channel in the transmission node.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to an optical transmission device, an optical transmission system, and a transmission optical power control method. [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] However, the transmission performance of a WDM signal depends on the wavelength. For this reason, pre-emphasis (or pre-equalization) is sometimes performed in WDM transmission systems. For example, the receiving node monitors the optical power or optical signal-to-noise ratio (OSNR) of each wavelength channel. The transmitting node then controls the transmission power of each wavelength channel so that the optical power or OSNR of each wavelength channel monitored at the receiving node becomes uniform. As a result, variations in the transmission performance of each wavelength channel are suppressed.

[0004] Incidentally, a device and a method for adjusting the transmission power of an optical signal in an optical transmission system have been proposed (for example, Patent Document 1). A method for identifying a reflection point (fault location) in a wavelength multiplexing transmission system has been proposed (for example, Patent Document 2). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-287649 [Patent Document 2] Japanese Patent Application Laid-Open No. 2012-124686 Summary of the Invention [Problem to be solved by the invention]

[0006] In recent years, the bandwidth of WDM transmission systems has expanded. For example, while signals were previously transmitted using only the C-band, WDM transmission systems that simultaneously use the C-band and L-band have been proposed. As a result, existing pre-emphasis techniques may not be able to sufficiently suppress the variations in transmission performance of each wavelength channel.

[0007] An object of one aspect of the present invention is to suppress variations in the transmission performance of wavelength channels in a wideband WDM transmission system. [Means for solving the problem]

[0008] An optical transmission system according to one embodiment 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 comprising: a first optical channel monitor in the first optical transmission device that detects the optical power of each wavelength channel of the WDM signal; a second optical channel monitor in the second optical transmission device that detects the optical power of each wavelength channel of the WDM signal; a first calculation unit that calculates a linear SNR of each wavelength channel based on the optical power of each wavelength channel detected by the second optical channel monitor; a second calculation unit that calculates a nonlinear SNR of each wavelength channel based on the optical power of each wavelength channel detected by the first optical channel monitor; a third calculation unit that calculates a GSNR of each wavelength channel using the linear SNR calculated by the first calculation unit and the nonlinear SNR calculated by the second calculation unit; and a power control unit in the first optical transmission device that controls the transmission power of each wavelength channel of the WDM signal transmitted from the first optical transmission device based on the GSNR of each wavelength channel calculated by the third calculation unit. [Effects of the Invention]

[0009] According to the above-described aspect, variations in the transmission performance of wavelength channels are suppressed in a wideband WDM transmission system. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a diagram illustrating an example of an optical transmission system according to an embodiment of the present invention. [Figure 2] FIG. 1 illustrates an example of GSNR at a receiving node. [Figure 3] FIG. 1 is a diagram illustrating the relationship between OSNR, nonlinear SNR, and GSNR. [Figure 4] 1 is a diagram illustrating an example of an optical transmission system according to a first embodiment of the present invention. [Figure 5] FIG. 1 illustrates an example of a method for calculating the linear SNR of a wavelength channel. [Figure 6] FIG. 10 illustrates another example of a method for calculating the linear SNR of a wavelength channel. [Figure 7] 10 is a flowchart illustrating an example of a process for calculating a GSNR. [Figure 8] 10 is a flowchart illustrating an example of a method for controlling the transmission power of each wavelength channel. [Figure 9] FIG. 1 is a diagram illustrating a variation of an optical transmission system according to a first embodiment of the present invention. [Figure 10] FIG. 10 is a diagram illustrating an example of an optical power profile of an optical fiber transmission line in which a lamp loss occurs. [Figure 11] FIG. 10 is a diagram illustrating an example of an optical transmission system according to a second embodiment of the present invention. [Figure 12] FIG. 10 is a diagram illustrating an example of a simulation of the effect of ramp loss on GSNR. [Figure 13] 10 is a flowchart illustrating an example of a process for calculating a GSNR in the second embodiment. [Figure 14] 10 is a flowchart showing a variation of the process for calculating the GSNR in the second embodiment. [Figure 15] 10 is a flowchart illustrating an example of transmission light power control. DETAILED DESCRIPTION OF THE INVENTION

[0011] FIG. 1 shows an example of an optical transmission system according to an embodiment of the present invention. The optical transmission system 100 according to the embodiment of the present invention includes an optical transmission device 1 and an optical transmission device 2. An optical fiber transmission line 3 connects the optical transmission device 1 and the optical transmission device 2. In FIG. 1, an optical signal is transmitted from the optical transmission device 1 to the optical transmission device 2, but it is also possible to transmit an optical signal from the optical transmission device 2 to the optical transmission device 1. That is, in FIG. 1, the optical receiving circuit of the optical transmission device 1 is omitted, and the optical transmitting circuit of the optical transmission device 2 is omitted.

[0012] The optical transmission system 100 transmits a WDM signal. That is, a WDM signal generated by an optical transmission device 1 is transmitted via an optical fiber transmission line 3. An optical transmission device 2 receives this WDM signal. In this embodiment, the WDM signal includes an optical signal in the C band and an optical signal in the L band.

[0013] The optical transmission device 1 includes a wavelength selective switch (WSS) 11, an optical amplifier circuit 12, and a control unit 13. The optical transmission device 1 may include other elements, circuits, or functions not shown in FIG.

[0014] The WSS 11 adjusts the optical power of each wavelength channel of the WDM signal in response to instructions given by the control unit 13. 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 control unit 13. In other words, the optical transmission device 1 may use, instead of the WSS 11, another type of optical circuit that controls the optical power of each wavelength channel of the WDM signal.

[0015] The optical amplifier circuit 12 amplifies the WDM signal in response to instructions given from the control unit 13. 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.

[0016] The optical amplifier circuit 12 performs automatic gain control (AGC) to maintain a constant gain between the input of the optical amplifier 12a and the output of the optical amplifier 12c. Here, the gain of the optical amplifier circuit 12 can be tilted with respect to wavelength by changing the attenuation of the VOA 12b. Specifically, increasing the attenuation of the VOA 12b increases the sum of the gains of the optical amplifiers 12a and 12c by the same amount as the increase in VOA loss. The optical amplifiers 12a and 12c are rare-earth doped amplification media such as EDF, and increasing the gain generates a short-wavelength-up tilt, where the short-wavelength side becomes higher. This allows a short-wavelength-up tilt to be generated in the output of the optical amplifier circuit 12. Conversely, decreasing the attenuation of the VOA 12b generates a tilt in the opposite direction.

[0017] The control unit 13 controls the WSS 11 and the optical amplifier circuit 12 based on feedback information received from the optical transmission device 2. In the example shown in Fig. 1, the optical amplifier circuit 12 is provided on the output side of the WSS 11, but the optical amplifier circuit 12 may also be provided on the input side of the WSS 11.

[0018] The optical transmission device 2 includes an optical amplifier 21, an optical channel monitor (OCM) 22, and a control unit 23. The optical transmission device 2 may include other elements, circuits, or functions not shown in FIG.

[0019] The optical amplifier 21 amplifies the WDM signal received via the optical fiber transmission line 3. The OCM 22 monitors the optical power of each wavelength channel of the received WDM signal. The control unit 23 generates feedback information based on the output signal of the OCM 22. The feedback information may be information representing the optical power of each wavelength channel of the received WDM signal. Alternatively, the feedback information may be information representing the OSNR of each wavelength channel of the received WDM signal. The feedback information is sent from the optical transmission device 2 to the optical transmission device 1 using, for example, an OSC (Optical Supervisory Channel). In this case, the OSC optical transmitter of the optical transmission device 2 transmits an optical signal representing the feedback information from the optical transmission device 2 to the optical transmission device 1 via the optical fiber that propagates light. Then, the OSC optical receiver of the optical transmission device 2 acquires the feedback information by receiving this optical signal.

[0020] In the optical transmission system 100 configured as above, the quality of each wavelength channel received by the optical transmission device 2 is expressed, for example, by OSNR. However, when the bandwidth of the WDM signal is wide, it is preferable to take into consideration nonlinear noise occurring in the optical fiber transmission line 3. In other words, it is preferable to express the quality of the optical signal as GSNR (Generalized SNR). Here, GSNR is expressed by equation (1).

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[0021] FIG. 2 shows an example of GSNR at a receiving node. In the example shown in FIG. 2(a), the optical power of each wavelength channel of a WDM signal transmitted from a transmitting node (optical transmission device 1 in FIG. 1) is constant. However, when the WDM signal propagates through an optical fiber transmission line 3, linear noise and nonlinear noise are generated. For this reason, the GSNR at the receiving node (optical transmission device 2 in FIG. 1) is not flat with respect to wavelength. In the example shown in FIG. 2, the GSNR is smallest in the short wavelength region of the C-band. Here, errors are more likely to occur in the wavelength channel with the smallest GSNR. Therefore, it is important to increase the minimum GSNR.

[0022] As described above, the optical transmission device 1 controls the transmission optical power of each wavelength channel of the WDM signal based on the feedback information generated by the optical transmission device 2. At this time, it is preferable that the optical transmission device 1 controls the optical power of each wavelength channel of the WDM signal so as to increase the minimum GSNR at the receiving node. For example, as shown in FIG. 2(b), the optical transmission device 1 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.

[0023] FIG. 3 is a diagram illustrating the relationship between OSNR, nonlinear SNR, and GSNR. Here, FIG. 3(a) shows the characteristics when the optical power of each wavelength channel of a WDM signal input to an optical fiber (in FIG. 1, the WDM signal transmitted from the optical transmission device 1) is flat with respect to wavelength. In an optical fiber transmission line, stimulated Raman scattering (SRS) causes a portion of the optical power of a channel in the short wavelength region to be absorbed by a channel in the long wavelength region. This reduces the optical power of the wavelength channel in the short wavelength region, and the OSNR in the short wavelength region decreases. On the other hand, the higher the optical power, the greater the nonlinear noise. Therefore, when the optical power of a wavelength channel in the long wavelength region increases due to SRS, the nonlinear noise increases, and the nonlinear SNR in the long wavelength region deteriorates.

[0024] To address this issue, the transmitting node may control the transmission power of each wavelength channel so that the OSNR of each wavelength channel at the receiving node is flat with respect to wavelength. That is, the OSNR at the receiving node can be made flat with respect to wavelength by using pre-emphasis. However, in this case, pre-emphasis is performed to increase the transmission power in the short wavelength region, which reduces the nonlinear SNR of the wavelength channel in the short wavelength region, as shown in Figure 3(b). Here, the GSNR depends on both the OSNR and the nonlinear SNR, as expressed by equation (1). Therefore, simply flattening the OSNR with respect to wavelength may not result in a flat GSNR with respect to wavelength.

[0025] Therefore, the transmission power control method according to the embodiment of the present invention calculates the GSNR of each wavelength channel. Then, the transmission power of each wavelength channel is controlled so that the GSNR of each wavelength channel becomes flat or nearly flat with respect to wavelength. As a result, the GSNR of each wavelength channel becomes flat with respect to wavelength at the receiving node, as shown in FIG. 2(b). Therefore, the minimum GSNR improves, and the quality of the WDM signal improves.

[0026] First Embodiment FIG. 4 shows an example of an optical transmission system according to a first embodiment of the present invention. In the first embodiment, the optical transmission device 1 may include an ASE (Amplified Spontaneous Emission) light source 14 in addition to the WSS 11, optical amplifier circuit 12, and control unit 13 described with reference to FIG. 1. The ASE light source 14 generates ASE light. Here, the ASE light source 14 is assumed to be capable of generating high-power, broadband ASE light. The ASE light generated by the ASE light source 14 is then guided to the WSS 11. The WSS 11 can use this ASE light to generate a pseudo optical signal to be transmitted via one or more desired wavelength channels.

[0027] When monitoring the characteristics of a WDM signal, it is preferable that the total optical power of the WDM signal be maintained at a predetermined level. Therefore, when the number of optical signals multiplexed into the WDM signal is small, the total optical power of the WDM signal may be maintained at a predetermined level by inserting a pseudo optical signal generated using ASE light into the WDM signal.

[0028] The optical transmission device 1 also includes an OCM 15. The OCM 15 monitors the optical power of each wavelength channel of the WDM signal output from the optical transmission device 1 to the optical fiber transmission line 3. The monitoring result of the OCM 15 is notified to the control unit 13.

[0029] In the receiving node (i.e., the optical transmission device 2), the control unit 23 includes a linear SNR calculation unit 31. The linear SNR calculation unit 31 calculates the linear SNR of each wavelength channel of the WDM signal using the output signal of the OCM 22. Here, although the linear SNR is not strictly the same as the OSNR, in this embodiment it is assumed to be equivalent to the OSNR.

[0030] 5 shows an example of a method for calculating the linear SNR of a wavelength channel. When calculating the linear SNR of a wavelength channel, the optical transmission device 1 transmits a WDM signal to the optical transmission device 2. At this time, it is preferable to insert a pseudo optical signal generated using an ASE light source 14 into wavelength channels that do not transmit a signal.

[0031] In the optical transmission device 2, the OCM 22 monitors the optical power of the WDM signal. That is, the spectrum of the WDM signal received by the optical transmission device 2 is detected. Then, the linear SNR calculation unit 31 calculates the linear SNR of each wavelength using the output signal of the OCM 22. Specifically, as shown in FIG. 5(a), the linear SNR calculation unit 31 detects the optical power of the center wavelength λ0 of the wavelength channel (Measurement 1). The linear SNR calculation unit 31 also detects the optical power of a wavelength shifted by Δλ from the wavelength λ0 (Measurement 2). Note that λ0 + Δλ is a wavelength where the signal component is sufficiently small. Therefore, the optical power detected at λ0 + Δλ corresponds to the power of the ASE noise. Therefore, the linear SNR calculation unit 31 can calculate the linear SNR using the procedure shown in FIG. 5(b).

[0032] In S1, the linear SNR calculation unit 31 detects the optical power of the central wavelength λ0 of the wavelength channel. This gives the optical power P_ch of the signal. In S2, the linear SNR calculation unit 31 detects the optical power of a wavelength shifted by Δλ from the wavelength λ0. This gives the optical power P_ASE of the ASE noise. Then, in S3, the linear SNR calculation unit 31 calculates the linear SNR from the optical power P_ch of the signal obtained in S1 and the optical power P_ASE of the ASE noise obtained in S2. The linear SNR (SNR_L) is expressed by equation (2).

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[0033] The linear SNR can be calculated while transmitting a signal using the method shown in Fig. 5. However, when the wavelength channel spacing of a WDM signal is narrow, it is difficult to accurately measure the optical power of ASE noise.

[0034] 6 shows another example of a method for calculating the linear SNR of a wavelength channel. In this method, as shown in FIG. 6(a), the optical power of the center wavelength λ0 of the wavelength channel is detected while a signal is being transmitted (measurement 1). Also, the optical power of wavelength λ0 is detected while the signal is stopped (measurement 2). Specifically, the linear SNR calculation unit 31 can calculate the linear SNR using the procedure shown in FIG. 6(b).

[0035] In S11, the transmitting node transmits a signal via the wavelength channel to be measured. In S12, the linear SNR calculation unit 31 implemented in the receiving node detects the optical power of the center wavelength λ0 of the wavelength channel. This obtains the optical power P_ch of the signal. In S13, the transmitting node controls the WSS 11 to stop the wavelength channel to be measured. In S14, the linear SNR calculation unit 31 detects the optical power of the wavelength λ0. This obtains the optical power P_ASE of the ASE noise. Then, in S15, the linear SNR calculation unit 31 calculates the linear SNR from the optical power P_ch of the signal obtained in S12 and the optical power P_ASE of the ASE noise obtained in S14. The linear SNR is expressed by equation (2), similar to the method shown in FIG. 5.

[0036] 5 or 6. Then, the optical transmission device 2 transmits information indicating the linear SNR calculated by the linear SNR calculation unit 31 as feedback information to the optical transmission device 1. In addition, the optical transmission device 2 may transmit information indicating the optical power of each wavelength channel detected by the OCM 22 as feedback information to the optical transmission device 1.

[0037] The control unit 13 implemented in the optical transmission device 1 includes a nonlinear SNR calculation unit 32, a GSNR calculation unit 33, and a power control unit 34. The control unit 13 also includes an acquisition unit (or a receiver) (not shown) that acquires information indicating the linear SNR calculated in the optical transmission device 2. The control unit 13 then calculates the GSNR based on feedback information received from the optical transmission device 2, and controls the transmission optical power of each wavelength channel of the WDM signal so as to flatten the GSNR with respect to wavelength or to increase the minimum GSNR.

[0038] The nonlinear SNR calculation unit 32 calculates the nonlinear SNR of each wavelength channel based on the optical power detected by the OCM 15. Here, the intensity of the nonlinear noise is proportional to the cube of the optical power input to the optical fiber transmission line 3. That is, the nonlinear noise P_NLI is expressed by equation (3).

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[0039] 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).

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[0040] Therefore, the nonlinear SNR is expressed by equation (5).

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[0041] Here, the proportionality coefficient ηd is a known value determined by the type of fiber in the transmission path, etc. The bandwidth of the wavelength channel is also known. Therefore, if the optical power P_CH(T) of the wavelength channel is detected using the OCM15, the nonlinear SNR can be calculated. Note that "P_CH(T) / B_CH" corresponds to the optical fiber input power per unit bandwidth.

[0042] The GSNR calculation unit 33 calculates the GSNR based on the linear SNR notified from the receiving node (i.e., the optical transmission device 2) and the nonlinear SNR calculated by the nonlinear SNR calculation unit 32. In this embodiment, the GSNR is calculated by equation (6).

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[0043] 7 is a flowchart illustrating an example of a process for calculating the GSNR. In this embodiment, the process of this flowchart is executed by the control unit 13 of the transmitting node (that is, the optical transmission device 1).

[0044] In S21, the nonlinear SNR calculation unit 32 measures the optical power of the center wavelength of the target wavelength channel using the OCM 15. That is, the transmitted optical power (or the fiber input power) is measured. In S22, the nonlinear SNR calculation unit 32 calculates the nonlinear SNR of the target wavelength channel based on the optical power measured in S21. The nonlinear SNR is calculated, for example, using the above-mentioned equation (5). Note that the coefficient ηd and the bandwidth of the wavelength channel are assumed to be given.

[0045] In S23, the GSNR calculation unit 33 acquires information representing the linear SNR of the target wavelength channel. The information representing the linear SNR is generated as feedback information in the receiving node. In S24, the GSNR calculation unit 33 calculates the GSNR based on the nonlinear SNR calculated in S22 and the linear SNR acquired in S23. The GSNR is calculated using equation (6). The control unit 13 calculates the GSNR for each wavelength channel of the WDM signal.

[0046] In the optical transmission device 1, the power control unit 34 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 calculation unit 33. Specifically, the power control unit 34 may control the transmission optical power of each wavelength channel of the WDM signal, for example, to reduce the variation in GSNR of each wavelength channel or to increase the minimum GSNR. At this time, the power control unit 34 controls the attenuation amount for each wavelength channel in the WSS 11 and also controls the attenuation amount of the VOA 12b in the optical amplification circuit 12.

[0047] 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).

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[0048] 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).

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[0049] Alternatively, the adjustment value of the target value of the fiber input power of wavelength channel i may be determined based on the linear SNR of wavelength channel i, the average value of the linear SNR, the nonlinear SNR of wavelength channel i, and the average value of the nonlinear SNR, as shown in equation (9).

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[0050] For example, when attempting to compensate for the wavelength characteristics of the target adjustment value ΔP_CH(i) of wavelength channel i by tilting the output of the optical amplifier circuit 12, the power control unit 34 calculates a change amount ΔVOA of the VOA 12b in the optical amplifier circuit 12 that most closely approximates the wavelength characteristics of ΔP_CH(i), and changes the attenuation of the VOA 12b by this amount. The remaining components that cannot be compensated for by this gain tilt are then compensated for by changing the attenuation of each channel of the WSS 11. This achieves a change in the wavelength characteristics of ΔP_CH(i). The optical amplifier circuit 12 includes an optical amplifier 12a, a VOA 12b, and an optical amplifier 12c, as shown in FIG. 1 or FIG. 4.

[0051] 8 is a flowchart showing an example of a method for controlling the transmission power of each wavelength channel. The process of this flowchart may be executed periodically, for example. Alternatively, the process of this flowchart may be executed in response to an instruction from a network administrator.

[0052] In S31, the control unit 23 implemented in the receiving node calculates the linear SNR of each wavelength channel. In S32, the control unit 13 implemented in the transmitting node calculates the nonlinear SNR of each wavelength channel. In S33, the control unit 13 calculates the GSNR of each wavelength channel based on the linear SNR obtained in S31 and the nonlinear SNR obtained in S32. In S34, the control unit 13 controls the WSS 11 and optical amplifier circuit 12 implemented in the transmitting node based on the GSNR of each wavelength channel. The above-mentioned transmission power control reduces the variation in GSNR of each wavelength channel, as shown in Figure 2(b). As a result, the minimum GSNR increases, improving the quality of the WDM signal.

[0053] In the example shown in FIG. 4, the linear SNR is calculated at the receiving node (i.e., the optical transmission device 2) and feedback information representing the linear SNR is notified to the transmitting node (i.e., the optical transmission device 1). However, the embodiment of the present invention is not limited to this configuration. For example, the optical transmission device 2 may transmit spectral information detected by the OCM 22 to the optical transmission device 1. This spectral information represents the spectrum of the WDM signal received by the optical transmission device 2. In this case, the linear SNR calculation unit 31 shown in FIG. 4 is implemented in the control unit 13 of the optical transmission device 1 and calculates the linear SNR of each wavelength channel using the spectral information received from the optical transmission device 2.

[0054] 9 shows a variation of the optical transmission system according to the first embodiment of the present invention. The optical transmission system 100 shown in FIG. 9 includes a network control device 50. The network control device 50 can control each communication device in the optical transmission system 100. The network control device 50 is also connected to optical transmission devices 1 and 2. The network control device 50 includes a linear SNR calculation unit 31, a nonlinear SNR calculation unit 32, and a GSNR calculation unit 33 shown in FIG. 4.

[0055] The optical transmission device 2 transmits the spectral information detected by the OCM 22 to the network control device 50. This spectral information represents the spectrum of the WDM signal received by the optical transmission device 2. The linear SNR calculation unit 31 implemented in the network control device 50 calculates the linear SNR of each wavelength channel based on the spectral information received from the optical transmission device 2.

[0056] The optical transmission device 1 transmits the spectral information detected by the OCM 12 to the network control device 50. This spectral information represents the spectrum of the WDM signal input from the optical transmission device 1 to the optical fiber transmission line 3. The nonlinear SNR calculation unit 32 implemented in the network control device 50 calculates the nonlinear SNR of each wavelength channel based on the spectral information received from the optical transmission device 1.

[0057] The GSNR calculation unit 33 implemented in the network control device 50 calculates the GSNR of each wavelength channel based on the linear SNR and nonlinear SNR. Furthermore, the network control device 50 generates control information for controlling the WSS 11 and the optical amplifier circuit 12 of the optical transmission device 1 based on the GSNR of each wavelength channel. The network control device 50 then transmits this control information to the optical transmission device 1. In response to this, the power control unit 34 in the optical transmission device 1 controls the WSS 11 and the optical amplifier circuit 12 according to the control information received from the network control device 50. As a result, as shown in FIG. 2(b), the GSNR variation of each wavelength channel is suppressed. The configuration shown in FIG. 9 can reduce the load on the optical transmission devices 1 and 2.

[0058] The linear SNR calculation unit 31, the nonlinear SNR calculation unit 32, the GSNR calculation unit 33, and the power control unit 34 are realized by, for example, a computer including a processor and a memory. In this case, the functions of the linear SNR calculation unit 31, the nonlinear SNR calculation unit 32, the GSNR calculation unit 33, and the power control unit 34 are provided by the processor executing a program stored in the memory.

[0059] 4, the function of the linear SNR calculator 31 is provided by a processor implemented in the optical transmission device 2, and the functions of the nonlinear SNR calculator 32, the GSNR calculator 33, and the power controller 34 are provided by a processor implemented in the optical transmission device 1. In the configuration shown in Fig. 9, the functions of the linear SNR calculator 31, the nonlinear SNR calculator 32, and the GSNR calculator 33 are provided by a processor implemented in the network control device 50, and the function of the power controller 34 is provided by a processor implemented in the optical transmission device 1.

[0060] <Second embodiment> The power of light propagating through an optical fiber depends on the propagation distance. That is, as the distance from the transmitting node increases, the optical power gradually decreases. However, unintended losses that do not depend on the characteristics of the optical fiber may occur on the optical fiber transmission line. For example, bending loss, splice loss, and coupling loss may occur. When such unintended losses occur, the error in the GSNR calculated by the above-mentioned method increases. Therefore, the second embodiment provides a procedure for suppressing this error. In the following description, unintended losses that occur on the optical fiber transmission line may be referred to as "ramp loss."

[0061] 10 shows an example of an optical power profile of an optical fiber transmission line where a ramp loss has occurred. In this example, the length (i.e., span length) of the optical fiber transmission line is LS, and the ramp loss has occurred at a position PL that is a distance L1 away from the transmitting node.

[0062] In this case, as light propagates from the transmitting node toward position PL, the optical power gradually decreases. Then, at position PL, the optical power decreases by δ1 due to ramp loss. Furthermore, as light propagates from position PL toward the receiving node, the optical power decreases further.

[0063] Here, when a lamp loss occurs, the optical power reaching the receiving node is lower than when there is no lamp loss. Specifically, when a lamp loss occurs, the optical power between the position PL and the receiving node is lower than when there is no lamp loss. Therefore, when calculating the nonlinear SNR, it is preferable to correct the coefficient ηd used in equation (4) or (5). For example, the coefficient ηd is corrected using equation (10).

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[0064] "α" represents the fiber loss coefficient of the optical fiber transmission line 3 (i.e., the loss per unit length). Function f3 generates a value that depends on the fiber loss coefficient α. For example, function f3 may be realized by a calculation formula in which the calculation result decreases as the fiber loss coefficient α increases. Furthermore, function f4 generates a value that depends on the ramp loss δ1, the distance L1 from the transmitting node to the ramp loss occurrence position PL, and the span length LS. For example, function f4 may be realized by a calculation formula in which the calculation result decreases as the ramp loss δ1 increases, and as the distance from the ramp loss occurrence position PL to the receiving node (i.e., "LS-L1") increases.

[0065] When ramp losses occur at multiple positions on the optical fiber transmission line, the coefficient ηd is corrected using equation (11).

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[0066] The fiber loss coefficient, the location where the lamp loss occurs, and the magnitude of the lamp loss can be detected using, for example, an optical time domain reflectometer (OTDR). An OTDR is an optical measuring instrument that detects discontinuities in optical fibers. Specifically, an OTDR emits an optical pulse into an optical fiber and detects the light reflected from the optical fiber. The OTDR can then detect the optical power profile of the optical fiber (including information indicating the location of the discontinuity in the optical fiber) based on the power and timing of the reflected light. Note that discontinuities in optical fibers include breaks and end points. Discontinuities in optical fibers also include splices between optical fibers. Therefore, an OTDR can detect the fiber loss coefficient, the location where the lamp loss occurs, and the magnitude of the lamp loss.

[0067] 11 shows an example of an optical transmission system according to a second embodiment of the present invention. In the second embodiment, the optical transmission device 1 includes an OTDR 16 in addition to a WSS 11, an optical amplifier circuit 12, a control unit 13, an ASE light source 14, and an OCM 15. The OTDR 16 detects the state of the optical fiber transmission line 3 connecting the optical transmission device 1 and the optical transmission device 2. That is, the OTDR 16 inputs an optical pulse into the optical fiber transmission line 3 and detects the reflected pulse, thereby creating an optical power profile of the optical fiber transmission line 3.

[0068] The control unit 13 detects the fiber loss coefficient, the location of the lamp loss, and the magnitude of the lamp loss based on the optical power profile created by the OTDR 16. Here, the span length LS (i.e., the distance between the optical transmission devices 1 and 2) is known. Therefore, the control unit 13 can correct the coefficient ηd for calculating the nonlinear SNR using equation (11).

[0069] 11, the state of the optical fiber transmission line 3 is detected using an OTDR installed in the transmitting node, but an OTDR installed in the receiving node may also be used to detect the state of the optical fiber transmission line 3. In this case, the optical transmission device 2 transmits information obtained by the OTDR to the optical transmission device 1.

[0070] Figure 12 shows an example of a simulation of the effect of ramp loss on GSNR. Figure 12(a) shows a case where parameters were set in advance assuming no ramp loss in the optical fiber. In other words, the wavelength characteristics of the linear SNR and the nonlinear SNR were set in advance so that the GSNR would be flat with respect to wavelength.

[0071] However, when transmit power control is performed based on the pre-design, the occurrence of ramp loss reduces the optical power in the optical fiber transmission line, resulting in a smaller SRS. In other words, the transition of optical power from the short-wavelength region to the long-wavelength region is reduced. As a result, compared to the pre-design, the optical power in the long-wavelength region decreases, and the linear SNR in the long-wavelength region deteriorates, as shown in Figure 12(b). Furthermore, because the power of the light propagating through the optical fiber transmission line decreases due to ramp loss, the nonlinear SNR increases overall. Because the SRS decreases, this tendency is more pronounced in the long-wavelength region. As a result, the balance between the wavelength characteristics of the linear SNR and the wavelength characteristics of the nonlinear SNR is lost, and the GSNR does not become flat with respect to wavelength. In the example shown in Figure 12, the GSNR in the short-wavelength region decreases relatively. When ramp loss occurs, the contribution of the wavelength characteristics of the nonlinear SNR to the wavelength characteristics of the GSNR becomes relatively small. However, even if the wavelength characteristics of the linear SNR are flat, the wavelength characteristics of the GSNR do not necessarily become flat, as shown in Figure 12(c).

[0072] Therefore, in the embodiment of the present invention, the WSS 11 and the optical amplifier circuit 12 are feedback-controlled so that the GSNR becomes flat with respect to the wavelength. In addition, in the second embodiment, the coefficient ηd for calculating the nonlinear SNR is corrected taking into account the lamp loss.

[0073] 13 is a flowchart showing an example of a process for calculating the GSNR in the second embodiment. In the second embodiment, the process of S41 is executed in addition to the procedure shown in FIG.

[0074] In S41, the control unit 13 detects the state of the optical fiber transmission line 3 using the OTDR 16. Then, based on the information obtained by the OTDR 16, the nonlinear SNR calculation unit 32 detects the fiber loss coefficient of the optical fiber transmission line 3, the position where the ramp loss occurred, and the magnitude of the ramp loss.

[0075] The control unit 13 calculates the nonlinear SNR and GSNR in the same procedure as S21 to S24 shown in Fig. 7. However, in the second embodiment, the nonlinear SNR calculation unit 32 corrects the coefficient ηd for calculating the nonlinear SNR in S42. At this time, the coefficient ηd is corrected based on the fiber loss coefficient, the position where the ramp loss occurred, and the magnitude of the ramp loss obtained in S41. Then, the nonlinear SNR calculation unit 32 calculates the nonlinear SNR using the corrected coefficient ηd. Thereafter, the GSNR calculation unit 33 calculates the GSNR.

[0076] Fig. 14 is a flowchart showing a variation of the process for calculating the GSNR in the second embodiment. In the procedure shown in Fig. 13, the fiber loss coefficient α of the optical fiber transmission line 3 is detected using the OTDR 16. In contrast, in the variation shown in Fig. 14, the fiber loss coefficient of the optical fiber transmission line 3 is detected by another method. Note that the control unit 13 does not need to detect the fiber loss coefficient of the optical fiber transmission line 3 using the OTDR 16 in S41.

[0077] The optical transmission device 1 generates a WDM signal and transmits it to the optical transmission device 2. At this time, the control unit 13 may use the ASE light source 14 and the WSS 11 to generate the WDM signal.

[0078] In S51, the OCM 15 of the transmitting node (i.e., optical transmission device 1) measures the transmission optical power of each wavelength channel of the WDM signal. That is, the fiber input power of each wavelength channel is measured. In S52, the OCM 22 of the receiving node (i.e., optical transmission device 2) measures the reception optical power of each wavelength channel of the WDM signal. In S53, the control unit 23 transmits information indicating the reception optical power of each wavelength channel to the optical transmission device 1. In S54, the control unit 13 calculates the fiber loss coefficient for each wavelength channel based on the transmission optical power measured in S51 and the reception optical power measured in S52. In this case, the fiber loss coefficient for wavelength channel i is expressed by equation (12). In the procedure shown in FIG. 14, the same measurement is performed in S51 and S21. This is to update the measurement value if there is a change over time. If no change over time is expected, the measurement value of S51 can be used as is.

number

[0079] P_CH(T)(i) represents the transmitted optical power of wavelength channel i. P_CH(R)(i) represents the received optical power of wavelength channel i. δ represents the magnitude of the lamp loss detected by the OTDR. When multiple lamp losses exist on the optical fiber transmission line, δ represents the sum of the lamp losses. LS represents the span length. After this, S21 to S24 are executed to calculate the GSNR.

[0080] 15 is a flowchart showing an example of transmission light power control, and the process of this flowchart is repeatedly executed at a predetermined interval.

[0081] In S61, the control unit 13 of the transmitting node calculates the GSNR of each wavelength channel. At this time, the control unit 13 calculates the GSNR of each wavelength channel in cooperation with the control unit 23 of the receiving node. Specifically, the GSNR is calculated, for example, according to the procedure shown in FIG. 7, FIG. 13, or FIG. 14.

[0082] In S62, based on the GSNR of each wavelength channel, the control unit 13 controls the WSS 11 and the optical amplifier circuit 12. At this time, the control unit 13 controls the attenuation of each wavelength channel in the WSS 11 and the gain of the optical amplifier circuit 12, for example, so as to reduce the variation in GSNR of each wavelength channel.

[0083] 15 is repeated at a predetermined interval, the variation in GSNR of each wavelength channel relative to the wavelength is reduced, and the minimum GSNR of the WDM signal is stabilized, thereby realizing stable WDM transmission. [Explanation of symbols]

[0084] 1, 2 Optical transmission equipment 3 Optical fiber transmission line 11 Wavelength Selective Switch (WSS) 12 Optical amplifier circuit 12a, 12c Optical amplifier 12b Variable Optical Attenuator (VOA) 13 Control Unit 14 ASE light source 15 Optical Channel Monitor (OCM) 16 Optical Time Domain Reflectometer (OTDR) 22 Optical Channel Monitor (OCM) 23 Control Unit 31 Linear SNR calculation section 32 Nonlinear SNR calculation unit 33 GSNR calculation section 34 Power control section 50 Network control device 100 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, a first optical channel monitor in the first optical transmission device that detects the optical power of each wavelength channel of the WDM signal; a second optical channel monitor in the second optical transmission device that detects the optical power of each wavelength channel of the WDM signal; a first calculation unit that calculates a linear SNR of each wavelength channel based on the optical power of each wavelength channel detected by the second optical channel monitor; a second calculation unit that calculates a nonlinear SNR of each wavelength channel based on the optical power of each wavelength channel detected by the first optical channel monitor; a third calculation unit that calculates a GSNR of each wavelength channel using the linear SNR calculated by the first calculation unit and the nonlinear SNR calculated by the second calculation unit; a power control unit in the first optical transmission device that controls the transmission power of each wavelength channel of the WDM signal transmitted from the first optical transmission device based on the GSNR of each wavelength channel calculated by the third calculation unit; An optical transmission system comprising:

2. The power control unit controls the transmission power of each wavelength channel of the WDM signal transmitted from the first optical transmission device so as to reduce the variation in GSNR of each wavelength channel calculated by the third calculation unit.

2. The optical transmission system according to claim 1.

3. the first optical transmission device, an optical circuit that controls the optical power of each wavelength channel of the WDM optical signal in response to an instruction given by the power control unit; an optical amplifier circuit that controls the optical power of the WDM signal in response to an instruction given from the power control unit; 2. The optical transmission system according to claim 1.

4. The second calculation unit calculates nonlinear noise for each wavelength channel by multiplying the cube of the optical power detected by the first optical channel monitor by a predetermined proportionality coefficient, and calculates the nonlinear SNR by calculating a ratio between the optical power detected by the first optical channel monitor and the nonlinear noise.

2. The optical transmission system according to claim 1.

5. The second calculation unit corrects the proportionality coefficient based on a fiber loss coefficient of the optical fiber transmission line, a position where a lamp loss occurs, and a magnitude of the lamp loss.

5. The optical transmission system according to claim 4.

6. An optical transmission device that transmits a WDM signal to a receiving node via an optical fiber transmission line, an acquisition unit that acquires information representing a linear SNR of each wavelength channel of the WDM signal, the information being calculated based on the received optical power of each wavelength channel of the WDM signal detected at the receiving node; an optical channel monitor for detecting the transmission optical power of each wavelength channel of the WDM signal; a nonlinear SNR calculation unit that calculates a nonlinear SNR of each wavelength channel based on the optical power of each wavelength channel detected by the optical channel monitor; a GSNR calculation unit that calculates a GSNR of each wavelength channel using the linear SNR and the nonlinear SNR; a power control unit that 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 calculation unit; An optical transmission device comprising:

7. a linear SNR of each wavelength channel of the WDM signal is calculated at the receiving node; The acquisition unit acquires information representing a linear SNR of each wavelength channel of the WDM signal from the receiving node.

7. The optical transmission device according to claim 6.

8. a linear SNR of each wavelength channel of the WDM signal is calculated in a network controller connected to the optical transmission device and the receiving node; The acquisition unit acquires information representing a linear SNR of each wavelength channel of the WDM signal from the network control device.

7. The optical transmission device according to claim 6.

9. 1. A transmission optical power control method for controlling transmission optical power of each wavelength channel of a WDM signal in 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, comprising: detecting a transmission optical power of each wavelength channel of the WDM signal using a first optical channel monitor in the first optical transmission device; detecting the received optical power of each wavelength channel of the WDM signal using a second optical channel monitor in the second optical transmission device; calculating a linear SNR for each wavelength channel based on the received optical power of each wavelength channel detected by the second optical channel monitor; calculating a nonlinear SNR of each wavelength channel based on the transmitted optical power of each wavelength channel detected by the first optical channel monitor; Calculating a GSNR for each wavelength channel using the linear SNR and the nonlinear SNR; In the first optical transmission device, the transmission power of each wavelength channel of the WDM signal is controlled based on the GSNR of each wavelength channel. A transmission light power control method comprising:

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