Optical network device and transmission path monitoring method

Abstract

To provide an optical network device and a method for monitoring an optical transmission line, which reduces a time required to identify a place where a polarization dependent loss occurs.SOLUTION: In an optical network device 1 which receives a polarized multiplex optical signal, a polarization separation unit separates an electric field information signal of the polarized multiplex optical signal into mutually orthogonal first and second polarization components. A polarization control unit controls the first and second polarization components in a coordinate system which indicates mutually orthogonal first and second polarization directions, so as to generate mutually orthogonal third and fourth polarization components. A feature extraction unit calculates an evaluation value corresponding to the power of the third polarization component for each of a plurality of positions on the transmission line. A change amount calculation unit calculates the change amount of the evaluation value with respect to a control amount by the polarization control unit, for each of the plurality of positions. Based on the comparison result of the change amount of the evaluation values at mutually neighboring first and second positions, a determination unit determines whether the first position is an extraction target.SELECTED DRAWING: Figure 9

Description

[Technical Field]

[0001] The present invention relates to an optical network device and a method for monitoring an optical transmission line. [Background technology]

[0002] The increase in communication traffic volume has created a demand for optical fiber communications with long-distance and large-capacity transmission. However, because communications are conducted at near-limits to the performance of transmission equipment, changes in the state of the optical transmission path can cause degradation of transmission characteristics. One of the causes of such degradation of transmission characteristics is polarization-dependent loss (PDL). PDL is a phenomenon in which the insertion loss varies depending on the polarization angle. It occurs not only during device manufacturing, but also during operation due to route changes at optical nodes, reconnection of optical fiber, bending of optical fiber, etc.

[0003] When PDL exists, for example, the received power of one of the polarizations is reduced, degrading transmission quality. Therefore, detecting the PDL state during operation is important for reducing the period during which the performance of the transmission system is degraded.

[0004] In measuring PDL, for example, a single-polarized test light is input from a transmitting node to an optical fiber transmission line while the polarization angle of the test light is gradually changed. Then, at a receiving node, the power of the light output from the optical fiber transmission line is measured for each polarization, and the PDL is calculated based on the maximum and minimum values ​​of these measurements.

[0005] Similarly to PDL, another phenomenon that causes a power difference between polarizations is polarization-dependent gain (PDG). PDG is a phenomenon in which the gain varies depending on the polarization state in an optical amplifier, and can occur in distributed Raman amplifiers, for example.

[0006] In addition, a method for identifying the location and cause of a fault in an optical transmission system has been proposed (for example, Patent Document 1). Also, a device for estimating nonlinearity within a channel has been proposed (for example, Patent Document 2). [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2020-088628 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-074625 Summary of the Invention [Problem to be solved by the invention]

[0008] In the above-mentioned method (i.e., calculating PDL from the maximum and minimum optical power values ​​at the receiving node while changing the polarization angle), it is difficult to detect the location of PDL on the optical transmission line. In addition, this method requires inputting a dedicated test light for measuring PDL into the optical transmission line, which requires communication services to be stopped when measuring PDL.

[0009] It may be possible to identify the location where PDL is occurring by splitting the optical signal at multiple locations on the optical transmission line and monitoring the optical power. However, this method requires a lot of time and effort to identify the location where PDL is occurring. In particular, investigating optical transmission lines of several hundred kilometers requires a huge amount of time and effort.

[0010] An object of one aspect of the present invention is to reduce the time required to identify the location where polarization dependent loss occurs on an optical transmission line. [Means for solving the problem]

[0011] An optical network device according to one embodiment of the present invention receives a polarization multiplexed optical signal transmitted from a transmitting node, and includes: a polarization splitter that splits an electric field information signal representing an electric field of the polarization multiplexed optical signal into a first polarization component and a second polarization component that are orthogonal to each other; a polarization controller that generates a third polarization component and a fourth polarization component that are orthogonal to each other by controlling the first polarization component and the second polarization component in a coordinate system representing the orthogonal first and second polarization directions; a feature extractor that calculates, for each of a plurality of positions on an optical transmission line between the transmitting node and the optical network device, an evaluation value corresponding to at least one of the power of the third polarization component or the fourth polarization component; a fluctuation calculator that calculates, for each of the plurality of positions, a fluctuation amount of the evaluation value with respect to a control amount by the polarization controller; and a determiner that determines whether the first position is a target for extraction based on a result of comparing the fluctuation amount of the evaluation value at a first position among the plurality of positions with the fluctuation amount of the evaluation value at a second position among the plurality of positions adjacent to the first position. [Effects of the Invention]

[0012] According to the above-described aspect, it is possible to reduce the time required to identify the location where polarization dependent loss occurs on the optical transmission line. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a diagram illustrating an example of a method for measuring the power of an optical signal at an arbitrary position on a transmission line. [Figure 2] FIG. 2 is a diagram illustrating an example of functions of a digital signal processing unit. [Figure 3] 10A and 10B are diagrams illustrating an example of changes in power and chromatic dispersion of an optical signal. [Figure 4] 10 is a flowchart illustrating an example of a process for measuring the power of an optical signal at a plurality of positions on a transmission line. [Figure 5] FIG. 10 is a diagram illustrating an example of a method for identifying the location where polarization dependent loss occurs. [Figure 6]FIG. 10 is a diagram illustrating a rotation transformation in a direction in which a loss occurs. [Figure 7] FIG. 10 is a diagram illustrating an example of a power profile. [Figure 8] 1 is a diagram illustrating an example of an optical network device according to an embodiment of the present invention. [Figure 9] 1 is a diagram illustrating an example of an optical network device according to an embodiment of the present invention; [Figure 10] FIG. 10 illustrates an example of a transmission waveform reconstruction unit. [Figure 11] 1A and 1B are diagrams illustrating rotation of polarization of an optical signal. [Figure 12] 10 is a flowchart illustrating an example of a method for identifying the location where polarization dependent loss occurs. [Figure 13] FIG. 10 is a diagram illustrating an example of a method for creating a power profile. [Figure 14] FIG. 10 illustrates an example of a position specifying unit. [Figure 15] FIG. 10 is a diagram illustrating an example of a method for identifying a position where polarization dependent loss occurs. [Figure 16] 10A and 10B are diagrams illustrating another example of a method for identifying the position where polarization dependent loss occurs. [Figure 17] FIG. 1 is a diagram illustrating an example of an optical transmission system configured with a plurality of spans. [Figure 18] FIG. 10 is a diagram illustrating the process of the optical network device. [Figure 19] FIG. 1 is a diagram illustrating a first variation of an optical network device according to an embodiment of the present invention. [Figure 20] FIG. 10 is a diagram illustrating a second variation of an optical network device according to an embodiment of the present invention. [Figure 21] FIG. 10 is a diagram illustrating a third variation of an optical network device according to an embodiment of the present invention. [Figure 22] FIG. 10 is a diagram illustrating a fourth variation of the optical network device according to the embodiment of the present invention. [Figure 23] FIG. 10 is a diagram illustrating an example of a change in the power of an optical signal output from a transmitting node. [Figure 24] FIG. 2 is a diagram illustrating amplification by an optical amplifier mounted on an optical transmission line. [Figure 25] FIG. 10 is a diagram illustrating an example of an optical network device according to a second embodiment of the present invention. [Figure 26] FIG. 10 illustrates an example of a span identification unit. [Figure 27] FIG. 10 is a diagram illustrating an example of a span and power profile. [Figure 28] 28 is a diagram showing an example of a power profile in the optical transmission line shown in FIG. 27. FIG. [Figure 29] FIG. 10 is a diagram (part 1) showing an example of a change in power profile with respect to a polarization rotation calculation. [Figure 30] FIG. 10 is a diagram (part 2) showing an example of a change in power profile with respect to a polarization rotation calculation. [Figure 31] FIG. 10 is a diagram showing the power difference and the determination result for each span. [Figure 32] 10 is a flowchart showing an example of a method for identifying the location where polarization dependent loss occurs in the second embodiment. [Figure 33] FIG. 1 is a diagram illustrating distributed Raman amplification. [Figure 34] FIG. 1 is a diagram illustrating PDG occurring in distributed Raman amplification. [Figure 35] FIG. 1 illustrates an example of a span identifier used in an optical transmission system equipped with a Raman amplifier. DETAILED DESCRIPTION OF THE INVENTION

[0014] This application relates to a technology for detecting polarization-dependent loss (PDL) and estimating its location. However, as described above, polarization-dependent gain (PDG) is known as a phenomenon that, like PDL, causes a power difference between polarized waves. Both PDL and PDG are phenomena in which a power difference occurs between polarized waves in a specific polarization state. Therefore, PDG can be detected using substantially the same method as the method for detecting PDL. Therefore, although a method for detecting PDL will be described below, the method for detecting PDL can also be applied to the case of detecting PDG.

[0015] The optical network equipment according to the embodiment of the present invention has a function of measuring the optical power at any position on a transmission line based on a received optical signal, and a function of identifying the position where polarization dependent loss occurs by using the measurement results. Therefore, before describing the function of identifying the position where polarization dependent loss occurs, the function of measuring the power of an optical signal at any position on a transmission line will be described.

[0016] 1 shows an example of a method for measuring the power of an optical signal at an arbitrary position on a transmission line. In this example, an optical signal transmitted from a transmitting node 100 is transmitted to an optical network device 1 via an optical transmission line (optical fiber).

[0017] The optical network device 1 includes a coherent receiver 11, an analog-to-digital converter (ADC) 12, a digital signal processing unit 13, a simulated transmitter 14, a memory circuit 15, and a feature extraction unit 16. The optical network device 1 may include other functions or circuits not shown in FIG.

[0018] The coherent receiver 11 includes a 90-degree optical hybrid circuit and generates an electric field information signal (or electric field data) representing the electric field of the received optical signal. The electric field information signal includes an in-phase (I) component and a quadrature (Q) component of the received optical signal. When the optical signal is a polarization multiplexed optical signal, the electric field information signal includes the I and Q components of the H polarization and the I and Q components of the V polarization. The ADC 12 converts the electric field information signal into a digital signal.

[0019] Fig. 2 shows an example of the functions of the digital signal processing unit 13. As shown in Fig. 2, the digital signal processing unit 13 includes a fixed equalizer 13a, an adaptive equalizer 13b, a phase regenerator 13c, and a discriminator 13d. The digital signal processing unit 13 processes the electric field information of the received optical signal.

[0020] The fixed equalizer 13a compensates for known waveform degradation components (e.g., chromatic dispersion in the transmission path). The adaptive equalizer 13b adaptively performs equalization. For example, the adaptive equalizer 13b can compensate for residual dispersion. When the received optical signal is a polarization-multiplexed optical signal, the adaptive equalizer 13b has a function of separating the received optical signal by polarization. The phase regenerator 13c estimates the frequency offset between the transmitting node 100 and the optical network unit 1. The phase regenerator 13c then regenerates the phase of the optical signal transmitted from the transmitting node 100. That is, a signal point on the constellation is regenerated for each symbol. The discriminator 13d regenerates the transmission data based on the constellation information (phase and amplitude) output from the phase regenerator 13c. The discriminator 13d may be included in the digital signal processing unit 13 or may be provided on the output side of the digital signal processing unit 13. The digital signal processing unit 13 may also have an error correction circuit on the output side of the discriminator 13d.

[0021] The simulated transmitter 14 generates an electric field information signal by mapping the transmission data recovered by the digital signal processing unit 13 (or the discriminator 13d) onto a constellation. Here, this mapping is the same as the mapping performed in the transmitting node 100. Therefore, the electric field information signal generated by the simulated transmitter 14 is substantially the same as the electric field information signal for generating an optical signal in the transmitting node 100. In other words, the output signal of the simulated transmitter 14 represents the electric field of the optical signal in the transmitting node 100.

[0022] The memory circuit 15 stores an electric field information signal that represents the electric field of the received optical signal. Note that this electric field information signal represents the state before chromatic dispersion in the transmission line is compensated for.

[0023] The feature extraction unit 16 includes a first dispersion compensator 16a, a nonlinear compensator 16b, a second dispersion compensator 16c, and a correlation calculator 16d, and compensates for chromatic dispersion and nonlinear distortion of an electric field information signal representing the electric field of the received optical signal. The first dispersion compensator 16a compensates for a portion of the chromatic dispersion of the transmission line (hereinafter referred to as the first chromatic dispersion) of the electric field information signal. The nonlinear compensator 16b compensates for the nonlinear distortion of the transmission line of the output signal from the first dispersion compensator 16a. The second dispersion compensator 16c compensates for the remaining chromatic dispersion of the transmission line (hereinafter referred to as the second chromatic dispersion) of the output signal from the nonlinear compensator 16b. The correlation calculator 16d calculates the correlation between the output signal from the second dispersion compensator 16c and the output signal from the dummy transmitter 14. Here, the output signal from the dummy transmitter 14 represents the electric field of the optical signal at the transmitting node 100, as described above. That is, the correlation calculation unit 16d calculates the correlation between the electric field information signal in which chromatic dispersion and nonlinear distortion have been compensated for and the electric field information signal that represents the electric field of the optical signal at the transmitting node 100. It is preferable that the output signal of the second dispersion compensating unit 16c and the output signal of the dummy transmitter 14 are each properly normalized.

[0024] This correlation value represents the power of the optical signal transmitted through the transmission line. That is, the optical network device 1 can measure the power of the optical signal transmitted through the transmission line by calculating this correlation value. Here, the relationship between the correlation value and the power of the optical signal will be described with reference to FIG. 3.

[0025] 3 shows an example of changes in the power and chromatic dispersion of an optical signal. In this example, an optical signal is transmitted from a transmitting node 100 to an optical network unit 1. An optical amplifier is provided on the transmission path.

[0026] The power of the optical signal attenuates as it moves away from the transmitting node 100. The optical signal is then amplified by the optical amplifier. After that, the power of the optical signal attenuates as it moves away from the optical amplifier. The accumulated chromatic dispersion added to the optical signal increases in proportion to the distance from the transmitting node 100. Note that CD in FIG. 3 represents the total chromatic dispersion of the transmission path between the transmitting node 100 and the optical network device 1.

[0027] Here, the optical network device 1 measures the power of the optical signal at position P shown in Fig. 3. The chromatic dispersion of the transmission path between the optical network device 1 and position P is CD1. The chromatic dispersion between position P and the sending node 100 is CD2. The sum of CD1 and CD2 is CD.

[0028] As described above, the feature extraction unit 16 compensates for chromatic dispersion and nonlinear distortion. That is, the first dispersion compensator 16a compensates for chromatic dispersion CD1 in the electric field information signal representing the received optical signal. The nonlinear compensator 16b compensates for nonlinear distortion in the output signal of the first dispersion compensator 16a. At this time, the nonlinear compensator 16b compensates for a predetermined amount of nonlinear distortion. Then, the second dispersion compensator 16c compensates for chromatic dispersion CD2 in the output signal of the nonlinear compensator 16b.

[0029] Here, the amount of nonlinear distortion occurring in the optical transmission path depends on the power of the optical signal. Specifically, the greater the power of the optical signal, the greater the amount of nonlinear distortion. In this example, the nonlinear compensator 16b is designed to compensate for nonlinear distortion occurring when the power of the optical signal is sufficiently large. As an example, which is not particularly limited, the nonlinear compensator 16b is designed to compensate for nonlinear distortion occurring with respect to the output optical power of the transmitting node 100.

[0030] On the other hand, the correlation value calculated by the correlation calculation unit 16d represents the correlation between the electric field information signal in which chromatic dispersion and nonlinear distortion have been compensated for in the feature extraction unit 16 and the electric field information signal that represents the electric field of the optical signal at the transmitting node 100. Therefore, when the nonlinear distortion is appropriately compensated for in the nonlinear compensation unit 16b, the correlation value calculated by the correlation calculation unit 16d becomes large.

[0031] That is, when the power of the optical signal at position P is large, the amount of nonlinear distortion at position P increases, and the difference between the amount of nonlinear distortion at position P and the amount of nonlinear distortion compensated for by nonlinear compensator 16b decreases. As a result, nonlinear distortion is appropriately compensated for by nonlinear compensator 16b, and the correlation value calculated by correlation calculator 16d increases. On the other hand, when the power of the optical signal at position P is small, the amount of nonlinear distortion at position P decreases, and the difference between the amount of nonlinear distortion at position P and the amount of nonlinear distortion compensated for by nonlinear compensator 16b increases. As a result, nonlinear distortion is not appropriately compensated for by nonlinear compensator 16b, and the correlation value calculated by correlation calculator 16d decreases. Therefore, the correlation value calculated by feature extractor 16 essentially represents the power of the optical signal at a predetermined position on the transmission path (position P in FIG. 3). Note that this correlation value is an example of an evaluation value corresponding to the power of the optical signal on the transmission path.

[0032] 3 is specified by a combination of chromatic dispersion CD1 and chromatic dispersion CD2. Therefore, the feature extraction unit 16 can measure the power of the optical signal at a desired position on the transmission line by changing the combination of chromatic dispersion CD1 and chromatic dispersion CD2 for the electric field information signal that represents the electric field of the received optical signal.

[0033] 4 is a flowchart showing an example of a process for measuring the power of an optical signal at multiple positions on a transmission line. This process is executed when the optical network device 1 receives an optical signal transmitted from the transmitting node 100 via the transmission line.

[0034] In S1, the feature extraction unit 16 acquires a transmission electric field information signal generated by the simulated transmitter 14. This transmission electric field information signal represents the electric field of the optical signal at the transmitting node 100. In S2, the feature extraction unit 16 acquires an electric field information signal of the received optical signal. This electric field information signal is assumed to have been generated by the coherent receiver 11 and stored in the memory circuit 15.

[0035] In S3, the feature extraction unit 16 initializes the chromatic dispersion CD1 to "zero." The value of the chromatic dispersion CD1 corresponds to the transmission distance based on the optical network unit 1. The chromatic dispersion CD2 is calculated from "CD1 + CD2 = CD." CD represents the total chromatic dispersion of the transmission path between the sending node 100 and the optical network unit 1, and its value is known. In S4, the feature extraction unit 16 determines whether the chromatic dispersion CD1 is equal to or less than CD. If the chromatic dispersion CD1 is equal to or less than CD, the process of the feature extraction unit 16 proceeds to S5.

[0036] In S5, the feature extraction unit 16 performs compensation for chromatic dispersion CD1, nonlinear compensation, and compensation for chromatic dispersion CD2 on the electric field information signal of the received optical signal in that order. In S6, the feature extraction unit 16 calculates the correlation between the electric field information signal compensated in S6 and the transmitted electric field information signal acquired in S1.

[0037] In S7, the feature extraction unit 16 adds ΔCD to the chromatic dispersion CD1. After this, the processing of the feature extraction unit 16 returns to S4. That is, in S4 to S7, the feature extraction unit 16 calculates the correlation value while increasing the chromatic dispersion CD1 by ΔCD each time until the chromatic dispersion CD1 becomes larger than CD. Here, the value of chromatic dispersion CD1 corresponds to the transmission distance based on the optical network device 1. Therefore, the step of increasing the chromatic dispersion CD1 by ΔCD each time is equivalent to the step of shifting the position on the transmission line by a distance corresponding to ΔCD. Therefore, the feature extraction unit 16 calculates the correlation values ​​at multiple positions on the transmission line by repeatedly executing the processing of S4 to S7.

[0038] When the chromatic dispersion CD1 becomes larger than CD, in S8, the feature extraction unit 16 outputs the correlation value calculated in S4 to S7. Here, the correlation value essentially represents the power of the optical signal at a predetermined position on the transmission path corresponding to the combination of chromatic dispersions CD1 and CD2. In other words, the feature extraction unit 16 can detect the power of the optical signal at multiple positions on the transmission path. In the following description, information representing the power of the optical signal at multiple positions on the transmission path may be referred to as a "power profile."

[0039] In this way, the optical network unit 1 can measure the power of an optical signal at a desired position on the transmission line, and then use this function to estimate the polarization dependent loss at the desired position on the transmission line.

[0040] 5 shows an example of a method for identifying the location of polarization dependent loss. In this example, a transmitting node 200 includes a polarization rotator 201, a digital-to-analog converter (DAC) 202, and a modulator 203.

[0041] The polarization rotation unit 201 controls the polarization of the optical signal. However, the polarization rotation unit 201 does not directly control the polarization of the optical signal, but rather controls the polarization of the optical signal by correcting the electric field information signal used to generate the optical signal. In this case, the polarization rotation is realized, for example, by multiplying the electric field information signal representing the main signal by a Jones matrix. Furthermore, the amount of polarization rotation is changed, for example, from zero degrees to 180 degrees in 10-degree increments.

[0042] The DAC 202 converts the output signal of the polarization rotator 201 into an analog signal. The modulator 203 generates a modulated optical signal based on the output signal of the DAC 202. Therefore, the polarization of this optical signal is controlled by the polarization rotator 201. The optical signal transmitted from the transmitting node 200 is propagated through a transmission line. The optical network device 300 receives this optical signal through the transmission line. Note that the amount of polarization rotation provided by the polarization rotator 201 is notified to the optical network device 300 by the transmitting node 200.

[0043] The optical network device 300 includes a coherent receiver 301, an ADC 302, a digital signal processing unit 303, a memory circuit 304, a feature extraction unit 305, and a position identification unit 306. The coherent receiver 301, the ADC 302, the digital signal processing unit 303, the memory circuit 304, and the feature extraction unit 305 correspond to the coherent receiver 11, the ADC 12, the digital signal processing unit 13, the memory circuit 15, and the feature extraction unit 16 shown in Fig. 1. The transmission electric field information signal is generated by a function equivalent to that of the simulated transmitter 14 shown in Fig. 1.

[0044] The digital signal processor 303 regenerates the main signal based on the electric field information signal representing the electric field of the received optical signal. This electric field information signal is stored in the memory circuit 304. The optical network device 300 acquires information representing the amount of polarization rotation from the transmitting node 200.

[0045] As described with reference to FIGS. 1 to 4, the feature extraction unit 305 can detect the power of an optical signal at any position on the transmission path by calculating the correlation between a transmission signal and a reception signal. In this embodiment, the feature extraction unit 305 detects the power of an optical signal at each of a plurality of positions on the transmission path. Furthermore, the feature extraction unit 305 detects the power of an optical signal for each amount of polarization rotation of the optical signal. That is, the feature extraction unit 305 detects the power of an optical signal for each amount of polarization rotation for each of a plurality of positions on the transmission path.

[0046] The position specifying unit 306 determines whether polarization dependent loss occurs at each position on the transmission line (i.e., measurement position). For example, if the difference between the maximum and minimum values ​​of optical power detected for multiple polarization rotation amounts is greater than a threshold, it is determined that polarization dependent loss occurs. Then, the position specifying unit 306 identifies the position where polarization dependent loss occurs based on this determination result.

[0047] 5, the location of polarization dependent loss is identified by rotating the polarization of the optical signal transmitted from the transmitting node 200. However, the optical network device 300 estimates the polarization dependent loss by acquiring electric field information of the received optical signal when the amount of rotation of the polarization of the optical signal is maintained at a constant value. Therefore, the method shown in FIG. 5 requires a long time to identify the location of polarization dependent loss.

[0048] <Embodiment> In an embodiment of the present invention, a polarization multiplexed optical signal is transmitted from a transmitting node to an optical network unit, and the polarization multiplexed optical signal is generated by multiplexing a set of optical signals having orthogonal polarizations.

[0049] When the electric field information of the transmitted optical signal is represented by Esx and Esy and the transfer function of the transmission line is represented by A, the electric field information Ex and Ey of the received optical signal are expressed by equation (1).

number

[0050] In this case, in the configuration shown in Fig. 1, electric field information Ecx and Ecy are generated by compensating for dispersion and nonlinear distortion in electric field information Ex and Ey, which represent the electric field of the received optical signal. Ex, Ey, Ecx, and Ecy are complex numbers each having information on the electric field strength and optical phase. In the method described with reference to Figs. 1 and 4, the optical power at any position on the transmission line is measured by calculating the correlation between the absolute value of Ecx and the absolute value of Esx and / or the correlation between the absolute value of Ecy and the absolute value of Esy.

[0051] Polarization-dependent loss is expressed by a rotational transformation in the direction in which loss occurs, the addition of loss, and a rotational transformation in the opposite direction to return to the original polarization state. That is, polarization-dependent loss is expressed by equation (2). φ represents the amount of rotation in the direction in which loss occurs, and Γ (0<Γ≦1) represents the loss.

number

[0052] Figure 6 shows the rotational transformation in the direction in which loss occurs. The direction in which loss occurs is represented by φ. In this case, the rotational transformation in the direction in which loss occurs is expressed by equation (3). Erx and Ery represent the polarization components after the rotational transformation. That is, Ery corresponds to the sum of the projected component of Esx and the projected component of Esy onto the PDL axis. Furthermore, Erx corresponds to the sum of the projected component of Esx and the projected component of Esy onto the axis orthogonal to the PDL axis. Erx, Ery, Esx, and Esy are complex numbers that contain information on the electric field intensity and optical phase, respectively.

number

[0053] Here, in the rotational transformation, the polarization direction of each electric field component (the direction of the arrow in Figure 6) changes, but the power does not change. Therefore, when measuring optical power using the method described with reference to Figures 1 and 4, Erx and Ery shown in Figure 6(b) can be used instead of Esx and Esy shown in Figure 6(a). In other words, the power of the optical signal at any position on the transmission line can be measured by calculating the correlation between the signals obtained by rotating the polarization of the electric field information Ecx and Ecy, which represent the electric field after compensating for the dispersion and nonlinear distortion of the received optical signal, and the signals obtained by rotating the polarization of the electric field information Esx and Esy, which represent the electric field of the transmitted optical signal.

[0054] Furthermore, when polarization-dependent loss occurs in the PDL axis direction shown in Fig. 6(b), the optical phase information of Erx does not change, but the optical phase information of Ery changes by the amount of phase rotation reduced by nonlinear phase noise due to the polarization-dependent loss. Therefore, polarization-dependent loss can be detected by measuring the optical power from the optical phase information including the Erx / Ery phase noise using the method described with reference to Figs. 1 and 4.

[0055] Figure 7 shows an example of a power profile. As shown in Figure 7(a), the power of an optical signal decreases gradually and almost linearly with the transmission distance. However, if a local loss occurs on the transmission path, the power of the optical signal drops suddenly. In Figure 7(b), a local loss occurs at position P.

[0056] When localized polarization-dependent loss occurs, if the polarization direction of the optical signal coincides with the PDL axis, the power profile shown in Figure 7(b) is obtained. On the other hand, when the polarization direction of the optical signal is orthogonal to the PDL axis, the power profile shown in Figure 7(a) is obtained. However, the direction in which polarization-dependent loss occurs (φ in Figure 6(b)) is unknown. Therefore, the optical network device according to the embodiment of the present invention performs rotational transformation on the electric field information signal using a plurality of different rotation amounts θi, and creates a power profile for each rotation amount θi. When the power profile shown in Figure 7(b) is found, the position where the power suddenly drops (i.e., position P) is detected.

[0057] FIG. 8 shows an example of an optical network device according to an embodiment of the present invention. In the optical network device 1 according to the embodiment of the present invention, an electric field information signal generated by the coherent receiver 11 is guided as a monitor signal to the monitor signal rotator 24. This electric field information signal represents the electric field of the received optical signal. The monitor signal rotator 24 performs the rotational transformation shown in FIG. 6 (i.e., the rotational transformation expressed by equation (3)) on the monitor signal. The rotation amount θ is specified by the rotational transformation control unit 26. The reference signal represents the electric field of the optical signal transmitted from the transmitting node 100 shown in FIG. 1. In the configuration shown in FIG. 1, the reference signal is obtained by, for example, the simulated transmitter 14. Then, the reference signal rotator 25 performs the rotational transformation shown in FIG. 6 (i.e., the rotational transformation expressed by equation (3)) on the reference signal. Here, the rotational transformation amount for the monitor signal and the rotational transformation amount for the reference signal are the same.

[0058] The feature extraction unit 27 creates a power profile for each rotation amount θ (θ1, θ2, ...). The position identification unit 28 detects the maximum power value and the minimum power value for each measurement position on the transmission path. For example, if, at position k, the power Pi calculated for θi is the maximum and the power Pj calculated for θj is the minimum, it is estimated that θi is the direction of the PDL axis. Furthermore, the difference D between the maximum power Pi and the minimum power Pj corresponds to the magnitude of the polarization dependent loss. In this case, the position where the difference D changes abruptly is considered to be the location where the polarization dependent loss occurs. Therefore, the position identification unit 28 can identify the location where the polarization dependent loss occurs by calculating the difference D at each position.

[0059] In this way, the position specifying unit 28 determines whether a predetermined position on the transmission line is a position where polarization dependent loss occurs. Then, the position specifying unit 28 outputs information indicating the position where polarization dependent loss occurs. Therefore, the position specifying unit 28 is an example of a determination unit that determines whether a predetermined position on the transmission line is a position where polarization dependent loss occurs. The information output from the position specifying unit 28 is stored in, for example, a server computer. In this case, a network administrator can use this server computer to check the state of the transmission line.

[0060] FIG. 9 shows an example of an optical network device according to an embodiment of the present invention. The optical network device 1 receives an optical signal transmitted from the transmitting node 100 shown in FIG. 1. The transmitting node 100 generates and transmits a polarization multiplexed optical signal. The polarization multiplexed optical signal is generated by multiplexing a pair of optical signals (i.e., an X-polarized optical signal and a Y-polarized optical signal) whose polarizations are orthogonal to each other. The polarization multiplexed optical signal output from the transmitting node 100 is transmitted via an optical fiber transmission line. The optical network device 1 then receives the polarization multiplexed optical signal via the optical fiber transmission line.

[0061] 9, the optical network equipment 1 includes a coherent receiver 11, a digital signal processing unit 13, a monitor signal storage unit 21, a transmission data storage unit 22, a transmission waveform reconstruction unit 23, a monitor signal rotation unit 24, a reference signal rotation unit 25, a rotation amount control unit 26, a feature extraction unit 27, and a position identification unit 28. Note that an ADC provided between the coherent receiver 11 and the digital signal processing unit 13 is omitted. The optical network equipment 1 may also include other circuits or functions not shown in FIG. 9.

[0062] Coherent receiver 11 generates electric field information (or electric field data) representing the electric field of the received optical signal. As described with reference to FIG. 2, digital signal processing unit 13 includes fixed equalizer 13a, adaptive equalizer 13b, phase regenerator 13c, and discriminator 13d. Therefore, digital signal processing unit 13 regenerates data transmitted from transmitting node 100 shown in FIG. 1 based on the electric field information representing the received optical signal. At this time, adaptive equalizer 13b separates the electric field information of the received polarization multiplexed optical signal into a pair of mutually orthogonal polarization components (Ex, Ey). This extracts an X-polarized optical signal and a Y-polarized optical signal. Then, digital signal processing unit 13 regenerates data from the X-polarized optical signal and the Y-polarized optical signal, respectively.

[0063] A set of electric field information (Ex, Ey) extracted by the adaptive equalizer 13b is output as a monitor signal. This monitor signal is then stored in a monitor signal storage unit 21. The monitor signal storage unit 21 is realized by, for example, a memory.

[0064] The transmission data storage unit 22 stores data reproduced by the digital signal processing unit 13. Here, as described above, the digital signal processing unit 13 reproduces the data transmitted from the transmitting node 100 shown in Fig. 1. That is, the transmission data is stored in the transmission data storage unit 22. The transmission data storage unit 22 is realized by, for example, a memory.

[0065] FIG. 10 shows an example of the transmission waveform reconstructor 23. As shown in FIG. 10, the transmission waveform reconstructor 23 includes a symbol mapper 23a, a sampling rate adjuster 23b, and a Nyquist filter 23c. The transmission waveform reconstructor 23 performs substantially the same signal processing as the transmitting node 100. Specifically, the symbol mapper 23a maps the transmission data stored in the transmission data storage unit 22 for each symbol. That is, electric field information representing the transmission data is generated according to a specified modulation method. The modulation method is the same in the transmitting node 100 and the symbol mapper 23a. The sampling rate adjuster 23b adjusts the sampling rate of the electric field information signal. The Nyquist filter 23c corrects the electric field information signal to suppress inter-symbol interference. As a result, the electric field information of the optical signal transmitted from the transmitting node 100 is reconstructed. Note that, in the following description, the signal representing the electric field of the transmission signal obtained by the sampling rate adjuster 23b may be referred to as a "reference signal." The transmission waveform reconstructor 23 corresponds to the simulated transmitter 14 shown in FIG. 1.

[0066] Figure 11 is a diagram explaining the rotation of polarization in an optical signal. Here, the direction of polarization is represented using a polarization plane coordinate system consisting of an H axis corresponding to horizontal polarization and a V axis corresponding to vertical polarization. The horizontal polarization H and the vertical polarization V are orthogonal to each other.

[0067] The monitor signal rotator 24 rotates the monitor signal in the coordinate system shown in Fig. 11. Here, it is assumed that a pair of polarization components (Ex, Ey) extracted by the adaptive equalizer 13b is in the state shown in Fig. 11(a). Furthermore, it is assumed that the monitor signal rotator 24 rotates the polarization of the monitor signal by θ. In this case, the signals Erx and Ery are obtained as shown in Fig. 11(b).

[0068] The signal Ery is represented by a component obtained by projecting the polarization component Ex onto the θ axis and a component obtained by projecting the polarization component Ey onto the θ axis, while the signal Erx is represented by a component obtained by projecting the polarization component Ex onto an orthogonal axis and a component obtained by projecting the polarization component Ey onto the orthogonal axis.

[0069] The monitor signal rotator 24 performs a rotation operation as shown in FIG. 11(c). That is, it multiplies the H polarization signal Ex and the V polarization signal Ey by a rotation matrix. The rotation matrix includes a parameter θ that represents the amount of polarization rotation. The H polarization signal Ex, the V polarization signal Ey, and the parameter θ may be written to a register once. In this case, the monitor signal rotator 24 reads Ex, Ey, and θ from the register for each symbol and performs the rotation operation. As a result, the signals Erx and Ery shown in FIG. 11(b) are obtained.

[0070] The configuration and processing of the reference signal rotator 25 are substantially the same as those of the monitor signal rotator 24. That is, the reference signal rotator 25 performs a rotation operation on the reference signal obtained by the transmission waveform reconstructor 23.

[0071] The rotation amounts of monitor signal rotator 24 and reference signal rotator 25 are specified by rotation amount control unit 26. Rotation amount control unit 26 has a plurality of rotation amount values ​​(0, 10, 20, ..., 180) within a range from zero to 180 degrees, for example. Rotation amount control unit 26 then sequentially supplies these values ​​to monitor signal rotator 24 and reference signal rotator 25. Alternatively, if the rotation amount change step Δθ is constant, rotation amount control unit 26 may instruct monitor signal rotator 24 and reference signal rotator 25 on a parameter n (n = 0, 1, 2, ...). In this case, monitor signal rotator 24 and reference signal rotator 25 each perform a rotation calculation of nΔθ.

[0072] The feature extraction unit 27 calculates the correlation value between the monitor signal and the reference signal by the method described with reference to FIGS. 3 to 5. Here, the monitor signal represents the field intensity and optical phase of the received optical signal. However, the monitor signal is compensated for dispersion and nonlinear distortion. The reference signal represents the field intensity and optical phase of the optical signal transmitted from the transmitting node 100. Here, the correlation value depends on the power of the optical signal, as described above. In this embodiment, the feature extraction unit 27 is designed so that the correlation value becomes larger when the optical power is large. Then, the feature extraction unit 27 detects the power of the optical signal by calculating the correlation value between the monitor signal and the reference signal.

[0073] Furthermore, the feature extraction unit 27 detects the power of the optical signal at multiple positions on the transmission line. The positions on the transmission line are specified by a combination of dispersion compensation amounts CD1 and CD2, for example, as described with reference to FIGS. 3 and 4. Additionally, the feature extraction unit 27 detects the power of the optical signal for each rotation amount θ controlled by the rotation amount control unit 26. That is, the power of the optical signal is detected for each position and for each rotation amount. Furthermore, the feature extraction unit 27 detects the power of the optical signal for each polarization component. For example, the optical power of the polarization component in the θ-axis direction and / or the polarization component in the orthogonal axis direction shown in FIG. 11 is detected.

[0074] The position specifying unit 28 determines whether polarization dependent loss occurs on the transmission path based on the optical power detected for each position and for each rotation amount. If polarization dependent loss occurs, the position specifying unit 28 specifies the position where the polarization dependent loss occurs.

[0075] 12 is a flowchart showing an example of a method for identifying the location where polarization dependent loss occurs. The processing of this flowchart is mainly performed by monitor signal rotation unit 24, reference signal rotation unit 25, rotation amount control unit 26, feature extraction unit 27, and position identification unit 28. The processing of this flowchart is performed for each symbol.

[0076] In S11, a monitor signal and a reference signal of a target symbol are input. The monitor signal represents the electric field of the received optical signal and is stored in the monitor signal storage unit 21. The reference signal represents the electric field of the transmitted optical signal of the transmitting node 100 and is generated by the transmitted waveform reconstruction unit 23.

[0077] In S12, the rotation amount control unit 26 sets the rotation amount θi. The rotation amount θi is set, for example, in 10-degree intervals within a range from 0 to 180 degrees. In this case, for example, the initial value of the rotation amount θi is zero, and the rotation amount θi is incremented by 10 degrees each time S12 to S15 are executed.

[0078] In S13, monitor signal rotator 24 rotates the monitor signal by θi in the polarization plane coordinate system. The monitor signal is represented by a pair of mutually orthogonal polarization components (H polarization signal and V polarization signal). Here, as shown in FIG. 11(a), the H polarization signal and V polarization signal of the monitor signal are assumed to be Ex and Ey, respectively. In this case, output signals Erx and Ery of monitor signal rotator 24 are expressed by equation (4). Note that Ex, Ey, Erx, and Ery are complex numbers representing the electric field intensity and optical phase, respectively.

number

[0079] Similarly, reference signal rotator 25 rotates the reference signal by θi in the polarization plane coordinate system. Here, when the H polarization signal and V polarization signal of the reference signal are Esx and Esy, respectively, output signals Esrx and Esry of reference signal rotator 25 are expressed by equation (5). Note that Esx, Esy, Esrx, and Esry are complex numbers that represent the electric field intensity and optical phase, respectively.

number

[0080] In S14, the feature extraction unit 27 calculates a correlation value between the monitor signal and the reference signal while changing the position on the transmission path between the transmitting node 100 and the optical network device 1. As described above, the position on the transmission path is specified by a combination of the dispersion compensation amounts CD1 and CD2. Here, this correlation value represents the power of the optical signal at each position. Therefore, the feature extraction unit 27 calculates the power of the optical signal at multiple positions on the transmission path. Note that the feature extraction unit 27 calculates the optical power of the polarization component in the θ-axis direction and / or the polarization component in the orthogonal axis direction. In other words, the feature extraction unit 27 calculates the power of the optical signal in the θ-axis direction and / or the power of the orthogonal axis direction at multiple positions on the transmission path. Note that in the following description, the position at which the power of the optical signal is calculated may be referred to as the "measurement position."

[0081] For example, as shown in Figure 13(a), the sending node 100 and the optical network unit 1 are connected by a 5 km optical fiber. The chromatic dispersion of this optical fiber is 20 ps / nm / km. In other words, the total chromatic dispersion of the transmission path between the sending node 100 and the optical network unit 1 is 100 ps / nm.

[0082] The feature extraction unit 27 calculates correlation values ​​at positions 0 km (i.e., the transmitting end), 1 km, 2 km, 3 km, 4 km, and 5 km (i.e., the receiving end) relative to the transmitting node 100. Here, each position on the transmission path is represented by a combination of chromatic dispersions CD1 and CD2. For example, a position 1 km from the transmitting node 100 toward the optical network unit 1 is represented by "CD1=80, CD2=20." Also, a position 2 km from the transmitting node 100 toward the optical network unit 1 is represented by "CD1=60, CD2=40." As a result, for example, when the rotation amount set by the rotation amount control unit 26 is "θ0," the feature extraction unit 27 obtains correlation values ​​C00 to C50 shown in FIG. 13(b). Note that C00, C10, C20, C30, C40, and C50 represent correlation values ​​(i.e., optical signal power) corresponding to positions 0 km, 1 km, 2 km, 3 km, 4 km, and 5 km from the transmitting node 100 toward the optical network unit 1, respectively, when the rotation amount is θ0. Note that if there is no polarization dependent loss, the power of the optical signal on the transmission path will gradually attenuate almost linearly according to the transmission distance, as shown in Figure 13(a). In this way, in S14, a power profile for one rotation amount θi is generated.

[0083] In S15, the feature extraction unit 27 determines whether or not a power profile has been created for each of all rotation amounts (θ0 to θN). If there are any rotation amounts for which a power profile has not been created, the process of the feature extraction unit 27 returns to S12. That is, the feature extraction unit 27 creates a power profile for each rotation amount. As a result, as shown in FIG. 13(b), optical power corresponding to multiple measurement positions on the transmission line is obtained for each rotation amount θ0 to θ4. At this time, the feature extraction unit 27 creates a power profile for the polarization component in the θ-axis direction and / or the polarization component in the orthogonal axis direction. Note that in the example shown in FIG. 13(b), the constant N used in the flowchart is 4.

[0084] In S21, the position specifying unit 28 calculates the difference (dk) between the maximum power value and the minimum power value for measurement position k. For example, when the process of S21 is executed for position X2, the position specifying unit 28 extracts the maximum and minimum values ​​from among the correlation values ​​C20 to C24. Then, the position specifying unit 28 calculates the difference between the extracted maximum and minimum values. That is, a power difference dk representing the difference between the maximum power value and the minimum power value is calculated. Note that the difference between the maximum power value and the minimum power value calculated in S21 (or the difference between the maximum and minimum values ​​extracted from a plurality of correlation values) is an example of the amount of fluctuation in the evaluation value.

[0085] Here, if polarization-dependent loss occurs on the transmitting side at position X2, the power of the optical signal detected at position X2 may decrease depending on the polarization angle of the optical signal. For example, if the polarization direction of one of the optical signals constituting the polarization multiplexed optical signal coincides or nearly coincides with the direction in which polarization-dependent loss occurs, the power of that optical signal decreases. Therefore, when polarization-dependent loss occurs, rotating the electric field information signals representing the monitor signal and the reference signal increases the fluctuation in the power of the optical signal detected at position X2. Therefore, if the correlation values ​​C20 to C24 representing the power of the optical signal change significantly when the monitor signal and the reference signal are rotated, it is estimated that polarization-dependent loss is occurring on the transmitting side at position X2. In other words, if the difference between the maximum and minimum values ​​of the correlation values ​​C20 to C24 is large, it is estimated that polarization-dependent loss is occurring on the transmitting side at position X2.

[0086] The difference between the maximum and minimum correlation values ​​obtained when the amount of rotation in the rotation calculation for each measurement position is changed within a predetermined range corresponds to the power difference between two signals when one signal (e.g., X-polarized signal) of a pair of signals constituting a polarization multiplexed optical signal is set in the PDL axis direction and the other signal (e.g., Y-polarized signal) is set in the orthogonal axis direction. Therefore, the power difference calculated in S21 corresponds to the power difference between polarizations that occurs due to polarization dependent loss.

[0087] In S22, the position identification unit 28 calculates the difference (ddk) between the power difference dk calculated for measurement position k and the power difference dk-1 previously obtained for measurement position k-1. Measurement position k-1 represents a measurement position adjacent to measurement position k on the transmitting side.

[0088] S23 is provided to execute S21 to S22 for all measurement positions. Then, when the processes of S21 to S22 are completed for all measurement positions, the process of the position specifying unit 28 proceeds to S24.

[0089] In S24, the position specifying unit 28 extracts a measurement position k for which the difference ddk is greater than a threshold value. Here, if the difference ddk calculated for the measurement position k is greater than the threshold value, the position specifying unit 28 determines that a polarization dependent loss is occurring between the measurement position k and the measurement position k-1. Thereafter, in S25, the position specifying unit 28 outputs information indicating the position where the polarization dependent loss is estimated to be occurring (actually, the range where the polarization dependent loss is estimated to be occurring). The position information output from the position specifying unit 28 is stored, for example, in a server computer. In this case, a network administrator can check the state of the transmission path using this server computer. Note that if the difference ddk is smaller than the threshold value, the position specifying unit 28 determines that no polarization dependent loss is occurring between the adjacent measurement position and the target measurement position. In this case, the position specifying unit 28 may output information indicating that no polarization dependent loss is occurring.

[0090] The threshold value of S24 may be zero. However, if this threshold value is zero (or a value close to zero), an erroneous determination result may occur due to noise, etc. Therefore, it is preferable to set this threshold value to a predetermined positive value taking noise, etc. into consideration.

[0091] FIG. 14 shows an example of the position identification unit 28. The position identification unit 28 includes a power profile storage unit 28a, multiple difference calculation units 28b (#0 to #K), and a change point search unit 28c. The power profile storage unit 28a stores the power profile generated by the feature extraction unit 27. As shown in FIG. 13(b), the power profile represents an optical power value (or a correlation value) for a combination of position and rotation amount. The multiple difference calculation units 28b are provided corresponding to multiple measurement positions on the transmission path. Each difference calculation unit 28b acquires multiple optical power values ​​obtained for the corresponding measurement position and extracts the maximum power value and the minimum power value from among them. Each difference calculation unit 28b then calculates a power difference representing the difference between the maximum power value and the minimum power value. The power difference calculated by each difference calculation unit 28b corresponds to the polarization dependent loss at the corresponding position. The processing performed by each difference calculation unit 28b corresponds to S21 shown in FIG. 12.

[0092] The change-point searcher 28c sorts the power differences (d1, d2, . . . dN) obtained by the difference calculator 28b in order based on the transmission distance from the transmitting node 100. Then, the change-point searcher 28c searches for a position where the change in power difference between adjacent measurement positions is greater than a threshold. For example, in FIG. 13(a), when the difference dd2 between the power difference d1 obtained for position X1 (i.e., the PDL detected at position X1) and the difference value d2 obtained for position X2 (i.e., the PDL detected at position X2) is greater than the threshold, the change-point searcher 28c determines that a polarization dependent loss is occurring between positions X1 and X2.

[0093] 15 shows an example of a method for identifying the location where polarization dependent loss occurs. In this example, the polarization rotation amounts of the monitor signal and the reference signal are θ1, θ2, and θ3. Here, the correlation value obtained by the feature extraction unit 27 may be referred to as a "power value."

[0094] Fig. 15(a) shows power values ​​C11 to C31 calculated for positions X1 to X3 when the amount of rotation is θ1. Similarly, Fig. 15(b) shows power values ​​C12 to C32 calculated for positions X1 to X3 when the amount of rotation is θ2. Fig. 15(c) shows power values ​​C13 to C33 calculated for positions X1 to X3 when the amount of rotation is θ3. The position specifying unit 28 then uses the power values ​​calculated for each measurement position (X1 to X3 in this case) to determine whether or not polarization dependent loss is occurring.

[0095] At position X1, power values ​​C11, C12, and C13 are obtained for rotation amounts θ1, θ2, and θ3, respectively. In this example, the power values ​​C11, C12, and C13 are approximately the same. In this case, the difference between the maximum power value and the minimum power value (i.e., power difference d1) is approximately zero, so it is determined that no polarization dependent loss is occurring.

[0096] At position X2, power values ​​C21, C22, and C23 are obtained for rotation amounts θ1, θ2, and θ3, respectively. In this embodiment, power values ​​C21 and C23 are approximately the same, but power value C22 is smaller than the other power values. The difference between the maximum and minimum power values ​​obtained at position X2 (i.e., power difference d2) is greater than power difference d1 obtained at position X1. In other words, power difference d2 obtained at position X2 is greater than power difference d1 obtained at a measurement position adjacent to the transmitting side of position X2. Therefore, "position X2" is extracted in S24.

[0097] At position X3, power values ​​C31, C32, and C33 are obtained for rotation amounts θ1, θ2, and θ3, respectively. In this embodiment, power values ​​C31 and C33 are approximately the same, but power value C32 is smaller than the other power values. However, the difference between the maximum and minimum power values ​​obtained at position X3 (i.e., power difference d3) is approximately the same as the power difference d2 obtained at position X2. Therefore, "position X3" is not extracted in S24. As a result, in the case shown in FIG. 15, it is determined that polarization dependent loss occurs between positions X1 and X2.

[0098] Fig. 16 shows another example of a method for identifying the location where polarization dependent loss occurs. In this example, as shown in Fig. 13(a), the transmitting node 100 and the optical network unit 1 are connected by a 5 km optical fiber. The measurement results shown in Fig. 16 are obtained. The values ​​shown in Fig. 16 represent correlation values ​​(or optical power of the optical signal) obtained for combinations of positions and rotation amounts θ. In the following description, the values ​​shown in Fig. 16 may be referred to as "evaluation values." The "difference" shown in Fig. 16 represents the difference between the maximum and minimum evaluation values ​​calculated for each measurement position.

[0099] At position X0, the maximum and minimum evaluation values ​​are "81" and "80," respectively, and the difference d0 is "1." At position X1, the maximum and minimum evaluation values ​​are "71" and "69," respectively, and the difference d1 is "2." In this example, the threshold value of S24 is assumed to be "5." Then, the result of subtracting difference d0 from difference d1 is smaller than the threshold value. Therefore, the position identification unit 28 determines that no polarization dependent loss has occurred on the transmitting side of position X1.

[0100] At position X2, the maximum and minimum evaluation values ​​are "60" and "50," respectively, and the difference d2 is "10." Here, the result of subtracting difference d1 from difference d2 is "8," which is greater than the threshold. Therefore, in this case, the position identifying unit 28 determines that a polarization dependent loss is occurring between positions X1 and X2.

[0101] At position X3, the maximum and minimum evaluation values ​​are "50" and "40," respectively, and the difference d3 is "10." However, the result of subtracting difference d2 from difference d3 is "0," which is smaller than the threshold value. Therefore, the position identification unit 28 determines that no polarization dependent loss occurs between positions X2 and X3. Subsequently, the same determination results as for position X3 are obtained for positions X4 and X5.

[0102] In this way, the optical network device 1 can identify the location where polarization dependent loss occurs by analyzing the electric field information of the received optical signal, thereby reducing the effort and / or time required to identify the location where polarization dependent loss occurs.

[0103] The method shown in FIG. 5 can also identify the location where polarization dependent loss occurs by analyzing the electric field information of the received optical signal. However, in the method shown in FIG. 5, the polarization of the optical signal is actually rotated at the transmitting node 200. The optical network device 300 estimates the polarization dependent loss by acquiring the electric field information of the received optical signal while the amount of rotation of the polarization of the optical signal is maintained at a constant value. Therefore, the method shown in FIG. 5 requires a long time to identify the location where polarization dependent loss occurs. In contrast, in the embodiment of the present invention, polarization dependent loss can be detected by performing a rotational transformation on the electric field information of the received optical signal without actually rotating the polarization of the optical signal. Therefore, according to the embodiment of the present invention, the time required to identify the location where polarization dependent loss occurs can be shorter than the method shown in FIG. 5.

[0104] 17 shows an example of an optical transmission system consisting of multiple spans. In this example, the length of the transmission path between the transmitting node (Tx) 100 and the optical network unit (Rx) 1 is 14 km, and optical amplifiers are installed at positions X5 and X10. Local polarization dependent loss occurs between positions X6 and X7.

[0105] The optical network device 1 measures the optical power at each measurement position (X1 to X14) based on the electric field information of the received optical signal. Then, the optical network device 1 estimates the position where the polarization dependent loss occurs based on the change in power between adjacent measurement positions. Therefore, the position where the polarization dependent loss occurs can be estimated using the same method regardless of the number of spans in the optical transmission system.

[0106] 18 schematically shows the processing of the optical network device 1. As described above, the transmitting node 100 generates and transmits a polarization multiplexed optical signal. The electric fields of the polarization multiplexed optical signal are represented by Esx and Esy. Esx and Esy are orthogonal to each other.

[0107] The polarization of the polarization multiplexed optical signal rotates in the optical transmission line. The polarization multiplexed optical signal is also affected by polarization dependent loss in the transmission line. The coherent receiver 11 of the optical network unit 1 generates electric field information Ex0 and Ey0.

[0108] The adaptive equalizer 13b separates the electric field information signal representing the electric field of the polarization multiplexed optical signal into a first polarization component Ex and a second polarization component Ey that are orthogonal to each other, by correcting the output signal of the coherent receiver 11. In this embodiment, Ex and Ey are used as monitor signals.

[0109] The discriminator 13d recovers the transmission data based on the output signal of the adaptive equalizer 13b. Then, the transmission waveform reconstructor 23 generates electric field information by mapping the recovered transmission data. This electric field information is substantially the same as the electric field of the polarization multiplexed optical signal generated in the transmitting node 100. That is, the electric field information Esx and Esy are reconstructed. In this embodiment, the reconstructed Esx and Esy are used as reference signals.

[0110] Monitor signal rotator 24 generates electric field information Erx and Ery by rotating monitor signal (Ex, Ey) by θ around the origin of the polarization plane coordinate system. Similarly, reference signal rotator 25 generates electric field information Esrx and Esry by rotating reference signal (Esx, Esy) by θ around the origin of the polarization plane coordinate system. The rotation amount θ is specified by rotation amount controller 26. Feature extractor 27 calculates the optical power for each rotation amount θ by calculating the correlation between the rotationally transformed monitor signal (Erx, Ery) and the rotationally transformed reference signal (Esrx, Esry). Feature extractor 27 may calculate the optical power of the X-polarized signal by calculating the correlation between the polarization component Erx of the rotationally transformed monitor signal and the polarization component Esrx of the rotationally transformed reference signal. Feature extractor 27 may also calculate the optical power of the Y-polarized signal by calculating the correlation between the polarization component Ery of the rotationally transformed monitor signal and the polarization component Esry of the rotationally transformed reference signal. At this time, the optical power is calculated for each of multiple measurement positions on the transmission line. As a result, a power profile is created for each rotation amount θ. In addition, power profiles for each rotation amount θ are created for both the X-polarized signal and the Y-polarized signal.

[0111] Note that the flowchart shown in Fig. 12 is one example, and the present invention is not limited to this procedure. For example, in the flowchart shown in Fig. 12, polarization dependent loss is detected based on the difference between the maximum power value and the minimum power value, but polarization dependent loss may also be detected based on the ratio of the maximum power value to the minimum power value. Alternatively, the difference between the optical power of the X polarization signal and the optical power of the Y polarization signal may be monitored.

[0112] 12 is executed for one symbol, but the present invention is not limited to this method. That is, when noise on the transmission path is taken into consideration, it is preferable to calculate correlation values ​​(i.e., optical power) for multiple symbols and identify the location where polarization dependent loss occurs based on the average of these values.

[0113] 9, the monitor signal represents the electric field of the optical signal whose dispersion has been compensated for by the fixed equalizer 13a (and the adaptive equalizer 13b). Meanwhile, the feature extractor 27 specifies a position on the transmission line by utilizing the dispersion added to the received optical signal. Therefore, the optical network device 1 may be provided with a function to add the dispersion compensated for in the fixed equalizer 13a to the input or output side of the monitor signal rotator 24.

[0114] The digital signal processing unit 13, the monitor signal rotation unit 24, the reference signal rotation unit 25, the rotation amount control unit 26, the feature extraction unit 27, and the position identification unit 28 are realized, for example, by one or more processors. In this case, a program describing the functions of the digital signal processing unit 13, the monitor signal rotation unit 24, the reference signal rotation unit 25, the rotation amount control unit 26, the feature extraction unit 27, and the position identification unit 28 is stored in a memory (not shown). The processor then executes the program to provide the functions of the digital signal processing unit 13, the monitor signal rotation unit 24, the reference signal rotation unit 25, the rotation amount control unit 26, the feature extraction unit 27, and the position identification unit 28. Note that these functions may be realized by an ASIC, an FPGA, or the like. Alternatively, these functions may be realized by a combination of software and hardware circuits.

[0115] <Variations> Fig. 19 shows a first variation of an optical network device according to an embodiment of the present invention, in which the coherent receiver 11 and the digital signal processing unit 13 shown in Fig. 9 are omitted.

[0116] The optical network device 1B includes a plurality of calculation units 30 (#1 to #N). Each calculation unit 30 includes a monitor signal rotation unit 24, a reference signal rotation unit 25, and a feature extraction unit 27. The monitor signal rotation unit 24, the reference signal rotation unit 25, and the feature extraction unit 27 are substantially the same in FIGS. 9 and 19. Therefore, the calculation unit 30 performs a rotational transformation on the monitor signal and the reference signal. Then, the calculation unit 30 calculates the correlation value between the rotationally transformed monitor signal and the reference signal, thereby calculating the optical power at each measurement position.

[0117] A calculation unit 30 is provided for each rotation amount of the rotation transformation. For example, calculation unit 30#1 performs transformation using rotation amount θ1, and calculation unit 30#2 performs transformation using rotation amount θ2. Therefore, when calculating optical power at 10-degree intervals in the range of 0 to 180 degrees, 19 calculation units 30 are implemented in parallel. Here, most of the processing load for identifying the location of occurrence of polarization dependent loss is the processing load of the feature extraction unit creating a power profile. Therefore, by implementing N calculation units 30 in parallel, the calculation time is reduced to approximately 1 / N. For example, in the optical network device 1B, the processes of S12 to S14 are executed in parallel by multiple calculation units 30. Note that the multiple calculation units 30 may be implemented by, for example, assigning a CPU core to each calculation unit 30. Furthermore, since the rotation amount θ of each calculation unit 30 is predetermined, the optical network device 1B does not need to include a rotation amount control unit 26.

[0118] Figure 20 shows a second variation of an optical network device according to an embodiment of the present invention. In the optical network device 1 shown in Figure 9, the output signal of the adaptive equalizer 13b is used as the monitor signal. On the other hand, in the optical network device 1C shown in Figure 20, the output signal of the fixed equalizer 13a is used as the monitor signal. That is, the electric field information signal before being processed by the adaptive equalizer 13b is used as the monitor signal.

[0119] Here, the adaptive equalizer 13b corrects the polarization rotation occurring in the transmission path. That is, in the second variation, a monitor signal with uncorrected polarization rotation is generated. Therefore, the optical network device 1C includes a polarization rotation correction unit 41. The polarization rotation correction unit 41 corrects the polarization rotation occurring in the transmission path of the monitor signal. At this time, the polarization rotation correction unit 41 may acquire parameters used in the adaptive equalizer 13b and correct the polarization rotation based on the parameters. For example, if the adaptive equalizer 13b is realized by a digital filter, the parameters representing the amount of polarization rotation correction correspond to tap coefficients of the digital filter. In this case, the polarization rotation correction unit 41 acquires the tap coefficients from the adaptive equalizer 13b and corrects the polarization rotation. Alternatively, the polarization rotation correction unit 41 may calculate a correlation value between the reference signal before polarization rotation and the monitor signal while gradually rotating the polarization of the monitor signal, and correct the polarization of the monitor signal by the amount of rotation that maximizes the correlation value.

[0120] Furthermore, the polarization rotation corrector 41 may determine the amount of correction for polarization rotation and notify the determined amount of correction to the monitor signal rotator 24. In this case, the monitor signal rotator 24 adds the amount of correction determined by the polarization rotation corrector 41 to the amount of rotation θ notified by the rotation amount controller 26, and performs a polarization rotation calculation.

[0121] Incidentally, adaptive equalizer 13b not only corrects polarization rotation but also compensates for residual dispersion. At this time, nonlinear distortion components in the electric field information signal of the received optical signal are affected. On the other hand, feature extraction unit 27 calculates the optical power at a desired position on the transmission path using the nonlinear distortion components in the electric field information signal of the received optical signal. Therefore, in the configuration shown in FIG. 9, the optical power is calculated based on electric field information in which the nonlinear distortion components have been corrected. In contrast, in the second variation shown in FIG. 20, the optical power is calculated using electric field information in which the nonlinear distortion components have not been corrected. Therefore, compared to the configuration shown in FIG. 9, the second variation shown in FIG. 20 is expected to improve the accuracy of optical power calculation.

[0122] Fig. 21 shows a third variation of an optical network device according to an embodiment of the present invention. In the optical network device 1 shown in Fig. 9, the output signal of the adaptive equalizer 13b is used as the monitor signal. On the other hand, in the optical network device 1D shown in Fig. 21, the output signal of the coherent receiver 11 is used as the monitor signal. That is, the electric field information signal before being processed by the fixed equalizer 13a and the adaptive equalizer 13b is used as the monitor signal.

[0123] The optical network device 1D includes a polarization rotation corrector 41 and a dispersion compensator 42 in addition to the configuration shown in Fig. 9. The polarization rotation corrector 41 is substantially the same in Fig. 20 and Fig. 21, and therefore description thereof will be omitted.

[0124] The configuration and function of the dispersion compensator 42 are substantially the same as those of the fixed equalizer 13a. That is, the fixed equalizer 13a and the dispersion compensator 42 compensate for known waveform degradation components (e.g., chromatic dispersion in the transmission path). However, the fixed equalizer 13a must process continuous input signals in real time. Therefore, the accuracy of the compensation process by the fixed equalizer 13a is not necessarily high. In contrast, the dispersion compensator 42 processes signals stored in the monitor signal storage unit 21. Here, the process of monitoring the polarization-dependent loss does not necessarily need to be performed in real time. That is, compared to the fixed equalizer 13a, the dispersion compensator 42 can have a longer processing time per symbol. Therefore, the dispersion compensator 42 can compensate for dispersion with higher accuracy than the fixed equalizer 13a. For example, the frequency resolution of the dispersion compensator 42 may be increased, or the number of taps of the digital filter may be increased. Therefore, a highly accurate monitor signal (i.e., electric field information of the received optical signal) can be obtained, allowing the location of the polarization-dependent loss to be identified with high accuracy.

[0125] Fig. 22 shows a fourth variation of an optical network device according to an embodiment of the present invention. Note that Fig. 22 only illustrates a feature extraction unit and functions connected to the feature extraction unit, and omits other circuits or functions.

[0126] In the fourth variation, the monitor signal rotator 24 and the reference signal rotator 25 are implemented in the feature extractor 27. The monitor signal is compensated for by the first dispersion compensator 16a, the nonlinear compensator 16b, and the second dispersion compensator 16c, and then processed by the monitor signal rotator 24. Note that the method of performing the rotation operation after the compensation operation and the method of performing the compensation operation after the rotation operation are mathematically equivalent to each other.

[0127] <Second embodiment> As described above, the optical network device according to the embodiment of the present invention creates a power profile on the optical transmission line using a received optical signal, and estimates the location where polarization dependent loss occurs by identifying the location where the optical power changes suddenly. However, in the embodiment described with reference to Figures 6 to 22 (hereinafter, sometimes referred to as the "first embodiment"), the occurrence of polarization dependent loss may not be detected accurately.

[0128] Figure 23 shows an example of the change in power of an optical signal output from a transmitting node. The power of the optical signal decreases as the transmission distance from the transmitting node increases. In the example shown in Figure 23(a), a 1 mW optical signal is transmitted from the transmitting node, and the optical power gradually decreases as the transmission distance increases.

[0129] When polarization dependent loss occurs on an optical transmission line, the optical power in the direction in which the loss occurs drops suddenly, as shown in Fig. 23(b). In the first embodiment, the optical network device 1 calculates the power of the optical signal at predetermined intervals and identifies the position where the optical power changes suddenly, thereby estimating the position where the polarization dependent loss occurs.

[0130] In contrast, when polarization dependent loss occurs at a position where the power of the optical signal is low, the amount of change in optical power ΔP due to the polarization dependent loss is also small, as shown in Figure 23(c). Here, the optical signal propagating through the optical transmission line contains noise. Therefore, when the amount of change in optical power ΔP due to the polarization dependent loss is small, the optical network device 1 may not be able to accurately detect the occurrence of polarization dependent loss.

[0131] On the other hand, in an optical transmission system that realizes long-distance transmission, one or more optical amplifiers are installed between a transmitting node and a receiving node. In the example shown in Figure 24, two optical amplifiers are installed between the transmitting node and the receiving node. The optical power has a peak at the location where the optical amplifier is installed, and the optical power gradually decreases as the transmission distance from the optical amplifier increases. In the following description, the section from the peak of the optical power generated by each optical amplifier to the peak of the optical power generated by the adjacent optical amplifier may be referred to as a "span."

[0132] Here, assume that polarization dependent loss occurs on the optical transmission line. In the example shown in Figure 24, the optical power of the X polarization signal at the receiving port of the optical amplifier is 0.1 mW, and the optical power of the Y polarization signal is 0.05 mW. In this case, the power difference between the polarizations is small, making it difficult to detect the polarization dependent loss.

[0133] However, when polarization-multiplexed optical signals are transmitted between nodes, optical amplifiers typically amplify the X-polarized and Y-polarized signals without separating the polarizations. As a result, for example, at the output port of the optical amplifier, the optical power of the X-polarized signal is amplified to 1 mW, and the optical power of the Y-polarized signal is amplified to 0.5 mW. In this case, the power difference between the polarizations is large enough that polarization-dependent loss can be easily detected.

[0134] Therefore, the optical network device of the second embodiment estimates the occurrence of polarization dependent loss based on the optical state at the point where the power of the optical signal has a peak in an optical transmission system in which one or more optical amplifiers are installed between a transmitting node and a receiving node. Note that, according to this method, the accuracy of detecting polarization dependent loss is on a span-by-span basis. In other words, it is possible to detect in which span among multiple spans the polarization dependent loss is occurring.

[0135] Fig. 25 shows an example of an optical network device according to the second embodiment of the present invention. The configuration of the optical network device 2 according to the second embodiment of the present invention is almost the same as that of the optical network device 1 shown in Fig. 8. That is, the optical network device 2 includes a coherent receiver 11, a digital signal processing unit 13, a monitor signal rotator 24, a reference signal rotator 25, a rotation amount controller 26, and a feature extractor 27. However, the optical network device 2 includes a span identifier 51 instead of the position identifier 28 shown in Fig. 8.

[0136] Similar to the position specifying unit 28, the span specifying unit 51 specifies the location of occurrence of polarization dependent loss based on the power profile generated by the feature extracting unit 27. Therefore, the span specifying unit 51 is an example or variation of the position specifying unit 28. However, the span specifying unit 51 specifies the location of occurrence of polarization dependent loss on a span-by-span basis. That is, the span specifying unit 51 sets multiple spans by dividing the optical transmission line based on the positions of optical amplifiers provided on the optical transmission line, and detects in which of the multiple spans polarization dependent loss is occurring.

[0137] 26 shows an example of the span identifying section 51. In this embodiment, the span identifying section 51 includes a power profile storage section 51a, a peak detection section 51b, a power difference calculation section 51c, and a determination section 51d.

[0138] The power profile storage unit 51a stores the power profile generated by the feature extraction unit 27. The peak detection unit 51b detects the position where the power peak of the optical signal appears on the optical transmission path based on the power profile. In the following description, this position may be referred to as the "peak position." The power difference calculation unit 51c calculates the difference between the power of the X-polarized signal and the power of the Y-polarized signal for each amount of polarization rotation at each peak position. In this case, the power difference calculation unit 51c may calculate the maximum value of the difference between the power of the X-polarized signal and the power of the Y-polarized signal at each peak position. When a power difference greater than a predetermined threshold is detected, the determination unit 51d estimates that polarization-dependent loss is occurring in the span including the peak position where the power difference was detected.

[0139] Fig. 27 shows an example of span and power profile. Note that the position on the optical transmission line (i.e., the transmission distance from the transmitting node) is expressed by the dispersion compensation amount, as explained with reference to Fig. 3 or Fig. 13. In this embodiment, the position on the optical transmission line is expressed using the dispersion compensation amount CD2 compensated by the second dispersion compensator 16c shown in Fig. 1. For example, if the chromatic dispersion of the optical fiber is 20 ps / nm / km, "CD2=20" corresponds to "1 km."

[0140] In the example shown in Figure 27, optical amplifiers are installed at the positions corresponding to "CD2=100" and "CD=200". The power of the optical signal decreases as the transmission distance from the transmitting node increases, and is amplified in the optical amplifier. The power of the optical signal also decreases as the transmission distance from the optical amplifier increases.

[0141] As described above, a "span" represents a section from one optical power peak to the adjacent peak. In this embodiment, therefore, the section between the transmitting node and optical amplifier A1 is span S1, the section between optical amplifier A1 and optical amplifier A2 is span S2, and the section between optical amplifier A2 and the receiving node is span S3.

[0142] FIG. 28 shows an example of a power profile in the optical transmission path shown in FIG. 27. In this example, the optical power of the X-polarized signal and the Y-polarized signal at each measurement position P0 to P15 is estimated. Each measurement position is actually represented by a dispersion compensation amount CD1 or CD2. As described with reference to FIGS. 1 to 4, the power of the optical signal is estimated from a correlation value that represents the correlation between the received signal (monitor signal) and the transmitted signal (reference signal) after compensation for chromatic dispersion and nonlinear distortion. Furthermore, a power profile is created for each of multiple polarization rotation amounts. In this example, power profiles are created for 0 degrees, 45 degrees, 90 degrees, and 135 degrees. The polarization rotation amount corresponds to the rotation amount calculated by monitor signal rotator 24 and reference signal rotator 25.

[0143] For example, when the amount of polarization rotation is zero, the estimated optical power values ​​of the X polarization signal at positions P0 to P15 are x00 to x0f, and the estimated optical power values ​​of the Y polarization signal at positions P0 to P15 are y00 to y0f. The same applies to the other amounts of polarization rotation.

[0144] Next, we will explain the operation of the span identification unit 51 when the power profile shown in Fig. 28 is created for the optical transmission path shown in Fig. 27. It is assumed that the power profile is created by the feature extraction unit 27 and stored in the power profile storage unit 51a.

[0145] The peak detector 51b sets a plurality of spans by dividing the optical transmission line based on the span information. In the example shown in Fig. 27, a transmitting node and a receiving node are implemented at positions P0 and P15, respectively. Furthermore, an optical amplifier A1 and an optical amplifier A2 are implemented at positions P5 and P10, respectively. Therefore, positions P0 to P5 correspond to span S1, positions P5 to P10 correspond to span S2, and positions P10 to P15 correspond to span S3. The optical network device 2 according to the embodiment of the present invention is implemented in the receiving node.

[0146] In this example, the span information indicates the locations where the optical amplifiers A1 and A2 are installed, and is provided to the optical network device 2 by, for example, a network administrator.

[0147] Next, the peak detector 51b detects the position where the optical power peak appears on the optical transmission line based on the power profile. At this time, the peak detector 51b may identify the position where the maximum optical power is obtained for each span. However, in this embodiment, the position on the optical transmission line is estimated using the dispersion compensation amount, and therefore includes an error. For this reason, it is preferable that the range used to search for the position where the maximum optical power is obtained for each span is shifted toward the transmitting node relative to the actual span. For example, in the example shown in FIG. 27, the search range corresponding to span S1 is positions P0 to P4, the search range corresponding to span S2 is positions P4 to P9, and the search range corresponding to span S3 is positions P9 to P14. In this case, the peak detector 51b identifies the position where the maximum optical power is obtained from positions P0 to P4 for span S1, the position where the maximum optical power is obtained from positions P4 to P9 for span S2, and the position where the maximum optical power is obtained from positions P9 to P14 for span S3.

[0148] Peak detector 51b may detect the peak position based on the optical power of the X-polarized signal, the optical power of the Y-polarized signal, or the sum of the optical power of the X-polarized signal and the Y-polarized signal. Peak detector 51b can also detect the peak position based on the optical power obtained for any amount of polarization rotation.

[0149] 29 and 30 show examples of changes in power profiles with respect to polarization rotation calculations. Note that FIGS. 29(a), 29(b), 30(a), and 30(b) respectively show power profiles when the polarization rotation amounts are 0°, 45°, 90°, and 135° for a polarization multiplexed optical signal transmitted through the optical transmission path shown in FIG. 27. The solid line represents the power profile of the X-polarized signal, and the dashed line represents the power profile of the Y-polarized signal. Note that in FIGS. 29(b) and 30(b), the two power profiles match each other over the entire section. Also, in FIGS. 29(b) and 30(b), the two power profiles match each other at positions P0 to P7.

[0150] In this embodiment, polarization dependent loss occurs between positions P6 and P7. Furthermore, loss occurs in the direction of X polarization. In this case, when the polarization rotation amount by monitor signal rotator 24 and reference signal rotator 25 is "0 degrees," loss occurs in the X polarization signal, as shown in FIG. 29(a). At this time, no loss occurs in the Y polarization signal. That is, a power difference occurs between the X polarization signal and the Y polarization signal. Furthermore, when the optical signal is amplified by the optical amplifier at position P10, the power difference also increases.

[0151] When the polarization rotation is 45 degrees, the loss for the X-polarized signal and the loss for the Y-polarized signal are the same, so no power difference occurs between the X-polarized signal and the Y-polarized signal, as shown in Figure 29(b).

[0152] When the polarization rotation is "90 degrees," loss occurs in the Y-polarized signal, but no loss occurs in the X-polarized signal. That is, as shown in Figure 30(a), a power difference occurs between the X-polarized signal and the Y-polarized signal. However, when the polarization rotation is "0 degrees," the optical power of the X-polarized signal is smaller than the optical power of the Y-polarized signal, but when the polarization rotation is "90 degrees," the optical power of the Y-polarized signal is smaller than the optical power of the X-polarized signal.

[0153] The power profile when the polarization rotation is 135 degrees is substantially the same as the power profile when the polarization rotation is 45 degrees. That is, as shown in Figure 30(b), there is no difference in power between the X polarization signal and the Y polarization signal.

[0154] For such a power profile, the peak detection unit 51b detects the peak position for each span. Specifically, as described with reference to Fig. 27, the peak detection unit 51b detects the peak positions in each of the sections P0 to P4, P4 to P9, and P9 to P14. As a result, peak positions P0, P5, and P10 are obtained for the spans S1, S2, and S3, respectively.

[0155] 31(a), the power difference calculation unit 51c calculates the power difference for each amount of polarization rotation at each peak position. For example, as the power difference at peak position P10, "x0a-y0a" is calculated for a polarization rotation amount of "0 degrees," "x1a-y1a" is calculated for a polarization rotation amount of "45 degrees," "x2a-y2a" is calculated for a polarization rotation amount of "90 degrees," and "x3a-y3a" is calculated for a polarization rotation amount of "135 degrees." The same applies to the power differences at peak positions P0 and P5.

[0156] The determination unit 51d compares each power difference obtained by the power difference calculation unit 51c with a predetermined threshold. If a power difference greater than the threshold is detected, the determination unit 51d determines that a polarization-dependent loss is occurring on the optical transmission line. In the example shown in FIGS. 29 and 30, at peak position P0, the difference between the optical power of the X-polarized signal and the optical power of the Y-polarized signal (i.e., the power difference) is almost zero for all amounts of polarization rotation. That is, no power difference greater than the threshold occurs at peak position P0. The same is true for peak position P5. In contrast, at peak position P10, when the amount of polarization rotation is "0 degrees," a large power difference "x0a-y0a" occurs, as shown in FIG. 29(a). Even when the amount of polarization rotation is "90 degrees," a large power difference "x2a-y2a" occurs, as shown in FIG. 30(a). This results in the determination result shown in FIG. 31(b).

[0157] Furthermore, the determination unit 51d identifies the location where the polarization dependent loss occurs. Specifically, the determination unit 51d determines that the polarization dependent loss occurs between the peak position where a power difference greater than the threshold value is detected and the peak position adjacent to that peak position on the transmitting node side. In this example, a power difference greater than the threshold value is detected at peak position P10. Furthermore, the peak position adjacent to peak position P10 on the transmitting node side is "P5." Therefore, in this case, it is determined that the polarization dependent loss occurs in the section between position P5 and position P10.

[0158] 32 is a flowchart showing an example of a method for identifying the location where polarization dependent loss occurs in the second embodiment. Note that it is assumed that the feature extraction unit 27 has created a power profile for each amount of polarization rotation before the processing of this flowchart starts.

[0159] In S31, the peak detection unit 51b sets multiple spans on the optical transmission path between the transmitting node and the optical network device 2 based on the span information. In S32, a power profile corresponding to the polarization rotation amount nΔθ is acquired. The variable n specifies the polarization rotation amount by the monitor signal rotation unit 24 and the reference signal rotation unit 25. It is also assumed that the variable n is initialized to zero at the start of this flowchart. Δθ represents the step of the polarization rotation amount. In the examples shown in FIGS. 29 and 30, n is 0, 1, 2, or 3, and Δθ is 45 degrees. In S33, the peak detection unit 51b detects the peak position for each span. At this time, the peak detection unit 51b detects, for example, the measurement position at which the maximum optical power is detected within the search range set for each span.

[0160] In S34, power difference calculation unit 51c calculates the power difference at each peak position, that is, the difference between the optical power of the X polarization signal and the optical power of the Y polarization signal at each peak position.

[0161] In S35 to S36, the span determiner 51 increments the variable n. If the variable n is smaller than the threshold N, the process of the span determiner 51 returns to S32. The threshold N specifies the range of polarization rotation by the monitor signal rotator 24 and the reference signal rotator 25. In the example shown in FIGS. 29 and 30, N is 4. If the variable n reaches the threshold N, the process of the span determiner 51 proceeds to S41. That is, the span determiner 51 repeatedly executes S31 to S36 to calculate the power difference at each peak position for each amount of polarization rotation.

[0162] In S41, the determination unit 51d obtains the power difference corresponding to the k-th span. Specifically, the power difference corresponding to each polarization rotation amount detected at the peak position corresponding to the k-th span is obtained. The variable k specifies the span on the optical transmission line. It is also assumed that the variable k is initialized to 1 at the start of this flowchart. In other words, at the start of this flowchart, a span including the transmitting node is selected.

[0163] In S42, the determination unit 51d compares each power difference acquired in S41 with a predetermined PDL threshold. For example, in the case shown in FIG. 31(a), for span S1, four power differences "x00-y00," "x10-y10," "x20-y20," and "x30-y30" are compared with the PDL threshold. If no power difference greater than the PDL threshold is found in this comparison, the determination unit 51d increments the variable k in S43 and S44. If there are any unprocessed spans remaining, the process of the determination unit 51d returns to S41. That is, the determination unit 51d selects the next span. In this way, the determination unit 51d selects spans one by one in order from the transmitting node toward the receiving node and performs the corresponding process until a power difference greater than the PDL threshold is found. If no power difference greater than the PDL threshold is found in any span, the determination unit 51d determines in S45 that polarization dependent loss is not occurring.

[0164] When a power difference greater than the PDL threshold is found in the processing for the k-th span, the determination unit 51d determines in S46 that a polarization dependent loss has occurred. Furthermore, the determination unit 51d identifies the location where the polarization dependent loss has occurred. Specifically, information indicating that a polarization dependent loss has occurred in the section between the peak position corresponding to the k-1-th span and the peak position corresponding to the k-th span is output.

[0165] In this way, in the second embodiment, the presence or absence of polarization dependent loss is determined based on the power difference between the polarizations detected in the section where the optical power is large. Therefore, even if noise is added to the optical signal, the presence or absence of polarization dependent loss can be determined with high accuracy.

[0166] 25 to 32, the polarization dependent loss is detected using the difference between the optical power of the X polarization signal and the optical power of the Y polarization signal, but the second embodiment is not limited to this method. That is, the optical network device 2 according to the second embodiment may identify the location of the polarization dependent loss by using either the optical power of the X polarization signal or the optical power of the Y polarization signal. Alternatively, the optical network device 2 according to the second embodiment may identify the location of the polarization dependent loss based on the polarization component in the θ axis direction or the polarization component in the orthogonal axis direction shown in FIG. 11(b).

[0167] <Raman amplification> The optical amplifier described above is, for example, a rare-earth doped fiber amplifier such as an erbium-doped fiber amplifier (EDFA). However, the optical amplifier provided on the optical transmission line is not particularly limited, and may be, for example, a distributed Raman amplifier.

[0168] FIG. 33 is a diagram explaining distributed Raman amplification. As shown in FIG. 33(a), a Raman amplifier supplies pump light to a transmission optical fiber that propagates an optical signal. The optical signal is then amplified by the energy of the pump light. In many cases, the pump light is supplied to the optical transmission line so that it propagates in the opposite direction to the optical signal. In this case, the optical signal is amplified in a region close to the Raman amplifier, as shown in FIG. 33(b). That is, a power profile is obtained in which the optical power gradually decreases as the distance from the transmitting node increases and the optical power increases within the range reached by the pump light. In the following description, the region in which the optical signal is amplified by the pump light may be referred to as the "amplification section." Furthermore, other regions of the optical transmission line may be referred to as the "transmission section."

[0169] In an optical transmission line equipped with a Raman amplifier, if polarization dependent loss occurs, a power profile with a "step" in the power of the optical signal is obtained. For example, if polarization dependent loss occurs in the transmission section, the power profile shown in Figure 33(c) is obtained, and if polarization dependent loss occurs in the amplification section, the power profile shown in Figure 33(d) is obtained. In either case, a power difference occurs, making it possible to identify the location where polarization dependent loss occurs.

[0170] In distributed Raman amplification, PDG, in which the gain varies depending on the polarization state, can occur. For example, if PDG occurs due to distributed Raman amplification in the absence of polarization-dependent loss, the difference in optical power between the X-polarized signal and the Y-polarized signal gradually increases in the amplification section, as shown in Figure 34. Therefore, by controlling the amount of polarization rotation by monitor signal rotator 24 and reference signal rotator 25, the power profile shown by the solid line and the power profile shown by the dashed line in Figure 34 can be obtained.

[0171] Here, the optical network device 1 according to the first embodiment detects a "step" in the power profile. Therefore, the optical network device 1 can detect polarization dependent loss, but it is difficult to detect PDG caused by distributed Raman amplification. In contrast, the optical network device 2 according to the second embodiment detects the optical power of each polarization at the peak position where the optical power peak appears. Therefore, the optical network device 2 can detect a power difference when a power difference occurs due to PDG. In other words, the optical network device 2 can detect a power difference when a power difference occurs due to polarization dependent loss or PDG.

[0172] On the other hand, the location where the Raman amplifier is installed is known. In addition, the power profile obtained in the optical transmission line where the Raman amplifier is installed has a characteristic shape, as shown in Figure 33(b). Therefore, when a power difference is detected at the peak position, by acquiring the power profile, it is possible to determine whether the cause of the power difference is polarization-dependent loss or PDG.

[0173] 35 shows an example of a span identifier used in an optical transmission system equipped with a Raman amplifier. In this case, the span identifier 51R includes a profile selector 51e in addition to the configuration shown in FIG.

[0174] The power profile storage unit 51a, peak detection unit 51b, power difference calculation unit 51c, and determination unit 51d are substantially the same in the span determination unit 51 shown in Fig. 26 and the span determination unit 51R shown in Fig. 35. However, when the determination unit 51d implemented in the span determination unit 51R detects a power difference greater than a predetermined threshold, it outputs information indicating the amount of polarization rotation that caused the power difference. For example, in a case where a power difference greater than the threshold is detected when the amount of polarization rotation by the monitor signal rotation unit 24 and the reference signal rotation unit 25 is "45 degrees," the determination unit 51d outputs "amount of polarization rotation=45 degrees."

[0175] The profile selection unit 51e selects a corresponding power profile from the power profile storage unit 51a based on the information indicating the amount of polarization rotation output from the determination unit 51d. When the determination unit 51d outputs "amount of polarization rotation=45 degrees," the profile selection unit 51e selects and outputs a power profile corresponding to "amount of polarization rotation=45 degrees" from the power profile storage unit 51a. This allows the network administrator to determine whether the power difference is caused by polarization dependent loss or PDG based on the shape of the power profile selected by the profile selection unit 51e.

[0176] <Polarization rotation> As described above, the optical network devices 1 and 2 according to the embodiments of the present invention perform a calculation to rotate the polarization of the monitor signal and the reference signal. However, the calculation to control the polarization of the signal is not limited to a rotation calculation. That is, the optical network devices 1 and 2 may perform a calculation to control the phase difference between two polarized waves in addition to the rotation calculation. Specifically, the calculation of equation (6) may be performed on the monitor signal instead of equation (4). Note that Ex and Ey represent the polarization components of the monitor signal. θ represents the amount of polarization rotation. δ represents the phase difference between the two polarized waves.

number

[0177] In this case, the reference signal is calculated using equation (7) instead of equation (5). Esx and Esy represent the polarization components of the reference signal.

number

[0178] The optical network devices 1 and 2 then create a power profile while controlling the polarization rotation θ and the phase difference δ. At this time, the monitor signal rotator 24 controls the polarization rotation θ and the phase difference δ of the monitor signal. Thus, the monitor signal rotator 24 is an example of a polarization control unit that generates third and fourth polarization components that are orthogonal to each other by controlling the first and second polarization components that represent the monitor signal. Furthermore, the reference signal rotator 25 controls the polarization rotation θ and the phase difference δ of the reference signal. Thus, the reference signal rotator 25 is an example of a second polarization control unit that controls the polarization of the reference signal. [Explanation of symbols]

[0179] 1, 1B~1D, 2 Optical network equipment 11 Coherent receiver 13 Digital signal processing section 13a fixed equalizer 13b Adaptive Equalizer 16a First dispersion compensation unit 16b Nonlinear compensation section 16c Second dispersion compensation section 16d Correlation calculation unit 21 Monitor signal storage unit 22 Transmission data storage unit 23 Transmit waveform reconstruction unit 24 Monitor signal rotation section 25 Reference signal rotation unit 26 Rotation amount control section 27 Feature Extraction Unit 28 Location identification part 28a Power Profile Storage 28b Difference calculation part 28c Change point search section 30 Calculation section 41 Polarization rotation correction unit 42 Dispersion compensation section 51, 51R Span specific part 51a Power profile storage section 51b Peak detector 51c Power difference calculation section 51d Judgment section 51e Profile selection section 100 sending nodes

Claims

1. An optical network device that receives a polarization multiplexed optical signal transmitted from a transmitting node, a polarization splitter that splits an electric field information signal representing an electric field of the polarization multiplexed optical signal into a first polarization component and a second polarization component that are orthogonal to each other; a polarization control unit that generates a third polarization component and a fourth polarization component that are orthogonal to each other by controlling the first polarization component and the second polarization component in a coordinate system that represents a first polarization direction and a second polarization direction that are orthogonal to each other; a feature extraction unit that calculates an evaluation value corresponding to at least one of the power of the third polarization component and the power of the fourth polarization component for each of a plurality of positions on an optical transmission path between the transmitting node and the optical network device; a fluctuation amount calculation unit that calculates a fluctuation amount of the evaluation value with respect to a control amount by the polarization control unit for each of the plurality of positions; a determination unit that determines whether a first position is an extraction target based on a comparison result between an amount of variation in an evaluation value at a first position among the plurality of positions and an amount of variation in an evaluation value at a second position among the plurality of positions that is adjacent to the first position; An optical network device comprising:

2. the polarization control unit generates the third polarization component and the fourth polarization component by performing a rotation operation on the first polarization component and the second polarization component in the coordinate system; The fluctuation amount calculation unit calculates the fluctuation amount of the evaluation value with respect to the rotation amount of the rotation calculation performed by the polarization control unit.

2. The optical network device according to claim 1.

3. the second location is adjacent to the first location on the transmitting node side, The determination unit determines that the first position is an extraction target when a variation in the evaluation value at the first position is greater than a variation in the evaluation value at the second position.

2. The optical network device according to claim 1.

4. The amount of change in the evaluation value is the difference between the maximum and minimum values ​​of the evaluation value obtained when the polarization control unit rotates the first polarization component and the second polarization component in the coordinate system.

2. The optical network device according to claim 1.

5. The determination unit outputs information representing the first position when a difference between an amount of change in the evaluation value at the first position and an amount of change in the evaluation value at the second position is greater than a predetermined threshold.

2. The optical network device according to claim 1.

6. The feature extraction unit a first dispersion compensating unit that compensates for a first chromatic dispersion of the chromatic dispersion of the optical transmission line with respect to the electric field information signal; a nonlinear compensation unit that compensates for nonlinear distortion of the optical transmission line with respect to an output signal of the first dispersion compensation unit; a second dispersion compensator that compensates for the remaining chromatic dispersion of the chromatic dispersion of the optical transmission line with respect to the output signal of the nonlinear compensator; a calculation unit that calculates an evaluation value corresponding to a combination of the first chromatic dispersion and the remaining chromatic dispersion based on an output signal of the second dispersion compensator, The feature extraction unit calculates the evaluation value at each of a plurality of positions on the optical transmission line by changing the amount of the first chromatic dispersion.

2. The optical network device according to claim 1.

7. The evaluation value represents a correlation between a reference signal representing an electric field of the polarization multiplexed optical signal at the transmitting node and an output signal of the second dispersion compensator.

7. The optical network device according to claim 6.

8. a second polarization control unit that controls polarization of the reference signal in the coordinate system; the control amount by the polarization control unit and the control amount by the second polarization control unit are the same; The evaluation value represents a correlation between a reference signal that has been polarization-controlled by the second polarization control unit and an output signal of the second dispersion compensation unit.

8. The optical network device according to claim 7.

9. the polarization control unit includes a plurality of rotation processing units that rotate the first polarization component and the second polarization component in the coordinate system by different rotation amounts; The feature extraction unit includes a plurality of evaluation value calculation units that calculate corresponding evaluation values ​​from the output signals of the plurality of rotation processing units.

2. The optical network device according to claim 1.

10. a coherent receiver that generates an electric field information signal representing the electric field of the polarization multiplexed optical signal; a fixed equalizer that corrects the electric field information signal generated by the coherent receiver to compensate for chromatic dispersion of the optical transmission line; a regenerator configured to regenerate transmission data based on the first polarization component and the second polarization component generated by the polarization separator when the polarization separator generates the first polarization component and the second polarization component from the output signal of the fixed equalizer.

2. The optical network device according to claim 1.

11. a coherent receiver that generates an electric field information signal representing the electric field of the polarization multiplexed optical signal; a fixed equalizer that corrects the electric field information signal generated by the coherent receiver to compensate for chromatic dispersion of the optical transmission line; an adaptive equalizer that corrects the output signal of the fixed equalizer to compensate for residual dispersion; a reproduction unit that reproduces transmission data based on the output signal of the adaptive equalizer, The polarization splitter generates the first polarization component and the second polarization component from the output signal of the fixed equalizer.

2. The optical network device according to claim 1.

12. a coherent receiver that generates an electric field information signal representing the electric field of the polarization multiplexed optical signal; a fixed equalizer that corrects the electric field information signal generated by the coherent receiver to compensate for chromatic dispersion of the optical transmission line; an adaptive equalizer that corrects the output signal of the fixed equalizer to compensate for residual dispersion; a reproduction unit that reproduces transmission data based on the output signal of the adaptive equalizer; a dispersion compensator that corrects the electric field information signal generated by the coherent receiver to compensate for chromatic dispersion of the optical transmission line, The polarization splitter generates the first polarization component and the second polarization component from the output signal of the coherent receiver.

2. The optical network device according to claim 1.

13. a peak detection unit that detects a position at which a power peak of at least one of the third polarization component and the fourth polarization component appears, based on the evaluation value calculated by the feature extraction unit for each of a plurality of positions on the optical transmission line; The determination unit determines whether a section between a first peak position and a second peak position is an extraction target section based on a comparison result between an amount of change in an evaluation value at a first peak position among a plurality of peak positions detected by the peak detection unit and an amount of change in an evaluation value at a second peak position adjacent to the first peak position among the plurality of peak positions.

2. The optical network device according to claim 1.

14. When the amount of change in the evaluation value at the first peak position is greater than a predetermined threshold, a first control amount by the polarization control unit is specified, which makes the difference between the power of the third polarization component and the power of the fourth polarization component greater than a predetermined value, and an evaluation value corresponding to the first control amount at each of the plurality of positions is output.

14. The optical network device according to claim 13.

15. 1. A transmission path monitoring method for monitoring an optical transmission path between a transmitting node and an optical network device receiving a polarization multiplexed optical signal transmitted from the transmitting node, the method comprising: separating an electric field information signal representing an electric field of the polarization multiplexed optical signal into a first polarization component and a second polarization component which are orthogonal to each other; generating third and fourth polarization components that are orthogonal to each other by rotating the first and second polarization components in a coordinate system that represents first and second polarization directions that are orthogonal to each other; calculating an evaluation value corresponding to at least one of the power of the third polarization component and the power of the fourth polarization component for each of a plurality of positions on the optical transmission line; calculating a variation of the evaluation value with respect to a rotation amount for each of the plurality of positions; Whether the first position is to be output is determined based on a comparison result between an amount of change in the evaluation value at a first position among the plurality of positions and an amount of change in the evaluation value at a second position adjacent to the first position among the plurality of positions. A transmission line monitoring method comprising:

16. 1. A transmission path monitoring method for monitoring an optical transmission path between a transmitting node and an optical network device receiving a polarization multiplexed optical signal transmitted from the transmitting node, the method comprising: an electric field information signal representing the electric field of the polarization multiplexed optical signal is separated into a first polarization component and a second polarization component which are orthogonal to each other; generating a third polarization component and a fourth polarization component that are orthogonal to each other by performing a rotation operation on the first polarization component and the second polarization component in a coordinate system that represents a first polarization direction and a second polarization direction that are orthogonal to each other; determining whether or not a difference between the power of the third polarization component and the power of the fourth polarization component exceeds a predetermined threshold when the rotation amount of the rotation calculation is changed within a predetermined range for each of a plurality of positions on the optical transmission line; When the difference at a first position among the plurality of positions exceeds the threshold, it is determined whether the first position is to be output based on a comparison between the difference at the first position and the difference at a second position among the plurality of positions that is adjacent to the first position. A transmission line monitoring method comprising:

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