Transmission line monitoring device and transmission line monitoring method

Abstract

To provide a transmission line monitoring device and a transmission line monitoring method that estimate the characteristics of an optical transmission line using a relatively small number of symbols.SOLUTION: A transmission line monitoring device monitors the characteristics of an optical transmission line. The transmission line monitoring device includes a generation unit that generates predetermined information including optical power of an optical signal at multiple positions on the optical transmission line based on an electric field signal that indicates an optical electric field component of the optical signal received from the optical transmission line and a specific signal that indicates an electric field obtained by data demodulated based on the electric field signal, and an update unit that updates the predetermined information based on a first error between the electric field signal after linear distortion of the optical signal and nonlinear distortion of the optical signal are compensated for based on the predetermined information, and the specific signal, and a second error between a derivative of the optical power at the multiple positions and a reference value including a transmission loss of the optical transmission line.SELECTED DRAWING: Figure 6

Description

[Technical field]

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

[0002] A digital coherent optical transmission system including an optical transmitter and an optical receiver is known. The optical receiver receives an optical signal from an optical transmission line by coherent detection. The optical receiver converts the received optical signal into an electric field signal for each polarization component, and compensates for degradation of each electric field signal, for example, due to chromatic dispersion of the optical transmission line and nonlinear optical effect of the optical transmission line. An optical fiber, for example, tens to thousands of kilometers, extends between the optical transmitter and the optical receiver as the optical transmission line. One or more optical amplifiers may be provided on the optical transmission line. When maintaining and managing such an optical transmission line, it is difficult for an operator to check the state of the entire optical transmission line. For this reason, various techniques for monitoring the state of the optical transmission line have been proposed (for example, see Patent Documents 1 to 4).

[0003] For example, DLM (Digital Longitudinal Monitoring) has been proposed, which is based on a digital coherent optical transmission system. DLM is a technology that detects the characteristics of various devices (e.g., optical fibers, optical amplifiers, etc.) in an optical transmission system by digital signal processing on the receiving side of the optical transmission system. DLM monitors the optical power and other characteristics of the optical transmission line by performing digital signal processing on the electric field signal obtained by coherently detecting the optical signal propagating through the optical transmission line (see, for example, Patent Document 5). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2018-133725 A [Patent Document 2] US Patent Application Publication No. 2018 / 0234184 [Patent Document 3] JP 2021-048542 A [Patent Document 4] U.S. Pat. No. 1,140,5104 [Patent Document 5] International Publication No. 2023 / 139749 Summary of the Invention [Problem to be solved by the invention]

[0005] Incidentally, when an optical amplifier is provided on an optical transmission line, the optical transmission line is divided into a plurality of transmission sections by the optical amplifier. In this case, it may be said that the optical transmission line includes a plurality of transmission sections. When the characteristics of an optical transmission line including such a plurality of transmission sections are estimated by digital signal processing on the receiving side of an optical transmission system, the characteristics of the optical transmission line may not be estimated accurately. Specifically, the amount of nonlinear phase rotation proportional to the optical power at each position in the optical transmission line may not be estimated accurately.

[0006] For example, when estimating the characteristics of an optical transmission line, if the number of symbols (signal points) contained in the electric field signal is small, the amount of nonlinear phase rotation may vary due to noise caused by amplified spontaneous emission (ASE) generated in the optical transmission line. If the amount of nonlinear phase rotation varies, the characteristics of the optical transmission line cannot be estimated accurately, making it difficult to calculate the chromatic dispersion coefficient used to estimate the fiber type of an optical fiber, for example.

[0007] In order to suppress the variation in the amount of nonlinear phase rotation, a method of smoothing the amount of nonlinear phase rotation by, for example, moving average processing is considered. However, this method may not be able to suppress the variation in the amount of nonlinear phase rotation completely, and the characteristics of the optical transmission line may not be generated with high accuracy.

[0008] For example, it is conceivable that the variation in the amount of nonlinear phase rotation can be suppressed by acquiring a large number of symbols, such as millions or tens of millions, and averaging the acquired symbols. However, in this case, the symbol acquisition time and the calculation load may increase.

[0009] Therefore, in one aspect, an object of the present invention is to provide a transmission path monitoring device and a transmission path monitoring method that estimate characteristics of an optical transmission path with a relatively small number of symbols. [Means for solving the problem]

[0010] In one embodiment, the transmission line monitoring device is a transmission line monitoring device that monitors characteristics of an optical transmission line, and includes a generation unit that generates specified information including the optical power of the optical signal at multiple positions on the optical transmission line based on an electric field signal that indicates an optical electric field component of the optical signal received from the optical transmission line and a specific signal that indicates an electric field obtained by data demodulated based on the electric field signal, and an update unit that updates the specified information based on a first error between the electric field signal and the specific signal after linear distortion of the optical signal and nonlinear distortion of the optical signal are compensated for based on the specified information, and a second error between the derivative of the optical power at the multiple positions and a reference value including a transmission loss of the optical transmission line. Effect of the Invention

[0011] The characteristics of the optical transmission line can be estimated with a relatively small number of symbols. [Brief description of the drawings]

[0012] [Figure 1] 1 is an example of an optical transmission system. [Diagram 2] 1 is an example of an optical receiving device. [Diagram 3] This is an example of a DSP (Digital Signal Processor). [Figure 4] 1 is an example of an optical transmission line monitoring device. [Diagram 5] (a) is an example of an electric field signal, and (b) is an example of a reference signal. [Figure 6] 2 is an example of a profile generating unit. [Figure 7] 1 is an example of a distribution of a nonlinear phase rotation amount. [Figure 8] 1A is another example of the electric field signal, and FIG. 1B is an example of the reference signal. [Figure 9] 1 is an example of a differential quantity distribution. [Figure 10] 1 is an example of a data storage unit. [Figure 11] 13 is a diagram illustrating an example of distribution of nonlinear phase rotation amount before and after updating. [Figure 12] 13 is an example of a differential amount distribution before and after updating. [Figure 13] 2 is an example of a power profile. [Figure 14] 4 is a flowchart showing an example of an operation of the optical transmission line monitoring device. [Figure 15] 11A and 11B are diagrams illustrating an example of a process executed by a span detection unit; [Figure 16] 11A and 11B are diagrams illustrating another example of the process executed by the span detector; [Figure 17] 13 is a flowchart illustrating an example of a generation process. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0014] As shown in Fig. 1, the optical transmission system ST includes an optical transmitting device 100 and an optical receiving device 200. The optical transmitting device 100 and the optical receiving device 200 are connected by an optical transmission line 50. When transmission data is input, the optical transmitting device 100 transmits an optical signal obtained by modulating the transmission data to the optical transmission line 50. The optical signal propagates through the optical transmission line 50. The optical receiving device 200 receives the optical signal transmitted from the optical transmitting device 100 via the optical transmission line 50, demodulates it, and outputs the demodulated data.

[0015] A plurality of optical amplifiers 51A, 52A, 53A, 54A, and 55A are provided on the optical transmission line 50. Therefore, the optical transmission line 50 is divided into a plurality of transmission sections (hereinafter referred to as spans) SP#1, SP#2, SP#3, SP#4, and SP#5 by the optical amplifiers 51A, 52A, 53A, 54A, and 55A. In other words, the optical transmission line 50 is a multi-span optical transmission line including a plurality of spans SP#1, SP#2, SP#3, SP#4, and SP#5 (hereinafter referred to as SP#1,...,SP#5 as appropriate).

[0016] Various types of optical fibers 51F, 52F, 53F, 54F, and 55F are laid in each of the spans SP#1, ..., SP#5. For example, SSMF (Standard Single Mode Fiber) is laid as the optical fiber in the spans SP#1, SP#2, SP#4, and SP#5. NZ-DSF (Non-Zero Dispersion Shifted Fiber) is laid as the optical fiber in the span SP#3.

[0017] Although the lengths of the optical fibers 51F, 52F, 53F, 54F, and 55F are not particularly limited, in this embodiment, the lengths of the optical fibers 51F, 52F, 53F, 54F, and 55F are all described as 100 km (kilometers) as an example. That is, the path length of each of the multiple spans SP#1, . . . , SP#5 is 100 km, and the total path length obtained by adding up the individual path lengths is 500 km.

[0018] An optical transmission line monitoring device 300 separate from the optical receiving device 200 is connected to the optical receiving device 200. The optical transmission line monitoring device 300 may be integrally included in the optical receiving device 200. The optical transmission line monitoring device 300 is an example of a transmission line monitoring device. The optical transmission line monitoring device 300 monitors the characteristics of the optical transmission line 50. Although details will be described later, the optical transmission line monitoring device 300 obtains from the optical receiving device 200 an electric field signal indicating an optical electric field component of the optical signal received by the optical receiving device 200 and demodulated data demodulated based on this electric field signal.

[0019] When the optical transmission line monitoring device 300 acquires the electric field signal and the demodulated data, it generates a power profile including the optical power of the optical signal at multiple positions on the optical transmission line 50, based on the electric field signal and the demodulated data. The power profile is an example of predetermined information, and can represent the characteristics of the optical transmission line 50. If the optical transmission line monitoring device 300 can generate the power profile with high accuracy, it can accurately calculate, based on the power profile, the chromatic dispersion coefficient used in estimating the fiber type of the optical fiber.

[0020] Next, the optical receiving device 200 will be described in detail with reference to FIG. 2 and FIG.

[0021] The optical receiving device 200 includes an ICR (Integrated Coherent Receivers) 210 and an ITLA (Integrable Tunable Laser Assembly) 220. Although not shown, the ICR 210 includes a 90-degree optical hybrid circuit and a BPD (Balanced Photo Diode). The ICR 210 is an integrated circuit that houses the 90-degree optical hybrid circuit and the BPD in one package. The optical receiving device 200 also includes an ADC (Analogue Digital Converter) 230 and a DSP 240.

[0022] An optical signal is input to the ICR 210 via the optical amplifier 55A. The ITLA 220 is a local light source that outputs local light (i.e., laser light). The ICR 210 receives an optical signal by local light, converts the received optical signal into an electric field signal (specifically, an electric field information signal) corresponding to the optical signal, and outputs the electric field signal to the ADC 230. The ADC 230 converts the electric field signal from analog to digital format, and outputs the signal to the DSP 240 and the optical transmission line monitoring device 300.

[0023] The DSP 240 receives the electric field signal output from the ADC 230 and performs various digital signal processing on the received electric field signal. As shown in Fig. 3, the DSP 240 includes a fixed equalization unit 241, an adaptive equalization unit 242, a frequency correction unit 243, a phase estimation unit 244, a discrimination unit 245, and an error correction unit 246.

[0024] The fixed equalization unit 241 compensates for chromatic dispersion suffered by the optical signal propagating through the optical transmission path 50, for the electric field signal received by the DSP 240. The fixed equalization unit 241 outputs the electric field signal after compensating for chromatic dispersion to the adaptive equalization unit 242. The adaptive equalization unit 242 adaptively compensates for residual dispersion in the electric field signal output from the fixed equalization unit 241. Residual dispersion is chromatic dispersion that remains after not being completely compensated for by the fixed equalization unit 241. The adaptive equalization unit 242 outputs the electric field signal after compensating for residual dispersion to the frequency correction unit 243.

[0025] The frequency correction unit 243 compensates for the frequency offset of the electric field signal output from the adaptive equalization unit 242. The frequency offset is the difference (or deviation) between the optical frequency of a transmission light source (not shown) included in the optical transmission device 100 and the optical frequency of the ITLA 220. The frequency correction unit 243 outputs the electric field signal after compensating for the frequency offset to the phase estimation unit 244. The phase estimation unit 244 compensates for the phase offset of the electric field signal output from the frequency correction unit 243 and estimates the phase of the optical signal. The phase offset is the difference (or deviation) between the phase of the transmission light source and the ITLA 220. By compensating for the phase offset, the phase estimation unit 244 reproduces each symbol on the constellation. The phase estimation unit 244 outputs a constellation map in which the symbols are reproduced to the identification unit 245.

[0026] The discrimination unit 245 demodulates the transmission data by discriminating the value of each symbol based on the constellation map output from the phase estimation unit 244, and outputs the demodulated data to the error correction unit 246. The error correction unit 246 corrects bit errors in the demodulated data, and outputs the demodulated data after the errors are corrected.

[0027] Next, the optical transmission line monitoring device 300 will be described in detail with reference to FIGS.

[0028] The optical transmission line monitoring device 300 includes a memory 310 and a processor 320. The memory 310 temporarily stores the electric field signal output from the optical receiving device 200 (specifically, the ADC 230). As a result, as shown in Fig. 5(a), the memory 310 stores the electric field signal after it has been output from the ADC 230 and before it has been input to the DSP 240. The electric field signal includes an I (In-phase) component and a Q (Quadrature) component of the optical signal received by the optical receiving device 200. In the electric field signal, a relatively small number of symbols 10 (for example, tens of thousands) corresponding to combinations of the I and Q components appear at various positions on a constellation map.

[0029] The processor 320 includes a transmission electric field restoration unit 321, a profile generation unit 322, a span detection unit 323, and a dispersion coefficient calculation unit 324. The transmission electric field restoration unit 321, the profile generation unit 322, the span detection unit 323, and the dispersion coefficient calculation unit 324 are realized by the processor 320 executing a program according to a flowchart described later.

[0030] The transmission electric field restoration unit 321 generates, as a reference signal, an electric field signal similar to the electric field signal generated by the optical transmitting device 100 based on the transmission data, based on the demodulated data output from the optical receiving device 200. That is, the reference signal is substantially the same as the electric field signal for generating an optical signal in the optical transmitting device 100. For example, as shown in FIG. 5(b), the transmission electric field restoration unit 321 generates a reference signal by mapping a symbol 20 corresponding to the demodulated data on a constellation map. After generating the reference signal, the transmission electric field restoration unit 321 outputs the reference signal to the profile generation unit 322. Note that the reference signal is an example of a specific signal.

[0031] The profile generating unit 322 generates a power profile based on the electric field signal and the reference signal. After generating the power profile, the profile generating unit 322 outputs the power profile to the span detecting unit 323. The profile generating unit 322 will be described in detail later.

[0032] The span detection unit 323 detects multiple spans SP#1,...,SP#5 included in the optical transmission path 50 based on the power profile output from the profile generation unit 322. For example, the span detection unit 323 detects the portions between the peaks as spans SP#1,...,SP#5 based on each of the multiple peaks appearing in the power profile and the peaks adjacent to each peak. When the span detection unit 323 detects the multiple spans SP#1,...,SP#5, it outputs the detected multiple spans SP#1,...,SP#5 to the dispersion coefficient calculation unit 324.

[0033] The dispersion coefficient calculation unit 324 calculates the amount of chromatic dispersion of each of the multiple spans SP#1,...,SP#5 output from the span detection unit 323. The amount of chromatic dispersion corresponds to the difference between the accumulated chromatic dispersion amount at which a peak appears and the accumulated chromatic dispersion amount adjacent to that accumulated chromatic dispersion amount in the power profile. Then, the dispersion coefficient calculation unit 324 calculates the chromatic dispersion coefficient of each of the multiple spans SP#1,...,SP#5 based on the amount of chromatic dispersion of each of the multiple spans SP#1,...,SP#5.

[0034] For example, the dispersion coefficient calculation unit 324 calculates the chromatic dispersion coefficient by dividing the amount of dispersion estimated based on the power profile by the span length. The span lengths of the multiple spans SP#1,...,SP#5 are prepared in advance as span length data. After calculating the chromatic dispersion coefficient, the dispersion coefficient calculation unit 324 outputs the chromatic dispersion coefficient.

[0035] Next, the above-mentioned profile generating unit 322 will be described in detail with reference to FIGS.

[0036] First, as shown in Fig. 6, the profile generating unit 322 includes a distribution estimating unit 350, a first error calculating unit 360, a second error calculating unit 370, and a total error calculating unit 380. The distribution estimating unit 350 is an example of a generating unit and an updating unit, and estimates a nonlinear phase rotation amount distribution based on an electric field signal and a reference signal. More specifically, the distribution estimating unit 350 estimates a nonlinear phase rotation amount distribution 31 as shown in Fig. 7 based on an electric field signal, a reference signal, and an estimation algorithm for estimating a nonlinear phase rotation amount distribution from the electric field signal and the reference signal. Note that the estimation algorithm can refer to, for example, the following documents 1 to 3.

[0037] Reference 1: Takeo Sasai et al., “Proposal of Linear Least Squares for Fiber-Nonlinearity-Based Longitudinal Power Monitoring in Multi-Span Link”, 2022 27th OptoElectronics and Communications Conference (OECC) and 2022 International Conference on Photonics in Switching and Computing (PSC), Toyama, Japan, August. 2022. Reference 2: Takeo Sasai et al., “0.77-dB Anomaly Loss Localization Based on DSP-Based Fiber-Longitudinal Power Estimation Using Linear Least Squares”, 2023 Optical Fiber Communications Conference and Exhibition (OFC), San Diego, CA, USA, May. 2023. Reference 3: Inwoong Kim et al., “Robust Longitudinal Power Profile Estimation in Optical Networks using MMSE with Complex Scaling Factor”, Optical Fiber Communication Conference 2023, San Diego, CA, USA, March. 2023.

[0038] As shown in Fig. 7, the horizontal axis of the nonlinear phase rotation distribution 31 represents the accumulated chromatic dispersion CD (ps / nm) from the optical transmitting device 100 (represented as Tx in Fig. 7) to the optical receiving device 200 (represented as Rx in Fig. 7). The accumulated chromatic dispersion CD is the total amount of dispersion value of chromatic dispersion suffered by the optical signal propagating through the optical transmission line 50. As the optical signal propagates through the optical transmission line 50, the accumulated chromatic dispersion CD increases, and therefore the accumulated chromatic dispersion CD corresponds to the distance of the optical transmission line 50.

[0039] On the other hand, the vertical axis of the nonlinear phase rotation amount distribution 31 represents the nonlinear phase rotation amount φ occurring in the optical transmission line 50. The nonlinear phase rotation amount φ is proportional to the optical power of the optical signal. As shown in FIG. 7, in the nonlinear phase rotation amount distribution 31, the nonlinear phase rotation amount φ varies widely at each position from the optical transmitter 100 to the optical receiver 200. In this manner, the distribution estimation unit 350 estimates the nonlinear phase rotation amount distribution 31 based on the electric field signal including a large number of symbols 10 (see FIG. 5(a)) and the above-mentioned reference signal (see FIG. 5(b)). However, based on these signals, the nonlinear phase rotation amount φ in the nonlinear phase rotation amount distribution 31 tends to vary widely due to noise caused by ASE light, resulting in a noisy state.

[0040] The first error calculation unit 360 calculates and outputs the first error. The first error is a signal error between the electric field signal after the linear distortion and nonlinear distortion of the optical signal are compensated for based on the nonlinear phase rotation amount distribution estimated by the distribution estimation unit 350, and the above-mentioned reference signal. As shown in FIG. 6, the first error calculation unit 360 includes a first linear compensation unit 361, a nonlinear compensation unit 362, a second linear compensation unit 363, and a signal error calculation unit 364.

[0041] The first linear compensator 361 acquires the electric field signal from the distribution estimation unit 350, and converts the time domain electric field signal into a frequency domain electric field signal by Fourier transform. After conversion into the frequency domain electric field signal, the first linear compensator 361 compensates for a part of the chromatic dispersion occurring in the optical transmission line 50 with respect to this electric field signal in order to compensate for linear distortion of the optical signal.

[0042] 7, the first linear compensating unit 361 acquires, as the dispersion compensation amount, a chromatic dispersion amount CD1 from a predetermined position P in the nonlinear phase rotation amount distribution 31 to the optical receiving device 200, out of the accumulated chromatic dispersion amount CD in the nonlinear phase rotation amount distribution 31. When the first linear compensating unit 361 acquires the dispersion compensation amount, it compensates for chromatic dispersion in the frequency domain electric field signal by the amount of this dispersion compensation amount. After compensating for chromatic dispersion, the first linear compensating unit 361 converts the compensated frequency domain electric field signal into a time domain electric field signal by inverse Fourier transform, and outputs the time domain electric field signal to the nonlinear compensating unit 362.

[0043] The nonlinear compensating unit 362 compensates for degradation due to the nonlinear optical effect of the optical transmission path 50 in order to compensate for nonlinear distortion in the electric field signal output from the first linear compensating unit 361. For example, the nonlinear compensating unit 362 compensates for degradation due to the nonlinear optical effect based on the nonlinear phase rotation amount distribution estimated by the distribution estimating unit 350. The nonlinear compensating unit 362 outputs the electric field signal after compensating for degradation due to the nonlinear optical effect to the second linear compensating unit 363.

[0044] The second linear compensation unit 363 converts the time domain electric field signal into a frequency domain electric field signal by Fourier transform for the electric field signal output from the nonlinear compensation unit 362. After conversion into the frequency domain electric field signal, the second linear compensation unit 363 compensates for the remaining chromatic dispersion of the chromatic dispersion generated in the optical transmission line 50 for this electric field signal in order to compensate for the linear distortion of the optical signal.

[0045] For example, as shown in FIG. 7, the second linear compensation unit 363 acquires, as the dispersion compensation amount, a chromatic dispersion amount CD2 from the optical transmitting device 100 to a predetermined position P in the nonlinear phase rotation amount distribution 31, out of the accumulated chromatic dispersion amount CD in the nonlinear phase rotation amount distribution 31. The chromatic dispersion amount CD2 corresponds to a difference obtained by subtracting the chromatic dispersion amount CD1 from the accumulated chromatic dispersion amount CD. When the second linear compensation unit 363 acquires the dispersion compensation amount, it compensates for the chromatic dispersion of the frequency domain electric field signal by the amount of this dispersion compensation amount. After compensating for the chromatic dispersion, the second linear compensation unit 363 converts the compensated frequency domain electric field signal into a time domain electric field signal by inverse Fourier transform, and outputs the signal error calculation unit 364.

[0046] The signal error calculation unit 364 calculates a signal error between the electric field signal compensated by each of the first linear compensation unit 361, the nonlinear compensation unit 362, and the second linear compensation unit 363, and the reference signal transferred from the distribution estimation unit 350. The signal error calculation unit 364 may obtain the reference signal from the distribution estimation unit 350.

[0047] For example, as shown in FIG. 8(a), when an electric field signal after various compensations is input from the second linear compensation unit 363 to the signal error calculation unit 364, the signal error calculation unit 364 calculates a signal error between this electric field signal and a reference signal. As shown in FIG. 8(b), the reference signal is generated by the transmission electric field restoration unit 321 mapping the symbol 20 corresponding to the demodulated data on a constellation map. That is, the reference signal is the same as the reference signal described with reference to FIG. 5(b). The signal error calculation unit 364 calculates a signal error between such an electric field signal and the reference signal, and outputs the signal error to the total error calculation unit 380 as a first error.

[0048] The signal error calculation unit 364 can calculate the signal error by measuring the similarity between the corresponding symbols 10 and 20 in the electric field signal and the reference signal. For example, the least squares method (i.e., sum of squares error) or the residual sum of squares can be used to measure the similarity.

[0049] Here, the degree of degradation due to the nonlinear optical effect occurring in the optical transmission line 50 (i.e., nonlinear distortion) depends on the optical power of the optical signal. Therefore, the degree of degradation due to the nonlinear optical effect also depends on the amount of nonlinear phase rotation, which is proportional to the optical power of the optical signal. Specifically, the larger the amount of nonlinear phase rotation, the greater the degradation due to the nonlinear optical effect. The nonlinear compensation unit 362 in this embodiment is designed to compensate for degradation due to the nonlinear optical effect that occurs when the amount of nonlinear phase rotation is sufficiently large. For example, the nonlinear compensation unit 362 is designed to compensate for degradation due to the nonlinear optical effect that occurs with respect to the optical power of the optical signal output by the optical transmission device 100.

[0050] On the other hand, the signal error calculated by the signal error calculation unit 364 represents the error between the electric field signal after the degradation due to chromatic dispersion (i.e., linear distortion) and nonlinear optical effect has been compensated for in the first error calculation unit 360 and the reference signal. Therefore, when the degradation due to nonlinear optical effect is appropriately compensated for in the nonlinear compensation unit 362, the similarity between the electric field signal after the degradation due to chromatic dispersion and nonlinear optical effect has been compensated for increases, and the signal error calculated by the signal error calculation unit 364 decreases.

[0051] The second error calculation unit 370 calculates and outputs a second error. The second error is a differential error between the differential amount of the nonlinear phase rotation amount and a predetermined differential amount reference value including the transmission loss of the optical transmission line 50. As shown in FIG. 6, the second error calculation unit 370 includes a differential amount calculation unit 371, a differential error calculation unit 372, and a data storage unit 373. The data storage unit 373 is associated with the differential error calculation unit 372. The differential error calculation unit 372 is an example of a calculation unit.

[0052] The differential amount calculation unit 371 calculates the differential amount of the nonlinear phase rotation amount φ for each position on the optical transmission line 50 for the nonlinear phase rotation amount distribution 31 estimated by the distribution estimation unit 350. As a result, as shown in Fig. 9, the differential amount calculation unit 371 generates a differential amount distribution 41 by differentiating the nonlinear phase rotation amount distribution 31. If the nonlinear phase rotation amount φ at two adjacent positions increases in an upward trend, the differential amount of the nonlinear phase rotation amount φ also increases. Conversely, if the nonlinear phase rotation amount φ at two adjacent positions decreases in an downward trend, the differential amount of the nonlinear phase rotation amount φ also decreases.

[0053] The differential error calculation unit 372 calculates the differential error between the differential amount calculated by the differential amount calculation unit 371 and the above-mentioned differential amount reference value, and outputs the calculated differential error as a second error to the total error calculation unit 380. Here, the differential error calculation unit 372 calculates the differential amount reference value based on various data stored in the data storage unit 373, and calculates the differential error between the differential amount and the differential amount reference value.

[0054] The differential error calculation unit 372 can calculate the differential error by measuring the similarity between the differential amount in the common accumulated chromatic dispersion amount and the differential amount reference value. For measuring the similarity, for example, the L1 norm can be adopted.

[0055] 10, loss coefficient data DT1, dispersion data DT2, and span length data DT3 are stored in advance in the data storage unit 373. When the optical transmission line 50 is provided with an optical amplifier 51A, an optical amplifier 52A, etc., amplification data including the positions and amplification amounts of the optical amplifiers 51A, 52A, etc. may be stored in the data storage unit 373.

[0056] The loss coefficient data DT1 includes a loss coefficient at each position from the optical transmitting device 100 to the optical receiving device 200. For example, the loss coefficient data DT1 includes a transmission loss of 0.2 dB / km as a loss coefficient at each position. The loss coefficient is determined in advance based on the system design value of the optical transmission system ST.

[0057] The dispersion data DT2 includes the total amount of chromatic dispersion from the optical transmitting device 100 to the optical receiving device 200. The total amount of chromatic dispersion is measured and determined in advance. For example, the dispersion data DT2 includes the total amount of chromatic dispersion "6800" (ps / nm). The span length data DT3 includes the length (or distance) of each span and the total path length. For example, the span length data DT3 includes the length of each of the multiple spans SP#1, ..., SP#5, "100" (km), and the total path length, "500" (km).

[0058] Based on these data and the following formula (1), the differential error calculation section 372 calculates the differential reference value (dB / (ps / nm)). Derivative reference value = α / (D / L) (1) Here, α represents the loss coefficient (dB / km) for each position, D represents the total chromatic dispersion (ps / nm), and L represents the total path length (km).

[0059] For example, the differential error calculation unit 372 calculates −0.0147 (dB / (ps / nm)) as the differential amount reference value. Considering that the loss coefficient is a loss, when the differential error calculation unit 372 calculates the differential amount reference value, a minus sign is added to the loss coefficient. For this reason, in FIG. 9, the differential amount reference value RV is shown near the differential amount “0” of the nonlinear phase rotation amount φ. Using such a differential amount reference value RV, the differential error calculation unit 372 calculates a differential error between the differential amount calculated by the differential amount calculation unit 371 and the differential amount reference value, and outputs the calculated differential error to the total error calculation unit 380 as a second error.

[0060] The total error calculation unit 380 calculates a total error based on the first error and the second error. Specifically, the total error calculation unit 380 calculates a total error by adding up the first error and the second error. After calculating the total error, the total error calculation unit 380 outputs the total error to the distribution estimation unit 350.

[0061] When the total error is output from the total error calculation unit 380, the distribution estimation unit 350 judges whether or not this total error is less than a preset threshold value. If the total error is not less than the threshold value, the distribution estimation unit 350 calculates new dispersion compensation amounts and nonlinear compensation amounts that minimize the total error. Then, the distribution estimation unit 350 updates the nonlinear phase rotation amount distribution based on the calculated dispersion compensation amount and nonlinear compensation amount, and recalculates the total error based on the updated nonlinear phase rotation amount distribution. In this way, the distribution estimation unit 350 continuously updates the nonlinear phase rotation amount distribution until the total error becomes less than the threshold value.

[0062] Thereafter, when the total error falls below the threshold, the distribution estimation unit 350 generates a power profile based on the updated nonlinear phase rotation amount distribution, and outputs the generated power profile to the span detection unit 323. For example, when the distribution estimation unit 350 continuously updates the nonlinear phase rotation amount distribution, the total error may fall below the threshold.

[0063] 11, the original nonlinear phase rotation amount distribution 31 with the nonlinear phase rotation amount φ varying transitions to a nonlinear phase rotation amount distribution 32 according to the nonlinear phase rotation amount φ proportional to the optical power of the optical signal propagating through the optical transmission line 50. In the nonlinear phase rotation amount distribution 32, the nonlinear phase rotation amount φ proportional to the optical power decreases, for example, from position P0 of the optical transmitting device 100 to position P1 of the optical amplifier 51A. In the nonlinear phase rotation amount distribution 32, it is shown that the nonlinear phase rotation amount φ proportional to the optical power of the optical signal increases rapidly due to the optical amplifier 51A.

[0064] Furthermore, when the derivative of the nonlinear phase rotation φ for each position on the optical transmission line 50 is calculated for the nonlinear phase rotation distribution 32, most of the original derivative distribution 41 is updated to a new derivative distribution 42 and concentrated near the derivative reference value RV, as shown in Fig. 12. This is because the nonlinear phase rotation φ decreases linearly from the position P0 of the optical transmitter 100 to the position P1 of the optical amplifier 51A, for example.

[0065] Here, as described above, the distribution estimator 350 generates a power profile based on the updated nonlinear phase rotation distribution. For example, the distribution estimator 350 can generate a power profile by converting the amount of nonlinear phase rotation φ specified on the vertical axis of the updated distribution of nonlinear phase rotation into optical power. That is, the distribution of nonlinear phase rotation after converting the amount of nonlinear phase rotation φ specified on the vertical axis into optical power can be output as a power profile. As a result, a power profile is generated as shown in FIG. 13 and output from the distribution estimator 350 to the span detector 323.

[0066] Next, the operation of the optical transmission line monitoring device 300 will be described with reference to FIGS.

[0067] First, as shown in Fig. 14, the profile generating unit 322 executes a generating process (step S1). The generating process is a process for generating the above-mentioned power profile. The generating process will be described in detail later. After the profile generating unit 322 finishes executing the generating process, the span detecting unit 323 detects a power peak (step S2).

[0068] 15, the span detector 323 detects the power peak Pt immediately before the optical power starts to decrease among the multiple optical power peaks Pt, Pb appearing in the power profile. In other words, the power peak Pb immediately before the optical power starts to increase is excluded from the detection target. When the power peak Pt is detected, the span detector 323 identifies the parts corresponding to each of the spans SP#1, . . . , SP#5.

[0069] Specifically, the span detection unit 323 identifies the section between two adjacent power peaks Pt as portions X1, . . . , X5 corresponding to spans SP#1, . . . , SP#5, respectively. For example, the span detection unit 323 identifies the section between the power peak Pt with the smallest amount of accumulated chromatic dispersion and a power peak Pt adjacent to this power peak Pt as portion X1 corresponding to span SP#1.

[0070] 14, when the power peak is detected, the span detector 323 calculates the amount of chromatic dispersion (step S3). More specifically, the span detector 323 reads the CD coordinates of the power peaks corresponding to the start ends of the spans SP#1, . . . , SP#5, and calculates the amount of chromatic dispersion for each of the spans SP#1, . . . , SP#5.

[0071] 16, the span detector 323 reads CD "0" as the CD coordinate corresponding to the start end of span SP#1, and reads CD "1736" as the CD coordinate corresponding to the start end of span SP#2. The span detector 323 subtracts CD "0" from CD "1736" to calculate the amount of chromatic dispersion "1736" (ps / nm) of span SP#1. Spans SP#2,...,SP#4 (see FIG. 1) are basically the same as span SP#1, so detailed description will be omitted.

[0072] Here, for span SP#5, the span detection unit 323 reads CD "5335" as the CD coordinate corresponding to the start end of span SP#5, but in this embodiment, there is no CD coordinate corresponding to the start end of span SP#6. In this case, the span detection unit 323 obtains the total chromatic dispersion amount "6800" (ps / nm) included in the dispersion data DT2 stored in the data storage unit 373, and uses this total chromatic dispersion amount as the CD coordinate corresponding to the start end of span SP#6. Therefore, the span detection unit 323 subtracts CD "5335" from CD "6800" to calculate the chromatic dispersion amount "1465" (ps / nm) of span SP#5. In this way, the span detection unit 323 can calculate the chromatic dispersion amount for each of spans SP#1, . . . , SP#5.

[0073] 14, when the span detection unit 323 calculates the amount of chromatic dispersion, the dispersion coefficient calculation unit 324 calculates the chromatic dispersion coefficient (step S4) and ends the process. For example, the dispersion coefficient calculation unit 324 obtains the lengths of the spans SP#1, . . . , SP#5 included in the span length data DT3 stored in the data storage unit 373, and calculates the chromatic dispersion coefficient based on these lengths.

[0074] For example, for span SP#1, the dispersion coefficient calculation unit 324 divides the chromatic dispersion amount of span SP#1, "1736" (ps / nm), by the length of span SP#1, "100" (km), to calculate a chromatic dispersion coefficient of "17.36" (ps / nm / km). For span SP#5, the dispersion coefficient calculation unit 324 divides the chromatic dispersion amount of span SP#5, "1465" (ps / nm), by the length of span SP#5, "100" (km), to calculate a chromatic dispersion coefficient of "14.56" (ps / nm / km).

[0075] In this way, the dispersion coefficient calculation unit 324 can calculate the chromatic dispersion coefficient for each of the spans SP#1,...,SP#5, and can grasp the characteristics of each of the spans SP#1,...,SP#5. Therefore, for example, if the optical transmission line monitoring device 300 holds a table that manages the relationship between the chromatic dispersion coefficient and the fiber type, the optical transmission line monitoring device 300 can estimate the fiber type of each of the spans SP#1,...,SP#5 from the chromatic dispersion coefficient.

[0076] The above-mentioned generation process executed by the profile generating unit 322 will be described in detail with reference to FIG.

[0077] First, the distribution estimation unit 350 acquires an electric field signal and a reference signal (step S11). After acquiring the electric field signal and the reference signal, the distribution estimation unit 350 estimates a nonlinear phase rotation amount distribution (see FIG. 7) (step S12). After estimating the nonlinear phase rotation amount distribution, the first error calculation unit 360 calculates a first error (step S13). Specifically, the first linear compensation unit 361 compensates for a part of the linear distortion, the nonlinear compensation unit 362 compensates for the nonlinear distortion, and the second linear compensation unit 363 compensates for the remaining part of the linear distortion, and the signal error calculation unit 364 acquires the electric field signal after the first linear compensation unit 361 compensates for a part of the linear distortion, the nonlinear compensation unit 362 compensates for the nonlinear distortion, and the second linear compensation unit 363 compensates for the remaining part of the linear distortion. Then, when the signal error calculation unit 364 acquires the reference signal, the first error calculation unit 360 calculates a first error by calculating a signal error between the compensated electric field signal and the reference signal.

[0078] After calculating the first error, the second error calculation unit 370 calculates a derivative (step S14) and calculates a second error (step S15). Specifically, the derivative calculation unit 371 calculates the derivative of the amount of nonlinear phase rotation, the derivative error calculation unit 372 calculates a derivative reference value, and the second error calculation unit 370 calculates the second error by calculating a derivative error between the derivative and the derivative reference value.

[0079] After calculating the second error, the total error calculation unit 380 calculates the total error (step S16). After calculating the total error, the distribution estimation unit 350 judges whether the total error is less than a threshold value (step S17). If the total error is equal to or greater than the threshold value (step S17: NO), the distribution estimation unit 350 updates the nonlinear phase rotation amount distribution (step S18) and repeats the processes of steps S13 to S17 until the total error becomes less than the threshold value. This updates the nonlinear phase rotation amount distribution so that the total error becomes less than the threshold value (see FIG. 10). If the total error is less than the threshold value, the distribution estimation unit 350 generates a power profile (step S19) and ends the generation process.

[0080] As described above, the optical transmission line monitoring device 300 according to this embodiment generates a power profile representing the characteristics of the optical transmission line 50 in a multi-span optical transmission line including a plurality of spans SP#1, . . . , SP#5. In particular, the optical transmission line monitoring device 300 generates a power profile based not only on the above-mentioned signal error but also on the differential error between a differential amount appearing in a differential amount distribution obtained by differentiating a nonlinear phase rotation amount distribution and a differential amount reference value including a loss coefficient. Since the optical transmission line monitoring device 300 generates a power profile based on such a differential error, it is possible to generate a power profile with high accuracy even if the number of symbols is relatively small.

[0081] Although the preferred embodiment of the present invention has been described in detail above, the present invention is not limited to the specific embodiment, and various modifications and variations are possible within the scope of the gist of the present invention described in the claims.

[0082] For example, by calculating the differential reference value using at least one of the position and the amplification amount of the optical amplifier in addition to the above-mentioned loss coefficient, the optical transmission line monitoring device 300 can generate a power profile with even higher accuracy.

[0083] Also, a non-volatile memory connectable to the optical transmission line monitoring device 300 may be prepared, and the above-mentioned numerical value of the loss coefficient (i.e., α) and the numerical value of the average chromatic dispersion coefficient (i.e., D / L) may be stored in advance in the same or separate non-volatile memories. In this case, the differential error calculation unit 372 can calculate a quotient obtained by dividing the loss coefficient by the average dispersion coefficient based on these two numerical values ​​input from the non-volatile memory. Then, the distribution estimation unit 350 can use this quotient calculated by the differential error calculation unit 372 as a differential quantity reference value. On the other hand, the distribution estimation unit 350 may store such a quotient in advance in the non-volatile memory. In this case, the distribution estimation unit 350 can use the quotient stored in advance in the non-volatile memory as a differential quantity reference value.

[0084] In addition, the following supplementary notes are provided in relation to the above description. (Supplementary Note 1) A transmission line monitoring device for monitoring characteristics of an optical transmission line, comprising: a generation unit that generates predetermined information including optical power of the optical signal at multiple positions on the optical transmission line based on an electric field signal indicating an optical electric field component of an optical signal received from the optical transmission line and a specific signal indicating an electric field obtained by data demodulated based on the electric field signal; and an update unit that updates the predetermined information based on a first error between the electric field signal and the specific signal after linear distortion of the optical signal and nonlinear distortion of the optical signal are compensated for based on the predetermined information, and a second error between a differential amount of the optical power at the multiple positions and a reference value including a transmission loss of the optical transmission line. (Supplementary Note 2) The transmission path monitoring device according to Supplementary Note 1, characterized in that the update unit continuously updates the specified information until the sum of the first error and the second error becomes less than a threshold value. (Supplementary Note 3) The transmission line monitoring device according to Supplementary Note 1 or 2, characterized in that the update unit updates the predetermined information based on a minimum sum of the first error and the second error. (Supplementary Note 4) The transmission line monitoring device according to Supplementary Note 1 or 2, further comprising a calculation unit that calculates the reference value based on a loss coefficient representing the transmission loss at each of the plurality of positions, a total amount of dispersion value of chromatic dispersion generated in the optical transmission line, and a distance of the optical transmission line. (Appendix 5) The transmission path monitoring device according to appendix 1 or 2, further comprising a calculation unit that calculates a quotient obtained by dividing a loss coefficient by a dispersion coefficient based on a value of a dispersion coefficient pre-stored in a memory and a value of a loss coefficient pre-stored in a memory, and the update unit sets the quotient calculated by the calculation unit to the reference value, or sets the quotient stored in advance in the memory to the reference value. (Supplementary Note 6) The transmission line monitoring device according to Supplementary Note 4, wherein the calculation unit further calculates the reference value based on at least one of a position and an amplification amount of an optical amplifier provided on the optical transmission line. (Supplementary Note 7) The transmission path monitoring device according to Supplementary Note 1 or 2, characterized in that the update unit calculates an amount of dispersion compensation and an amount of nonlinear compensation that minimizes the sum of the first error and the second error, and updates the specified information based on the amount of dispersion compensation and the amount of nonlinear compensation. (Supplementary Note 8) The transmission line monitoring device according to Supplementary Note 1 or 2, further comprising: a first error calculation unit that calculates the first error; and a second error calculation unit that calculates the second error. (Supplementary Note 9) A transmission line monitoring method for monitoring characteristics of an optical transmission line, the method generating predetermined information including optical power of the optical signal at multiple positions on the optical transmission line based on an electric field signal indicating an optical electric field component of an optical signal received from the optical transmission line and a specific signal indicating an electric field obtained by data demodulated based on the electric field signal, and updating the predetermined information based on a first error between the electric field signal and the specific signal after linear distortion of the optical signal and nonlinear distortion of the optical signal are compensated for based on the predetermined information, and a second error between a differential amount of the optical power and a reference value including a transmission loss of the optical transmission line at the multiple positions. (Supplementary Note 10) The transmission path monitoring method according to Supplementary Note 9, characterized in that the predetermined information is continuously updated until the sum of the first error and the second error becomes less than a threshold value. (Supplementary Note 11) The transmission line monitoring method according to Supplementary Note 9 or 10, further comprising updating the predetermined information based on a minimum sum of the first error and the second error. (Supplementary Note 12) The transmission path monitoring method according to Supplementary Note 9 or 10, characterized in that the reference value is calculated based on a loss coefficient representing the transmission loss at each of the multiple positions, a total amount of dispersion value of chromatic dispersion generated in the optical transmission path, and a distance of the optical transmission path. (Appendix 13) The transmission path monitoring method according to appendix 9 or 10, characterized in that a quotient is calculated by dividing a loss coefficient by a dispersion coefficient based on a value of a dispersion coefficient pre-stored in a memory and a value of a loss coefficient pre-stored in a memory, and the calculated quotient is set as the reference value, or the quotient pre-stored in a memory is set as the reference value. (Supplementary Note 14) The transmission line monitoring method according to Supplementary Note 12, further comprising the step of calculating the reference value based on at least one of a position and an amplification amount of an optical amplifier provided on the optical transmission line. (Supplementary Note 15) The transmission path monitoring method according to Supplementary Note 9 or 10, characterized in that an amount of dispersion compensation and an amount of nonlinear compensation that minimize the sum of the first error and the second error are calculated, and the specified information is updated based on the amount of dispersion compensation and the amount of nonlinear compensation. [Explanation of symbols]

[0085] ST Optical Transmission System 50 Optical transmission line 100 Optical transmitter 200 Optical receiving device 300 Optical transmission line monitoring device 321 Transmission field restoration unit 322 Profile Generation Unit 323 Span detector 324 Dispersion coefficient calculation unit 350 Distribution estimation part 360 1st error calculation section 370 Second error calculation section 380 Total error calculation section

Claims

1. A transmission line monitoring device for monitoring characteristics of an optical transmission line, comprising: a generator that generates predetermined information including optical power of the optical signal at a plurality of positions on the optical transmission line based on an electric field signal indicating an optical electric field component of the optical signal received from the optical transmission line and a specific signal indicating an electric field obtained by data demodulated based on the electric field signal; an update unit that updates the predetermined information based on a first error between the electric field signal and the specific signal after linear distortion and nonlinear distortion of the optical signal are compensated for based on the predetermined information, and a second error between the differential amount of the optical power and a reference value including a transmission loss of the optical transmission line at the multiple positions; A transmission line monitoring device comprising:

2. The update unit continuously updates the predetermined information until a sum of the first error and the second error becomes less than a threshold value.

2. The transmission line monitoring device according to claim 1.

3. The update unit updates the predetermined information based on a minimum sum of the first error and the second error.

3. The transmission line monitoring device according to claim 1 or 2.

4. a calculation unit that calculates the reference value based on a loss coefficient that indicates the transmission loss at each of the plurality of positions, a total amount of dispersion value of chromatic dispersion generated in the optical transmission path, and a distance of the optical transmission path, 3. The transmission line monitoring device according to claim 1 or 2.

5. a calculation unit that calculates a quotient obtained by dividing the loss factor by the dispersion coefficient based on a value of the dispersion coefficient pre-stored in a memory and a value of the loss factor pre-stored in a memory, The update unit sets the quotient calculated by the calculation unit as the reference value, or sets the quotient stored in advance in a memory as the reference value.

3. The transmission line monitoring device according to claim 1 or 2.

6. the calculation unit further calculates the reference value based on at least one of a position and an amplification amount of an optical amplifier provided on the optical transmission line.

5. The transmission line monitoring device according to claim 4.

7. the update unit calculates an amount of dispersion compensation and an amount of nonlinear compensation that minimizes the sum of the first error and the second error, and updates the predetermined information based on the amount of dispersion compensation and the amount of nonlinear compensation.

3. The transmission line monitoring device according to claim 1 or 2.

8. A transmission line monitoring method for monitoring characteristics of an optical transmission line, comprising the steps of: generating predetermined information including optical power of the optical signal at a plurality of positions on the optical transmission line based on an electric field signal indicating an optical electric field component of the optical signal received from the optical transmission line and a specific signal indicating an electric field obtained by data demodulated based on the electric field signal; updating the predetermined information based on a first error between the electric field signal and the specific signal after the linear distortion and the nonlinear distortion of the optical signal are compensated for based on the predetermined information, and a second error between the differential amount of the optical power and a reference value including a transmission loss of the optical transmission line at the multiple positions; A transmission line monitoring method.

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