Optical receiver, optical transmitter, and optical communication fault monitoring method

Abstract

One aspect of the present invention is an optical receiver comprising a monitoring unit that: rotates both a reception signal and a reference signal received so as to approach the reception signal to optimize the polarization state; calculates the power profile of each polarization for the signals having optimized polarization states; calculates the inter-polarization power difference from the power profile of each polarization; and identifies the position at which a polarization-dependent failure has occurred, and the associated value, from the position where the calculated difference has occurred and the amount of the difference.

Description

Optical receiving device, optical transmitting device, and optical communication fault monitoring method

[0001] The present invention relates to techniques for an optical receiving device, an optical transmitting device, and an optical communication fault monitoring method.

[0002] Polarization-dependent impairments, such as polarization-dependent loss (PDL) and polarization-mode dispersion (PMD), are one of the major limiting factors that degrade signal quality in polarization-multiplexed digital coherent systems required for high-capacity, long-distance transmission (see, for example, Non-Patent Document 1).

[0003] Polarization mode dispersion (PMD) is a type of dispersion where light propagating in two different polarization modes along a waveguide experiences velocity changes between these modes due to external factors such as inherent defects in the waveguide, temperature, or stress. The presence of locally high PMD in a transmission link, such as an installed fiber, limits the transmission rate of the system for proper operation. Therefore, reliable PMD location detection and measurement techniques are crucial for maintaining secure networks.

[0004] Polarization-dependent loss, defined as the ratio of maximum output power to minimum output power for all polarization states, is a polarization-dependent loss that appears in various link components, such as fiber couplers, wavelength selective switches (WSSs), and erbium-doped fiber amplifiers (EDFAs). Because the polarization state of an optical signal is generally not maintained over time along the fiber, the polarization angles between the polarization state of the optical signal and various devices are randomly oriented. Therefore, PDL behaves as a stochastically varying loss, resulting in stochastic fluctuations in system performance. Therefore, to ensure reliability and avoid system outages during the operational lifetime, it is necessary to design margins based on worst-case assumptions. The accumulated margins of individual PDL components increase the ambiguity of signal quality estimates and limit transmission rates. Accurate monitoring of random and simultaneous dispersive PDL in multi-span links can avoid worst-case designs and maximize transmission rates.

[0005] Takeo Sasai, Masanori Nakamura, et al., “0.77-dB Anomaly Loss Localization Based on DSP-Based Fiber-Longitudinal Power Estimation Using Linear Least”, 2023 Optical Fiber Communications Conference and Exhibition (OFC), San Diego, CA, USA, 2023, W1H.4

[0006] However, polarization-dependent power and dispersion require analysis for each polarization, and because they are impairments that depend on the polarization state, measurement is difficult because statistical processing and polarization state control are required. Furthermore, it has been difficult to accurately obtain the true value of loss and its location using only signal processing on the receiving side without using dedicated measuring equipment. In light of the above, the present invention aims to provide a technology that can estimate polarization-dependent loss and modal dispersion in components in an optical transmission link.

[0007] One aspect of the present invention is an optical receiving device that includes a reference signal received so as to approximate a received signal, and a monitoring unit that rotates each of the received signals to optimize the polarization state, calculates a power profile for each polarization for the signal with the optimized polarization state, calculates a power difference between the polarizations from the power profile for each polarization, and identifies the location and value of a polarization-dependent fault from the location where the calculated power difference occurs and the amount of the difference.

[0008] One aspect of the present invention is an optical transmission device that includes a polarization state optimization unit that optimizes the polarization state for an optical transmission signal, and the polarization state optimization unit acquires the polarization state in the vicinity where the difference between polarizations becomes large through feedback from the receiving side, and changes the polarization state back and forth using the acquired polarization state as a reference, optimizing the polarization state by setting the amount of change to the maximum value when the amount of change falls below a lower threshold.

[0009] One aspect of the present invention is an optical communication fault monitoring method in which a polarization multiplexed optical receiving device rotates a received reference signal to bring it closer to the received signal, optimizes the polarization state, calculates a power profile for each polarization for the signal with the optimized polarization state, calculates a power difference between the polarizations from the power profile for each polarization, and identifies the location and value of a fault related to optical communication from the location where the calculated power difference occurs and the amount of the difference.

[0010] The present invention makes it possible to estimate polarization-dependent loss and modal dispersion in components in an optical transmission link.

[0011] FIG. 1 is a diagram illustrating a first exemplary configuration of an optical transmission system according to a first embodiment. FIG. 2 is a diagram illustrating a first exemplary configuration of a digital signal processing unit according to the first embodiment. FIG. 3 is a diagram illustrating a second exemplary configuration of a digital signal processing unit according to the first embodiment. FIG. 4 is a diagram illustrating an exemplary configuration of a PMD / PDL monitoring unit according to the first embodiment. FIG. 5 is a diagram for explaining mathematical expressions used in the embodiments, principles by which the present embodiments can be realized, and the like. FIG. 6 is a diagram illustrating a first exemplary configuration of a digital signal processing unit including a PMD / PDL monitoring unit according to the first embodiment. FIG. 7 is a diagram illustrating a second exemplary configuration of a digital signal processing unit including a PMD / PDL monitoring unit according to the first embodiment. FIG. 8 is a diagram illustrating an exemplary configuration of an optical transmission system according to a second embodiment. FIG. 9 is a diagram illustrating an example of a position where PDL is applied and an example of a power profile for each polarization in which the polarization state is optimized when a PDL of 1 dB is applied. FIG. 10 is a diagram illustrating estimated PDL values ​​when various PDLs are applied. FIG. 11 is a diagram for explaining a method of optimizing the polarization state. FIG. 12 is a diagram illustrating an example of a rotation angle of a reference signal and changes in PDL. FIG. 13 is a diagram illustrating an example of a reference signal and a received signal before polarization state optimization. FIG. 14 is a diagram illustrating a method of optimizing the polarization state when the rotation angle of the reference signal is approximately 45 degrees.

[0012] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail with reference to the drawings.

[0013] 1 is a diagram showing a first configuration example of an optical transmission system according to this embodiment. The optical transmission system 1 includes a polarization multiplexed optical transmitter 2, a polarization multiplexed optical receiver 3, and an optical transmission line 4. The polarization multiplexed optical transmitter 2 includes, for example, a data generator 21 and an optical signal transmitter 22. The polarization multiplexed optical receiver 3 includes, for example, an optical signal receiver 31 and a digital signal processor 32.

[0014] The optical transmission path 4 includes, for example, an optical fiber, an optical amplifier, and the like.

[0015] The polarization multiplexed optical transmitter 2 generates polarization multiplexed signal data, converts the generated polarization multiplexed signal data into an optical signal, and transmits it. The transmitted polarization multiplexed signal data is modulated using a predetermined modulation method (e.g., QPSK method), and is a polarization multiplexed signal that is transmitted by multiplexing different modulated signals onto vertically polarized and horizontally polarized signal light. The data generator 21 generates the polarization multiplexed signal data. The optical signal transmitter 22 converts the generated polarization multiplexed signal data into an optical signal and transmits it.

[0016] The polarization multiplexed optical receiving device 3 receives the optical signal transmitted by the polarization multiplexed optical transmitting device 2 via the optical transmission path 4, converts the received optical signal into polarization multiplexed signal data, and performs predetermined signal processing on the converted polarization multiplexed signal data. The signal processing will be described later. The optical signal receiving unit 31 receives the optical signal transmitted by the polarization multiplexed optical transmitting device 2 via the optical transmission path 4 and converts the received optical signal into polarization multiplexed signal data. The digital signal processing unit 32 performs predetermined signal processing on the converted polarization multiplexed signal data.

[0017] Next, we will explain a configuration example of the digital signal processing unit 32 of the polarization multiplexed optical receiving device 3. Fig. 2 is a diagram showing a first configuration example of the digital signal processing unit of this embodiment. The digital signal processing unit 32 includes, for example, a chromatic dispersion compensator 321, a frequency offset compensator 322, a polarization separator 323, a phase noise compensator 324, a chromatic dispersion load unit 325, and a PMD / PDL monitor 326.

[0018] The chromatic dispersion compensator 321 performs chromatic dispersion compensation processing on the polarization multiplexed signal data. Note that the signal waveform is distorted by chromatic dispersion when the optical signal propagates through the optical transmission line 4. For this reason, the chromatic dispersion compensator 321 performs chromatic dispersion compensation processing by, for example, estimating the magnitude of the distortion from the received signal and compensating for the distortion of the received signal due to chromatic dispersion.

[0019] The frequency offset compensator 322 performs frequency offset compensation processing on the polarization multiplexed signal data. The frequency offset compensator 322 performs frequency offset compensation processing by estimating and compensating for the frequency offset using, for example, two types of preambles (short training symbols and long training symbols).

[0020] The polarization separation unit 323 performs polarization separation processing on the polarization multiplexed signal data to separate the orthogonal polarization signal into a vertical polarization signal and a horizontal polarization signal.

[0021] The phase noise compensator 324 performs phase noise compensation processing on the polarization multiplexed signal data. Note that, for example, the polarization multiplexed optical transmitter 2 has an oscillator, and this oscillator has phase noise (frequency fluctuation). The phase noise compensator 324 performs phase noise compensation processing by compensating for such phase noise using a well-known method.

[0022] The wavelength dispersion load unit 325 performs wavelength dispersion load processing to load a predetermined value of PDL on the polarization multiplexed signal data.

[0023] The PMD / PDL monitor 326 monitors the polarization dependent loss (PDL) and polarization mode dispersion (PMD) of the polarization multiplexed signal data.

[0024] 3 is a diagram showing a second configuration example of the digital signal processing unit of this embodiment. The digital signal processing unit 32A includes, for example, a chromatic dispersion compensator 321, a frequency offset compensator 322, a polarization separator 323, a phase noise compensator 324, and a PMD / PDL monitor 326. In other words, the digital signal processing unit 32A does not include a chromatic dispersion load unit 325.

[0025] (Configuration and Processing of PMD / PDL Monitoring Unit) Fig. 4 is a diagram showing an example of the configuration of the PMD / PDL monitoring unit of this embodiment. As shown in Fig. 4, the PMD / PDL monitoring unit 326 includes, for example, a polarization state optimization unit 3261, a power profile calculation unit 3262, and a PDL / PMD calculation unit 3263.

[0026] The polarization state optimization unit 3261 optimizes the polarization state using a method described below.

[0027] The power profile calculation unit 3262 calculates the power profile for each polarization.

[0028] The PDL / PMD calculation unit 3263 calculates PMD. The PDL / PMD calculation unit 3263 calculates PDL. Since the PDL / PMD calculation unit 3263 can calculate the power profile of each polarization, it takes the difference between them. Then, the PDL / PMD calculation unit 3263 calculates the difference value that appears when PMD (or PDL) is present as the true value of PMD (or PDL). The calculation principles and methods for PMD and PDL will be described later.

[0029] The data generator 21 of the polarization multiplexed optical transmitter 2 and the digital signal processor 32 of the polarization multiplexed optical receiver 3 are configured using a processor such as a CPU (Central Processing Unit) and a memory. The data generator 21 functions as the data generator 21 when the processor executes a program. The digital signal processor 32 functions as a chromatic dispersion compensator 321, a frequency offset compensator 322, a polarization separator 323, a phase noise compensator 324, a chromatic dispersion load unit 325, and a PMD / PDL monitor 326 when the processor executes the program. Note that all or part of the functions of the data generator 21 and the digital signal processor 32 may be realized using hardware such as an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device), or an FPGA (Field Programmable Gate Array). The above programs may be recorded on a computer-readable recording medium. Examples of computer-readable recording media include portable media such as flexible disks, magneto-optical disks, ROMs, CD-ROMs, and semiconductor storage devices (e.g., SSDs: Solid State Drives), as well as storage devices such as hard disks and semiconductor storage devices built into computer systems. The above programs may be transmitted via telecommunications lines.

[0030] (Principle) Here, we will explain the mathematical expressions used in this embodiment, the principles by which this embodiment can be realized, etc. Fig. 5 is a diagram for explaining the mathematical expressions used in this embodiment, the principles by which this embodiment can be realized, etc.

[0031] The polarization multiplexed optical signal at position z and time t is expressed as A(z, t) = [A ｘ (z, t), A ｙ (z, t)], and α ｘ (z), α ｙ (z) is defined. This allows the propagation state to be calculated from the Manakov equation, and the power profile for each polarization to be found by the least squares method. β is the group velocity dispersion of the fiber. ２When γ is used as a nonlinear coefficient, the variables can be transformed as shown in the following equation (1).

[0032]

[0033] In equation (1), E(z, t) is an optical pulse propagating through a fiber. ｘ [z], E ｙ [z]], and will be described as a signal at position z (0≦z≦L) on the transmission path (transmission distance: L (km)). As shown in FIG. 5, when distance z=0, it is the transmitted signal E[0] (symbol g11), and when distance z=L, it is the received signal E[L] (symbol g15). Also, the optical transmission path g12 is represented by a digital twin link as shown by symbol g13.

[0034] From equation (1), the transfer equation can be expressed as the following equation (2).

[0035]

[0036] In equation (2), E (bold) is the electric field vector of the optical pulse propagating in the fiber. Here, γ' ｘ／ｙ (z) is the following formula (3) (E (bold) is normalized to 1). In the embodiment, x or y is expressed as x / y.

[0037]

[0038] Note that P is the initial power when z=0. ｘ , α ｙ is a variable related to loss and amplification. Therefore, γ' ｘ／ｙ If (z) can be calculated, the signal power P ｘ／ｙ (z) can be obtained. ｘ／ｙ (z) = [γ' ｘ , γ' ｙ ] can be obtained by solving the linear least squares problem of the following equation (4) (reference symbol g16 in FIG. 5).

[0039]

[0040] In equation (4), E[L] is the received discrete signal (hereinafter referred to as the "received signal"), and E ｒｅｆ [L] = [(E ｘ ) ｒｅｆ [L], (E ｙ ) ｒｅｆ [L] Ｔ (T represents transposition) is the reference signal (symbol g14 in FIG. 5) propagated through the digitally simulated optical transmission path. Then, the received signal E[L] is approximated by the following equation (5) using a first-order perturbation, and the reference signal is approximated by the following equation (6).

[0041]

[0042]

[0043] In addition, in the formulas (5) and (6), (E ０ [L], (E ０ ) ｒｅｆ [L] is the linear solution (or linear term), and E １ [L], (E １ ) ｒｅｆ [L] is a perturbation solution. If the linear term satisfies the Nyquist theorem, the cost function I is expressed by the following equation (7).

[0044]

[0045] The perturbation term can be expressed as the following equation (8).

[0046]

[0047] Here, G ｐｑ The k-th column of (p, qε{x, y}) is the following equation (9).

[0048]

[0049] In formula (8), D ｚｋＬ is the wavelength dispersion operator of the distance from position zk to L. As a result, the cost function I becomes the following equation (10), and the solution is determined by the linear least squares method as the following equation (11). Note that in equation (11), Re[*] represents the real part in [*]. Also, in equation (11), † (tagger) is a symbol that takes a complex conjugate and converts it into a transposed matrix.

[0050]

[0051]

[0052] From the above, it is possible to obtain a power profile for each polarization. Using this, it is possible to find PDL / PMD from the power difference and time (distance) difference. However, since PDL / PMD depends on the polarization state, it is necessary to find the optimal polarization state. First, PDL can be modeled using a Jones matrix, as shown in the following equation (12), using a unitary matrix R that represents the change in polarization state. In equation (12), "in" represents the input signal and "Out" represents the output signal.

[0053]

[0054] ρ represents the loss caused by the PDL source. This allows the polarization state where the power difference between the polarized waves is maximum to be determined, and the PDL is estimated. Reference symbol g17 in FIG. 5 shows the power profile for each polarization, optimizing the polarization state when a predetermined value is loaded as PDL. The horizontal axis represents the transmission distance (km), and the vertical axis represents the power (dBm). In the example of reference symbol g17, the L at the PDL load point is １ (km) to check the PDL.

[0055] Similarly, PMD can be expressed by the Jones matrix. If the local PMD between two polarized waves is τ [seconds], it can be expressed as in the following equation (13).

[0056]

[0057] As a result, according to this embodiment, it is possible to obtain the polarization state in which the delay difference between the polarizations is maximum, and estimate the PMD.

[0058] (Functional Example of First Configuration Example) Fig. 6 is a diagram showing a first configuration example of a digital signal processing unit including a PMD / PDL monitoring unit of this embodiment. Note that this configuration corresponds to the configuration of the digital signal processing unit 32 in Fig. 2. As shown in Fig. 6, the digital signal processing unit 32 includes a chromatic dispersion load unit 325, a polarization state optimization unit 3261-1, a decoding unit 3273, a polarization state optimization unit 3261-2, a transmission waveform restoration unit 3275, a linear solution estimator 3276, a perturbation term estimator 3277, a matrix G calculator 3278, a power profile calculator 3262, and a PDL / PMD calculator 3263.

[0059] In this example, as can be seen from equation (4), it is assumed that the signal comparison is performed at a position z = L (km) (receiving side). In the following, the optical pulse (signal data) E[z] = [E ｘ [z], E ｙ [z]], and will be described as a signal at position z (0≦z≦L) on a transmission path (transmission distance: L (km)).

[0060] The polarization separation unit 323 (Figure 2) outputs the separated polarization multiplexed signal data E[0] (hereinafter, "polarization multiplexed signal data" will also be simply referred to as "signal data") to the wavelength dispersion load unit 325 and the decoding unit 3273 (step S1).

[0061] The chromatic dispersion load processing unit 325 performs chromatic dispersion load processing on the input signal data, and outputs the signal data that has undergone chromatic dispersion load processing to the polarization state optimization unit 3261-1 (step S1).

[0062] The polarization state optimization unit 3261-1 optimizes the polarization state for the input signal data, and outputs the received discrete signal E[L] with the optimized polarization state to the perturbation term estimation unit 3277 (step S1).

[0063] The decoder 3273 performs decoding processing on the input polarization multiplexed signal data in accordance with the modulation method, and outputs the decoded signal data to the polarization state optimization unit 3261-2 (step S1).

[0064] The polarization state optimization unit 3261-2 optimizes the polarization state of the input signal data and outputs the signal data with the optimized polarization state to the transmission waveform restoration unit 3275 (step S1). The polarization state optimization method will be described later.

[0065] The transmission waveform restoration unit 3275 performs restoration processing on the transmission waveform of the input signal data using a well-known method, and the restored signal data is input to the linear solution estimation unit 3276 and the matrix G calculation unit 3278 (step S1).

[0066] The linear solution estimation unit 3276 uses the restored signal data to derive a linear solution signal E ０ [L] (Equation (5)) is estimated, and the estimated signal E ０ [L] is output to the perturbation term estimation unit 3277 (step S1).

[0067] The perturbation term estimation unit 3277 receives the received discrete signal E[L] from the polarization state optimization unit 3261-1 and the estimated signal E[L] from the linear solution estimation unit 3276. ０ The perturbation term estimation unit 3277 receives the received discrete signal E[L] and the signal E ０ [L] is used to calculate the perturbation term signal E １ [L] (equation (7)) is estimated, and the estimated perturbation term, signal E １ [L] is output to the power profile calculation unit 3262 (step S1).

[0068] The matrix G calculation unit 3278 calculates the matrix G (equation (7)) using equation (8) using the signal data input from the transmission waveform restoration unit 3275, and outputs the calculated matrix G to the power profile calculation unit 3262 (step S1).

[0069] The power profile calculation unit 3262 receives the signal E １ [L] is input, and matrix G is input from matrix G calculation unit 3278. In power profile calculation unit 3262, since the relationship of equation (9) exists, the input signal E １ Using [L] and the calculated matrix G, γ is estimated by the linear least squares method (equation (10)), and the power profile (P ｘ(z), P ｙ (z) is calculated (step S2).

[0070] The power profile for each polarization is input to the PDL / PMD calculation unit 3263 from the power profile calculation unit 3262. The PDL / PMD calculation unit 3263 calculates the power difference between the polarizations from the input power profile for each polarization (P[z]), and identifies the PDL / PMD fault occurrence position and its value from the difference occurrence position and difference amount (step S3).

[0071] (Functional Example of Second Configuration Example) In this example, signal comparison is performed at z=0 (km) (transmission side) as in the following equation (14).

[0072]

[0073] Fig. 7 is a diagram showing a second configuration example of a digital signal processing unit including a PMD / PDL monitoring unit according to this embodiment. This configuration corresponds to the configuration of the digital signal processing unit 32A in Fig. 3. As shown in Fig. 7, the digital signal processing unit 32A includes a perturbation term estimator 3277A, a polarization state optimizer 3261A-1, a decoder 3273, a transmission waveform restorer 3275A, a polarization state optimizer 3261A-2, a matrix G' calculator 3278A, a power profile calculator 3262, and a PDL / PMD calculator 3263.

[0074] The decoding unit 3273 receives the polarization multiplexed signal data E[0] separated by the polarization separation unit 323. The decoding unit 3273 performs decoding processing on the input polarization multiplexed signal data in accordance with the modulation method, and outputs the decoded signal data to the transmission waveform restoration unit 3275A (step S11).

[0075] The transmission waveform restoration unit 3275A restores the transmission waveform of the input signal data using a known method, and outputs the restored signal data E ０ [0] is output to the perturbation term estimation unit 3277A and the polarization state optimization unit 3261A-2 (step S11).

[0076] The perturbation term estimation unit 3277A receives the polarization multiplexed signal data E[0] separated by the polarization separation unit 323 and the E[0] output from the transmission waveform restoration unit 3275A.０ The perturbation term estimation unit 3277 receives the separated polarization multiplexed signal data E[0] and the signal E ０ Using [0], E １ [0]=E[0]-E ０ [0] by E １ [0] is estimated, and the estimated perturbation term is the signal data E １ [0] is output to the polarization state optimization unit 3261A-1 (step S11).

[0077] The polarization state optimization unit 3261A-1 receives the signal data E １ The polarization state is optimized for [0], and the signal data with the optimized polarization state is output to the power profile calculation unit 3262 (step S11).

[0078] The polarization state optimization unit 3261A-2 receives the signal data E ０ The polarization state is optimized for [0], and the signal data with the optimized polarization state is output to the matrix G' calculation unit (step S11).

[0079] The matrix G' calculation unit 3278A calculates the matrix G' using the signal data input from the polarization state optimization unit 3261A-2, and outputs the calculated matrix G' to the power profile calculation unit 3262. The matrix G' is expressed by equation (15). The matrix G' calculation unit 3278A calculates the matrix G' by performing the calculation of the following equation (16) (step S11).

[0080]

[0081]

[0082] The power profile calculation unit 3262 receives the perturbation term E estimated from the polarization state optimization unit 3261A-1. １ A signal with an optimized polarization state for [0] is input, and a matrix G' is input from the matrix G' calculation unit 3278A. The power profile calculation unit 3262 estimates γ' by the linear least squares method (the following equation (17)) using the input signal with an optimized polarization state and the calculation result of the matrix G' calculation unit 3278A, and calculates the power profile (P ｘ (z), P ｙ(z) is calculated (step S12).

[0083]

[0084] The power profile for each polarization is input to the PDL / PMD calculation unit 3263 from the power profile calculation unit 3262. The PDL / PMD calculation unit 3263 calculates the power difference between the polarizations from the input power profile (P[z]) for each polarization, and identifies the PDL / PMD fault occurrence position and its value from the difference occurrence position and difference amount (step S13).

[0085] 8 is a diagram showing an example of the configuration of an optical transmission system according to this embodiment. The optical transmission system 1B includes a polarization multiplexed optical transmitter 2B, a polarization multiplexed optical receiver 3, and an optical transmission line 4. The polarization multiplexed optical transmitter 2B includes, for example, a data generator 21, an optical signal transmitter 22, and a polarization state optimizer 23. The polarization multiplexed optical receiver 3 includes, for example, an optical signal receiver 31 and a digital signal processor 32.

[0086] The polarization state optimization unit 23 performs polarization state optimization processing on the optical transmission signal. The polarization state optimization unit 23 may change the polarization state at a constant rate by determining a range, increment, and time interval. Alternatively, the polarization state optimization unit 23 may obtain a polarization state in the vicinity where the difference between polarizations becomes large through feedback from the digital signal processing unit 32 on the receiving side, change the polarization state back and forth based on that polarization state, and optimize the polarization state by setting the change amount to a maximum value when it falls below a lower threshold. Furthermore, the means for changing the polarization state at this time may be digital signal processing or a polarization controller (not shown).

[0087] For feedback during operation, for example, a GCC, a DSP Com-ch, a control channel of the control plane (a part that controls how data is transferred within the network), a control channel of the network operation system (NW-OpS), etc. are used. Note that the GCC, the DSP Com-ch, a control channel of the control plane, a control channel of the network operation system (NW-OpS), etc. are examples of channels that can send feedback from the receiving side to the transmitting side. Therefore, means for sending feedback from the receiving side to the transmitting side are not limited to these.

[0088] The configuration example shown in Fig. 8 is merely an example, and is not limited to this. For example, in the configuration of Fig. 8, the PMD / PDL monitor 326 of the polarization multiplexed optical receiving device 3 may or may not include a polarization state optimization unit 3261.

[0089] In each of the above-described embodiments, in order to avoid a specific situation in which a fault cannot be distinguished from a fault that is not dependent on the polarization state, the polarization state of the reference signal may be set to different states before and after the entire data length. Alternatively, the reference signal may be changed to multiple polarization states on the transmitting side and averaged.

[0090] In each of the above-described embodiments, even when a plurality of measurement targets are assumed, it is possible to detect a plurality of PDLs and PMDs by optimizing the polarization state for each measurement target.

[0091] [Evaluation] Figure 9 shows an example of a power profile for each polarization with the polarization state optimized when a PDL load of 1 dB is applied, and the position where a PDL load is applied. Reference symbol g21 represents the optical transmission path. Reference symbol g30 represents the position (km) on the horizontal axis and the power (dBm) on the vertical axis. Reference symbol g31 represents the x-axis component (x-pol), and reference symbol g32 represents the y-axis component (y-pol).

[0092] For the evaluation, a simulation was conducted using a single-channel 128 GBd polarization-multiplexed probabilistic constellation-shaped (PCS) 64QAM signal with an input power of 5 dBm, covering a transmission distance of 150 km with three 50 km spans (assuming an optical amplifier noise figure of 5 dB). As shown by symbol g21 in Figure 9, PDL loads of 1, 0.7, 0.4, 0.2, and 0.1 dB were applied at a point 75 km away. Symbol g30 shows an example in which a PDL load of 1 dB was applied at a point 75 km away, and PDL was confirmed at the loaded point 75 km away.

[0093] 10 is a diagram showing estimated PDL values ​​when various PDLs are applied. The horizontal axis represents the set PDL value (dB), and the vertical axis represents the estimated PDL (dB). The circles (symbol g41) represent PDL values, and the line (symbol g42) represents the correct curve.

[0094] From the above evaluation results, it was confirmed that the configuration and method of this embodiment can estimate both the position and the value with high accuracy at a step size of 1 (km).

[0095] (Polarization Optimization) Fig. 11 is a diagram for explaining a method for optimizing the polarization state, where symbol g51 is a transmission signal, symbol g52 is a PDL, and symbol g53 is a receiving side.

[0096] 12 is a diagram showing an example of changes in the rotation angle of the reference signal and the PDL. In the symbols g61 to g63, the horizontal axis is the x-axis and the vertical axis is the y-axis. ｓｉｇ , y ｓｉｇ represent the x and y components of the reference signal. θ is the rotation angle of the reference signal. Symbol g61 represents the case where the rotation angle of the reference signal is 0 degrees (no rotation), symbol g62 represents the case where the rotation angle of the reference signal is approximately 45 degrees, and symbol g63 represents the case where the rotation angle of the reference signal is approximately 30 degrees. Arrow g64 represents the PDL axis, symbol g65 represents the signal, and symbols g66 to g68 represent PDL values.

[0097] 13 is a diagram showing an example of a reference signal and a received signal before the polarization state is optimized, where symbol g71 is the reference signal and symbol g72 is the received signal.

[0098] 14 is a diagram showing a method for optimizing the polarization state when the rotation angle of the reference signal is approximately 45 degrees (reference symbol g62 in FIG. 12 ). The polarization state optimization unit 3261 rotates the reference signal so that it receives the PDL at an angle θ as shown by reference symbol g62 in FIG. 12 , thereby obtaining an optimal polarization state that approaches the received signal obtained by reference symbol g62 in FIG. 12 . However, during this process, the received signal also needs to be rotated in the same way, so the polarization state optimization unit 3261 also rotates the received signal in the same way. Note that the above-described polarization state optimization method is merely an example and is not limited to this.

[0099] In the optical transmission system 1 configured as described above, the polarization multiplexed optical receiving device 3 is provided with a PMD / PDL monitoring unit 326, which makes it possible to estimate polarization-dependent loss and modal dispersion in components in the optical transmission link and identify the amount and location of the fault.

[0100] Alternatively, in the optical transmission system 1B, the polarization multiplexed optical transmitter 2B is provided with a polarization state optimization unit 23, which makes it possible to estimate polarization-dependent loss and modal dispersion in components in the optical transmission link and identify the amount and location of the fault.

[0101] Although an embodiment of the present invention has been described above in detail with reference to the drawings, the specific configuration is not limited to this embodiment, and includes designs within the scope of the gist of the present invention.

[0102] The present invention is applicable to optical transmission and reception systems, optical transmission devices, optical reception devices, and the like.

[0103] 1, 1B...optical transmission system, 2, 2A...polarization multiplexed optical transmitter, 3...polarization multiplexed optical receiver, 4...optical transmission path, 21...data generator, 22...optical signal transmitter, 23...polarization state optimizer, 31...optical signal receiver, 32...digital signal processor, 321...chromatic dispersion compensator, 322...frequency offset compensator, 323...polarization splitter, 324...phase noise compensator, 325...chromatic dispersion load unit, 326...PM D / PDL monitoring unit, 3261, 3261-1, 3261-2, 3261A-1, 3261A-2...polarization state optimization unit, 3262...power profile calculation unit, 3263...PDL / PMD calculation unit, 3273...decoding unit, 3275, 3275A...transmission waveform restoration unit, 3276...linear solution estimation unit, 3277, 3277A...perturbation term estimation unit, 3278...matrix G calculation unit, 3278A...matrix G' calculation unit

Claims

1. An optical receiving device comprising: a monitoring unit that optimizes a polarization state by rotating a received reference signal and the received signal so as to approximate the received signal; calculates a power profile for each polarization for the signal with the optimized polarization state; calculates a power difference between the polarizations from the power profile for each polarization; and identifies the location of occurrence of a polarization-dependent fault and its value from the location of occurrence and amount of the calculated power difference.

2. An optical transmitting device comprising a polarization state optimization unit that optimizes the polarization state for an optical transmission signal, wherein the polarization state optimization unit acquires the nearby polarization state where the difference between polarizations becomes large through feedback from the receiving side, and optimizes the polarization state by changing the polarization state back and forth based on the acquired polarization state and setting the maximum value when the amount of change falls below a lower threshold.

3. An optical communication fault monitoring method, in which a polarization multiplexed optical receiving device optimizes the polarization state by rotating a received reference signal so as to bring it closer to a received signal, calculates a power profile for each polarization for the signal with the optimized polarization state, calculates a power difference between the polarizations from the power profile for each polarization, and identifies the location of an optical communication fault and its value from the location of the calculated power difference and the amount of the difference.

4. The optical communication fault monitoring method according to claim 3, further comprising the steps of: optimizing a polarization state; estimating a perturbation term using the optimized signal and a signal that is an estimated linear solution; and calculating a power profile for each polarization using the estimated perturbation term.