Optical transmission line monitoring device, optical transmission line monitoring method, and optical transmission system

Abstract

To provide a method for specifying a generation position of a polarization fluctuation on an optical transmission line with high accuracy.SOLUTION: An optical transmission line monitoring device comprises: a delay measurement value acquisition part; a refraction index calculation part; a transfer time calculation part; and a distance calculation part, and specifies a polarization fluctuation position that expresses a position where a polarization fluctuation occurs in an optical transmission line between a first end terminal node and a second end terminal node. The delay measurement value acquisition part acquires a delay measurement value expressing a result of a delay measurement between the nodes in a plurality of spans constructing the optical transmission line. The refraction index calculation part calculates, for each of the plurality of spans, a refraction index of an optical fiber distributed in each span on the basis of a span length that expresses a length of each span and the delay measurement value. The transfer time calculation part calculates a transfer time of light between the first end terminal node and the polarization fluctuation position. The distance calculation part calculates a distance between the first end terminal node and the polarization fluctuation position on the basis of the transmission time thus calculated and the refraction index of each span.SELECTED DRAWING: Figure 8

Description

[Technical field]

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

[0002] In optical communication systems, coherent transmission has become mainstream due to the increasing transmission speed. In coherent transmission, signals are transmitted using the phase and polarization of light. Therefore, if the polarization changes suddenly on the optical transmission line, burst errors may occur at the receiving node.

[0003] Furthermore, as the transmission capacity of networks increases, modulation methods that allow each symbol to transmit a large number of bits are being adopted. However, in optical communications using such modulation methods, polarization fluctuations caused by the vibration of optical fibers or lightning strikes can have a significant impact on communication quality. For this reason, technology that identifies the position where polarization fluctuations occur on an optical transmission path has attracted attention (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2023-043154 A Summary of the Invention [Problem to be solved by the invention]

[0005] The position where the polarization fluctuation occurs (polarization fluctuation position) is identified, for example, based on the timing at which the polarization fluctuation is detected at a pair of terminal nodes connected to both ends of the optical transmission line. In this case, the propagation time of light from the terminal node to the polarization fluctuation position is calculated, and the polarization fluctuation position is identified from the propagation time. Here, the speed of light propagating through the optical transmission line depends on the refractive index of the optical fiber. In other words, the refractive index of the optical fiber is used in the calculation to identify the polarization fluctuation position.

[0006] However, the refractive index varies from optical fiber to optical fiber. For example, the refractive index of a typical optical fiber is 1.468, but the refractive index of optical fibers commercially available from several vendors varies by about ±1.5% (1.45 to 1.49). For this reason, when the polarization fluctuation position is identified using conventional techniques, there may be a large error.

[0007] An object of one aspect of the present invention is to provide a method for accurately identifying the position of polarization fluctuation on an optical transmission line by improving the estimation accuracy of the refractive index of an optical fiber that constitutes the optical transmission line. [Means for solving the problem]

[0008] An optical transmission line monitoring device according to one aspect of the present invention identifies a polarization fluctuation position indicating a position where a polarization fluctuation occurs in an optical transmission line between a first terminal node and a second terminal node. The optical transmission line monitoring device includes a delay measurement acquisition unit that acquires a delay measurement value indicating a result of delay measurement between nodes for each of a plurality of spans constituting the optical transmission line, a refractive index calculation unit that calculates a refractive index of an optical fiber laid in each of the plurality of spans based on a span length indicating a length of the span and the delay measurement value, a propagation time calculation unit that calculates a propagation time of light between the first terminal node and the polarization fluctuation position based on timing information indicating a timing at which a polarization fluctuation occurring on the optical transmission line was detected at each of the first terminal node and the second terminal node, and a distance calculation unit that calculates a distance between the first terminal node and the polarization fluctuation position based on the propagation time and the refractive index calculated for each of the plurality of spans. Effect of the Invention

[0009] According to the above-described aspect, it is possible to improve the estimation accuracy of the refractive index of the optical fiber that constitutes the optical transmission line, and to accurately identify the position of polarization fluctuation on the optical transmission line. [Brief description of the drawings]

[0010] [Figure 1] FIG. 11 is a diagram illustrating an example of a method for identifying the location where polarization fluctuation occurs. [Diagram 2] FIG. 1 is a diagram illustrating an example of an optical transmission system in which an optical transmission line is configured with a plurality of spans. [Diagram 3] 1 is a diagram for explaining an outline of a method for determining the refractive index of an optical fiber for each span. [Figure 4] FIG. 2 is a diagram illustrating an example of an arrangement of OSCs. [Diagram 5] FIG. 13 is a diagram illustrating an example of a function for detecting polarization fluctuation. [Figure 6] FIG. 1 illustrates an example of span delay measurement using OSC. [Figure 7] FIG. 13 is a diagram for explaining points at which delays may occur. [Figure 8] 1 is a diagram illustrating an example of a functional configuration of an optical transmission line monitoring device according to an embodiment of the present invention. [Figure 9] 10 is a flowchart showing an example of a method for calculating the refractive index of an optical fiber laid in each span of an optical transmission line. [Figure 10] 13 is a flowchart illustrating an example of a process for identifying a polarization fluctuation position. [Figure 11] 13 is a flowchart illustrating an example of a process for calculating a distance from a terminal node to a polarization fluctuation position. [Figure 12] FIG. 13 is a diagram showing an example of a method for calculating the distance from a terminal node to a polarization fluctuation position. [Figure 13] 1A to 1C are diagrams illustrating effects according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] FIG 1 shows an example of a method for identifying the location where polarization fluctuation occurs. In this example, an optical transmission system includes an optical transmission device (NE) 1A and an optical transmission device (NE) 1B. The optical transmission device 1A and the optical transmission device 1B are connected to each other by an optical transmission path 2. That is, the optical transmission device 1A and the optical transmission device 1B are provided at both ends of the optical transmission path 2. Each of the optical transmission devices 1A and 1B includes a transponder. The transponder includes an optical transmitter, an optical receiver, and a processor such as a DSP (Digital Signal Processor).

[0012] The optical transmission path 2 is composed of a pair of optical fibers 2x and 2y. The optical fiber 2x propagates an optical signal from the optical transmission device 1A to the optical transmission device 1B, and the optical fiber 2y propagates an optical signal from the optical transmission device 1B to the optical transmission device 1A. The optical fiber 2x and the optical fiber 2y are laid adjacent to each other. Although not particularly limited, the optical fiber 2x and the optical fiber 2y are housed in the same cable.

[0013] Here, as shown in Fig. 1(a), it is assumed that at time T0, the state of polarization (SOP) of the optical transmission line 2 changes suddenly. In Fig. 1, P0 represents the position where the polarization change occurs. In the following description, the position where the polarization change occurs may be referred to as the "polarization change position."

[0014] Fluctuations in polarization in the optical transmission line 2 affect the polarization state of the light propagating through the optical fibers 2x and 2y. In the following description, the optical component whose polarization state has been affected in the optical fiber 2x may be referred to as a polarization fluctuation component 3x, and the optical component whose polarization state has been affected in the optical fiber 2y may be referred to as a polarization fluctuation component 3y.

[0015] The polarization fluctuation component 3x propagates from position P0 toward the optical transmission device 1B through the optical fiber 2x. The polarization fluctuation component 3y propagates from position P0 toward the optical transmission device 1A through the optical fiber 2y. Therefore, as shown in FIG. 1(b), the polarization fluctuation components 3x and 3y each reach a position a predetermined distance away from position P0 at time T1.

[0016] Thereafter, the polarization fluctuation component 3x arrives at the optical transmission device 1B at time T2, as shown in FIG. 1(c). Moreover, the polarization fluctuation component 3y arrives at the optical transmission device 1A at time T3, as shown in FIG. 1(d). Then, the position P0 can be calculated based on the difference ΔT between the time T2 and the time T3. Here, it is assumed that the propagation time TL of the light between the optical transmission devices 1A and 1B is obtained by measurement or calculation. In this case, the propagation time "T(A-P0)" between the optical transmission device 1A and the position P0 is expressed by the following formula (1).

number

[0017] Furthermore, the distance "D(A-P0)" between the optical transmission device 1A and the position P0 is expressed by the following formula (2): n represents the refractive index of the optical fiber, and c represents the speed of light in a vacuum.

number

[0018] In this way, the polarization fluctuation position P0 can be specified by measuring the time when the polarization fluctuation is detected at the nodes on both ends of the optical transmission line 2. At this time, the refractive index n of the optical fiber is used.

[0019] In the case shown in FIG. 1, time synchronization is established between the optical transmission device 1A and the optical transmission device 1B. However, even if the times (or counters) between the optical transmission device 1A and the optical transmission device 1B are not synchronized with each other, the polarization fluctuation position P0 can be identified by detecting the polarization fluctuation at the nodes at both ends of the optical transmission path 2. For example, Patent Document 1 (JP 2023-043154 A) discloses a method for identifying the polarization fluctuation position by detecting the polarization fluctuation at each node in a configuration in which the times (or counters) at the nodes at both ends of the optical transmission path are not synchronized with each other.

[0020] Figure 2 shows an example of an optical transmission system in which an optical transmission line is composed of multiple spans. In this example, a WDM (Wavelength Division Multiplexing) signal is transmitted through the optical transmission line 2. The WDM signal transmits multiple optical signals using multiple wavelength channels. Therefore, each node is provided with a ROADM (Reconfigurable Optical Add Drop Multiplexer) 4. The ROADM 4 can drop an optical signal of a desired wavelength from the WDM signal. The ROADM 4 can also add an optical signal to an empty channel of the WDM signal.

[0021] An optical transmission device (NE) 1A and a ROADM 4a are provided at one end node of the optical transmission line 2, and an optical transmission device (NE) 1B and a ROADM 4d are provided at the other end node. In this embodiment, the optical transmission device 1A is connected to the ROADM 4a by a short optical fiber, and the optical transmission device 1B is connected to the ROADM 4d by a short optical fiber. In addition, ROADMs 4 (4b, 4c) are provided at each node between the ROADM 4a and the ROADM 4d. In-line amplifiers (ILAs) may be provided instead of the ROADMs 4b and 4c. The in-line amplifiers collectively amplify the WDM signals with a specified gain.

[0022] In the optical transmission system having the above configuration, optical signals transmitted between the optical transmission device 1A and the optical transmission device 1B propagate through the optical transmission path 2 without being converted into electrical signals in each ROADM (4a to 4d). Therefore, when a polarization fluctuation occurs on the optical transmission path 2, the optical transmission device 1A and the optical transmission device 1B can each detect the polarization fluctuation. Therefore, the position of the polarization fluctuation can be identified by the method described with reference to FIG. 1.

[0023] In the method described with reference to FIG. 1, the refractive index of the optical fiber is used when identifying the polarization fluctuation position. However, the refractive index of the optical fiber varies. Therefore, if a general refractive index value (for example, 1.468) is used, the calculated polarization fluctuation position will include an error. Specifically, if the refractive index of commercially available optical fibers varies by about ±1.5% (1.45 to 1.49), the calculated polarization fluctuation position will also include an error of up to ±1.5%. In this case, for example, if the distance obtained by calculation is 100 km, the calculated polarization fluctuation position will include an error of ±1.5 km.

[0024] Therefore, in order to accurately identify the polarization fluctuation position, the optical transmission line monitoring device according to the embodiment of the present invention has a function of calculating the refractive index of the optical fiber laid in each span constituting the optical transmission line. By utilizing this function, in this embodiment, the refractive indexes of the optical fibers laid in the spans Sa, Sb, and Sc are calculated to be 1.48, 1.47, and 1.46, respectively. Then, the optical transmission line monitoring device calculates the polarization fluctuation position using these refractive index values. As a result, the polarization fluctuation position is accurately identified.

[0025] 3 is a diagram for explaining an outline of a method for calculating the refractive index of an optical fiber for each span. In this example, a ROADM (or an ILA) 4 is provided at each node of the optical transmission system. Each ROADM 4 includes a DM (Delay Measurement) frame processing unit 5 and an OSC (Optical Supervisory Channel) circuit 6. A transponder (TRPN) 7A is connected to the ROADM provided at one end node, and a transponder (TRPN) 7B is connected to the ROADM provided at the other end node. The transponders 7A and 7B correspond to the optical transmission devices 1A and 1B in the example shown in FIG. 1.

[0026] The DM frame processing unit 5 generates a DM frame and transmits it to an adjacent node. The DM frame has a predetermined format and is used to measure the delay time of the span between the adjacent node. When transmitting the DM frame, the DM frame processing unit 5 assigns a time stamp indicating the transmission time to the DM frame. When the DM frame processing unit 5 receives a DM frame from an adjacent node, it returns the DM frame to the sender. Therefore, the DM frame processing unit 5 can measure the delay time between the adjacent node (in this embodiment, RTT (Round Trip Time)) by receiving the DM frame transmitted by itself. The DM frame processing unit 5 is not particularly limited, but is preferably realized by a hardware circuit such as an FPGA (Field Programmable Gate Array). In this case, the processing time by the DM frame processing unit 5 is sufficiently short, so that the delay time can be measured with high accuracy. The DM frame processing unit 5 is an example of a delay measurement unit that measures the delay time of the span between the adjacent node.

[0027] The OSC circuit 6 transmits control signals between nodes using the OSC. The above-mentioned DM frame is transmitted to an adjacent node via the OSC by the OSC circuit 6. Here, the OSC is set using a wavelength different from the wavelength region (e.g., C-band) for transmitting data signals, as shown in FIG. 4. As an example, the wavelength of the OSC is 1511 nm (or 198.4 THz).

[0028] For example, the DM frame processing unit 5 of the ROADM 4a transmits a DM frame to the ROADM 4b. A timestamp indicating the transmission time is attached to this DM frame. Then, this DM frame is transmitted to the ROADM 4b via the OSC. The DM frame processing unit 5 of the ROADM 4b returns the received DM frame to the ROADM 4a. Then, this DM frame is transmitted to the ROADM 4a via the OSC. Then, the DM frame processing unit 5 of the ROADM 4a can measure the delay time between the ROADM 4a and the ROADM 4b by referring to the timestamp attached to the DM frame received from the ROADM 4b.

[0029] Similarly, the delay times for the other spans are measured, and the delay times for each span are notified to the optical transmission line monitoring device 100.

[0030] The transponders (7A, 7B) mounted on the nodes at both ends of the optical transmission line 2 are equipped with a polarization fluctuation monitor 8. The polarization fluctuation monitor 8 monitors the state of polarization (SOP) of the received optical signal. When the polarization fluctuation monitor 8 detects a polarization fluctuation greater than a predetermined threshold level, it notifies the optical transmission line monitoring device 100 that a polarization fluctuation has occurred. For example, in the case shown in FIG. 1, the polarization fluctuation monitor 8 mounted on NE 1B notifies the optical transmission line monitoring device 100 that it has detected a polarization fluctuation at time T2. Moreover, the polarization fluctuation monitor 8 mounted on NE 1A notifies the optical transmission line monitoring device 100 that it has detected a polarization fluctuation at time T3.

[0031] Fig. 5 shows an example of a function for detecting polarization fluctuation. The function for detecting polarization fluctuation is realized by using a receiver that recovers symbols from a received optical signal. In this example, the receiver includes a 90-degree optical hybrid 31, a fixed equalizer 32, an adaptive equalizer 33, a phase estimation unit 34, a determination unit 35, and a fluctuation determination unit 36.

[0032] The 90-degree optical hybrid 31 generates an electrical signal representing the electric field of the received optical signal by using a local light source (not shown). The fixed equalizer 32 equalizes the output signal of the 90-degree optical hybrid 31. For example, the fixed equalizer 32 compensates for chromatic dispersion and the like. The adaptive equalizer 33 includes a digital filter such as an FIR filter, and adaptively equalizes the output signal of the fixed equalizer 32. The adaptive equalizer 33 also performs polarization separation. At this time, the coefficient of each tap of the digital filter is updated based on the input signal and output signal of the adaptive equalizer 33.

[0033] The phase estimator 34 compensates for the phase offset of the output signal of the adaptive equalizer 33. This recovers the phase of each symbol. The determiner 35 recovers the data assigned to each symbol in the transmitting node based on the output signal of the phase estimator 34.

[0034] The fluctuation determining unit 36 ​​monitors the polarization fluctuation occurring in the optical transmission path 2 between the ROADM 4a and the ROADM 4d, based on the output signal of the adaptive equalizer 33 or the tap coefficient of the digital filter constituting the adaptive equalizer 33. Then, the fluctuation determining unit 36 ​​outputs a polarization fluctuation detection flag when the polarization fluctuation is greater than a predetermined threshold level. That is, the polarization fluctuation detection flag indicates that a polarization fluctuation greater than a predetermined threshold level has occurred in the optical transmission path 2.

[0035] The fluctuation determining unit 36 ​​is realized, for example, by a digital signal processing circuit, which is a hardware circuit. Alternatively, the fluctuation determining unit 36 ​​is realized by a processor system including a processor and a memory. In this case, the processor provides the function of the fluctuation determining unit 36 ​​by executing a program that outputs a detection flag when a polarization fluctuation is detected.

[0036] The polarization fluctuation monitor 8 shown in Fig. 3 corresponds to the fluctuation determining unit 36 ​​shown in Fig. 5. Alternatively, the polarization fluctuation monitor 8 corresponds to the 90-degree optical hybrid 31, the fixed equalizer 32, the adaptive equalizer 33, and the fluctuation determining unit 36. The polarization fluctuation monitor 8 outputs a polarization fluctuation detection flag when a polarization fluctuation greater than a predetermined threshold level occurs in the optical transmission line 2. This allows the optical transmission line monitoring device 100 to be notified of the time when the polarization fluctuation is detected.

[0037] The optical transmission line monitoring device 100 calculates the refractive index of the optical fiber laid in each span based on the delay time measured for each span. At this time, it is preferable to calculate the refractive index based on the minimum value of the delay times measured multiple times for each span. The optical transmission line monitoring device 100 also calculates the propagation time of light between the terminal node of the optical transmission line 2 and the polarization fluctuation position. This propagation time is not particularly limited, but is calculated, for example, by the method described with reference to FIG. 1. Furthermore, the optical transmission line monitoring device 100 specifies the distance between the terminal node of the optical transmission line 2 and the polarization fluctuation position (i.e., the polarization fluctuation position) based on the refractive index of each span and the propagation time of light between the terminal node of the optical transmission line 2 and the polarization fluctuation position.

[0038] FIG. 6 shows an example of span delay measurement using OSC. In this example, the propagation delay between ROADM 4i and ROADM 4j is measured. Each of ROADM 4i and 4j represents a ROADM 4 (or an ILA) provided at an arbitrary node of the optical transmission path 2. However, ROADM 4i and 4j are provided at nodes adjacent to each other. A pair of optical fibers is laid in the span between ROADM 4i and 4j. Optical fiber 2x propagates an optical signal from ROADM 4i to ROADM 4j, and optical fiber 2y propagates an optical signal from ROADM 4j to ROADM 4i.

[0039] Each ROADM 4i, 4j includes an optical amplifier that amplifies the received WDM signal. An optical device that extracts an OSC signal from the received optical signal is provided on the input side of the optical amplifier. An optical device that multiplexes the WDM signal and the OSC signal is provided on the output side of the optical amplifier. Although omitted for clarity of the drawing, the ROADM further includes a wavelength selective switch (WSS). The WSS can process optical signals for each wavelength channel.

[0040] The CPU 11 controls the operation of the ROADM. The CPU 11 also generates a control signal for controlling the optical transmission system. This control signal is transmitted to the ROADM provided in the other node by the OSC circuits 6W and 6E. The DM frame processing unit 5 processes the DM frame used for delay measurement as described above. Then, the DM frame processing unit 5 uses the DM frame to perform delay measurement to obtain the propagation delay between adjacent nodes.

[0041] The switch 12 controls the connection between the CPU 11 and the OSC circuits 6W, 6E, and also controls the connection between the DM frame processing unit 5 and the OSC circuits 6W, 6E. The switch 12 is, for example, an L2 switch that processes layer 2 frames.

[0042] When measuring the propagation delay between the ROADMs 4i and 4j, for example, the DM frame processing unit 5 of the ROADM 4i generates a DM frame. A timestamp indicating the transmission time is added to this DM frame. This DM frame is also guided to the OSC circuit 6E by the switch 12. The OSC circuit 6E then transmits the DM frame to the ROADM 4j using OSC. The DM frame is then propagated via the optical fiber 2x and arrives at the ROADM 4j.

[0043] In the ROADM 4j, the OSC circuit 6W extracts a DM frame from the OSC. This DM frame is guided to the DM frame processing unit 5 by the switch 12. The DM frame processing unit 5 returns this DM frame to the ROADM 4i. At this time, this DM frame is guided to the OSC circuit 6W by the switch 12. The OSC circuit 6W transmits the DM frame to the ROADM 4i using the OSC. Then, the DM frame is propagated via the optical fiber 2y and arrives at the ROADM 4i.

[0044] In the ROADM 4i, the OSC circuit 6E extracts the DM frame from the OSC. This DM frame is guided to the DM frame processing unit 5 by the switch 12. The DM frame processing unit 5 then calculates the difference between the timestamp added to the DM frame and the current time. This provides the propagation delay value between the ROADMs 4i and 4j.

[0045] In the embodiment shown in FIG. 6, the optical fiber that transmits the DM frame from the ROADM4j to the ROADM4i may be the same as the optical fiber that transmits the DM frame from the ROADM4i to the ROADM4j. In this case, however, OSC signals of the same wavelength are mixed in one optical fiber, and the signal quality deteriorates. Therefore, to avoid this problem, OSC signals of different wavelengths may be used. In the embodiment shown in FIG. 6, the RTT of the span is measured, but when the time of each node is synchronized, the transmission delay may be measured in the ROADM4j by transmitting a DM frame from the ROADM4i to the ROADM4j.

[0046] As described above, the propagation delay between nodes is measured using DM frames. However, in the configuration shown in Fig. 6, in each ROADM, the DM frames are processed by the switch 12. Here, the switch 12 processes not only the DM frames but also the control frames generated by the CPU 11. Therefore, when the amount of control frames generated by the CPU 11 is large, the switch 12 may become congested. And when the switch 12 becomes congested, the propagation time of the DM frames may not be measured accurately.

[0047] Therefore, in the optical transmission line monitoring method according to the embodiment of the present invention, multiple delay measurements are performed for each span, and the optical transmission line monitoring device 100 adopts the smallest value among the multiple measured values ​​as the propagation delay value for each span.

[0048] Here, the switch 12 is not always congested, but becomes congested when there is contention with other frames (for example, control frames generated by the CPU 11). For example, assume that the processing capacity of the switch 12 is 1 Gbps and the average rate of signals passing through the switch 12 is 800 Mbps. In this case, the usage rate of the switch 12 is 80 percent, and if a DM frame arrives during the remaining 20 percent of the time, the DM frame will not be delayed in the switch 12.

[0049] On the other hand, in this embodiment, the DM frame is processed four times by the switch 12 in one delay measurement, as shown in Fig. 7. That is, the DM frame passes through four congestion points in one delay measurement. Note that the method of making a DM frame go back and forth between a pair of nodes to determine the propagation delay is supported, for example, in Ethernet (registered trademark).

[0050] In this case, the probability that the DM frame is not affected by congestion is 0.2 to the fourth power, which is 0.16 percent. That is, the probability that the DM frame is affected by congestion is 99.84 percent. Therefore, for example, when 10,000 delay measurements are performed, the probability that all delay measurements are affected by congestion is 0.9984 to the 10,000 power, which is approximately 0.0001 percent. In other words, when 10,000 delay measurements are performed, the probability that at least one propagation delay value that is not affected by congestion is obtained is 99.9999 percent. Therefore, by detecting the minimum value among the 10,000 measured values, there is a very high probability that a delay measurement value that is not affected by congestion can be obtained.

[0051] The delay measurement is performed for 10 seconds at 1000 frames / second. Also, assume that the length of the DM frame is 128 bytes. In this case, the bandwidth used by the DM frame is about 1 Mbps, and it does not affect the transmission of other control frames.

[0052] In addition, if the period for transmitting DM frames is synchronized with other controls (for example, the period for transmitting control frames by the CPU 11), all DM frames may be affected by congestion. To solve this problem, the DM frame processing unit 5 may transmit DM frames at random periods when performing multiple delay measurements.

[0053] FIG. 8 shows an example of a functional configuration of the optical transmission line monitoring device 100 according to the embodiment of the present invention. In this embodiment, the optical transmission line monitoring device 100 includes a delay measurement value acquisition unit 101, a refractive index calculation unit 102, a propagation time calculation unit 103, and a distance calculation unit 104. The optical transmission line monitoring device 100 may further include other functions not shown in FIG. 8. For example, the optical transmission line monitoring device 100 includes a function of communicating with an optical transmission device (in this embodiment, ROADM / ILA 4) implemented in each node of the optical transmission line 2. The optical transmission line monitoring device 100 also includes a function of communicating with transponders (7A, 7B) provided in each end node. However, the optical transmission line monitoring device 100 may communicate with the transponders (7A, 7B) via ROADMs (4a, 4b). Then, the optical transmission line monitoring device 100 identifies a polarization fluctuation position that indicates a position where a polarization fluctuation occurs in the optical transmission line 2 between the first end node (ROADM 4a) and the second end node (ROADM 4d).

[0054] The delay measurement value acquiring unit 101 acquires a delay measurement value indicating a result of delay measurement between nodes for each of a plurality of spans constituting the optical transmission line 2. The ROADM 4 implemented in each node performs delay measurement of adjacent spans in response to an instruction from the delay measurement value acquiring unit 101.

[0055] The refractive index calculation unit 102 calculates the refractive index of the optical fiber for each span based on the span length and the delay measurement value. When multiple delay measurements are performed for each span, the refractive index calculation unit 102 calculates the refractive index of the optical fiber based on the span length and the minimum value of multiple delay measurement values ​​obtained by the multiple delay measurements. This method can substantially avoid the influence of congestion of the switch in the optical transmission device installed in each node.

[0056] The propagation time calculation unit 103 calculates the propagation time of light between the first terminal node and the polarization fluctuation position based on timing information indicating the timing at which the polarization fluctuation occurring on the optical transmission path 2 was detected at each of the first terminal node and the second terminal node. The distance calculation unit 104 calculates the distance between the first terminal node and the polarization fluctuation position based on the propagation time of light between the first terminal node and the polarization fluctuation position and the refractive index calculated for each span.

[0057] According to this configuration, the polarization fluctuation position is identified using the refractive index calculated based on the delay measurement of each span. Therefore, even if the characteristics (here, the refractive index) of the optical fiber laid in each span vary, the polarization fluctuation position can be identified with high accuracy.

[0058] Fig. 9 is a flowchart showing an example of a method for calculating the refractive index of an optical fiber laid in each span of an optical transmission line. The process of this flowchart is executed before identifying the polarization variation position. The process of this flowchart is also executed for each span constituting the optical transmission line 2. In the following description, the span for which the process of the flowchart shown in Fig. 9 is executed may be referred to as a "target span".

[0059] In S1, the optical transmission line monitoring device 100 instructs the execution of delay measurement of the target span. The delay measurement instruction is given to the ROADM 4 implemented at one end of the target span. The ROADM 4 measures the propagation delay time between adjacent nodes using a DM frame in response to the delay measurement instruction. At this time, the ROADM 4 executes multiple delay measurements. The number of delay measurements to be executed may be notified from the optical transmission line monitoring device 100. The number of delay measurements to be executed may be determined according to the usage rate of the L2 switch that processes the signal in the ROADM 4. In this case, the number of delay measurements to be executed is determined, for example, so that the probability that the influence of congestion of the L2 switch occurs in all delay measurements is lower than a predetermined threshold. Alternatively, the number of delay measurements to be executed is determined so that the DM frame is propagated without being influenced by congestion of the L2 switch in at least one delay measurement. Then, the ROADM 4 transmits the result of the delay measurement to the optical transmission line monitoring device 100. That is, multiple delay measurement values ​​are transmitted to the optical transmission line monitoring device 100.

[0060] In S2, the delay measurement value acquisition unit 101 acquires a measurement result from the ROADM 4. That is, the delay measurement value acquisition unit 101 acquires a plurality of delay measurement values ​​for the target span. Then, in S3, the delay measurement value acquisition unit 101 identifies the minimum value among the plurality of delay measurement values ​​for the target span. Note that the minimum delay measurement value is an accurate delay measurement value that is not affected by congestion.

[0061] In S4, the refractive index calculation unit 102 acquires span length data representing the span length of the target span. It is assumed that the span length of each span is known and that the span length data is stored in the memory of the optical transmission line monitoring device 100.

[0062] In S5, the refractive index calculation unit 102 calculates the refractive index of the optical fiber laid in the target span. The refractive index n is calculated by equation (3). Here, c represents the speed of light in a vacuum. L represents the span length of the target span. The RTT is the minimum value (minimum measured delay value) among multiple delay measurement values ​​obtained by delay measurement.

number

[0063] In S6, the refractive index calculation unit 102 corrects the refractive index obtained in S5. That is, in this embodiment, as shown in FIG. 4, the wavelength of the OSC that transmits the DM frame is different from the wavelength of the C-band that transmits the data signal. As an example, the wavelength of the OSC is 1511 nm, and the central wavelength of the C-band is about 1550 nm. On the other hand, the refractive index of the optical fiber depends on the wavelength. Therefore, in order to obtain the refractive index for the wavelength that transmits the data signal, it is preferable to correct the refractive index calculated for the OSC light.

[0064] When the chromatic dispersion D of the optical fiber is expressed by equation (4), the relationship in equation (5) is obtained.

number

number

[0065] Furthermore, by multiplying both sides of equation (5) by c, we obtain equation (6).

number

[0066] In the embodiment of the present invention, the delay measurement is performed using the OSC. Therefore, the refractive index at the wavelength of the OSC is calculated. Therefore, the refractive index calculation unit 102 corrects the refractive index obtained in S5 based on the chromatic dispersion D of the optical fiber. This allows the refractive index in the wavelength region (in this example, the C band) in which the data signal is transmitted to be obtained.

[0067] The chromatic dispersion characteristics of an optical fiber depend on the type of fiber. For example, the chromatic dispersion characteristics of a single mode optical fiber (SMF), a dispersion shifted single mode optical fiber (DSF), and a non-zero dispersion shifted single mode optical fiber (NZ-DSF) are different from one another. Therefore, it is preferable that the optical transmission line monitoring device 100 has a function of identifying or estimating the type of optical fiber laid in each span. In this case, even if the type of optical fiber laid in each span is unknown, it is possible to identify the chromatic dispersion characteristics and obtain the correction amount of formula (6).

[0068] In this embodiment, the refractive index is calculated based on the RTT of each span. In the example shown in FIG. 6, the DM frame passes through the optical fiber 2x and the optical fiber 2y. Therefore, the refractive index calculated by the formulas (3) to (6) is the average value of the refractive index of the optical fiber 2x and the refractive index of the optical fiber 2y. However, a plurality of optical fibers laid in the same span are usually manufactured by the same vendor in the same manufacturing process and housed in one cable. For this reason, the physical properties (here, the refractive index) of a plurality of optical fibers (here, the optical fibers 2x and 2y) laid in the same span are expected to be almost the same as each other. Therefore, the refractive index calculated by the formulas (3) to (6) is substantially the refractive index of the optical fiber 2x and the refractive index of the optical fiber 2y.

[0069] Fig. 10 is a flowchart showing an example of a process for identifying a polarization fluctuation position. Note that, before the process of this flowchart is executed, it is assumed that the refractive index of the optical fiber laid in each span is calculated by the procedure shown in Fig. 9.

[0070] In S11, the optical transmission line monitoring device 100 acquires span length data indicating the span length of each span. In S12, the optical transmission line monitoring device 100 acquires refractive index data indicating the refractive index of the optical fiber laid in each span. Note that the span length data and the refractive index data are assumed to be stored in advance in the memory of the optical transmission line monitoring device 100.

[0071] In S13, the optical transmission line monitoring device 100 waits for a detection flag transmitted from a pair of terminal nodes located at both ends of the optical transmission line 2. Each terminal node is equipped with a transponder (transponders 7A and 7B in the example shown in FIG. 3). Each of the transponders 7A and 7B includes a polarization fluctuation monitor 8 and constantly monitors the polarization fluctuation in the optical transmission line 2. The polarization fluctuation is detected, for example, by a receiver shown in FIG. 5. In this case, when a polarization fluctuation greater than a predetermined threshold level is detected, the fluctuation determining unit 36 ​​transmits a detection flag to the optical transmission line monitoring device 100. The detection flag is provided with timing information indicating the timing at which the fluctuation determining unit 36 ​​detects the polarization fluctuation. Then, when the detection flag is received from the pair of terminal nodes, the process of the optical transmission line monitoring device 100 proceeds to S14.

[0072] In S14, the propagation time calculation unit 103 calculates the propagation time required for light to propagate from the terminal node to the polarization fluctuation position via the optical transmission path 2 based on timing information acquired from a pair of terminal nodes. This propagation time is not particularly limited, but is calculated, for example, using the above-mentioned formula (1). Then, in S15, the distance calculation unit 104 calculates the distance from the terminal node to the polarization fluctuation position based on the propagation time calculated by the propagation time calculation unit 103, the span length of each span, and the refractive index of each span.

[0073] 11 is a flowchart showing an example of a process for calculating the distance from a terminal node to a polarization fluctuation position. The process of this flowchart corresponds to S15 shown in FIG.

[0074] In this embodiment, as shown in FIG. 12, the optical transmission system includes nodes N(0) to N(K), and the optical transmission line 2 is composed of K spans Si (i=1 to K). Node N(0) and node N(K) are terminal nodes of the optical transmission line 2. The span length Li of each span Si is assumed to be known. The refractive index ni of the optical fiber laid in each span Si is calculated by the procedure shown in FIG. 9. The propagation time T for light to propagate from the terminal node N(0) to the polarization variation position via the optical transmission line 2 is calculated by the propagation time calculation unit 103 in S14 of FIG. 10.

[0075] In S21, the distance calculation unit 104 initializes a variable i to “1.” The variable i identifies each span.

[0076] In S22, the distance calculation unit 104 calculates the propagation time Ti that light takes to propagate from the terminal node N(0) to the node N(i) via the optical transmission line 2. The propagation time Ti is calculated by equation (7).

[0077]

number

[0078] In S23, the distance calculation unit 104 compares the propagation time Ti with the propagation time T. Here, as described above, the propagation time Ti represents the time it takes for light to propagate from the terminal node N(0) to the node N(1) via the optical transmission path 2. The propagation time T represents the time it takes for light to propagate from the terminal node N(0) to the polarization fluctuation position via the optical transmission path 2. Then, when the propagation time Ti is smaller than the propagation time T, the variable i is incremented in S24, and the process of the distance calculation unit 104 returns to S22. Therefore, the processes of S22 to S24 are repeatedly executed until the propagation time Ti becomes larger than the propagation time T.

[0079] For example, when the variable i is "2", the propagation time T2 that it takes for light to propagate from the terminal node N(0) to the node N(2) via the optical transmission line 2 is calculated. That is, the sum of the propagation time of the span S1 and the propagation time of the span S2 is calculated.

[0080] Then, when the propagation time Ti becomes greater than the propagation time T, the process of the distance calculation unit 104 proceeds to S25. In the following description, it is assumed that when the variable i is "M", as shown in Fig. 12, the propagation time Ti becomes greater than the propagation time T. In this case, the distance calculation unit 104 determines that a polarization fluctuation has occurred between node N(M-1) and node N(M).

[0081] In S25, the distance calculation unit 104 calculates the distance D(pol) from the termination node N(0) to the polarization fluctuation position using equation (8).

[0082]

number

[0083] In this way, according to the embodiment of the present invention, the refractive index of the optical fiber laid in each span constituting the optical transmission line 2 is used to calculate the distance from the terminal node to the polarization variation position. Here, the refractive index of each optical fiber is calculated based on a delay measurement performed for each span using the OSC. In addition, the value of each refractive index is corrected taking into account the difference between the wavelength of the OSC and the wavelength that transmits the data signal. Therefore, the distance from the terminal node to the polarization variation position can be calculated with high accuracy, and the polarization variation position can be identified with high accuracy.

[0084] The optical transmission line monitoring device 100 is realized by, for example, a computer including a processor and a memory. In this case, an optical transmission line monitoring program describing the procedures of the flowcharts shown in Figs. 9 to 11 is stored in the memory. Then, the processor executes the optical transmission line monitoring program to provide the functions of a delay measurement value acquisition unit 101, a refractive index calculation unit 102, a propagation time calculation unit 103, and a distance calculation unit 104. In addition, this computer includes an interface for communicating with each node of the optical transmission system.

[0085] 13 is a diagram for explaining the effect of an embodiment of the present invention. In this embodiment, as shown in FIG. 13(a), the optical transmission system includes a terminal node N0, relay nodes N1 to N8, and a terminal node N9. That is, the optical transmission path is made up of spans S1 to S9. The span length of each of the spans S1 to S9 is 80 km.

[0086] In the optical transmission system having the above configuration, when polarization fluctuation occurs on the optical transmission line 2, the optical transmission line monitoring device 100 calculates the propagation time when light propagates from the terminal node N0 to the position where the polarization fluctuation occurs through the optical transmission line 2. In this embodiment, it is assumed that this propagation time is 2000 μsec.

[0087] Fig. 13(b) shows the refractive index and delay in a comparative example for explaining the effect of an embodiment of the present invention. Fig. 13(c) shows the refractive index and delay in an embodiment of the present invention. In Fig. 13(b) and Fig. 13(c), the "delay" is half the RTT obtained by transmitting a DM frame via OSC. The "accumulated delay" is the sum of the delays of each span starting from the end node N0.

[0088] In the comparative example shown in Fig. 13(b) and the embodiment shown in Fig. 13(c), the procedure for calculating the distance from the terminal node N0 to the polarization fluctuation position is substantially the same. That is, the optical transmission line monitoring device 100 calculates the distance from the terminal node N0 to the polarization fluctuation position according to the procedure of S15 shown in Fig. 10 (S21 to S25 shown in Fig. 11). Then, in this embodiment, by repeatedly executing S21 to S24, it is detected that the accumulated delay exceeds "2000 μsec" in span S6. In this case, it is determined that polarization fluctuation has occurred in span S6 (i.e., between node N5 and node N6).

[0089] After that, the optical transmission line monitoring device 100 executes the calculation of formula (8) in S25. In the comparative example, a representative value of "1.4675" is used as the refractive index of the optical fiber of each span. That is, in the calculation of formula (8), n i (i=1~5) and n M (M=6) is 1.4675 in both cases. As a result, the distance from the end node N0 to the polarization fluctuation position is obtained as "412.6 km". In contrast, in the embodiment of the present invention, in the calculation of equation (8), n i (i=1~5) and n M The values ​​obtained by delay measurement are used for (M=6). As a result, the distance from the end node N0 to the polarization fluctuation position is obtained as "409.6 km".

[0090] Here, the refractive index of commercially available optical fibers has a variation of about ±1.5% (±0.02 when converted into a refractive index value). That is, when the distance between the terminal node and the polarization fluctuation position is about 400 km, the error in the polarization fluctuation position is about ±6 km in the comparative example. In contrast, in the embodiment of the present invention, for example, when the measurement of the transmission delay using a DM frame is ±1 μsec, the error in the refractive index is about ±0.004. That is, compared to the comparative example, the monitoring method of the embodiment of the present invention has five times the accuracy. Therefore, the error in the polarization fluctuation position is about ±1.2 km.

[0091] In the above embodiment, a DM frame supported by Ethernet or the like is transmitted in delay measurement, but the embodiment of the present invention is not limited to this configuration. For example, in a configuration in which an OTN frame is transmitted via the optical transmission line 2, delay measurement may be performed by setting a DM bit provided in the overhead of the OTN frame. [Explanation of symbols]

[0092] 1A, 1B Optical transmission equipment 2. Optical Transmission Line 2x, 2y optical fiber 4 (4a~4d, 4i, 4j) ROADM 5 DM frame processing section 6 OSC circuit 7A, 7B Transponders 8 Polarization fluctuation monitor 36 Fluctuation judgment section 100 Optical transmission line monitoring device 101 Delay measurement value acquisition unit 102 Refractive index calculation section 103 Propagation time calculation unit 104 Distance calculation section

Claims

1. An optical transmission line monitoring device that identifies a polarization fluctuation position indicating a position where a polarization fluctuation occurs in an optical transmission line between a first terminal node and a second terminal node, a delay measurement value acquisition unit that acquires delay measurement values ​​representing results of delay measurement between nodes for each of a plurality of spans constituting the optical transmission line; a refractive index calculation unit that calculates, for each of the plurality of spans, a refractive index of an optical fiber laid in the span based on a span length representing the length of the span and the delay measurement value; a propagation time calculation unit that calculates a propagation time of light between the first terminal node and the polarization fluctuation position based on timing information indicating timings at which polarization fluctuations occurring on the optical transmission line are detected at the first terminal node and the second terminal node, respectively; a distance calculation unit that calculates a distance between the first terminal node and the polarization variation position based on the propagation time and the refractive index calculated for each of the plurality of spans; An optical transmission line monitoring device comprising:

2. When the delay measurement is performed a plurality of times for each of the plurality of spans, the refractive index calculation unit calculates the refractive index of the optical fiber laid in each of the plurality of spans based on the span length of the span and the minimum value of the plurality of delay measurement values ​​obtained by the plurality of delay measurements.

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

3. A plurality of optical transmission devices provided on an optical transmission path between a first terminal node and a second terminal node; an optical transmission line monitoring device for identifying a polarization fluctuation position indicating a position where a polarization fluctuation has occurred in the optical transmission line, the optical transmission line is composed of a plurality of spans, Each optical transmission device includes a delay measurement unit that measures a delay time of a span between adjacent nodes, The optical transmission line monitoring device comprises: a delay measurement value acquiring unit that acquires, for each of the plurality of spans, a delay measurement value representing a result of delay measurement between nodes from a corresponding optical transmission device; a refractive index calculation unit that calculates, for each of the plurality of spans, a refractive index of an optical fiber laid in the span based on a span length representing the length of the span and the delay measurement value; a propagation time calculation unit that calculates a propagation time of light between the first terminal node and the polarization fluctuation position based on timing information indicating timings at which polarization fluctuations occurring on the optical transmission line are detected at the first terminal node and the second terminal node, respectively; a distance calculation unit that calculates a distance between the first terminal node and the polarization variation position based on the propagation time and the refractive index calculated for each of the plurality of spans.

1. An optical transmission system comprising:

4. the optical transmission line transmits a data signal using a first wavelength; The delay measurement unit is transmitting a delay measurement frame to an adjacent node via the optical transmission line using a second wavelength different from the first wavelength; receiving the delay measurement frame from the adjacent node via the optical transmission path; Measure the delay time of the span between the adjacent node based on the transmission time of the delay measurement frame and the reception time of the delay measurement frame.

4. The optical transmission system according to claim 3.

5. The refractive index calculation unit corrects the calculated refractive index for each of the plurality of spans based on a difference between the first wavelength and the second wavelength.

5. The optical transmission system according to claim 4.

6. The refractive index calculation unit corrects the calculated refractive index for each of the plurality of spans based on a difference between the first wavelength and the second wavelength and a type of optical fiber laid in the span.

5. The optical transmission system according to claim 4.

7. The refractive index calculation unit corrects the calculated refractive index for each of the plurality of spans based on a difference between the first wavelength and the second wavelength and a chromatic dispersion characteristic of an optical fiber laid in the span.

5. The optical transmission system according to claim 4.

8. 1. An optical transmission line monitoring method for identifying a polarization fluctuation position indicating a position where a polarization fluctuation occurs in an optical transmission line between a first end node and a second end node, comprising: acquiring delay measurement values ​​representing results of delay measurement between nodes for each of a plurality of spans constituting the optical transmission line; calculating, for each of the plurality of spans, a refractive index of an optical fiber installed in the span based on a span length representing a length of the span and the delay measurement value; calculating a propagation time of light between the first terminal node and the polarization fluctuation position based on timing information indicating timings at which polarization fluctuation occurring on the optical transmission line was detected at the first terminal node and the second terminal node, respectively; Calculating a distance between the first terminal node and the polarization variation location based on the propagation time and the calculated refractive index for each of the plurality of spans.

2. An optical transmission line monitoring method comprising:

Images