Optical transmission line monitoring device and optical transmission line monitoring method

Abstract

To accurately estimate optical power near a transmitting end.SOLUTION: An optical transmission line monitoring device includes a first compensating unit that compensates for a part of the chromatic dispersion of an optical transmission line with respect to an electric field signal that indicates an optical field component of an optical signal digitally coherently received from the optical transmission line, a nonlinear compensating unit that compensates for degradation due to nonlinear optical effects of the optical transmission line with respect to the electric field signal after compensation by the first compensating unit, a second compensating unit that compensates for the remaining chromatic dispersion, excluding a part of the chromatic dispersion of the optical transmission line, with respect to the electric field signal after compensation by the nonlinear compensating unit, and additionally compensates for chromatic dispersion at a virtual position outside the optical transmission line with respect to the compensated electric field signal, an adjusting unit that adjusts the amount of compensation additionally compensated by the second compensating unit, a third compensating unit that compensates for the chromatic dispersion of a reference signal that indicates the optical field component of the optical signal at a transmitting end of the optical transmission line with the compensation amount, and an estimating unit that estimates optical power near the transmitting end based on the correlation between the amplitudes of the signal output from the second compensating unit and the signal output from the third compensating unit.SELECTED DRAWING: Figure 4

Description

[Technical Field]

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

[0002] A transmission line monitoring device is known that has a first compensator that compensates for part of the chromatic dispersion of the transmission line for an electric field signal that indicates the optical field component of an optical signal digitally coherently received from the transmission line. Another transmission line monitoring device is known that has a second compensator that compensates for degradation due to nonlinear optical effects of the transmission line for the electric field signal after compensation by the first compensator. Another transmission line monitoring device is known that has a third compensator that compensates for the remaining chromatic dispersion of the transmission line for the electric field signal after compensation by the second compensator (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2018-133725 Summary of the Invention [Problem to be solved by the invention]

[0004] The second compensation unit compensates for degradation due to nonlinear optical effects by applying a phase inverse rotation proportional to the power of the input electric field signal, thereby compensating for phase rotation proportional to the power of the electric field signal itself, known as self-phase modulation (SPM).

[0005] In the second compensator, only the argument of the complex number changes. Therefore, the magnitudes of the first electric field signal input to the second compensator and the second electric field signal output from the second compensator remain unchanged and are equivalent. The waveform of the second electric field signal changes as the subsequent third compensator compensates for chromatic dispersion. In other words, the magnitude of the second electric field signal output from the second compensator and input to the third compensator differs from the magnitude of the third electric field signal output from the third compensator.

[0006] However, when the compensation amount of the third compensator is zero or close to zero, the magnitudes of the first electric field signal, the second electric field signal, and the third electric field signal remain unchanged and are equivalent. That is, when the compensation amount of the third compensator is zero or close to zero, the compensation effect of the second compensator is reduced. Therefore, there is a risk that the accuracy of estimating the optical power will decrease near the transmitting end where the compensation amount of the third compensator is zero or close to zero.

[0007] Therefore, in one aspect, an object of the present invention is to provide an optical transmission line monitoring device and an optical transmission line monitoring method that accurately estimate the optical power near the transmitting end. [Means for solving the problem]

[0008] In one embodiment, an optical transmission line monitoring device includes: a first compensating unit that compensates for a part of the chromatic dispersion of the optical transmission line with respect to an electric field signal indicating an optical field component of an optical signal digitally coherently received from the optical transmission line; a nonlinear compensating unit that compensates for degradation due to a nonlinear optical effect of the optical transmission line with respect to the electric field signal after compensation by the first compensating unit; a second compensating unit that compensates for the remaining chromatic dispersion of the optical transmission line excluding the part with respect to the chromatic dispersion of the optical transmission line with respect to the electric field signal after compensation by the nonlinear compensating unit, and additionally compensates for chromatic dispersion at a virtual position outside the optical transmission line with respect to the compensated electric field signal; an adjusting unit that adjusts the amount of compensation additionally compensated by the second compensating unit; a third compensating unit that compensates for the chromatic dispersion of a reference signal indicating an optical field component of the optical signal at a transmitting end of the optical transmission line with the compensation amount; and an estimating unit that estimates optical power near the transmitting end based on the correlation between the amplitudes of the first signal output from the second compensating unit and the second signal output from the third compensating unit. [Effects of the Invention]

[0009] The optical power near the transmitting end can be estimated with high accuracy. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is an example of an optical transmission system. [Figure 2] 2 is an example of a hardware configuration of an optical receiving device and an optical transmission line monitoring device. [Figure 3] 2 is an example of a functional configuration of a DSP (Digital Signal Processor) and an FPGA (Field Programmable Gate Array) according to the first embodiment. [Figure 4] FIG. 4 is a diagram illustrating an example of additional compensation according to the first embodiment. [Figure 5] FIG. 4 is a diagram illustrating an example of setting values ​​according to the first embodiment. [Figure 6] 10 is an example of a power profile according to a comparative example. [Figure 7] 10 is an example of a power profile according to an embodiment. [Figure 8] 4 is a flowchart showing an example of the operation of the optical transmission line monitoring device according to the first embodiment. [Figure 9] 10 is a diagram illustrating an example of a functional configuration of a DSP and an FPGA according to a modified example of the first embodiment. [Figure 10] 10 is a part of a flowchart showing an example of the operation of the optical transmission line monitoring device according to the second embodiment. [Figure 11] FIG. 10 is a diagram illustrating an example of setting values ​​according to the second embodiment. [Figure 12] FIG. 10 is a diagram illustrating an example of additional compensation according to the second embodiment. [Figure 13] 10 is an example of a partial functional configuration of an FPGA according to a third embodiment. [Figure 14] 10A is an example of the functional configuration of an estimated sensitivity correction unit according to the third embodiment, and FIG. 10B is an example of a coefficient DB (Data Base). [Figure 15] 10 is an example of a power profile according to the third embodiment. [Figure 16] 10A is an example of the functional configuration of an estimated sensitivity correction unit according to the fourth embodiment, and FIG. 10B is an example of a correction function. [Figure 17] 13 is a diagram illustrating an example of a functional configuration of a DSP and an FPGA according to a modification of the fifth embodiment. [Figure 18] 13 is a part of a flowchart showing an example of the operation of the optical transmission line monitoring device according to the fifth embodiment. [Figure 19] 13 is a diagram illustrating an example of a functional configuration of a DSP and an FPGA according to a modification of the sixth embodiment. [Figure 20] 13 is a part of a flowchart showing an example of the operation of the optical transmission line monitoring device according to the sixth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0012] (First embodiment) As shown in Fig. 1, the optical transmission system ST includes an optical transmitting device 100 as a transmitting end and an optical receiving device 200 as a receiving end. The optical transmitting device 100 and the optical receiving device 200 are connected by an optical transmission line 50. When transmission data is input to the optical transmitting device 100, the optical transmitting device 100 modulates the transmission data and transmits the modulated optical signal to the optical transmission line 50. The optical signal propagates through the optical transmission line 50. The optical receiving device 200 receives the optical signal transmitted from the optical transmitting device 100 via the optical transmission line 50, demodulates it, and outputs the demodulated data.

[0013] A plurality of optical amplifiers 51A, 52A, and 53A are provided on the optical transmission line 50. Therefore, the optical transmission line 50 is divided into a plurality of transmission sections (hereinafter referred to as spans) SP#1, SP#2, SP#3, and SP#4 by the optical amplifiers 51A, 52A, and 53A. In other words, the optical transmission line 50 is a multi-span optical transmission line including a plurality of spans SP#1, SP#2, SP#3, and SP#4 (hereinafter referred to as SP#1, . . . , SP#4 as appropriate).

[0014] Optical fibers 51F, 52F, 53F, and 54F are laid in each of the spans SP#1, ..., SP#4. For example, SSMF (Standard Single Mode Fiber) is laid as the optical fiber in the spans SP#1, ..., #SP#4. DSF (Dispersion Shifted Fiber) may be laid as the optical fiber in some or all of the spans SP#1, ..., #SP#4.

[0015] Although the lengths of the optical fibers 51F, 52F, 53F, and 54F are not particularly limited, the present embodiment will be described assuming that the lengths of the optical fibers 51F, 52F, 53F, and 54F are all several tens of kilometers (km) as an example. That is, the path length of each of the multiple spans SP#1, ..., SP#4 is several tens of kilometers, and the total path length obtained by adding up the individual path lengths is several hundreds of kilometers.

[0016] The optical receiving device 200 includes an optical transmission line monitoring device 300. The optical transmission line monitoring device 300 may be provided as a separate device from the optical receiving device 200. In this case, the optical transmission line monitoring device 300 may be included in an optical network controller that manages the optical transmission system ST. The optical transmission line monitoring device 300 monitors the characteristics of the optical transmission line 50. As will be described in detail later, the optical transmission line monitoring device 300 acquires an electric field signal that indicates an optical electric field component of the optical signal received by the optical receiving device 200.

[0017] When the optical transmission line monitoring device 300 acquires the electric field signal, it estimates the optical power of the optical signal at multiple positions on the optical transmission line 50 based on the electric field signal. The optical transmission line monitoring device 300 can generate a power profile including the estimated optical power based on the estimated optical power. The power profile can represent the characteristics of the optical transmission line 50. If the power profile can be generated accurately, the optical transmission line monitoring device 300 can accurately detect the position of an anomalous loss that has occurred near the optical transmitting device 100 based on the power profile. Furthermore, a monitoring monitor that displays the power profile, the position of the anomalous loss, etc. may be connected to the optical transmission line monitoring device 300. This allows, for example, an operator of the optical transmission system ST to check the power profile, the position of the anomalous loss, etc.

[0018] Next, the optical receiving device 200 and the optical transmission line monitoring device 300 will be described in detail with reference to FIGS.

[0019] As shown in Fig. 2, the optical receiving device 200 includes an ICR (Integrated Coherent Receiver) 210 and an ITLA (Integrable Tunable Laser Assembly) 220. Although not shown, the ICR 210 includes a 90-degree optical hybrid circuit and a BPD (Balanced Photo Diode). The ICR 210 is an integrated circuit that houses the 90-degree optical hybrid circuit and the BPD in a single package. The optical receiving device 200 also includes an ADC (Analogue Digital Converter) 230 and a DSP 240.

[0020] An optical signal is input to the ICR 210 via the optical fiber 54F. The ITLA 220 is a local light source that outputs local light (i.e., laser light). The ICR 210 receives the optical signal via the local light, converts the received optical signal into an electric field signal (specifically, an electric field information signal) corresponding to the optical signal, and outputs the electric field signal to the ADC 230. The ADC 230 converts the electric field signal from analog format to digital format and outputs the signal to the DSP 240.

[0021] The DSP 240 receives the electric field signal output from the ADC 230 and performs various digital signal processing on the received electric field signal. As shown in Fig. 3, the DSP 240 includes a fixed equalization unit 241, an adaptive equalization unit 242, a frequency compensation unit 243, a phase estimation unit 244, a discrimination unit 245, and an error correction unit 246.

[0022] The fixed equalizer 241 compensates for chromatic dispersion suffered by the optical signal propagating through the optical transmission line 50, for the electric field signal received by the DSP 240. The fixed equalizer 241 outputs the electric field signal after compensating for chromatic dispersion to the adaptive equalizer 242. The adaptive equalizer 242 adaptively compensates for residual dispersion in the electric field signal output from the fixed equalizer 241. Residual dispersion is chromatic dispersion that remains after not being completely compensated for by the fixed equalizer 241. The adaptive equalizer 242 outputs the electric field signal after compensating for residual dispersion to the frequency compensator 243.

[0023] The frequency compensation unit 243 compensates for a frequency offset in the electric field signal output from the adaptive equalization unit 242. The frequency offset is the difference (or deviation) between the optical frequency of a transmission light source (not shown) provided in the optical transmission device 100 and the optical frequency of the ITLA 220. The frequency compensation unit 243 outputs the electric field signal after compensating for the frequency offset to the phase estimation unit 244. The phase estimation unit 244 compensates for a phase offset in the electric field signal output from the frequency compensation unit 243 and estimates the phase of the optical signal. The phase offset is the difference (or deviation) between the phases of the transmission light source and the ITLA 220. The phase estimation unit 244 outputs the electric field signal after compensating for the phase offset to the discrimination unit 245.

[0024] The discrimination unit 245 demodulates the transmission data by discriminating the value of each symbol based on the electric field signal output from the phase estimation unit 244, and outputs the demodulated data to the error correction unit 246. The error correction unit 246 corrects bit errors in the demodulated data and outputs the demodulated data after the errors are corrected.

[0025] 2, the optical transmission line monitoring device 300 has an FPGA 310 as a hardware circuit. The optical transmission line monitoring device 300 may have an ASIC (Application Specific Integrated Circuit) as a hardware circuit instead of the FPGA 310. The optical transmission line monitoring device 300 may have a CPU (Central Processing Unit) as a processor instead of the FPGA 310.

[0026] The FPGA 310 receives the electric field signal output from the DSP 240 and performs various digital signal processing on the received electric field signal. As shown in FIG. 3, the FPGA 310 includes a capture memory 311, a first compensation unit 312, a nonlinear compensation unit 313, and a second compensation unit 314. The FPGA 310 also includes a discrimination unit 315, a third compensation unit 316, an optical power estimation unit 317, and an adjustment unit 320. The adjustment unit 320 includes a range determination unit 321, a compensation amount determination unit 322, and an addition unit 323. The first compensation unit 312, the nonlinear compensation unit 313, the second compensation unit 314, the third compensation unit 316, the optical power estimation unit 317, and the adjustment unit 320 are realized by the FPGA 310 executing a program according to a flowchart described below. The optical transmission path monitoring method of the present invention is realized by the FPGA 310 executing a program according to a flowchart described below.

[0027] The capture memory 311 holds the electric field signal output from the phase estimation unit 244 as a capture signal. The electric field signal output from the phase estimation unit 244 is a signal after chromatic dispersion has been compensated for by the fixed equalization unit 241. Therefore, the amount of chromatic dispersion contained in the capture signal is 0 (zero) ps / nm (picoseconds / nanometer).

[0028] As shown in FIG. 3, the first compensating unit 312 acquires a capture signal from the capture memory 311. Upon acquiring the capture signal, the first compensating unit 312 compensates for part of the chromatic dispersion of the optical transmission line 50 to the capture signal. More specifically, as shown in FIG. 4, the first compensating unit 312 adds chromatic dispersion to the entire optical transmission line 50, that is, from the transmitting end to the receiving end, and compensates for chromatic dispersion from the receiving end to the monitoring position (denoted as first compensation in FIG. 4). However, because chromatic dispersion can be added, it can also be said that the first compensating unit 312 adds chromatic dispersion from the transmitting end to the monitoring position. This is because the difference between compensation and addition is simply the sign of the amount of dispersion.

[0029] As shown in Fig. 3, first information is set in the first compensator 312. The first information is the sum of the first dispersion compensation amount input value and the dispersion addition amount of the entire optical transmission line 50. For example, 100 ps / nm is used as the dispersion addition amount. The dispersion addition amount corresponds to the transmission line dispersion amount calculated in advance based on the distance and dispersion coefficient of the optical transmission line 50, for example. The first information is given as the first dispersion compensation amount setting value by the following formula (1): <Formula (1)> First dispersion compensation amount setting value = First dispersion compensation amount input value - Dispersion addition amount In this way, 100 ps / nm is added to the first dispersion compensation amount input value as the additional dispersion amount of chromatic dispersion occurring throughout the optical transmission line 50 .

[0030] Therefore, as shown in FIGS. 4 and 5, if the first dispersion compensation amount input value is 75 ps / nm, for example, the first dispersion compensation amount setting value is calculated as −25 ps / nm based on Equation (1). The minus sign in the dispersion compensation amount indicates the addition of dispersion. As shown in FIG. 5, the first compensator 312 reads all first dispersion compensation amount input values ​​at monitor positions incremented by 25 ps / nm from 0 ps / nm to 100 ps / nm and calculates the first dispersion compensation amount setting value. After calculating the first dispersion compensation amount setting value, the first compensator 312 sets the first dispersion compensation amount setting value itself. The first compensator 312 may read the first dispersion compensation amount input value from a lookup table included in the optical receiving device 200 or from an external device connected to the optical receiving device 200. The first compensator 312 outputs the capture signal after adding (or compensating for) the dispersion of the optical transmission line 50 to the nonlinear compensator 313 as a monitor signal.

[0031] As shown in FIG. 3, the nonlinear compensator 313 compensates for degradation of the monitor signal compensated for by the first compensator 312 due to the nonlinear optical effect of the optical transmission line 50. An example of a nonlinear optical effect is the Kerr effect. When the Kerr effect occurs, the refractive index of the optical fiber of the optical transmission line 50 changes in proportion to the square of the power of the optical signal. As a result, self-phase modulation occurs in the optical signal, causing the pulse width to narrow due to a change in the phase velocity of light, which can result in signal errors. The nonlinear compensator 313 compensates for degradation due to the nonlinear optical effect of the optical transmission line 50 by performing a phase rotation equal to the square of the amplitude of the monitor signal multiplied by a predetermined value.

[0032] The second compensating unit 314 compensates for the remaining chromatic dispersion in the optical transmission line 50 for the monitor signal after compensation by the nonlinear compensating unit 313. Furthermore, the second compensating unit 314 additionally compensates for the chromatic dispersion at a virtual position off the optical transmission line 50 for the monitor signal after compensation by the second compensating unit 314. More specifically, as shown in FIG. 4, the second compensating unit 314 compensates for the chromatic dispersion from the monitor position to the transmitting end (denoted as second compensation in FIG. 4). Furthermore, the second compensating unit 314 compensates for the chromatic dispersion from the transmitting end to the virtual position (denoted as additional compensation in FIG. 4).

[0033] Here, as shown in Fig. 3, second information is set in the range determination unit 321. The second information is a second dispersion compensation amount input value. As shown in Fig. 5, the range determination unit 321 reads all second dispersion compensation amount input values ​​at the monitor position, which are incremented by 25 ps / nm from 0 ps / nm to 100 ps / nm. After reading the second dispersion compensation amount input value, the range determination unit 321 checks the minimum and maximum values ​​of the dispersion amount from the monitor position to the transmitting end, thereby obtaining the range of the dispersion amount on the optical transmission line 50.

[0034] 4, the additional dispersion compensation amount is outside the range of dispersion amounts that can be taken in the optical transmission line 50. Therefore, the compensation amount determination unit 322 determines the additional dispersion compensation amount so that it is a value that is a predetermined value smaller than the minimum value of the dispersion amount acquired by the range determination unit 321, or a predetermined value larger than the maximum value. The predetermined value may be determined appropriately through design, experiment, or the like, within a range in which the effects of this embodiment can be realized. In the first embodiment, an example will be described in which the compensation amount determination unit 322 determines a value that is a predetermined value smaller than the minimum value of the dispersion amount.

[0035] 5, the minimum value of the second dispersion compensation amount input value is 0 ps / nm. Therefore, the compensation amount determination unit 322 determines the additional dispersion compensation amount to be 150 ps / nm, which is a predetermined value, for example, 150 ps / nm smaller than that, so that the residual dispersion becomes −150 ps / nm. Note that the predetermined value may be set in the compensation amount determination unit 322 in advance. The compensation amount determination unit 322 outputs the determined additional dispersion compensation amount to the adder 323 and also sets it in the third compensator 316.

[0036] The adder 323 adds the second dispersion compensation amount input value and the additional dispersion compensation amount to calculate the second dispersion compensation amount setting value. That is, the adder 323 calculates the second dispersion compensation amount setting value based on the following mathematical formula (2). <Formula (2)> Second dispersion compensation amount setting value = Second dispersion compensation amount input value + Additional dispersion compensation amount Therefore, in the first embodiment, the adder 323 sets the compensation amount in the second compensator 314 by adding 150 ps / nm as the additional dispersion compensation amount to the second dispersion compensation amount input value.

[0037] The decision unit 315 acquires the capture signal held in the capture memory 311, recovers symbols from the capture signal, and demodulates the capture signal by identifying the value of each symbol to reproduce the transmission signal (specifically, a replica of the transmission signal). The decision unit 315 may use a transmission signal prepared in advance without reproducing the transmission signal. After reproducing the transmission signal, the decision unit 315 outputs the transmission signal to the third compensation unit 316 as a reference signal.

[0038] The third compensator 316 compensates the reference signal with the compensation amount set by the compensation amount determiner 322. The reference signal input to the third compensator 316 does not contain any dispersion. That is, the dispersion amount of the reference signal is 0 ps / nm. When no compensation is performed on the reference signal, the dispersion amount included in the monitor signal output from the second compensator 314 differs from the dispersion amount included in the reference signal. When the dispersion amounts of the monitor signal and the reference signal differ, the dispersion amounts do not correspond in the downstream optical power estimator 317, and the correlation between the complex amplitudes of the monitor signal and the reference signal cannot be calculated accurately. For this reason, the third compensator 316 performs the same compensation on the reference signal as on the monitor signal. In the first embodiment, the third compensator 316 compensates the reference signal with the compensation amount of 150 ps / nm set by the compensation amount determiner 322. The third compensator 316 outputs the compensated reference signal to the downstream optical power estimator 317.

[0039] The optical power estimator 317 calculates a correlation value between the complex amplitudes of the monitor signal and the reference signal for each dispersion amount (specifically, the cumulative dispersion amount) based on the monitor signal output from the second compensator 314 and the reference signal output from the third compensator 316. After calculating the correlation value, the optical power estimator 317 outputs the calculated correlation value as an estimate of the optical power for each dispersion amount. Because the magnitude of the self-phase modulation corresponds to the optical power at the monitor position, the optical power estimator 317 can output the correlation value as an estimate of the optical power.

[0040] For example, as shown in FIG. 5, when 75 ps / nm is given as the first dispersion compensation amount input value and 25 ps / nm is given as the second dispersion compensation amount input value, the optical power estimator 317 outputs c10 as the estimated value of the optical power. The optical power estimator 317 can generate a power profile based on the estimated value of the optical power. For example, Japanese Patent Application Laid-Open No. 2023-178193 can be referred to for generating the power profile. As shown in FIGS. 6 and 7, the horizontal axis of the power profile represents the distance from the transmitting end, and the vertical axis of the power profile represents the estimated value of the optical power. The horizontal axis of the power profile may also represent the accumulated dispersion amount from the transmitting end. That is, the horizontal axis of the power profile may represent the accumulated dispersion amount, or the accumulated dispersion amount may be converted into the distance from the transmitting end and represented.

[0041] As a result, as shown by the dashed line in Fig. 6, in the comparative example in which no abnormal loss occurs in the optical transmission line 50, the optical power near the transmitting end drops once and then rises, resulting in a decrease in the accuracy of optical power estimation. In contrast, as shown by the dashed line in Fig. 7, in the example in which no abnormal loss occurs in the optical transmission line 50, the optical power near the transmitting end gradually rises without ever dropping, resulting in an improvement in the accuracy of optical power estimation. In this way, according to the first embodiment, the optical transmission line monitoring device 300 can accurately estimate the optical power near the transmitting end.

[0042] Although not shown, a detection unit that detects the position of anomalous loss occurring near the transmitting end may be provided downstream of the optical power estimator 317. For example, if an anomalous loss is intentionally generated at a position belonging to span SP#1 that is 3 km away from the transmitting end of the optical transmission line 50, the shape of the power profile changes as shown by the solid lines in FIGS.

[0043] As shown in the comparative example of Figure 6, there is a difference between the power profile indicated by the solid line where anomalous loss occurs and the power profile indicated by the dashed line where anomalous loss does not occur. The detector calculates the difference between these two power profiles and detects the differential value of this difference as an index value for the location where anomalous loss occurs. Since this difference becomes large where anomalous loss occurs, if the change point where this difference becomes large is found, that location corresponds to the location where anomalous loss occurred. In the comparative example, the detector detected the location where anomalous loss occurred as 4.5 km away. Because anomalous loss occurred at a position 3 km away from the transmitting end of the optical transmission line 50, the error was 1.5 km.

[0044] 7, the detection unit detected the location where the anomalous loss occurred as 3.5 km away. Because the anomalous loss occurred at a location 3 km away from the transmitting end of the optical transmission line 50, the error was 0.5 km. According to the first embodiment, the error is reduced to about one-third, improving the detection accuracy of the location where the anomalous loss occurred by about three times.

[0045] The operation of the optical transmission line monitoring device 300 will be described with reference to FIG.

[0046] First, the first compensating unit 312 reads the first information (step S1). More specifically, the first compensating unit 312 reads all the first dispersion compensation amount input values ​​at the monitoring positions as the first information. After reading the first information, the first compensating unit 312 calculates a first setting value (step S2). More specifically, the first compensating unit 312 calculates the first dispersion compensation amount setting value as the first setting value based on the above-mentioned formula (1).

[0047] After calculating the first setting value, the first compensating unit 312 sets the first setting value (step S3). That is, the first compensating unit 312 sets the first setting value to itself. After the first compensating unit 312 sets the first setting value, the range determining unit 321 reads the second information (step S4). More specifically, the range determining unit 321 reads all the second dispersion compensation amount input values ​​at the monitoring positions as the second information.

[0048] When the second information is read, the range determination unit 321 searches for the minimum value (step S5). In the first embodiment, the range determination unit 321 reads the second dispersion compensation amount input values ​​ranging from 0 ps / nm to 100 ps / nm, incremented by 25 ps / nm. Therefore, when the range determination unit 321 searches for the minimum value, the range determination unit 321 specifies 0 ps / nm as the search result.

[0049] After searching for the minimum value, the compensation amount determination unit 322 calculates the amount of chromatic dispersion (step S6). More specifically, the compensation amount determination unit 322 calculates the amount of chromatic dispersion by subtracting the above-mentioned predetermined value from the minimum value. As a result, for example, the compensation amount determination unit 322 calculates the amount of chromatic dispersion to be −150 ps / nm. After calculating the amount of chromatic dispersion, the compensation amount determination unit 322 calculates the additional compensation amount (step S7). Specifically, the compensation amount determination unit 322 calculates the additional compensation amount by inverting the sign of the amount of chromatic dispersion. In this way, the compensation amount determination unit 322 determines the additional compensation amount and sets the additional compensation amount in the third compensator 316.

[0050] When the compensation amount determination unit 322 determines the additional compensation amount, the adder 323 calculates a second setting value (step S8). More specifically, the adder 323 adds the additional compensation amount to the second dispersion compensation amount input value based on the above-mentioned formula (2) to calculate a second setting value as the second dispersion compensation amount setting value. When the second setting value is calculated, the adder 323 sets the second setting value (step S9). More specifically, the adder 323 sets the second setting value in the second compensation unit 314.

[0051] When the adder 323 sets the second setting value, the first compensator 312 performs first dispersion compensation (step S10). More specifically, the first compensator 312 acquires a capture signal from the capture memory 311. Upon acquiring the capture signal, the first compensator 312 compensates the capture signal with the first setting value set therein and outputs the compensated capture signal as a monitor signal.

[0052] After the first compensation unit 312 performs first dispersion compensation, the nonlinear compensation unit 313 performs nonlinear compensation (step S11). More specifically, the nonlinear compensation unit 313 performs phase rotation by an amount equal to the square of the amplitude of the monitor signal multiplied by a predetermined value, thereby compensating for degradation due to the nonlinear optical effect of the optical transmission line 50. After the nonlinear compensation unit 313 performs nonlinear compensation, the second compensation unit 314 performs second dispersion compensation (step S12). More specifically, the second compensation unit 314 compensates the monitor signal with a second setting value set by the adder 323 and outputs the compensated monitor signal.

[0053] After the second compensation unit 314 performs the second dispersion compensation, the decision unit 315 performs decision and symbol recovery processing (step S13). More specifically, the decision unit 315 acquires the capture signal held in the capture memory 311, recovers symbols from the capture signal, and demodulates the capture signal by identifying the value of each symbol to recover the transmission signal. After recovering the transmission signal, the decision unit 315 outputs the transmission signal as a reference signal. After the decision unit 315 outputs the reference signal, the third compensation unit 316 performs third dispersion compensation (step S14). More specifically, the third compensation unit 316 compensates the reference signal with the additional compensation amount set by the compensation amount determination unit 322 and outputs the reference signal.

[0054] After the third compensation unit 316 performs the third dispersion compensation, the optical power estimation unit 317 calculates a correlation value (step S15). More specifically, the optical power estimation unit 317 calculates the correlation value between the complex amplitudes of the monitor signal and the reference signal. After calculating the correlation value, the optical power estimation unit 317 outputs the calculated correlation value as an optical power estimation value (step S16), and ends the process. Note that the optical transmission line monitoring device 300 may perform the processes of steps S10 to S12 and the processes of steps S13 and S14 in parallel.

[0055] (Modification of the first embodiment) A modification of the first embodiment will be described with reference to Fig. 9. As shown in Fig. 9, the capture memory 311 holds the electric field signal output from the adaptive equalization unit 242 as a capture signal. The electric field signal output from the adaptive equalization unit 242 is a signal after chromatic dispersion has been compensated for by the fixed equalization unit 241. Therefore, the amount of chromatic dispersion included in the capture signal is 0 ps / nm.

[0056] The modification of the first embodiment also differs from the first embodiment in that the FPGA 310 includes a dispersion adding unit 318, a frequency compensating unit 319, and a phase estimating unit 319A. The dispersion adding unit 318 acquires a capture signal from the capture memory 311, and compensates for the amount of chromatic dispersion of the capture signal with the above-mentioned dispersion addition amount of the entire optical transmission line 50. For this reason, in this modification, the above-mentioned first dispersion compensation amount input value is set alone as first information in the first compensating unit 312.

[0057] The frequency compensation unit 319 performs the same processing as the frequency compensation unit 243 described in the first embodiment. Furthermore, the phase estimation unit 319A performs the same processing as the phase estimation unit 244 described in the first embodiment. As described above, in the modified example, the capture memory 311 holds the electric field signal output from the adaptive equalization unit 242 as a capture signal. Therefore, the frequency compensation unit 319 and the phase estimation unit 319A perform processing similar to the processing performed in the subsequent stage of the adaptive equalization unit 242. Even with the configuration of this modified example, the optical transmission line monitoring device 300 can ensure the same effects as those of the first embodiment.

[0058] (Second embodiment) A second embodiment of the present invention will be described with reference to Figures 10 to 12. In Figure 10, processes similar to those described with reference to Figure 8 are denoted by the same reference numerals, and detailed description thereof will be omitted. The same applies to the embodiments described below. In the first embodiment and its modifications, a case in which the dispersion coefficient is positive has been described as an example of an optical fiber, but in the second embodiment, an optical fiber with a negative dispersion coefficient will be described as an example. For example, a dispersion-shifted fiber with a negative dispersion coefficient is known as an optical fiber with a negative dispersion coefficient.

[0059] First, as shown in Fig. 10, when the process of step S4 is executed, the range determination unit 321 searches for the maximum value (step S21). In the second embodiment, as shown in Fig. 11, the range determination unit 321 reads the second dispersion compensation amount input value ranging from 0 ps / nm to -100 ps / nm, incremented by -25 ps / nm. Therefore, when the range determination unit 321 searches for the maximum value, the range determination unit 321 specifies 0 ps / nm as the search result.

[0060] After searching for the maximum value, the compensation amount determination unit 322 calculates the amount of chromatic dispersion (step S22). More specifically, the compensation amount determination unit 322 calculates the amount of chromatic dispersion by adding a predetermined value to the maximum value. In the second embodiment, 100 ps / nm is used as an example of the predetermined value. As a result, for example, the compensation amount determination unit 322 calculates 100 ps / nm as the amount of chromatic dispersion. After calculating the amount of chromatic dispersion, the compensation amount determination unit 322 executes the process of step S7 described above, as in the first embodiment. That is, the compensation amount determination unit 322 calculates the additional compensation amount by inverting the sign of the amount of chromatic dispersion. Therefore, in the second embodiment, -100 ps / nm is calculated as the additional compensation amount.

[0061] As a result, as shown in Fig. 12, an estimated value of optical power is calculated at a virtual position near the transmitting end that is off the optical transmission line 50. In Fig. 12, contrary to Fig. 4, the left-pointing arrow indicates a positive amount of chromatic dispersion compensation, and the right-pointing arrow indicates a negative amount of chromatic dispersion compensation. According to the second embodiment, even in the case of a dispersion-shifted fiber with a negative dispersion coefficient, the optical transmission line monitoring device 300 can accurately estimate the optical power near the transmitting end.

[0062] In particular, dispersion compensation in the frequency domain requires a large number of sample points because FFT (Fast Fourier Transform) is used. This increases the amount of calculation when an estimated value of optical power is calculated in the same way as in the first embodiment. However, according to the second embodiment, even for a dispersion-shifted fiber with a negative dispersion coefficient, an estimated value of optical power can be calculated by the same process as in the first embodiment, simply by changing from subtracting a predetermined value from the minimum value to adding a predetermined value to the maximum value. This allows the optical transmission line monitoring device 300 to suppress an increase in the amount of calculation.

[0063] (Third embodiment) A third embodiment of the present invention will be described with reference to Fig. 13 to Fig. 15. In the third embodiment, as shown in Fig. 13, an estimated sensitivity correcting unit 330 is provided in an FPGA 310. The estimated sensitivity correcting unit 330 corrects the estimated sensitivity of the estimated value of optical power output by the optical power estimating unit 317. This allows the optical transmission line monitoring device 300 to estimate the optical power near the transmitting end with higher accuracy than in the first embodiment.

[0064] As shown in Fig. 14(a), the estimated sensitivity correction unit 330 includes a coefficient DB 331, an extraction unit 332, an acquisition unit 333, and a calculation unit 334. As shown in Fig. 14(b), the coefficient DB 331 stores a correction coefficient for each amount of dispersion from the transmitting end (i.e., the accumulated dispersion amount). According to the coefficient DB 331, the correction coefficient decreases as the amount of dispersion from the transmitting end increases.

[0065] The extraction unit 332 acquires the second information described in the first embodiment. That is, the extraction unit 332 acquires the second dispersion compensation amount input value (see FIG. 5) as the second information. Upon acquiring the second information, the extraction unit 332 extracts the dispersion amount at the monitor position and outputs it to the acquisition unit 333.

[0066] The acquisition unit 333 acquires a correction coefficient corresponding to the amount of dispersion from the coefficient DB 331 based on the amount of dispersion output from the extraction unit 332. Upon acquiring the correction coefficient, the acquisition unit 333 outputs the correction coefficient associated with the amount of dispersion to the calculation unit 334.

[0067] The calculation unit 334 acquires the estimated value of optical power for each amount of dispersion output from the optical power estimator 317. Upon acquiring the estimated value of optical power, the calculation unit 334 corrects the estimated sensitivity of the estimated value of optical power based on the correction coefficient output from the acquisition unit 333 and outputs the corrected value. For example, the calculation unit 334 multiplies the estimated value of optical power by the correction coefficient to correct the estimated sensitivity of the estimated value of optical power and outputs the corrected value. The calculation unit 334 may also correct the estimated sensitivity of the estimated value of optical power by adding the correction coefficient to the estimated value of optical power and output the corrected value.

[0068] As a result, as shown in FIG. 15, the optical transmission line monitoring device 300 according to the third embodiment can estimate the optical power near the transmitting end with higher accuracy than when there is no correction by the estimation sensitivity correcting unit 330.

[0069] (Fourth embodiment) A fourth embodiment of the present invention will be described with reference to Figures 16(a) and (b). Unlike the third embodiment, the estimated sensitivity correction unit 330 may include a coefficient DB 335 and a calculation unit 336, as shown in Figure 16(a). The coefficient DB 335 stores coefficients α and β of the correction function. The coefficients α and β are determined in advance depending on the design, etc. The calculation unit 336 calculates the amount of dispersion as Cd_i and the estimated value of the optical power as R i The correction function R i ´=R i e -αCd_i +β. Once the coefficients α and β of the correction function are determined, the correction function can be expressed as a function in which the correction coefficient decreases as the amount of dispersion from the transmitting end increases, as shown in Figure 16(b).

[0070] When a combination of the dispersion amount and the estimated value of the optical power is input from the optical power estimator 317 to the calculator 336, the calculator 336 acquires the coefficients α and β of the correction function from the coefficient DB 335. Upon acquiring the coefficients α and β of the correction function, the calculator 336 calculates the estimated value R of the corrected optical power based on the coefficients α and β, the combination of the dispersion amount and the estimated value of the optical power input from the optical power estimator 317, and the correction function. iIn this way, by performing correction using the correction function, the optical transmission line monitoring device 300 can estimate the optical power near the transmitting end with higher accuracy, as in the third embodiment.

[0071] (Fifth embodiment) A fifth embodiment of the present invention will be described with reference to Fig. 17 and Fig. 18. As shown in Fig. 17, an adjustment unit 320 according to the fifth embodiment includes a positive / negative determination unit 324 instead of a range determination unit 321.

[0072] The positive / negative determination unit 324 receives as input the amount of transmission path dispersion used when performing dispersion compensation in the fixed equalization unit 241. The positive / negative determination unit 324 may also receive as input the amount of transmission path compensation used when performing dispersion compensation in the fixed equalization unit 241. The sign of the amount of transmission path dispersion may be positive or negative. When the amount of transmission path dispersion is input, the positive / negative determination unit 324 determines whether the amount of transmission path dispersion is positive or negative, and outputs either the positive or negative sign to the compensation amount determination unit 322.

[0073] The compensation amount determination unit 322 is preset with additional compensation amounts Cp and Cn corresponding to the sign of the transmission path dispersion amount. For example, if the sign of the transmission path dispersion amount is positive, the additional compensation amount Cp is set, and if the sign of the transmission path dispersion amount is negative, the additional compensation amount Cn is set. The compensation amount determination unit 322 selects and determines either the additional compensation amount Cp or Cn based on the sign, positive or negative, input from the positive / negative determination unit 324. For example, if a positive sign is input, the compensation amount determination unit 322 selects and determines the additional compensation amount Cp. If a negative sign is input, the compensation amount determination unit 322 selects and determines the additional compensation amount Cn. The compensation amount determination unit 322 outputs either the determined additional compensation amount Cp or Cn to the third compensation unit 316 and the adder 323.

[0074] 18, for example, when the adder 323 executes the process of step S4, the compensation amount determiner 322 sets the additional compensation amounts Cp and Cn to itself (step S31). The compensation amount determiner 322 may set the additional compensation amounts Cp and Cn from a lookup table included in the optical receiving device 200, or may set them from an external device connected to the optical receiving device 200. The compensation amount determiner 322 may set the additional compensation amounts Cp and Cn before executing the process of step S4. For example, the compensation amount determiner 322 may set the additional compensation amounts Cp and Cn before executing the process of step S1.

[0075] When the compensation amount determination unit 322 sets the additional compensation amounts Cp and Cn, the positive / negative determination unit 324 reads the transmission path dispersion amount Cdt (step S32). The positive / negative determination unit 324 may set the transmission path dispersion amount Cdt from a lookup table included in the optical receiving device 200, or may set it from an external device connected to the optical receiving device 200. When the positive / negative determination unit 324 reads the transmission path dispersion amount Cdt, it determines whether the transmission path dispersion amount Cdt is positive or not (step S33).

[0076] If the transmission path dispersion amount Cdt is positive (step S33: YES), the compensation amount determination unit 322 selects and determines the additional compensation amount Cp from the additional compensation amounts Cp and Cn (step S34). On the other hand, if the transmission path dispersion amount Cdt is negative (step S33: NO), the compensation amount determination unit 322 selects and determines the additional compensation amount Cn from the additional compensation amounts Cp and Cn (step S35). After determining either the additional compensation amount Cp or Cn, the compensation amount determination unit 322 sets either the determined additional compensation amount Cp or Cn in the third compensation unit 316. Furthermore, after determining either the additional compensation amount Cp or Cn, the compensation amount determination unit 322 outputs either the determined additional compensation amount Cp or Cn to the adder 323. When either the additional compensation amount Cp or Cn is output from the compensation amount determination unit 322, the adder 323 executes the process of step S8 and calculates a second setting value.

[0077] As described above, according to the fifth embodiment, the additional compensation amount is determined by the positive or negative sign representing the transmission line dispersion amount used when performing dispersion compensation in the fixed equalization unit 241. This eliminates the processes of steps S5 to S7 described in the first embodiment. That is, the search process and calculation process are omitted. This makes it possible to reduce the processing load of the optical transmission line monitoring device 300 compared to the first embodiment.

[0078] (Sixth embodiment) The sixth embodiment of the present invention will be described with reference to Fig. 19 and Fig. 20. As shown in Fig. 19, an adjustment unit 320 according to the sixth embodiment differs from the fifth embodiment in that it further includes a dispersion amount calculation unit 325.

[0079] Design information of the optical transmission line 50 is input to the dispersion amount calculation unit 325. The design information includes, for example, the fiber length and dispersion coefficient of each of the optical fibers 51F, 52F, 53F, and 54F. The design information may include the dispersion amount of each of the optical fibers 51F, 52F, 53F, and 54F instead of the fiber length and dispersion coefficient. The dispersion amount calculation unit 325 calculates the transmission line dispersion amount, which is the dispersion amount of the entire optical transmission line 50, based on the design information. After calculating the transmission line dispersion amount, the dispersion amount calculation unit 325 outputs the calculated transmission line dispersion amount to the positive / negative determination unit 324.

[0080] 20, when the compensation amount determination unit 322 executes the process of step S31, the dispersion amount calculation unit 325 reads the design information (step S41). The dispersion amount calculation unit 325 may read the design information from a memory included in the optical receiving device 200, or may read the design information from an external device connected to the optical receiving device 200. After reading the design information, the dispersion amount calculation unit 325 calculates the transmission path dispersion amount Cdt (step S42). For example, if the design information includes a fiber length and a dispersion coefficient, the dispersion amount calculation unit 325 calculates the sum of the products of the fiber length and the dispersion coefficient as the transmission path dispersion amount Cdt. After calculating the transmission path dispersion amount Cdt, the positive / negative determination unit 324 executes the process of step S33 and determines whether the transmission path dispersion amount Cdt is positive or negative.

[0081] In this way, the optical transmission line monitoring device 300 according to the sixth embodiment can use the amount of transmission line dispersion calculated based on the design information, instead of the amount of transmission line dispersion used when performing dispersion compensation in the fixed equalizer 241. Even in this sixth embodiment, the processes of steps S5 to S7 described in the first embodiment are omitted. Therefore, even in the sixth embodiment, the processing load of the optical transmission line monitoring device 300 can be reduced compared to the first embodiment.

[0082] Although the preferred embodiments of the present invention have been described in detail above, the present invention is not limited to the specific embodiments, and various modifications and variations are possible within the scope of the gist of the present invention as described in the claims.

[0083] In addition, the following supplementary notes are provided in relation to the above description. an adjustment unit that adjusts the amount of compensation additionally provided by the second compensator; a third compensator that compensates for the amount of chromatic dispersion of a reference signal indicating an optical field component of the optical signal at a transmitting end of the optical transmission line; and an estimation unit that estimates optical power near the transmitting end based on the correlation between the amplitudes of the first signal output from the second compensator and the second signal output from the third compensator. (Supplementary Note 2) The optical transmission line monitoring device according to Supplementary Note 1, further comprising a correction unit that, after estimating the optical power, corrects the estimated sensitivity of the optical power based on a correction coefficient that decreases in accordance with an increase in the amount of dispersion from the transmitting end. (Supplementary Note 3) The optical transmission line monitoring device according to Supplementary Note 2, wherein the correction unit includes a database that stores a correction coefficient that decreases in accordance with an increase in the amount of dispersion from the transmitting end. (Appendix 4) The optical transmission line monitoring device according to appendix 2, characterized in that the correction unit corrects the estimated sensitivity of the optical power based on a correction function in which a correction coefficient decreases as the amount of dispersion from the transmitting end increases. (Appendix 5) The optical transmission line monitoring device according to appendix 1 or 2, characterized in that the adjustment unit determines whether the input dispersion amount of the optical transmission line is positive or negative, and sets the compensation amount determined in accordance with the determination result in the second compensation unit. (Appendix 6) The optical transmission line monitoring device according to appendix 1 or 2, characterized in that the adjustment unit calculates a total dispersion amount of the optical transmission line based on input design information of the optical transmission line, determines whether the total dispersion amount is positive or negative, and sets the compensation amount determined in accordance with the determination result in the second compensation unit. (Supplementary Note 7) The optical transmission line monitoring device according to Supplementary Note 1 or 2, further comprising a detection unit that detects a position of an abnormal loss occurring near the transmitting end based on the optical power. (Supplementary Note 8) An optical transmission line monitoring method comprising: compensating an electric field signal indicating an optical field component of an optical signal digitally coherently received from an optical transmission line for a part of chromatic dispersion of the optical transmission line; compensating the compensated electric field signal for degradation due to nonlinear optical effects of the optical transmission line; compensating the compensated electric field signal for the remaining chromatic dispersion excluding the part from the chromatic dispersion of the optical transmission line; additionally compensating the compensated electric field signal for chromatic dispersion at a virtual position off the optical transmission line; adjusting the amount of compensation to be additionally compensated; compensating a reference signal indicating an optical field component of the optical signal at a transmitting end of the optical transmission line for the chromatic dispersion of the compensation amount; and estimating optical power near the transmitting end based on the correlation between the amplitudes of a first signal, which is the electric field signal after additional compensation, and a second signal, which is the reference signal after compensation for the chromatic dispersion of the compensation amount. [Explanation of symbols]

[0084] ST Optical Transmission System 50 Optical transmission line 100 Optical transmitter 200 Optical receiving device 300 Optical transmission line monitoring device 312 1st Compensation Department 313 Nonlinear Compensation Section 314 Second Compensation Department 316 Third Compensation Department 317 Optical Power Estimation Unit 320 Adjustment section

Claims

1. a first compensation unit that compensates for a part of chromatic dispersion of an optical transmission line with respect to an electric field signal that indicates an optical electric field component of an optical signal that is digitally coherently received from the optical transmission line; a nonlinear compensation unit that compensates for degradation of the electric field signal after compensation by the first compensation unit due to a nonlinear optical effect of the optical transmission line; a second compensating unit that compensates for the remaining chromatic dispersion of the optical transmission line, excluding the portion, of the electric field signal after compensation by the nonlinear compensating unit, and additionally compensates for chromatic dispersion at a virtual position deviated from the optical transmission line, with respect to the compensated electric field signal; an adjusting unit that adjusts the amount of compensation additionally provided by the second compensating unit; a third compensator that compensates for chromatic dispersion of the compensation amount with respect to a reference signal that indicates an optical field component of the optical signal at a transmitting end of the optical transmission line; an estimation unit that estimates optical power near the transmitting end based on correlation between amplitudes of a first signal output from the second compensation unit and a second signal output from the third compensation unit; An optical transmission line monitoring device having:

2. a correction unit that, after estimating the optical power, corrects the estimated sensitivity of the optical power based on a correction coefficient that decreases in accordance with an increase in the amount of dispersion from the transmitting end; 2. The optical transmission line monitoring device according to claim 1.

3. the adjustment unit determines whether the input dispersion amount of the optical transmission line is positive or negative, and sets the compensation amount determined in accordance with the determination result in the second compensation unit.

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

4. the adjustment unit calculates a total dispersion amount of the optical transmission line based on the input design information of the optical transmission line, determines whether the total dispersion amount is positive or negative, and sets the compensation amount determined in accordance with the determination result in the second compensation unit.

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

5. a detection unit that detects a position of an abnormal loss occurring near the transmitting end based on the optical power, 3. The optical transmission line monitoring device according to claim 1 or 2.

6. a part of chromatic dispersion of an optical transmission line is compensated for in an electric field signal indicating an optical electric field component of an optical signal digitally coherently received from the optical transmission line; Compensating for degradation of the compensated electric field signal due to nonlinear optical effects of the optical transmission line; Compensating for the remaining chromatic dispersion of the optical transmission line, excluding the portion, for the compensated electric field signal; additionally compensating for chromatic dispersion at a virtual position outside the optical transmission line with respect to the compensated electric field signal; Adjust the amount of additional compensation; Compensating for the chromatic dispersion of the compensation amount with respect to a reference signal indicating an optical field component of the optical signal at a transmitting end of the optical transmission line; estimating optical power near the transmitting end based on correlation between amplitudes of a first signal, which is the electric field signal after additional compensation, and a second signal, which is the reference signal after compensation for the chromatic dispersion of the compensation amount; Optical transmission line monitoring method.

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