Optical communication system and optical communication method

Abstract

One aspect of the present invention is an optical communication system comprising: an optical repeater that monitors signal quality on the basis of a pilot signal that is added to a transmission signal and frequency-multiplexed with the transmission signal in order to monitor the signal quality; and an insertion device that inserts one or more additional pilot signals into a predetermined range of frequencies of the pilot signal at a stage preceding the optical repeater.

Description

Optical communication system and optical communication method 【0001】 This invention relates to optical communication systems and optical communication methods. 【0002】 As shown in Figure 11, there is a system in which an electrical signal to be transmitted (transmission signal) is converted into an optical signal by an optical transmitter 901 and output, and then relayed in multiple stages by N optical repeaters 902 (902-1, 902-2, 902-3, ...) to an optical receiver 903. The optical repeaters 902 perform at least one of the following processes that contribute to improving signal quality, such as signal amplification, shaping, and retiming, or signal branching. Figure 11 is a diagram showing an example of the configuration of a conventional transmission system. In such a transmission system, there is a pilot signal frequency-multiplexed with the transmission signal in order to monitor signal quality. The optical repeaters and optical receivers measure the intensity of the pilot signal and, if a decrease in intensity occurs, they determine that there is an abnormality such as a malfunction in the system and issue an alarm (see, for example, Non-Patent Documents 1 and 2). 【0003】 Figure 12 is a diagram illustrating the levels of the pilot signal. Code g900 shows an example of the spectrum of the pilot signal g901 and the transmission signal, with frequency on the horizontal axis and intensity on the vertical axis. Code g910 shows an example of the pilot signal margin and intensity range, with pilot signal intensity (mW) on the vertical axis. The double arrow g911 shows an example of the pilot signal intensity range where an abnormality in service quality occurs (P1 (mW) or less). The double arrow g912 shows an example of the margin ((P2 - P1) (mW)). Code g913 shows an example of the pilot signal intensity range that is expected to be measured by the optical repeater 902 and optical receiver 903 when the system is operating normally (P3 to P2 (mW) or less). The dashed line g914, level P2, is an example of the alarm threshold set for the optical repeater 902 and optical receiver 903. The optical repeater 902 and optical receiver 903 compare the strength of the pilot signal with a threshold and determine that no alarm should be issued if the strength of the pilot signal is equal to or greater than the threshold. The optical repeater 902 and optical receiver 903 determine that an alarm should be issued if the strength of the pilot signal is less than the threshold. 【0004】"Transmission equipment for transferring multi-channel television signals over optical access networks by frequency modulation conversion", Rec.ITU-T J.185, International Telecommunication Union, June 2012"Transmission equipment for multi-channel television signals over optical access networks by sub-carrier multiplexing (SCM)", Rec.ITU-T J.186, International Telecommunication Union, Jun. 2008 【0005】 However, in the prior art, for example, when an optical transmitter is replaced with a successor model with a lower pilot signal level than the model before system update or the like, the pilot signal level measured by an optical repeater or an optical receiver decreases, so there is a problem that an alarm is issued even during normal operation. In addition, since the devices have already been introduced into the market, it is difficult to change the thresholds of the optical repeaters and optical receivers from the viewpoints of labor and cost. Also, it is difficult to change the internal specifications of the optical transmitter of the successor model. In view of the above circumstances, an object of the present invention is to provide a technology capable of relaxing the alarm generation conditions for pilot signals. 【0006】 One aspect of the present invention is an optical communication system including an optical repeater that monitors the signal quality based on a pilot signal that is added to a transmission signal and frequency multiplexed with the transmission signal to monitor the signal quality, and an insertion device that inserts one or more additional pilot signals in a predetermined range of the frequency of the pilot signal in front of the optical repeater. 【0007】One aspect of the present invention is an optical communication method in which an optical repeater monitors the signal quality based on a pilot signal that is added to the transmission signal and frequency multiplexed with the transmission signal to monitor the signal quality, and an insertion device inserts one or more additional pilot signals within a predetermined frequency range of the pilot signal before the optical repeater. 【0008】 This invention makes it possible to relax the alarm issuance conditions for pilot signals. 【0009】 This figure shows an example configuration of the optical communication system of the first embodiment. This figure shows an overview of the signal processing of the insertion device and optical transmitter of the first embodiment. This figure shows an overview of the signal processing of the optical repeater of the first embodiment. This figure shows a schematic of an example hardware configuration of an information processing device applied to the embodiment. This is a sequence diagram of the alarm issuance determination process of the communication system of the first embodiment. This is a diagram illustrating an example of alarm issuance determination using CNR. This figure shows an example configuration of the optical communication system of the second embodiment. This figure shows an overview of the signal processing of the insertion device and optical transmitter of the second embodiment. This is a diagram showing an overview of the signal processing of the optical repeater of the second embodiment. This is a sequence diagram of the alarm issuance determination process of the communication system of the second embodiment. This figure shows an example configuration of a conventional transmission system. This is a diagram illustrating the level of the pilot signal. 【0010】 Embodiments of the present invention will be described in detail with reference to the drawings. 【0011】<First Embodiment> In this embodiment, an example is described in which the strength of the pilot signal is used as an evaluation index for alarm issuance. Figure 1 is a diagram showing an example of the configuration of the optical communication system of this embodiment. The optical communication system 1 comprises, for example, an insertion device 2, an optical transmitter 3, and an optical repeater 4 (4-1). The insertion device 2 comprises, for example, an additional pilot signal generation unit 21, an adjustment unit 22, and an addition unit 23. The optical transmitter 3 comprises, for example, a pilot signal generation unit 31, an addition unit 32, a signal amplification unit 33, a confirmation unit 34, a signal processing unit 35, and an electro-optical converter 36. The optical repeater 4-1 includes, for example, a distribution unit 401, an optical-electric converter 402 (measurement and determination unit), a signal processing unit 403 (measurement and determination unit), a filter 404 (measurement and determination unit), an averaging unit 405 (measurement and determination unit), a measurement unit 406 (measurement and determination unit), a determination unit 407 (measurement and determination unit), and a relay processing unit 408 (measurement and determination unit). 【0012】 The insertion device 2 and the optical transmitter 3 are connected by a wired electrical signal line, a wired optical signal line, or a wireless line. The optical transmitter 3 and the optical repeater 4-1 are connected, for example, by an optical fiber. The optical repeater 4-1 is also connected to a subsequent optical repeater 4 or an optical receiver (not shown). 【0013】 (Insertion device) The additional pilot signal generation unit 21 generates an additional pilot signal and outputs the generated additional pilot signal to the adjustment unit 22. The intensity of the generated additional pilot signal may be any intensity predetermined, but for example, it is the same intensity as the pilot signal generated by the optical transmitter 3. Furthermore, the frequency of the pilot signal generated by the optical transmitter 3 is different from but close to (within a predetermined range) the frequency of the pilot signal generated by the optical transmitter 3, and is within the range detectable by the filter 404 of the optical repeater 4, and is a different frequency from the frequency of the transmission signal. 【0014】 The adjustment unit 22 fine-tunes the intensity and phase of the additional pilot signal based on the quality confirmation information output by the confirmation unit 34 of the optical transmitter 3, in order to avoid a degradation in the quality of the transmission signal due to the insertion of the additional pilot signal, so that the quality meets the standard. Note that the insertion device 2 does not necessarily have to be equipped with the adjustment unit 22. 【0015】The summing unit 23 adds an additional pilot signal, or an additional pilot signal, whose intensity and phase have been finely adjusted, to a transmission signal input from another device, and outputs the added electrical signal to the optical transmitter 3. 【0016】 (Optical Transmitter) The pilot signal generation unit 31 generates a pilot signal for quality confirmation in the optical communication system 1 and outputs the generated pilot signal to the summing unit 32. 【0017】 The adder 32 adds a pilot signal to the transmission signal input from the insertion device 2 and outputs the added electrical signal to the signal amplification unit 33. The electrical signal output by the adder 32 includes the pilot signal, an additional pilot signal, and the transmission signal. 【0018】 The signal amplification unit 33 amplifies the amplitude of the electrical signal output by the summing unit 32 at a predetermined amplification factor and outputs the amplified electrical signal to the signal processing unit 35. Note that the optical transmitter 3 does not necessarily have to be equipped with a signal amplification unit 33. 【0019】 The verification unit 34 checks the signal quality (e.g., S / N ratio) of the electrical signal output by the signal amplification unit 33 to the signal processing unit 35, and outputs the result of the check as signal quality information to the insertion device 2. Note that the input signal to the verification unit is not limited to the output signal of the signal amplification unit, but may also be the output signal of another processing block in the optical transmitter, such as the output of the signal processing unit 35. 【0020】 The signal processing unit 35 performs, for example, frequency modulation (FM) on the signal amplification unit 33 and outputs the modulated electrical signal to the electro-optical converter 36. 【0021】 The electro-optical converter 36 converts the electrical signal output by the signal processing unit 35 into an optical signal and transmits the converted optical signal to the optical repeater 4-1. Note that the insertion of the pilot signal does not necessarily have to be performed in the pilot signal generation unit 31 inside the optical transmitter 3; it may also be performed in a configuration prior to the optical transmitter 3 (inside the insertion device 2 or even further prior to the insertion device 2). 【0022】 (Optical Repeater) The distribution unit 401 distributes the optical signal received by the optical repeater 4 and outputs the distributed optical signal to the optical-electric converter 402 and the relay processing unit 408. 【0023】 The optical-electric converter 402 converts the optical signal input from the distribution unit 401 into an electrical signal and outputs the converted electrical signal to the signal processing unit 403. 【0024】 The signal processing unit 403 performs demodulation processing (e.g., frequency demodulation) on the electrical signal output by the optical-electric converter 402 in accordance with the modulation performed by the signal processing unit 35 of the optical transmitter 3, and outputs the demodulated electrical signal to the filter 404. 【0025】 The filter 404 is, for example, a BPF (Band-Pass Filter), an LPF (Low-Pass Filter), or an HPF (High-Pass Filter), and extracts the bandwidth in which the pilot signal exists and removes the transmission signal. The filter 404 outputs the extracted electrical signal, i.e., the electrical signal including the pilot signal and additional pilot signals, with the transmission signal removed, to the averaging unit 405. 【0026】 The averaging unit 405 averages the electrical signal input from the filter 404 over time and outputs the time-averaged electrical signal to the measurement unit 406. 【0027】 The measurement unit 406 measures the level of the averaged electrical signal output by the averaging unit 405 and outputs the measured level to the determination unit 407. 【0028】 The determination unit 407 stores a threshold value (alarm issuance determination criterion value). If the optical repeater 4-1 is equipped with a memory unit, the memory unit may also store the threshold value. The determination unit 407 compares the measured level output by the measurement unit 406 with the threshold value. The determination unit 407 determines that no alarm will be issued if the measured level is equal to or greater than the threshold value. The determination unit 407 determines that an alarm will be issued if the measured level is less than the threshold value. 【0029】 The relay processing unit 408 transmits the optical signal input from the distribution unit 401 to the optical repeater 4 or optical receiver connected to it via an optical fiber. 【0030】(Overview of signal processing for each device) Next, an overview of the signal processing for each device in the optical communication system 1 will be explained using Figures 2 and 3. Figure 2 is a diagram showing an overview of the signal processing for the insertion device and the optical transmitter in this embodiment. The symbol g10 represents the electrical signal output by the summing unit 23 of the insertion device 2, with frequency on the horizontal axis and intensity on the vertical axis, and is a state in which an additional pilot signal has been inserted into the transmission signal. The electrical signal includes the additional pilot signal g11 and the transmission signal g12. The intensity of the additional pilot signal is, for example, 5 (mW). In the figure, the frequency of the additional pilot signal g11 is shown to be lower than the frequency of the bandwidth of the transmission signal g12, but it may be higher than the frequency of the bandwidth of the transmission signal g12. 【0031】 The symbol g20 represents an image of the electrical signal output by the summing unit 32 of the optical transmitter 3, with frequency on the horizontal axis and intensity on the vertical axis. It shows the state after adding the pilot signal to the input signal (additional pilot signal + transmission signal). The electrical signal includes the additional pilot signal g11, the pilot signal g21, and the transmission signal g12. The intensity of the pilot signal is, for example, 5 (mW). In the example shown for symbol g20, the frequency of the additional pilot signal g11 is shown to be lower than the frequency of the pilot signal g21, but the frequency of the additional pilot signal g11 may also be higher than the frequency of the pilot signal g21. 【0032】 The symbol g30 represents an image of the electrical signal output by the signal amplification unit 33 of the optical transmitter 3, with frequency on the horizontal axis and intensity on the vertical axis. The electrical signal output by the signal amplification unit 33 includes the amplified pilot signal g32, the amplified additional pilot signal g31, and the amplified transmission signal g33. The intensity of the amplified pilot signal and the amplified additional pilot signal are, for example, 8.5 (mW). 【0033】Figure 3 is a diagram illustrating the signal processing overview of the optical repeater in this embodiment. The symbol g40 represents the electrical signal output by the filter 404 of the optical repeater 4-1, with frequency on the horizontal axis and intensity on the vertical axis. As shown by symbol g41, the transmission signal is removed by the filter. Furthermore, the amplified pilot signal g32 and the amplified additional pilot signal g31 are not cut by the filter 404 because their frequencies are within the passband of the filter 404. In other words, the insertion bandwidth of the inserted pilot (additional pilot signal) is within the bandwidth from which the pilot signal is extracted. 【0034】 The level measured by the measurement unit 406 of the optical repeater 4-1 is 17 mW (= 8.5 x 2), since it is the amplified pilot signal g32 and the amplified additional pilot signal g31. When the threshold is 15 mW, the determination unit 407 determines that the measured level of 17 mW is greater than or equal to the threshold of 15 mW, and therefore does not issue an alarm. 【0035】 The above description illustrates an example where the optical transmitter 3 is replaced with a successor unit. According to this embodiment, by inserting an additional pilot signal, the level measured after passing through the filter 404 can be raised, thus preventing unintended alarms caused by the replacement, even when the unit is replaced with a successor unit. 【0036】The functional units that handle electrical signals in the insertion device 2, the optical transmitter 3, and the optical repeater 4-1 are configured using a processor such as a CPU (Central Processing Unit) and memory. The insertion device 2 functions, for example, as an additional pilot signal generation unit 21, an adjustment unit 22, and an addition unit 23, when the processor executes a program. The optical transmitter 3 functions, for example, as a pilot signal generation unit 31, an addition unit 32, a signal amplification unit 33, a verification unit 34, and a signal processing unit 35, when the processor executes a program. The optical repeater 4-1 functions, for example, as a signal processing unit 403, a filter 404, an averaging unit 405, a measurement unit 406, and a determination unit 407, when the processor executes a program. The functional units that handle electrical signals in the insertion device 2, the optical transmitter 3, and the optical repeater 4-1 may be implemented using hardware such as ASICs (Application Specific Integrated Circuits), PLDs (Programmable Logic Devices), or FPGAs (Field Programmable Gate Arrays). The above program may be recorded on a computer-readable recording medium. Computer-readable recording media include, for example, portable media such as flexible disks, magneto-optical disks, ROMs, CD-ROMs, and semiconductor memory devices (e.g., SSDs: Solid State Drives), as well as storage devices such as hard disks and semiconductor memory devices built into computer systems. The above program may also be transmitted via a telecommunications line. 【0037】Figure 4 is a schematic diagram of an example hardware configuration of an information processing device applied to an embodiment. The information processing device 6 comprises a processor 601, a main memory 602, a communication interface 603, an auxiliary storage device 604, an input / output interface 605, and an internal bus 606. The processor 601, the main memory 602, the communication interface 603, the auxiliary storage device 604, and the input / output interface 605 are connected to each other via the internal bus 606 so as to be able to communicate with each other. The information processing device 6 may be applied to, for example, an insertion device 2, a functional unit that handles electrical signals of an optical transmitter 3, or a functional unit that handles electrical signals of an optical repeater 4-1. For example, if the optical repeater 4 has a storage unit, the storage unit may be configured using the main memory 602 and the auxiliary storage device 604. Also, for example, the averaging unit 405, the measurement unit 406, and the determination unit 407 may be configured using the processor 601, the main memory 602, and the auxiliary storage device 604. 【0038】 (Example of processing procedure) Figure 5 is a sequence diagram of the alarm issuance determination process of the communication system in this embodiment. 【0039】 (Step S1) The additional pilot signal generation unit 21 of the insertion device 2 generates an additional pilot signal and outputs the generated additional pilot signal to the adjustment unit 22. 【0040】 (Step S2) The adjustment unit 22 of the insertion device 2 fine-tunes the intensity and phase of the additional pilot signal based on the quality confirmation information output by the confirmation unit 34 of the optical transmitter 3. Subsequently, the addition unit 23 of the insertion device 2 adds the additional pilot signal with fine-tuned intensity and phase, or the additional pilot signal, to a transmission signal input from another device, and outputs the added electrical signal to the optical transmitter 3. 【0041】 (Step S11) The adder 32 of the optical transmitter 3 adds a pilot signal to the transmission signal input from the insertion device 2 and outputs the added electrical signal to the signal amplification unit 33. 【0042】 (Step S12) The signal amplification unit 33 of the optical transmitter 3 amplifies the amplitude of the electrical signal output by the summing unit 32 at a predetermined amplification rate, and outputs the amplified electrical signal to the signal processing unit 35. 【0043】(Step S13) The confirmation unit 34 of the optical transmitter 3 checks the signal quality of the electrical signal output from the signal amplification unit 33 to the signal processing unit 35, and outputs the checked result to the insertion device 2 as signal quality information. 【0044】 (Step S14) The signal processing unit 35 of the optical transmitter 3 performs, for example, frequency modulation on the signal amplification unit 33, and outputs the modulated electrical signal to the electro-optical converter 36. 【0045】 (Step S15) The electro-optical converter 36 of the optical transmitter 3 converts the electrical signal output from the signal processing unit 35 into an optical signal, and transmits the converted optical signal to the optical repeater 4-1. 【0046】 (Step S21) The opto-electrical converter 402 of the optical repeater 4-1 converts the optical signal input from the distribution unit 401 into an electrical signal, and outputs the converted electrical signal to the signal processing unit 403. 【0047】 (Step S22) The signal processing unit 403 of the optical repeater 4-1 performs demodulation processing on the electrical signal output from the opto-electrical converter 402 with respect to the modulation performed by the signal processing unit 35 of the optical transmitter 3, and outputs the demodulated electrical signal to the filter 404. 【0048】 (Step S23) The filter 404 of the optical repeater 4-1 extracts the band where the pilot signal exists and removes the transmission signal, and outputs the extracted electrical signal to the averaging unit 405. 【0049】 (Step S24) The averaging unit 405 of the optical repeater 4-进行时间平均化,并将时间平均化后的电気信号输出到测量部406。 【0050】 (Step S25) The measuring unit 406 of the optical repeater 4-1 measures the level of the averaged electrical signal, and outputs the measured level to the determination unit 407. 【0051】 (Step S26) The determination unit 407 of the optical repeater 4-1 compares the measured level output from the measuring unit 406 with a threshold value. The determination unit 407 determines not to issue an alarm when the measured level is greater than or equal to the threshold value. The determination unit 407 determines to issue an alarm when the measured level is less than the threshold value. 【0052】In the example described above, the frequency of the additional pilot signal was described as being within the bandwidth of filter 404 and in the vicinity of a predetermined range of the pilot signal frequency, but this is not limited to this. The frequency of the additional pilot signal may be within a predetermined range of the pilot signal frequency that includes the same frequency as the pilot signal frequency. Also, the number of additional pilot signals is not limited to one, but may be two or more. 【0053】 Furthermore, since the insertion of the additional pilot signal should not affect the quality of the transmission signal, the insertion device 2 may be configured to acquire signal quality information from the optical transmitter 3, adjust the frequency, intensity, and phase of the additional pilot signal in the adjustment unit 22 before insertion. If the optical transmitter 3 cannot output signal quality information externally, the insertion device 2 may output the generated additional pilot signal to the adder 23, or adjust it to a predetermined intensity and phase in the adjustment unit 22 before outputting it to the adder 23. In addition, the insertion of the additional pilot signal may be performed before the optical repeater, and the functions of the insertion device 2 may be provided by, for example, the optical transmitter 3. Furthermore, the adjustment unit 22 may be provided by a device that has the functions of the insertion device 2. In this case, the adjustment unit 22 may adjust at least one of the intensity and phase of the additional pilot signal based on the results of checking the signal quality of the transmission signal including the additional pilot signal and the pilot signal, so that the signal quality meets the standard. 【0054】In the example described above, the determination of whether to issue an alarm was performed by the optical repeater 4, but this is not the only example. For example, the optical transmitter 3 may be equipped with the necessary functional units for determining whether to issue an alarm (e.g., a filter, an averaging unit, a measurement unit, and a determination unit) and perform the determination. Alternatively, the optical receiver may be equipped with the necessary functional units for determining whether to issue an alarm (e.g., a filter, an averaging unit, a measurement unit, and a determination unit) and perform the determination. In the first embodiment, the explanation was given for the case where the transmission signal to which the pilot signal is multiplexed is an electrical signal, but this is not the only example, and the transmission signal may be an optical signal or a wireless signal. In that case, for example, the additional pilot signal of light or radio waves may be generated by the additional pilot signal generation unit 21 of the insertion device 2 for the optical signal or wireless signal, added by the addition unit 23, and the pilot signal of light or radio waves may be added by the addition unit 32 of the optical transmitter 3. 【0055】 <Second Embodiment> In this embodiment, an example is described in which the Carrier-to-noise ratio (CNR), which is the ratio of the intensity of the pilot signal to the amount of noise, is used as an evaluation index for alarm issuance. 【0056】 Figure 6 is a diagram illustrating an example of alarm issuance determination using CNR. Code g60 represents an example where the CNR is normal. In code g60, the horizontal axis is frequency and the vertical axis is intensity. For example, the signal input to the optical repeater includes a pilot signal g61, the spectrum g62 of the transmission signal, and noise g63. Here, for example, suppose the intensity of the pilot signal g61 is 20 dBm, the noise floor at the frequency of the pilot signal is 2 dBm, and the alarm issuance threshold is 15 dB. In this case, the CNR at the frequency of the pilot signal is 18 dB (= 20 dBm - 2 dBm), which is greater than or equal to the threshold of 15 dB, so it is determined that no alarm is issued. 【0057】Code g70 is an example where the CNR is low because the pilot signal intensity is low. In code g70, the horizontal axis is frequency and the vertical axis is intensity. For example, the signal input to the optical repeater includes the pilot signal g71, the spectrum g72 of the transmission signal, and noise g73. Here, for example, suppose the intensity of the pilot signal g71 is 10 dBm, the noise floor at the frequency of the pilot signal is 2 dBm, and the alarm threshold is 15 dB. In this case, the CNR at the frequency of the pilot signal is 8 dB (= 10 dBm - 2 dBm), which is less than the threshold of 15 dB, so it is determined that an alarm should be issued. 【0058】 Code g80 is an example where the CNR is low due to a high noise level. In code g80, the horizontal axis is frequency and the vertical axis is intensity. For example, the signal input to the optical repeater includes the pilot signal g81, the spectrum g82 of the transmission signal, and noise g83. Here, for example, suppose the intensity of the pilot signal g71 is 20 dBm, the noise floor at the frequency of the pilot signal is 11 dBm, and the alarm threshold is 15 dB. In this case, N in the CNR increases, and the CNR at the frequency of the pilot signal is 9 dB (= 20 dBm - 11 dBm), which is less than the threshold of 15 dB, so it is determined that an alarm should be issued. 【0059】 Even when using CNR to determine whether to issue an alarm, in conventional communication systems, for example, if the pilot signal is lower than that of the previous model when replacing the optical transmitter 3 with a successor model, an alarm will be issued due to the decrease in CNR, even if the replaced optical transmitter 3 is functioning correctly. Therefore, in this embodiment, the insertion device 2 inserts an additional pilot signal to increase the level of C in the CNR determination, thereby suppressing the issuance of false alarms. 【0060】Figure 7 shows an example configuration of the optical communication system of this embodiment. The optical communication system 1A includes, for example, an insertion device 2, an optical transmitter 3, and an optical repeater 4A (4A-1). The insertion device 2 includes, for example, an additional pilot signal generation unit 21, an adjustment unit 22, and an addition unit 23. The insertion device 2 does not necessarily have to include the adjustment unit 22. Alternatively, the adjustment unit 22 may be provided by a device that performs the functions of the insertion device 2. The optical transmitter 3 includes, for example, a pilot signal generation unit 31, an addition unit 32, a signal amplification unit 33, a verification unit 34, a signal processing unit 35, and an electro-optical converter 36. The optical transmitter 3 does not necessarily have to include the signal amplification unit 33. The optical repeater 4A-1 includes, for example, a distribution unit 401A, a pilot signal measurement unit 409 (measurement and determination unit), a noise measurement unit 410 (measurement and determination unit), and a determination unit 407A. The pilot signal measurement unit 409 includes, for example, a photoelectric converter 402, a signal processing unit 403, a filter 404, an averaging unit 405, and a measurement unit 406. The noise measurement unit 410 includes, for example, a photoelectric converter 411, a signal processing unit 412, a filter 413, an averaging unit 414, and a measurement unit 415. 【0061】 The strength of the additional pilot signal may be, for example, any strength predetermined beforehand, or the same strength as the pilot signal. Furthermore, the insertion of the pilot signal does not necessarily have to be done inside the optical transmitter 3; it may also be done in a configuration prior to the optical transmitter 3 (inside the insertion device 2, or even further prior to the insertion device 2). 【0062】 The pilot signal measurement unit 409 extracts the pilot signal and additional pilot signals and measures their levels. The optical-electric converter 402 converts the input optical signal into an electrical signal and outputs the converted electrical signal to the signal processing unit 403. The signal processing unit 403 performs demodulation processing (e.g., frequency demodulation processing) on ​​the electrical signal output by the optical-electric converter 402 in accordance with the modulation performed by the signal processing unit 35 of the optical transmitter 3, and outputs the demodulated electrical signal to the filter 413. 【0063】Filter 404 removes the transmission signal and allows the pilot signal and additional pilot signal to pass through. Averaging unit 405 averages the electrical signal input from filter 404 over time and outputs the time-averaged electrical signal to measurement unit 406. Measurement unit 406 measures the level of the averaged electrical signal output by averaging unit 405 and outputs the measured level to determination unit 407A. In the following explanation, it is assumed that the intensity of the pilot signal and additional pilot signal are both 18 dBm. Also, it is assumed that the measured level of the pilot signal was, for example, 21 dBm. 【0064】 The noise measurement unit 410 measures the noise level. The optical-electric converter 411 converts the input optical signal into an electrical signal and outputs the converted electrical signal to the signal processing unit 412. The signal processing unit 412 performs demodulation processing (e.g., frequency demodulation processing) on ​​the electrical signal output by the optical-electric converter 411 in accordance with the modulation performed by the signal processing unit 35 of the optical transmitter 3, and outputs the demodulated electrical signal to the filter 413. 【0065】 The filter 413 is, for example, a BPF, LPF, or HPF, and extracts a bandwidth set for noise level measurement. The filter 413 outputs the extracted noise to the averaging unit 414. The averaging unit 414 time-averages the noise input from the filter 413 and outputs the time-averaged electrical signal to the measurement unit 415. The measurement unit 415 measures the level of the averaged noise output by the averaging unit 414 and outputs the measured level to the determination unit 407A. Here, let's assume that the measured noise level was, for example, 4 (dBm). 【0066】The determination unit 407A calculates the CNR (e.g., 21 dBm - 4 dBm = 17 dB) by subtracting the noise level (e.g., 4 dBm) from the pilot signal level (e.g., 21 dBm). The determination unit 407A stores the threshold value (alarm issuance judgment criterion value). If the optical repeater 4-1 is equipped with a memory unit, the memory unit may store the threshold value. The determination unit 407A compares the calculated CNR (e.g., 17 dB) with the threshold value (e.g., 15 dB). The determination unit 407A determines that no alarm will be issued if the calculated CNR is greater than or equal to the threshold value. The determination unit 407A determines that an alarm will be issued if the CNR is less than the threshold value. 【0067】 (Overview of signal processing for each device) Next, an overview of the signal processing for each device in the optical communication system 1A will be explained using Figures 8 and 9. Figure 8 is a diagram showing an overview of the signal processing for the insertion device and the optical transmitter in this embodiment. The symbol g110 represents an image of the electrical signal output by the summing unit 23 of the insertion device 2, with frequency on the horizontal axis and intensity on the vertical axis, and represents a state in which an additional pilot signal has been inserted into the transmission signal. The electrical signal includes the additional pilot signal g111 and the transmission signal g112. The intensity of the additional pilot signal is, for example, 5 (dBm). The frequency of the additional pilot signal g111 is, for example, lower than the frequency of the bandwidth of the transmission signal g112. 【0068】 The symbol g120 represents an image of the electrical signal output by the summing unit 32 of the optical transmitter 3, with frequency on the horizontal axis and intensity on the vertical axis. It shows the state after adding the pilot signal to the input signal (additional pilot signal + transmission signal). The electrical signal includes the additional pilot signal g111, the pilot signal g121, the transmission signal g112, and noise g122. The intensity of the pilot signal is, for example, 5 (dBm). In the example shown for symbol g120, the frequency of the additional pilot signal g111 is shown to be lower than the frequency of the pilot signal g121, but the frequency of the additional pilot signal g111 may be higher than or the same as the frequency of the pilot signal g121. 【0069】The symbol g130 represents an image of the electrical signal output by the signal amplification unit 33 of the optical transmitter 3, with frequency on the horizontal axis and intensity on the vertical axis. The electrical signal output by the signal amplification unit 33 includes the amplified pilot signal g132, the amplified additional pilot signal g131, the amplified transmission signal g133, and the amplified noise g134. The intensity of the amplified pilot signal and the amplified additional pilot signal are, for example, 18 (dBm). 【0070】 Figure 9 is a diagram illustrating the signal processing overview of the optical repeater in this embodiment. The symbol g140 represents the electrical signal output by the filter 404 of the optical repeater 4A-1, with frequency on the horizontal axis and intensity on the vertical axis. As shown by symbol g141, the transmission signal is removed by the filter 404. The amplified pilot signal g132 and the amplified additional pilot signal g131 are not cut by the filter 404 because their frequencies are within the passband of the filter 404. The signal after passing through the filter 404 also includes noise g134. 【0071】 Here, we will explain why the measurement value at the pilot level detection unit increases when an additional pilot is inserted. By inserting an additional pilot, both the conventional pilot signal and the additional pilot signal exist simultaneously within the bandwidth of filter 404. Therefore, the measurement unit 406 measures the total power within the bandwidth (the sum of the conventional pilot and the additional pilot), causing the level to increase. However, the measurement value at the noise level detection unit of measurement unit 415 remains the same regardless of whether an additional pilot is inserted or not. This is because filter 413 is monitoring a bandwidth where only noise exists, and this bandwidth does not change with the insertion of the additional pilot. Therefore, the CNR, which is the ratio between the level measured by measurement unit 406 and the level measured by measurement unit 415, improves with the insertion of the additional pilot. 【0072】 The determination unit 407A calculates the CNR using the increased signal level of the pilot signal measured by the pilot signal measurement unit 409 and the noise level measured by the noise measurement unit 410. Then, the determination unit 407A determines whether or not to issue an alarm according to the determination result. 【0073】 (Example of processing procedure) Figure 10 is a sequence diagram of the alarm issuance determination process of the communication system in this embodiment. Note that the following processing example is an example in which the insertion device 2 does not perform signal adjustment. 【0074】 (Step S101) The additional pilot signal generation unit 21 of the insertion device 2 generates an additional pilot signal and outputs the generated additional pilot signal to the addition unit 23. Subsequently, the addition unit 23 of the insertion device 2 adds the additional pilot signal, or the additional pilot signal, whose intensity and phase have been finely adjusted, to a transmission signal input from another device, and outputs the added electrical signal to the optical transmitter 3. 【0075】 (Step S111) The adder 32 of the optical transmitter 3 adds a pilot signal to the transmission signal input from the insertion device 2 and outputs the added electrical signal to the signal amplification unit 33. 【0076】 (Step S112) The signal amplification unit 33 of the optical transmitter 3 amplifies the amplitude of the electrical signal output by the summing unit 32 at a predetermined amplification rate, and outputs the amplified electrical signal to the signal processing unit 35. 【0077】 (Step S113) The signal processing unit 35 of the optical transmitter 3 performs frequency modulation on the signal amplification unit 33 and outputs the modulated electrical signal to the electro-optical converter 36. 【0078】 (Step S114) The electro-optic converter 36 of the optical transmitter 3 converts the electrical signal output by the signal processing unit 35 into an optical signal, and transmits the converted optical signal to the optical repeater 4A-1. 【0079】 (Step S121) The optical repeater 4A-1's optical-electric converter 402 converts the optical signal input from the distribution unit 401 into an electrical signal. Subsequently, the signal processing unit 403 of the optical repeater 4A-1 demodulates the electrical signal output by the optical-electric converter 402 for the modulation performed by the signal processing unit 35 of the optical transmitter 3. Subsequently, the filter 404 of the optical repeater 4A-1 extracts the bandwidth in which the pilot signal exists and removes the transmission signal. Subsequently, the averaging unit 405 of the optical repeater 4A-1 averages the electrical signal from which the transmission signal has been removed over time. Subsequently, the measurement unit 406 of the optical repeater 4A-1 measures the level of the averaged electrical signal. 【0080】 (Step S122) The optical repeater 4A-1's optical-electric converter 411 converts the optical signal input from the distribution unit 401 into an electrical signal. Subsequently, the signal processing unit 412 of the optical repeater 4A-1 demodulates the electrical signal output by the optical-electric converter 402 for the modulation performed by the signal processing unit 35 of the optical transmitter 3. Subsequently, the filter 413 of the optical repeater 4A-1 extracts the noise. Subsequently, the averaging unit 414 of the optical repeater 4A-1 averages the extracted noise over time. Subsequently, the measurement unit 415 of the optical repeater 4A-1 measures the level of the averaged noise. 【0081】 (Step S123) The determination unit 407A of the optical repeater 4-1 calculates the CNR using the increased signal level of the pilot signal measured by the pilot signal measurement unit 409 and the noise level measured by the noise measurement unit 410. Subsequently, the determination unit 407A determines whether or not to issue an alarm according to the determination result. 【0082】 Furthermore, in a configuration without an additional pilot signal, if the CNR of the pilot signal and noise is measured to determine whether or not to issue an alarm, for example, if the pilot signal level is lower than that of the previous model due to the replacement of the optical transmitter, the measured CNR will decrease, leading to a problem of misjudgment. 【0083】 In contrast, in this embodiment, an additional pilot signal is inserted into the transmission signal. As a result, according to this embodiment, only the value of C in the CNR can be increased, thereby suppressing the issuance of alarms in the optical repeater 4 and the optical receiver (not shown). 【0084】 In this embodiment as well, the insertion of the additional pilot signal must not affect the quality of the transmitted signal. Therefore, the insertion device 2 may be configured to acquire signal quality information from the optical transmitter 3, and then adjust the frequency, intensity, and phase of the additional pilot signal in the signal adjustment unit before insertion. Furthermore, although the above example described an example where the signal-to-noise ratio (CNR) was used for determination, the signal-to-noise ratio (SNR) may also be calculated and used for determination. 【0085】Although embodiments of this invention have been described in detail above with reference to the drawings, the specific configuration is not limited to these embodiments and includes designs and the like that do not depart from the spirit of this invention. 【0086】 <Notes> (1) A communication method in an optical communication system that relaxes alarm activation conditions without modifying the alarm activation equipment by inserting one or more additional pilot signals near or at the same frequency as an existing pilot signal. (2) A communication method in which, in the manner of (1) above, the insertion bandwidth of the additional pilot signal is set to be within the bandwidth of the BPF in the alarm activation equipment. (3) A communication method in which, in the manner of (1) or (2) above, the additional pilot signal is inserted by a device inserted before the existing optical transmitter. (4) A communication method in which, in any one of the manners of (1) to (3) above, in order to not degrade the transmission signal quality when the additional pilot signal is inserted, one or more of the strength, frequency, and phase of the additional pilot signal are adjusted and then frequency multiplexed into the transmission signal as an additional pilot signal. 【0087】 The present invention is applicable to optical communication systems, optical signaling devices, optical receiving devices, optical transceivers, optical transmission systems, optical transmission devices, and the like. 【0088】 1...Optical communication system, 2...Insertion device, 3...Optical transmitter, 4, 4A, 4-1, 4A-1...Optical repeater, 21...Additional pilot signal generation unit, 22...Adjustment unit, 23...Addition unit, 31...Pilot signal generation unit, 32...Addition unit, 33...Signal amplification unit, 34...Verification unit, 35...Signal processing unit, 36...Electro-optic converter, 401, 401A...Distribution unit, 402, 411...Opto-electro-converter, 403, 412...Signal processing unit, 404, 413...Filter, 405, 414...Averaging unit, 406, 415...Measurement unit, 407, 407A...Determination unit, 408...Relay processing unit, 409...Pilot signal measurement unit, 410...Noise measurement unit

Claims

1. An optical communication system comprising: an optical repeater that monitors the signal quality based on a pilot signal that is added to the transmission signal and frequency multiplexed with the transmission signal to monitor the signal quality; and an insertion device that inserts one or more additional pilot signals into the optical repeater before the pilot signal within a predetermined frequency range of the pilot signal.

2. The optical communication system according to claim 1, wherein the insertion bandwidth of the additional pilot signal is within the bandwidth for extracting the pilot signal.

3. An optical communication system according to claim 1 or 2, comprising: an adjustment unit that adjusts at least one of the intensity and phase of the additional pilot signal based on the result of checking the signal quality of the transmission signal so that the signal quality meets a standard.

4. The optical communication system according to claim 1 or 2, wherein the optical repeater comprises a measurement and determination unit that extracts the additional pilot signal and the pilot signal, measures the combined signal level of the extracted additional pilot signal and the pilot signal, compares the measured signal level with a threshold, and determines the signal quality based on the result of the comparison.

5. An optical communication method comprising: an optical repeater monitoring the signal quality based on a pilot signal that is added to the transmission signal and frequency multiplexed with the transmission signal to monitor the signal quality; and an insertion device inserting one or more additional pilot signals within a predetermined frequency range of the pilot signal before the optical repeater.

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