Optical repeater, optical transmission system, and optical repeater method

Abstract

An optical transceiver (101) of this optical relay device comprises a coherent reception front end (110) that coherently detects an input optical signal on the basis of local oscillation light and outputs a coherently-detected first analog electric signal, a coherent transmission front end (120) that coherently modulates, on the basis of transmission light, a second analog electric signal obtained by turning around the first analog electric signal and outputs a coherently-modulated output optical signal, and an analog compensation unit (130) that performs analog signal processing on the first analog electric signal to compensate for signal quality according to signal characteristics between the input at the coherent reception front end (110) and the output at the coherent transmission front end (120) and thereby generates the second analog electric signal.

Description

[Technical field]

[0001] The present invention relates to an optical repeater, an optical transmission system, and an optical repeating method. [Background technology]

[0002] In recent years, the introduction of 5G wireless communication systems has been progressing, and in the post-5G era, there is a growing demand not only for ultra-high speeds but also for ultra-low latency and multiple simultaneous connections in the optical communication field as well as in wireless communication. For this reason, research on optical communication systems is being conducted with the expectation that they will be used for a variety of communication services and industrial applications.

[0003] For example, in backbone optical communication systems, a digital coherent method that combines optical phase modulation and polarization multiplexing / demultiplexing technology has been used to achieve a capacity of over 100 Gbps (Giga bit per second). Furthermore, research and development is being conducted on a transmission method that improves frequency utilization efficiency and enables multiple simultaneous connections by narrowing the signal band and using wavelength division multiplexing (WDM).

[0004] As a related technique, for example, Patent Document 1 is known. Patent Document 1 discloses a wavelength converter that converts the wavelength of an optical signal by a receiving end and a transmitting end using a coherent system. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Special Publication No. 2017-511036 Summary of the Invention [Problem to be solved by the invention]

[0006] In the wavelength converter disclosed in Patent Document 1, a receiving end including a coherent detection front-end module converts a received optical signal into an analog electrical signal, and a transmitting end including an optical modulation module converts the analog electrical signal into a transmitted optical signal. , special Patent Document 1 does not take into consideration the degradation of the quality of the optical signal due to passing through multiple optical repeaters. , special Patent Document 1 only converts an optical signal into an analog electrical signal and then converts the analog electrical signal into an optical signal, which causes a problem that signal quality may be degraded.

[0007] In view of the above problems, an object of the present disclosure is to provide an optical repeater, an optical transmission system, and an optical repeater method that are capable of suppressing degradation of signal quality. [Means for solving the problem]

[0008] The optical repeater according to the present disclosure comprises: a coherent optical receiving front-end means for coherently detecting an input optical signal based on a local optical signal and outputting the coherently detected first analog electrical signal; a coherent optical transmitting front-end means for coherently modulating a second analog electrical signal, which is a reflection of the first analog electrical signal, based on transmitted light and outputting the coherently modulated output optical signal; and an analog compensation means for performing analog signal processing on the first analog electrical signal so as to compensate for signal quality in accordance with signal characteristics between the input of the coherent optical receiving front-end means and the output of the coherent optical transmitting front-end means, thereby generating the second analog electrical signal.

[0009] The optical transmission system according to the present disclosure is an optical transmission system including a plurality of optical repeater devices, each of which includes: a coherent optical receiving front-end means for coherently detecting an input optical signal input from a previous optical repeater device based on local light and outputting the coherently detected first analog electrical signal; a coherent optical transmitting front-end means for coherently modulating a second analog electrical signal obtained by folding back the first analog electrical signal based on transmitted light and outputting the coherently modulated output optical signal to the optical repeater device of the next stage; and an analog compensation means for performing analog signal processing on the first analog electrical signal so as to compensate for signal quality in accordance with signal characteristics between the input to the coherent optical receiving front-end means and the output of the coherent optical transmitting front-end means, thereby generating the second analog electrical signal.

[0010] The optical relay method according to the present disclosure is an optical relay method in an optical relay device having a coherent optical receiving front-end means and a coherent optical transmitting front-end means, wherein the coherent optical receiving front-end means coherently detects an input optical signal based on local light and outputs the coherently detected first analog electrical signal, the coherent optical transmitting front-end means coherently modulates a second analog electrical signal obtained by folding back the first analog electrical signal based on transmitted light and outputs the coherently modulated output optical signal, and performs analog signal processing on the first analog electrical signal to compensate for signal quality in accordance with signal characteristics between the input to the coherent optical receiving front-end means and the output of the coherent optical transmitting front-end means to generate the second analog electrical signal. Effect of the Invention

[0011] According to the present disclosure, it is possible to provide an optical repeater, an optical transmission system, and an optical repeating method that are capable of suppressing degradation of signal quality. [Brief description of the drawings]

[0012] [Figure 1] 1 is a configuration diagram showing a configuration example of an optical transmission system according to a first embodiment. [Diagram 2] 1 is a configuration diagram showing a configuration example of an optical repeater according to a first embodiment. [Diagram 3] FIG. 1 is a diagram illustrating a configuration example of a related digital coherent optical transceiver. [Figure 4] FIG. 1 is a diagram for explaining a problem with a related digital coherent optical transceiver. [Diagram 5] FIG. 1 is a diagram for explaining a problem with a related digital coherent optical transceiver. [Figure 6] FIG. 1 is a diagram for explaining an overview of an optical repeater according to a first embodiment. [Figure 7] 1 is a configuration diagram showing a configuration example of an optical transceiver according to a first embodiment. [Figure 8] 1 is a configuration diagram showing a configuration example of an optical transceiver according to a first embodiment. [Figure 9] 2 is a configuration diagram showing a configuration example of a coherent receiving front-end unit according to the first embodiment. FIG. [Figure 10] 2 is a configuration diagram showing a configuration example of a coherent transmission front-end unit according to the first embodiment. FIG. [Figure 11] FIG. 11 is a configuration diagram showing a configuration example of an optical transceiver according to a second embodiment. [Figure 12] FIG. 11 is a configuration diagram showing an example of the configuration of a bandwidth monitor according to the second embodiment. [Figure 13A] FIG. 11 is a characteristic diagram showing an example of monitoring a band according to the second embodiment. [Figure 13B] FIG. 11 is a characteristic diagram showing an example of monitoring a band according to the second embodiment. [Figure 14] 11 is a configuration diagram showing a configuration example of a band adjusting circuit according to a second embodiment. FIG. [Figure 15] 11 is a configuration diagram showing a configuration example of a band adjusting circuit according to a second embodiment. FIG. [Figure 16] 10 is a characteristic diagram showing an example of characteristics of the band adjustment circuit according to the second embodiment. FIG. [Figure 17]11 is a configuration diagram showing a configuration example of a band adjusting circuit according to a second embodiment. FIG. [Figure 18] FIG. 11 is a configuration diagram showing a configuration example of an optical transceiver according to a third embodiment. [Figure 19] FIG. 11 is a configuration diagram showing an example of the configuration of a frequency offset monitor according to a third embodiment. [Figure 20A] FIG. 13 is a characteristic diagram showing an example of monitoring a frequency offset according to the third embodiment. [Figure 20B] FIG. 13 is a characteristic diagram showing an example of monitoring a frequency offset according to the third embodiment. [Figure 21] FIG. 11 is a configuration diagram showing an example of the configuration of a frequency offset monitor according to a third embodiment. [Figure 22A] FIG. 13 is a characteristic diagram showing an example of monitoring a frequency offset according to the third embodiment. [Figure 22B] FIG. 13 is a characteristic diagram showing an example of monitoring a frequency offset according to the third embodiment. [Diagram 23] FIG. 13 is a configuration diagram showing a configuration example of an optical transceiver according to a modified example of the third embodiment. [Figure 24] FIG. 13 is a configuration diagram showing an example of the configuration of a frequency offset monitor according to a modification of the third embodiment. [Diagram 25] FIG. 13 is a characteristic diagram showing an example of monitoring a frequency offset according to a modification of the third embodiment. [Figure 26] FIG. 13 is a configuration diagram showing a configuration example of an optical transceiver according to a fourth embodiment. [Figure 27] FIG. 13 is a configuration diagram showing an example of the configuration of a skew monitor according to a fourth embodiment. [Figure 28A] FIG. 13 is a characteristic diagram showing an example of monitoring a skew according to the fourth embodiment. [Figure 28B] FIG. 13 is a characteristic diagram showing an example of monitoring a skew according to the fourth embodiment. [Figure 29] FIG. 13 is a configuration diagram showing a configuration example of an optical transceiver according to a modified example of the fourth embodiment. [Diagram 30] FIG. 13 is a configuration diagram showing an example of the configuration of a skew monitor according to a modified example of the fourth embodiment. [Figure 31A]FIG. 13 is a characteristic diagram showing an example of monitoring a skew according to a modification of the fourth embodiment. [Figure 31B] FIG. 13 is a characteristic diagram showing an example of monitoring a skew according to a modification of the fourth embodiment. [Figure 32A] FIG. 13 is a signal waveform diagram showing an example of monitoring a skew according to a modification of the fourth embodiment. [Figure 32B] FIG. 13 is a signal waveform diagram showing an example of monitoring a skew according to a modification of the fourth embodiment. [Diagram 33] FIG. 13 is a configuration diagram showing a configuration example of an optical transceiver according to a fifth embodiment. [Diagram 34] FIG. 13 is a configuration diagram showing a configuration example of a coherent receiving front-end unit according to a fifth embodiment. [Diagram 35] FIG. 13 is a configuration diagram showing an example of the configuration of a skew monitor according to a fifth embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, the embodiments will be described with reference to the drawings. In each drawing, the same elements are given the same reference numerals, and duplicated explanations will be omitted as necessary. Note that the arrows in the configuration diagrams (block diagrams) are examples for explanation, and do not limit the types or directions of signals.

[0014] (Embodiment 1) Hereinafter, the first embodiment will be described with reference to the drawings. FIG. 1 shows an example of the configuration of an optical transmission system according to the present embodiment. The optical transmission system 1 according to the present embodiment is, for example, a backbone wavelength multiplexing optical transmission system, which performs wavelength multiplexing and digital coherent transmission with optical signals of each wavelength, thereby performing large-capacity communication of over 100 Gbps. Wavelength multiplexing makes it possible to improve the efficiency of optical frequency utilization, and can handle mobile traffic and wavelength defragmentation. In addition, wavelength multiplexing allows the transmission path (wavelength path) to be flexibly switched as it is with the optical signal, so that the failure can be avoided and the infrastructure can be maintained by switching the transmission path when a failure occurs. Furthermore, in this embodiment, real-time performance is improved and ultra-low latency can be supported toward the post-5G era.

[0015] 1, an optical transmission system 1 includes a plurality of optical repeaters 2 (e.g., 2-1 to 2-10) connected to each other so as to be capable of optical communication via an optical fiber transmission line 3. The optical repeater 2 is a photonic node capable of repeating wavelength-multiplexed optical signals, for example, a reconfigurable optical add / drop multiplexer (ROADM) device. In this example, the optical repeaters 2-1 to 2-10 form a ring network including three rings, but may form a network of other topologies.

[0016] A wavelength path is assigned to each optical repeater 2, and the optical repeater 2 transfers traffic of the local network it accommodates and traffic of other optical repeaters 2 via the assigned wavelength path. For example, the optical repeater 2-1 accommodates the network of the data center 4, and the optical repeater 2-2 accommodates the network of the data center 5, and transfers large volume traffic such as video distribution services that distribute high quality video (4k / 8k). When the optical repeater 2-1 and the optical repeater 2-2 are transferring traffic between the data center 4 and the data center 5 via the wavelength path P1, if a fault occurs in the wavelength path P1, the optical repeater 2-1 switches the wavelength path P1 to the wavelength path P2. This allows the transfer of traffic between the data center 4 and the data center 5 to be maintained via a detour route including the optical repeater 2-3 and the optical repeater 2-4.

[0017] For example, the optical repeater 2-5 accommodates an IoT sensor network of an IT service provider 6, and the optical repeater 2-8 accommodates a mobile network of an event venue 7. Traffic of the mobile network is traffic of spot demand from moving users. When the optical repeater 2-5 and the optical repeater 2-8 are transferring traffic between the IT service provider 6 and the event venue 7 via the wavelength path P3 including the optical repeater 2-6 and the optical repeater 2-7, if a user at the event venue 7 moves to the event venue 8, the wavelength path P3 is switched to the wavelength path P4. This makes it possible to maintain the transfer of traffic of the user who moved to the event venue 8 via the optical repeater 2-6, the optical repeater 2-4, and the optical repeater 2-10.

[0018] 2 shows a configuration example of an optical repeater 2 according to the present embodiment. The optical repeater 2 drops / adds a wavelength multiplexed signal, and coherently modulates / demodulates the signals of each wavelength to be dropped / added. As shown in FIG. 2, the optical repeater 2 includes a transmitting / receiving unit 100 and an optical switch unit 200.

[0019] The optical switch unit 200 transfers an optical signal of a predetermined wavelength path received from a previous-stage optical repeater 2 in the optical transmission system 1 to a subsequent-stage optical repeater 2, and also drops / adds the received optical signal for each wavelength. For example, the optical switch unit 200 includes a demultiplexer 201, a multiplexer 202, and a drop / add unit 203. The demultiplexer 201 separates an optical signal received from the optical fiber transmission line 3 into optical signals of multiple wavelengths. The multiplexer 202 multiplexes optical signals of multiple wavelengths into one optical signal and transmits it to the optical fiber transmission line 3. The drop / add unit 203 drops / adds the optical signal of each wavelength between the demultiplexer 201 and the multiplexer 202.

[0020] The transceiver unit (transponder) 100 receives optical signals of each wavelength branched from the add / drop unit 203 of the optical switch unit 200, outputs the coherently demodulated received data to a local device (network) that accommodates the data, and also inputs transmission data from the local device and transmits (inserts) the coherently modulated optical signals of each wavelength to the add / drop unit 203 of the optical switch unit 200. The transceiver unit 100 includes a plurality of optical transceivers 101 that transmit and receive optical signals of each wavelength. The optical transceivers 101 receive optical signals of a specified wavelength, and further transmit optical signals of a specified wavelength (same or different from the received wavelength).

[0021] Here, we will consider problems that arise when a digital coherent optical transceiver is used as the optical transceiver 101. Fig. 3 shows an example of the configuration of a related digital coherent optical transceiver. As shown in Fig. 3, the related digital coherent optical transceiver 102 includes a coherent receiving front-end unit 110, a coherent transmitting front-end unit 120, and a DSP 900.

[0022] The coherent receiving front-end unit 110 coherently detects an optical signal received from the optical repeater 2 at the previous stage by using a local light (local oscillator (LO) light) of a predetermined wavelength, and outputs the detected signal to the DSP 900. The coherent transmitting front-end unit 120 optically modulates (coherently modulates) the signal processed by the DSP 900 to a predetermined wavelength, and transmits the generated optical signal to the optical repeater 2 at the next stage. The DSP 900 converts the signal coherently detected by the coherent receiving front-end unit 110 into a digital signal and outputs the decoded received data, and also encodes inputted transmission data and outputs a signal converted for optical modulation to the coherent transmitting front-end unit 120.

[0023] Consider a case where optical signals of the same wavelength collide when an optical repeater 2 using such a digital coherent optical transceiver 102 is repeating an optical signal, as shown in Fig. 4. For example, assume that a wavelength path P5 is set between the optical repeater 2-2 and the optical repeater 2-5, and traffic is transferred between an IT service provider 6 and a data center 5, and a wavelength path P6 is set between the optical repeater 2-2 and the optical repeater 2-8, and traffic is transferred between an event venue 7 and a data center 5. In this case, if the wavelength slots of the wavelength paths P5 and P6 are λ1, an optical signal S1 of the wavelength path P5 and an optical signal S2 of the wavelength path P6 collide in the optical repeater 2-7.

[0024] In this case, it is possible to avoid the collision by switching wavelength path P5 or wavelength path P6 to another route, but there is no guarantee that the wavelength slot of the other route will be available. Even if the path is switched, there is a risk that the latency will increase due to the detour path. It is also possible to switch multiple wavelengths as optical signals collectively in units of wavelength grids including multiple wavelength slots (wavelength channels) in the optical switch of the optical repeater, but in this case, it is not possible to switch on a wavelength-by-wavelength basis.

[0025] Therefore, a method of converting an optical signal to an empty wavelength slot at the optical repeater 2-7 where collision occurs can be considered, as shown in Fig. 5. For example, in the digital coherent optical transceiver 102 of the optical repeater 2-7, the wavelength of the optical signal on the wavelength path P6 is converted from λ1 to λ2. This makes it possible to avoid collisions on the route from the optical repeater 2-7 to the optical repeater 2-2, since the wavelengths of the optical signal S1 on the wavelength path P5 and the optical signal S2 on the wavelength path P6 are different.

[0026] 5, there is a problem that the latency increases when the digital coherent optical transceiver 102 converts the wavelength and loops it back, because the digital coherent optical transceiver 102 regenerates and repeats the wavelength. That is, the latency is large because the DSP 900 of the digital coherent optical transceiver 102 performs complex digital signal processing and error correction processing. In addition, the circuit size for digital signal processing is large, and power consumption is also large.

[0027] Therefore, in this embodiment, the optical transceiver 101 loops back the optical signal without using a digital coherent optical transceiver, thereby suppressing an increase in latency. As shown in Fig. 6, in this embodiment, the optical transceiver 101 loops back and relays an analog signal output from the coherent receiver front-end unit 110 to the coherent transmitter front-end unit 120 without going through a DSP (before the DSP). Note that the optical transceiver 101 does not need to include a DSP, and may not use a DSP when looping back an optical signal. By not including a DSP, the circuit size of the optical transceiver 101 can be reduced.

[0028] Fig. 7 shows a configuration example of an optical transceiver 101 according to this embodiment. As shown in Fig. 7, the optical transceiver 101 (optical repeater) according to this embodiment includes a coherent receiving front-end unit 110, a coherent transmitting front-end unit 120, and an analog compensation unit 130.

[0029] The coherent receiver front-end unit 110 and the coherent transmitter front-end unit 120 are the same as those of the digital coherent optical transceiver 102. That is, the coherent receiver front-end unit 110 is an optical / electrical converter that converts an optical signal into an electrical signal, and a coherent detector that performs coherent detection. The coherent receiver front-end unit 110 coherently detects the input optical signal SO1 based on the local light r1, and outputs the generated analog electrical signal SA1 (first analog electrical signal).

[0030] The coherent transmission front-end unit 120 is an electrical / optical converter that converts an electrical signal into an optical signal, and a coherent modulator that performs coherent modulation. The coherent transmission front-end unit 120 coherently modulates an analog electrical signal SA2 (a second analog electrical signal) obtained by folding back the analog electrical signal SA1 based on a transmission light r2, and outputs the generated output optical signal SO2.

[0031] The input optical signal SO1 and the output optical signal SO2 are phase-modulated and polarization-multiplexed optical signals. The analog electrical signals SA1 and SA2 are four-lane (4ch) signals including an XI signal of the I component (in-phase component) of the X polarization, an XQ signal of the Q component (orthogonal component) of the X polarization, a YI signal of the I component of the Y polarization, and a YQ signal of the Q component of the Y polarization.

[0032] The frequency of the local light r1 is the frequency (carrier frequency) of the input optical signal SO1 to be received, and the frequency of the transmitted light r2 is the frequency of the output optical signal SO2 to be transmitted. For example, the local light r1 and the transmitted light r2 have different frequencies, but they may also be the same frequency. By changing the frequencies of the local light r1 and the transmitted light r2, it is possible to switch the wavelength of the optical signal to be returned. In other words, the input optical signal SO1 can be converted into an output optical signal SO2 with a different wavelength.

[0033] The analog compensation unit 130 is a circuit that performs predetermined analog signal processing on the analog electrical signal SA1 between the coherent reception front-end unit 110 and the coherent transmission front-end unit 120 to generate an analog electrical signal SA2. The analog compensation unit 130 performs analog signal processing on the analog electrical signal SA1 to compensate for the signal quality in accordance with the signal characteristics from the input of the coherent reception front-end unit 110 to the output of the coherent transmission front-end unit 120, and generates an analog electrical signal SA2. Either or both of the optical signal and the analog electric signal may be simply called a "signal". In the present embodiment, the compensation of the signal quality includes compensation for the deterioration of the optical signal that occurs each time the signal passes through an optical repeater, and compensation for the deterioration of the analog electric signal that occurs within the optical repeater. For example, the deterioration of the optical signal to be compensated for includes band deterioration (PBN: Pass Band Narrowing) that occurs when passing through an optical multiplexer / demultiplexer or an optical filter of each optical repeater, amplitude variation of four lanes due to variations in O / E or E / O conversion efficiency, optical frequency offset, and the like. The deterioration of the analog electric signal to be compensated for includes band deterioration due to characteristic deterioration and characteristic variations of the analog electric circuit of each of the four lanes, amplitude variation of four lanes, skew of four lanes, and the like. In other words, the compensation of the signal quality includes band compensation that compensates for band deterioration of the signal, frequency offset compensation that compensates for the shift in the frequency of the local light, skew compensation that compensates for the timing variations of each signal component included in the signal, amplitude compensation that compensates for the amplitude variations of each signal component included in the signal, and the like.

[0034] Fig. 8 shows a specific example of the optical transceiver 101 according to this embodiment. As shown in Fig. 8, the analog compensation unit 130 may include an analog signal processing unit 131, a control unit 132, and a monitor unit 133. The optical transceiver 101 may also include a reference light source 140 that generates local light r1 and a transmission light source 150 that generates transmission light r2. The reference light source 140 may be located inside the coherent receiver front-end unit 110, and the transmission light source 150 may be located inside the coherent transmitter front-end unit 120.

[0035] The analog signal processing unit 131 is an analog circuit that performs a predetermined analog signal processing for compensating for signal quality. The analog signal processing unit 131 processes the analog electrical signal SA1 as an analog signal and outputs an analog electrical signal SA2. The analog signal processing unit 131 performs only analog signal processing and does not perform digital signal processing that generates a large delay. This makes it possible to suppress the physical delay to, for example, a few nsec or less.

[0036] The monitor unit 133 monitors the signal characteristics of any of the input optical signal SO1, the analog electrical signal SA1, the analog electrical signal SA2, and the output optical signal SO2 between the input of the coherent receiver front-end unit 110 and the output of the coherent transmitter front-end unit 120. The signal characteristics to be monitored are, for example, the characteristics of the polarized signals (polarization multiplexed X polarization and Y polarization) included in the optical signal, the characteristics of the complex signals (phase-modulated I component and Q component) included in the analog electrical signal, etc.

[0037] The control unit 132 controls the operation of the analog signal processing of the analog signal processing unit 131 based on the monitoring result of the monitor unit 133. By optimizing the analog signal processing according to the monitored signal characteristics, band compensation, skew compensation, etc. are performed to suppress deterioration of signal quality. Since the signal processing speed of the control unit 132 and the monitor unit 133 does not affect the latency of the main signal, the time constant of the control unit 132 and the monitor unit 133 may be low. The control unit 132 and the monitor unit 133 may be an analog circuit or a digital circuit.

[0038] Fig. 9 shows an example of the configuration of the coherent receiver front-end unit 110 according to this embodiment. As shown in Fig. 9, the coherent receiver front-end unit 110 includes a polarization splitter 111, 90-degree hybrid circuits 112-1 to 112-2, O / E converters 113-1 to 113-4, and amplifiers 114-1 to 114-4.

[0039] The polarization splitter 111 splits the input optical signal SO1, which is an input polarization combined signal, into an X-polarized wave and a Y-polarized wave. The 90-degree hybrid circuits (coherent optical detectors) 112-1 to 112-2 perform coherent detection by causing the optical signal split by the polarization splitter 111 to interfere with the local light r1 of the reference light source 140, and the O / E converters 113-1 to 113-4, which are composed of photodiodes or the like, convert the detected signal into a four-lane analog electric signal. The 90-degree hybrid circuit 112-1 splits the X-polarized wave of the input optical signal SO1 into an I component and a Q component, and then the O / E converters 113-1 to 113-2 perform photoelectric conversion to generate an XI signal and an XQ signal. The 90-degree hybrid circuit 112-2 separates the Y polarization of the input optical signal SO1 into an I component and a Q component, and then performs photoelectric conversion using O / E conversion units 113-3 to 113-4 to generate a YI signal and a YQ signal. The amplifiers 114-1 to 114-4 amplify the generated XI signal, XQ signal, YI signal, and YQ signal, respectively, and output them as a four-lane analog electrical signal SA1 to the analog compensation unit 130. The analog compensation unit 130 performs analog signal processing on all or a part of the XI signal, XQ signal, YI signal, and YQ signal (X polarization or Y polarization).

[0040] 10 shows an example of the configuration of the coherent transmission front-end unit 120 according to this embodiment. As shown in FIG. 10, the coherent transmission front-end unit 120 includes amplifiers 121-1 to 121-4, MZ modulators (Mach-Zehnder Modulators) 122-1 to 122-4, and a polarization combining unit 123.

[0041] The amplifiers 121-1 to 121-4 respectively amplify the XI signal, XQ signal, YI signal, and YQ signal of the analog electrical signal SA2 output from the analog compensation unit 130, and drive the MZ modulators 122-1 to 122-4. The MZ modulators (IQ optical modulators) 122-1 to 122-4 perform IQ modulation on the transmission light r2 from the transmission light source 150 in accordance with the XI signal, XQ signal, YI signal, and YQ signal applied thereto. The MZ modulators 122-1 to 2 generate X-polarized IQ modulated optical signals based on the XI signal and XQ signal passed through the amplifiers 121-1 to 121-2. The MZ modulators 122-3 to 4 generate Y-polarized IQ modulated optical signals based on the YI signal and YQ signal passed through the amplifiers 121-3 to 121-4. The polarization combiner 123 polarization-combines the generated X-polarized IQ-modulated optical signal and Y-polarized IQ-modulated optical signal, and outputs the combined optical signal as an output optical signal SO2.

[0042] As described above, in an optical transmission system, low latency can be achieved by switching paths as optical signals from the sending end to the receiving end, and realizing an optically transparent network that minimizes unnecessary processing. However, in increasingly complex WDM networks, wavelength conflicts with other signals mean that it is not possible to lay the shortest path end-to-end with a single wavelength, so it is necessary to lay a path while converting the wavelength to an available wavelength slot. In related technology, a ROADM can temporarily extract a signal and then regenerate and repeat it to a different wavelength with a digital coherent optical transceiver, thereby realizing wavelength conversion on a channel-by-channel basis. However, this involves complex processing involving signal delays, such as error correction processing by DSP, which impairs low latency.

[0043] Therefore, in this embodiment, the analog signal output from the coherent optical receiving front end is subjected to analog signal processing without going through a digital coherent DSP, and is then folded back and relayed to the coherent optical transmitting front end. That is, the optical signal is converted to an electrical signal once, but only the minimum necessary analog signal processing is performed, and the signal is converted again to an optical signal of a different wavelength and relayed. Although an analog delay of the electrical circuit physically occurs, it is a few nsec or less at most and is negligibly small. Therefore, a wavelength conversion function with low latency can be realized on a channel-by-channel basis. In addition to wavelength conversion, it is also possible to relay with low latency at the same wavelength. Furthermore, in an optical transmission system, in addition to the deterioration of the quality of the optical signal caused by passing through multiple optical repeaters, the signal quality deteriorates due to the imperfection of the electrical circuit in the optical repeater. Therefore, in this embodiment, quality compensation is performed by processing these signal deteriorations as analog signals. As a result, it is possible to suppress the deterioration of signal quality while suppressing delay, and therefore it is possible to perform long-distance communication with low latency.

[0044] (Embodiment 2) The second embodiment will be described below with reference to the drawings. In an optical transmission system, optical signals passing through each optical repeater are subject to band narrowing due to the influence of optical filters included in the optical repeater. In particular, the more stages there are, the more band narrowing occurs, limiting the transmission distance. In addition, in an optical repeater, an optical signal is converted into an electrical signal once, so it is also affected by the bandwidth of the transmitter and receiver. Therefore, in this embodiment, band compensation is made possible in the analog compensation unit of the first embodiment.

[0045] Fig. 11 shows an example of the configuration of an optical transceiver 101 according to this embodiment. As shown in Fig. 11, in this embodiment, an analog compensation unit 130 includes an analog signal processing unit 131, a control unit 132, and a post-signal monitoring unit 134.

[0046] The analog signal processing unit 131 includes a band adjustment circuit 301 that adjusts the band (frequency components) of the analog electrical signal SA1 based on the control from the control unit 132. For example, the analog signal processing unit 131 includes four band adjustment circuits 301, and each band adjustment circuit 301 adjusts the power of each band of the four lanes of the analog electrical signal SA1. The band adjustment circuit 301 is composed of, for example, an analog FIR filter, a peaking variable amplifier, an LCR variable filter, etc., but may be any other analog circuit capable of band adjustment.

[0047] The rear signal monitor unit 134 is an example of the monitor unit 133 of the first embodiment, and monitors the analog electrical signal SA2 (rear signal) output from the analog signal processing unit 131. It can also be said that the rear signal monitor unit 134 monitors the signal input to the coherent transmission front-end unit 120. The rear signal monitor unit 134 includes a band monitor 302 that monitors the band (frequency characteristic) of the analog electrical signal SA2. The band monitor 302 monitors the band of the four lanes of the analog electrical signal SA2. The four band monitors 302 may be used to monitor the four lanes, or one band monitor 302 may be used to monitor the four lanes by switching the input signal.

[0048] The control unit 132 controls the output power for each band of the band adjustment circuit 301 based on the band (frequency characteristics) monitored by the band monitor 302. The control unit 132 controls the amount of band adjustment of the band adjustment circuit 301 corresponding to each signal according to the monitoring result of the band of the four lanes. For example, the control unit 132 increases the power of band components that are attenuated more than the desired spectrum as a result of monitoring the analog electrical signal SA2.

[0049] Fig. 12 shows an example of the configuration of the bandwidth monitor 302 according to this embodiment. As shown in Fig. 12, the bandwidth monitor 302 includes BPFs (Band Pass Filters) 303a and 303b, and power monitors 304a and 304b.

[0050] For example, the BPF 303a extracts low-frequency components (first band) of the analog electrical signal SA2 (any of the four lanes). The power monitor 304a monitors the power of the low-frequency components of the analog electrical signal SA2 extracted by the BPF 303a. The BPF 303b extracts high-frequency components (second band) of the analog electrical signal SA2 (any of the four lanes). The power monitor 304b monitors the power of the high-frequency components of the analog electrical signal SA2 extracted by the BPF 303b.

[0051] The control unit 132 controls the band adjustment circuit 301 of each lane according to the monitoring results of the low frequency components and high frequency components of the four lanes of the analog electrical signal SA2. For example, as shown in FIG. 13A, the power of the low frequency f11 and the power of the high frequency f12 are monitored, and the output power of the low frequency f11 or the high frequency f12 from the band adjustment circuit 301 is controlled according to the comparison result of these powers. The low frequency f11 and the high frequency f12 are frequencies that have the same power when the signal is not degraded (for example, a substantially trapezoidal spectrum). For this reason, the difference Δw1 between the power of the low frequency f11 and the power of the high frequency f12 is obtained, and the amount of band adjustment is controlled so that Δw1 is small. As shown in FIG. 13B, the amount of band lifting of the band adjustment circuit 301 is adjusted so that the power of the low frequency f11 and the power of the high frequency f12 are equal. For example, when the power of the high frequency f12 is lower than the power of the low frequency f11 by Δw1, the power of the high frequency f12 is controlled to be increased by Δw1.

[0052] Fig. 14 shows an example of the configuration of band adjustment circuit 301 according to this embodiment, which is configured using an analog FIR filter. As shown in Fig. 14, band adjustment circuit 301 includes an AGC (Automatic Gain Control) amplifier 311, an LPF (Low Pass Filter) 312, a plurality of delay circuits (T / 2) 313, a plurality of weighting circuits (g0 to gn) 314, an adder circuit (Σ) 315, and a driver circuit (Driver) 316, as an example of an analog FIR filter. Note that the configuration in Fig. 14 is only an example, and the analog FIR filter may be configured using other circuits.

[0053] The AGC amplifier 311 has its gain adjusted via an LPF 312 connected between its output terminal and its control terminal, and amplifies the input signal with the adjusted gain. The amplified signal is gradually delayed in predetermined time units by a plurality of delay circuits 313 connected in series. Each delayed signal is weighted by a weighting circuit 314, and the weighted signals are added together by an adder circuit 315. The addition result by the adder circuit 315 is output via a drive circuit 316. For example, the control unit 132 controls the power of a predetermined band by adjusting the weighting (coefficient) of the weighting circuit 314 according to the monitoring result of the band monitor 302.

[0054] FIG. 15 shows another example of the configuration of band adjustment circuit 301 according to the present embodiment, which is configured using a peaking variable amplifier. As shown in FIG. 15, band adjustment circuit 301 includes amplifier 321, BPF 322, variable gain amplifier 323, and adder circuit 324 as an example of a peaking variable amplifier. Note that the configuration in FIG. 15 is only an example, and the peaking variable amplifier may be configured using other circuits. Amplifier 321 amplifies an input signal and outputs an amplified signal G1 to adder circuit 324. BPF 322 extracts a predetermined band component (e.g., high-frequency component) of the input signal. Variable gain amplifier 323 amplifies a signal of the extracted band and outputs an amplified signal G2 to adder circuit 324. Adder circuit 324 adds signal G1 from amplifier 321 and signal G2 from variable gain amplifier 323, and outputs the added signal G3.

[0055] Fig. 16 shows the characteristics of each signal in band adjustment circuit 301 of Fig. 15. Signal G1, which is an input signal amplified by amplifier 321, has attenuated high-frequency components. When BPF 322 extracts the high-frequency components of the input signal, signal G2, which is amplified by variable gain amplifier 323, becomes a signal in which the gain of only the high-frequency components is increased. Then, signal G3, which is the sum of signals G1 and G2 by adder circuit 324, has characteristics in which the high-frequency components are increased from signal G1. For example, control unit 132 controls the power of a predetermined band by adjusting the pass band of BPF 322 and the gain of variable gain amplifier 323 according to the monitoring result of band monitor 302.

[0056] FIG. 17 shows another example of the configuration of band adjustment circuit 301 according to the present embodiment, which is configured using an LCR variable filter. As shown in FIG. 17, band adjustment circuit 301 includes resistor R1, inductor L1, resistor R2, and capacitor C1 as an example of an LCR variable filter. Note that the configuration in FIG. 17 is only an example, and the LCR variable filter may be configured using other circuits. Between the input terminal and the output terminal, resistor R1 and inductor L1 are connected in series, and between the output terminal and the ground terminal, resistor R2 and capacitor C1 are connected in parallel. For example, resistors R1 and R2 are variable resistors, capacitor C1 is a variable capacitance, and inductor L1 is a variable inductor, and control unit 132 controls the power of a predetermined band by adjusting these variable resistors, variable capacitance, and variable inductor.

[0057] As described above, in this embodiment, the spectrum of the post-analog signal (analog electrical signal SA2) of the analog signal processing section is monitored, and the analog signal processing section adjusts the band components according to the monitoring results, thereby performing band compensation by analog signal processing. This makes it possible to transmit an optical signal that has been appropriately band compensated by the analog signal processing section, enabling 2R (Receive / Reshape) relaying. This makes it possible to improve the signal quality at the final receiving end and extend the transmission distance.

[0058] (Embodiment 3) Hereinafter, the third embodiment will be described with reference to the drawings. In the second embodiment, band compensation was performed, but if there is a frequency offset in the local light of the reference light source, it affects the frequency characteristics of the analog electrical signal coherently detected by the local light, making it difficult to monitor and compensate for the correct band. Therefore, in this embodiment, the analog compensation unit of the second embodiment performs frequency offset compensation and band compensation. Note that only the frequency offset may be compensated for without performing band compensation.

[0059] Fig. 18 shows a configuration example of an optical transceiver 101 according to the present embodiment. As shown in Fig. 18, in this embodiment, an analog compensation unit 130 includes a front-end signal monitor unit 135 in addition to the configuration of the second embodiment. The other configurations are the same as those of the second embodiment.

[0060] The front-end signal monitor unit 135 is an example of the monitor unit 133, and monitors the analog electrical signal SA1 (front-end signal) input to the analog signal processing unit 131. It can also be said that the front-end signal monitor unit 135 monitors the signal output from the coherent reception front-end unit 110. The front-end signal monitor unit 135 includes a frequency offset monitor 401 that monitors the frequency offset of the local oscillation light r1 by the analog electrical signal SA1. The frequency offset monitor 401 may monitor the frequency offset by a four-lane signal of the analog electrical signal SA1, or may monitor the frequency offset by a two-lane signal (X-polarized wave or Y-polarized wave).

[0061] The control unit 132 controls the frequency of the local light r1 of the reference light source 140 based on the frequency offset monitored by the frequency offset monitor 401. The control unit 132 controls the amount of shift in the output frequency of the reference light source 140 according to the monitoring result of the four-lane or two-lane frequency offset. For example, the control unit 132 shifts the frequency of the local light r1 so as to return the deviation from the desired spectrum as a result of monitoring the analog electrical signal SA1.

[0062] Fig. 19 shows a configuration example of frequency offset monitor 401 according to this embodiment. In the example of Fig. 19, analog electrical signal SA1 is captured by a full-band ADC (Analog / Digital Converter) to restore the spectrum. In this example, a wideband ADC or the like is required, which is costly, but it is possible to monitor while the main signal is being conducted.

[0063] As shown in FIG. 19, the frequency offset monitor 401 includes a wideband ADC 411, a complex signal converter 412, an FFT (Fast Fourier Transform) unit 413, and an asymmetry monitor 414. The wideband ADC 411 converts the full band of the analog electrical signal SA1 into a digital signal. The wideband ADC 411 performs AD conversion on two lanes (XI signal and XQ signal, or YI signal and YQ signal) of one of four lanes of X-polarized and Y-polarized waves of the analog electrical signal SA1. The complex signal converter (complex signal generator) 412 converts the AD-converted digital I signal and Q signal into a complex signal (I+jQ). The FFT unit (FFT processor) 413 performs FFT processing on the converted complex signal to generate a spectrum in the frequency domain. This provides the spectrum of the input optical signal SO1. The asymmetry monitor 414 monitors the left-right asymmetry of the spectrum of the input optical signal SO1 obtained by the FFT processing with respect to the center frequency. Asymmetry monitor 414 may monitor the asymmetry using a spectrum obtained by accumulating and averaging the results of the FFT processing.

[0064] 20A and 20B are monitor results by the frequency offset monitor 401 of FIG. 19, and show examples of spectra without and with a frequency offset, respectively. As shown in FIG. 20A, when there is no frequency offset, the spectrum of the input optical signal SO1 is symmetrical with respect to the center frequency f0. As shown in FIG. 20B, when there is a frequency offset, the spectrum of the input optical signal SO1 is asymmetrical with respect to the center frequency f0. The control unit 132 controls the shift amount of the local light r1 so that the spectrum of the input optical signal SO1 is symmetrical with respect to the center frequency f0. For example, when Δf21 from f0 to the positive end of the spectrum is different from Δf22 from f0 to the negative end of the spectrum, the frequency of the local light r1 is shifted so that Δf21 and Δf22 are equal. The shift amount of the frequency of the local light r1 is controlled according to the difference between Δf21 and Δf22.

[0065] Fig. 21 shows another example of the configuration of frequency offset monitor 401 according to this embodiment. In the example of Fig. 21, the power of analog electrical signal SA1 is monitored to restore the spectrum while sweeping the frequency of local light r1, which is a reference light source. In this example, since the local light is monitored while being controlled, it is not possible to monitor while the main signal is conducted, but this can be realized at low cost. Note that a monitor light source correlated with the local light may be prepared, and the frequency offset may be monitored by the monitor light source.

[0066] As shown in FIG. 21, the frequency offset monitor 401 includes an LPF 421, a power monitor 422, a spectrum restoration unit 423, and an asymmetry monitor 424. For example, the monitoring is performed before the optical signal is conducted, but may be performed at any timing when the optical signal is not transmitted or received. When the monitoring is performed, the frequency of the local light r1 of the reference light source 140 is swept. That is, the frequency of the local light r1 is gradually increased from the lowest frequency to the highest frequency within the range of the spectrum to be generated. The power monitor 422 monitors the power of the analog electric signal SA1 generated by mixing the local light r1, the frequency of which has been changed, with the input optical signal SO1 via the LPF 421. That is, the analog electric signal SA1 is a signal obtained by cutting out the input optical signal SO1 in the band of the LPF 421 centered on the frequency of the local light r1. In other words, the frequency of the local light r1, which is the reference light, is changed and mixed with the input optical signal SO1, and the signal is extracted via the LPF 421, thereby playing a role equivalent to that of a variable optical bandpass filter. Therefore, the power monitor 422 can monitor the power of each band of the input optical signal SO1 by varying the local light r1. The signals of four lanes of the analog electrical signal SA1 may be monitored, or the signals of any lane may be monitored. The spectrum restoration unit (spectrum generation unit) 423 plots the power of each band of the input optical signal SO1 from the power monitor value of the analog electrical signal SA1 monitored while sweeping the local light r1, to restore (generate) a spectrum. The asymmetry monitor 424 monitors the asymmetry of the restored spectrum, similar to FIG. 19.

[0067] 21, the frequency offset monitor 401 may monitor the entire spectrum or only some frequency components. The frequency offset monitor 401 may monitor the asymmetry of the spectrum based on the power of the first band and the power of the second band in the input optical signal SO1, and the control unit 132 may control the frequency of the local light r1 so that the power of the first band and the power of the second band become equal.

[0068] For example, as shown in Figures 22A and 22B, the frequency of the local light r1 is varied to -Δf3 and +Δf3, and the frequency components of -Δf3 and +Δf3 ​​are monitored. As shown in Figure 22A, when there is no frequency offset, the spectrum of the input optical signal SO1 is symmetrical, so the power of -Δf3 and the power of +Δf3 ​​are equal. -Δf3 and +Δf3 ​​are frequencies near both ends in the frequency axis direction in the spectrum when there is no frequency offset, and are frequencies near the attenuation point where the power starts to attenuate from the maximum level. In other words, they are frequencies near both ends of the upper base (upper side) in a spectrum that is approximately trapezoidal.

[0069] As shown in Figure 22B, when there is a frequency offset, the spectrum of the input optical signal SO1 becomes asymmetric, and a difference Δw2 occurs between the power of -Δf3 and the power of +Δf3. Therefore, the frequency of the local light r1 is controlled so that Δw2 becomes small and the power of -Δf3 and the power of +Δf3 ​​become equal. For example, when the power of -Δf3 is lower than the power of +Δf3 ​​by Δw2, the frequency of the local light r1 is gradually lowered, and the frequency of the local light r1 is shifted until the power of Δf3 and the power of +Δf3 ​​become equal.

[0070] As described above, in this embodiment, in addition to the band compensation of the second embodiment, the frequency offset is compensated for by controlling the frequency of the local light of the reference light source according to the frequency offset of the local light r1 estimated from the front signal (analog electrical signal SA1) of the analog signal processing unit. This makes it possible to appropriately perform the band compensation of the second embodiment after suppressing the frequency offset of the local light. Therefore, it is possible to further improve the signal quality and extend the transmission distance without being affected by the frequency offset of the reference light source.

[0071] (Modification of the third embodiment) Fig. 23 shows a configuration of a modified example of the third embodiment. In the third embodiment, the analog electrical signal SA1 between the coherent receiver front-end unit 110 and the analog signal processing unit 131 is monitored, but as shown in Fig. 23, the input optical signal SO1, which is the input to the coherent receiver front-end unit 110, may be branched and monitored. For example, the frequency offset monitor 401 monitors the frequency offset using the input optical signal SO1 and the local light r1. That is, the frequency offset of the local light r1 is monitored based on the spectrum of the input optical signal SO1 and the spectrum of the local light r1.

[0072] 24 shows a configuration example of a frequency offset monitor 401 according to a modification of the embodiment 3. In this example, the spectra of both the input optical signal SO1 and the local light r1 are restored by a tunable optical BPF and an optical power monitor, and the frequency deviation is monitored.

[0073] As shown in FIG. 24, the frequency offset monitor 401 includes tunable optical BPFs 431 a and 431 b , optical power monitors 432 a and 432 b , spectrum restoration units 433 a and 433 b , and a spectrum comparison unit 434 .

[0074] The tunable optical BPF 431a, the optical power monitor 432a, and the spectrum restoration unit 433a monitor the spectrum of the input optical signal SO1. The tunable optical BPF 431b, the optical power monitor 432b, and the spectrum restoration unit 433b monitor the spectrum of the local light r1. The spectra of the input optical signal SO1 and the local light r1 may be monitored by switching the input signal using one tunable optical BPF, one optical power monitor, and one spectrum restoration unit, respectively.

[0075] The variable optical BPF (input optical BPF) 431a extracts each frequency component of the input optical signal SO1 (which may be X-polarized or Y-polarized) by varying the center frequency of the passband. The optical power monitor (input optical power monitor unit) 432a monitors the power of each frequency (band) of the extracted input optical signal SO1. The spectrum restoration unit (input optical spectrum generator) 433a restores the spectrum using the power of each frequency of the monitored input optical signal SO1. Similarly, the variable optical BPF (local optical BPF) 431b extracts each frequency component of the local optical light r1 by varying the center frequency of the passband. The optical power monitor (local optical light power monitor unit) 432b monitors the power of each frequency (band) of the extracted local optical light r1. The spectrum restoration unit (local optical light spectrum generator) 433b restores the spectrum using the power of each frequency of the monitored local optical light r1. The spectrum comparison unit 434 compares the spectrum of the restored input optical signal SO1 with the spectrum of the local optical signal r1, and monitors the frequency offset based on the comparison result.

[0076] Fig. 25 shows an example of the monitoring result by the frequency offset monitor 401 of Fig. 24. When there is a frequency offset, the center frequency f41 of the spectrum of the local light r1 shifts from the center frequency f42 of the spectrum of the input optical signal SO1. Therefore, the difference Δf5 between the center frequency f42 of the spectrum of the input optical signal SO1 and the center frequency f41 of the spectrum of the local light r1 is obtained, and the frequency of the local light r1 is shifted so that Δf5 becomes smaller. Note that, in principle, it is impossible to perform spectrum monitoring with a resolution equal to or smaller than the bandwidth of the tunable optical BPF, so the spectrum monitoring result of the local light r1 is broader than the original spectrum of a single frequency (single wavelength). However, it is still possible to monitor with sufficient accuracy for the purpose of monitoring the center frequency. In this example, there is no need to sweep the local light r1, which serves as the reference light source. The spectrum of each of the branched input optical signal SO1 and the local light r1 can be monitored by varying the center frequency of the tunable optical BPF, making it possible to control them even when the signal is conducting.

[0077] (Embodiment 4) Hereinafter, the fourth embodiment will be described with reference to the drawings. Ideally, it is desirable that a signal in which the skew and amplitude of four lanes (XI, XQ, YI, YQ) have been compensated for at the transmitting end reaches the receiving end. However, when an optical signal is converted into an analog electrical signal in an optical transceiver, if there is a skew or amplitude error between the four lanes, the signal quality will deteriorate and the transmission distance may be restricted. Therefore, in this embodiment, the analog compensation unit of the first to third embodiments performs amplitude compensation and skew compensation. Note that only either the amplitude compensation or the skew compensation may be performed.

[0078] Fig. 26 shows a configuration example of an optical transceiver 101 according to this embodiment. As shown in Fig. 26, in this embodiment, an analog compensation unit 130 includes an analog signal processing unit 131, a control unit 132, a front signal monitor unit 135, and a back signal monitor unit 134.

[0079] The analog signal processing unit 131 includes an amplitude adjustment circuit 501 that adjusts the amplitude of the analog electrical signal SA1 based on the control from the control unit 132, and a skew adjustment circuit 502 that adjusts the skew of the analog electrical signal SA1 based on the control from the control unit 132. For example, the analog signal processing unit 131 includes four amplitude adjustment circuits 501 and four skew adjustment circuits 502. Each amplitude adjustment circuit 501 adjusts the amplitude of the four lane signals of the analog electrical signal SA1, and each skew adjustment circuit 502 adjusts the skew of the four lane signals of the analog electrical signal SA1. In this example, the skew adjustment circuit 502 adjusts the skew of the signal whose amplitude has been adjusted by the amplitude adjustment circuit 501. The amplitude adjustment circuit 501 is, for example, an amplifier, but may be any other analog circuit capable of adjusting the amplitude. The skew adjustment circuit 502 is, for example, a delay adjustment device, a phase shifter (phase adjuster), or the like, but may be any other analog circuit capable of adjusting the skew.

[0080] The front-end signal monitor unit 135 includes an amplitude monitor 503 that monitors the amplitude of the analog electrical signal SA1. The amplitude monitor 503 monitors the amplitude of the four lanes of the analog electrical signal SA1. The amplitude of the four lanes may be monitored by four amplitude monitors 503, or the amplitude of the four lanes may be monitored by one amplitude monitor 503 by switching the input signal. For example, the amplitude monitor 503 is configured by a power monitor or the like.

[0081] The control unit 132 controls the output amplitude (power) of the amplitude adjustment circuit 501 based on the amplitude monitored by the amplitude monitor 503. The control unit 132 controls the amplitude adjustment amount of the amplitude adjustment circuit 501 corresponding to each signal according to the monitoring result of the amplitude of the four lanes. For example, the control unit 132 increases the amplitude of the lane that is attenuated as a result of monitoring the four lanes of the analog electrical signal SA1, thereby making the amplitude of each lane equal.

[0082] The rear signal monitor unit 134 includes a skew monitor 504 that monitors the skew of the analog electrical signal SA2. The skew monitor 504 may monitor the skew of the four-lane signal of the analog electrical signal SA2, or may monitor the skew of the two-lane signal (X-polarized wave or Y-polarized wave). The skew monitor 504 monitors at least the skew of the I component and the Q component of the X-polarized wave or the Y-polarized wave included in the analog electrical signal SA2.

[0083] The control unit 132 controls the amount of skew adjustment in the skew adjustment circuit 502 based on the skew monitored by the skew monitor 504. The control unit 132 controls the delay of the I component and Q component in the skew adjustment circuit 502 according to the monitoring result of the skew between the I component and the Q component (four lanes or two lanes). For example, the control unit 132 advances the phase of the lane that is delayed as a result of monitoring the I component and Q component of the analog electrical signal SA2, thereby aligning the phase (timing) of each lane.

[0084] 27 shows an example of the configuration of the skew monitor 504 according to this embodiment. In this example, the constellation of the analog electrical signal SA2 is monitored, and the skew is adjusted according to the constellation pattern. As shown in FIG. 27, the skew monitor 504 includes ADCs 505a and 505b, and a constellation monitor 506.

[0085] The ADC 505a samples and AD converts the I component (XI or YI) of the analog electrical signal SA2. The ADC 505b samples and AD converts the Q component (XQ or YQ) of the analog electrical signal SA2. The constellation monitor 506 monitors the constellation of the analog electrical signal SA2 using the AD converted I and Q components.

[0086] The control unit 132 controls the skew adjustment circuit 502 according to the monitoring result of the constellation of the analog electrical signal SA2. Fig. 28A shows the constellation when there is skew, and Fig. 28B shows the constellation when there is no skew. As shown in Fig. 28A, when there is skew, the shape (pattern) of the constellation becomes an ellipse. When there is a difference in the amplitude of the I signal and the Q signal or when there is skew, the constellation becomes an ellipse. In this example, the amplitude difference is compensated for in advance by the amplitude monitor and the amplitude adjustment circuit, so that the presence or absence of skew can be determined based on whether the constellation of the analog electrical signal SA2 is an ellipse.

[0087] For this reason, when the constellation is an ellipse as in Fig. 28A, the delay amount of skew adjustment circuit 502 is controlled so that the constellation becomes a perfect circle as in Fig. 28B. The degree of ellipse (for example, the difference between the major and minor axes) is monitored, and the delays of the I and Q components are controlled so that the constellation becomes a perfect circle. For example, the length ΔQ of the constellation in the Q axis direction is compared with the length ΔI of the constellation in the I axis direction, and control is performed so that the difference between ΔQ and ΔI becomes small. For example, the delay amount of either the Q signal or the I signal is gradually increased (or decreased) to adjust ΔQ and ΔI to be equal.

[0088] As described above, in this embodiment, the amplitude variation of the front signal of the analog signal processing unit is monitored, and the analog signal processing unit performs amplitude compensation according to the monitoring result, and the skew of the rear signal of the analog signal processing unit is monitored, and the analog signal processing unit performs skew compensation according to the monitoring result. This makes it possible to transmit an optical signal that has been appropriately amplitude compensated and skew compensated by the analog signal processing unit, and suppresses signal quality degradation due to skew and amplitude variation inside the optical repeater.

[0089] (Modification of the fourth embodiment) Fig. 29 shows the configuration of a modified example of the fourth embodiment. In the fourth embodiment, the analog electrical signal SA2 between the analog signal processing unit 131 and the coherent transmission front-end unit 120 is monitored, but as shown in Fig. 29, the output optical signal SO2 that is the output of the coherent transmission front-end unit 120 may be split and monitored. For example, the skew monitor 504 monitors the X-polarized or Y-polarized optical signal separated by the PBS (polarizing beam splitter) 510.

[0090] 30 shows an example of the configuration of a skew monitor 504 according to a modification of the fourth embodiment. In this example, the intensity of the output optical signal is monitored, and the skew is adjusted according to the amount of fluctuation in the intensity of the output optical signal.

[0091] 30, the skew monitor 504 includes an optical power monitor 511 and a fluctuation amount monitor 512. The optical power monitor 511 monitors the power of the X-polarized wave or the Y-polarized wave of the output optical signal SO2. The fluctuation amount monitor 512 monitors the amount of fluctuation in the power of the X-polarized wave or the Y-polarized wave of the output optical signal SO2.

[0092] The control unit 132 controls the skew adjustment circuit 502 according to the monitoring result of the amount of fluctuation in the power of the output optical signal SO2. Figures 31A and 31B show constellations with and without skew, respectively. Figures 32A and 32B show the optical signal intensities with and without skew, respectively.

[0093] When there is skew, the constellation becomes an ellipse as shown in Fig. 31A, and the intensity of the optical signal fluctuates as shown in Fig. 32A. When there is no skew, the constellation becomes a perfect circle as shown in Fig. 31B, and the intensity of the optical signal becomes constant as shown in Fig. 32B. For this reason, as shown in Fig. 32A, the skew is adjusted according to the fluctuation amount Δw3 of the intensity of the optical signal. For example, the delay amount of the I component and Q component of the skew adjustment circuit 502 is controlled so that the fluctuation amount Δw3 of the intensity is minimized and the intensity becomes constant.

[0094] (Embodiment 5) Hereinafter, the fifth embodiment will be described with reference to the drawings. In this embodiment, an example in which the skew is monitored by using a dither signal in the analog compensation unit of the fourth embodiment will be described.

[0095] 33 shows an example of the configuration of an optical transceiver 101 according to this embodiment. As shown in FIG. 33, the optical transceiver 101 includes a dither signal generating unit 136 in addition to the configuration of the fourth embodiment. The dither signal generating unit (dither signal superimposing unit) 136 generates a dither signal Sd for skew detection, and applies (superimposes) the dither signal on the analog electrical signal SA1 output from the coherent receiver front-end unit 110.

[0096] The dither signal is simultaneously applied to four lanes of the main signal (analog electrical signal SA1) in the coherent receiving front-end unit 110, and is detected by the skew monitor 504 of the rear signal monitor unit 134. A dither signal may be applied to the I component and Q component of at least two lanes for skew detection. The dither signal is a signal of a predetermined frequency that does not affect the transmission of the main signal. For example, the frequency (band) of the dither signal is different from the frequency of the main signal (outside the band of the main signal) and is a lower frequency than the main signal (a predetermined frequency away), such as 100 KHz or 50 KHz.

[0097] Fig. 34 shows a configuration example of the coherent receiver front-end unit 110 according to this embodiment, and shows a specific example of applying a dither signal. As shown in Fig. 34, for example, a dither signal Sd is applied from a dither signal generating unit 136 to amplifiers 114-1 to 114-4 at the output stage of the coherent receiver front-end unit 110. The dither signal may be applied (superimposed) by using the amplifiers 114-1 to 114-4 as TIAs (Trans-Impedance AMPs) capable of applying a dither signal. Also, the power supplies of the PDs (Photo Diodes) constituting the O / E conversion units 113-1 to 113-4 and the TIA power supplies of the amplifiers 114-1 to 114-4 may be slightly fluctuated to superimpose the dither signal on four lanes at the same time.

[0098] 35 shows an example of the configuration of skew monitor 504 according to this embodiment. Skew monitor 504 monitors the skew of the dither signal superimposed in coherent reception front-end unit 110. For example, skew monitor 504 includes a dither signal extractor 521 and a dither signal monitor 522. Dither signal extractor 521 is a low-speed ADC, a low-pass filter, or the like, and extracts a low-frequency dither signal from the four-lane (or two-lane) signal of analog electrical signal SA2. Dither signal monitor 522 monitors the skew by comparing the phases of the extracted four-lane (or two-lane) dither signals.

[0099] As described above, in this embodiment, when performing skew compensation as in embodiment 4, a dither signal for skew detection is applied in the coherent receiving front end unit, and the skew is monitored by the post-signal monitor unit. By making the dither signal a low-frequency signal outside the band of the main signal, it is possible to detect skew without affecting the main signal.

[0100] The present disclosure is not limited to the above-described embodiment, and may be modified as appropriate without departing from the spirit and scope of the present disclosure.

[0101] Although the present disclosure has been described above with reference to the embodiments, the present disclosure is not limited to the above-mentioned embodiments. Various modifications that can be understood by a person skilled in the art can be made to the configuration and details of the present disclosure within the scope of the present disclosure.

[0102] A part or all of the above-described embodiments can be described as, but is not limited to, the following supplementary notes. (Appendix 1) a coherent optical receiving front-end means for coherently detecting an input optical signal based on a local oscillator light and outputting a first analog electrical signal obtained by the coherent detection; a coherent optical transmission front-end means for coherently modulating a second analog electrical signal obtained by folding back the first analog electrical signal based on a transmission light, and outputting the coherently modulated output optical signal; an analog compensation means for performing analog signal processing on the first analog electrical signal so as to compensate for signal quality in accordance with a signal characteristic between an input of the coherent optical receiving front-end means and an output of the coherent optical transmitting front-end means, and generating the second analog electrical signal; An optical repeater device comprising: (Appendix 2) The signal characteristics include characteristics of a polarized signal or characteristics of a complex signal. 2. An optical repeater device as described in claim 1. (Appendix 3) The compensation of the signal quality includes any one of bandwidth compensation, frequency offset compensation, skew compensation, and amplitude compensation. 3. An optical repeater device according to claim 1 or 2. (Appendix 4) The analog compensation means comprises: an analog signal processing unit that processes the analog signal; a monitor unit that monitors a signal characteristic of any one of the input optical signal, the first analog electrical signal, the second analog electrical signal, and the output optical signal; a control unit that controls the operation of the analog signal processing based on the monitored result; 4. The optical repeater device according to claim 1, further comprising: (Appendix 5) The monitor unit monitors a frequency characteristic of the second analog electrical signal, the control unit controls an output power for each band in the analog signal processing based on the monitored frequency characteristics. 5. The optical repeater device according to claim 4. (Appendix 6) the monitor unit monitors a power of a first band and a power of a second band in the second analog electrical signal; the control unit controls an output power of the first band or the second band of the analog signal processing so that a power of the first band and a power of the second band are equal to each other. 6. The optical repeater device according to claim 5. (Appendix 7) the monitor unit monitors a frequency offset of the local light by the first analog electrical signal or the input optical signal; The control unit controls a frequency of the local light based on the monitored frequency offset. 7. An optical repeater device according to any one of claims 4 to 6. (Appendix 8) the monitor unit monitors left-right asymmetry with respect to a center frequency of a frequency spectrum of the first analog electrical signal; The control unit controls a frequency of the local light so that the frequency spectrum is symmetrical with respect to a center frequency. 8. The optical repeater device according to claim 7. (Appendix 9) The monitor unit includes: AD conversion means for converting the first analog electrical signal into a digital signal; a complex signal generating means for generating a complex signal from the converted digital signal; an FFT processing means for performing FFT processing on the generated complex signal; an asymmetry monitor that monitors the asymmetry of the frequency spectrum obtained by the FFT processing; 9. The optical repeater device of claim 8, comprising: (Appendix 10) The monitor unit includes: a power monitor means for monitoring the power of each band of the first analog electrical signal; a spectrum generating means for generating a frequency spectrum based on the monitored power; an asymmetry monitor that monitors asymmetry of the generated frequency spectrum; 9. The optical repeater device of claim 8, comprising: (Appendix 11) the monitor unit monitors asymmetry of the frequency spectrum based on a power of a first band and a power of a second band in the first analog electrical signal; the control unit controls a frequency of the local light so that a power of the first band and a power of the second band are equal to each other. 11. The optical repeater device of claim 10. (Appendix 12) the monitor unit monitors a frequency offset of the local light based on a spectrum of the input optical signal and a spectrum of the local light; 8. The optical repeater device according to claim 7. (Appendix 13) The monitor unit includes: an input optical power monitor means for monitoring the power of the input optical signal for each band; an input optical spectrum generating means for generating a frequency spectrum based on the power of the monitored input optical signal; a local light power monitor means for monitoring the power of the local light for each band; a local light spectrum generating means for generating a frequency spectrum based on the monitored power of the local light; a spectrum comparison unit that compares a frequency spectrum of the generated input optical signal with a frequency spectrum of a local oscillator light and monitors a frequency offset; 13. The optical repeater device of claim 12, comprising: (Appendix 14) The monitor unit monitors the amplitude of the first analog electrical signal, The control unit controls an output amplitude in the analog signal processing based on the monitored amplitude. 14. An optical repeater device according to any one of claims 4 to 13. (Appendix 15) The monitor unit monitors a skew of the second analog electrical signal or the output optical signal, The control unit controls a signal delay in the analog signal processing based on the monitored skew. 15. An optical repeater device according to any one of claims 4 to 14. (Appendix 16) The monitor unit monitors a constellation of the second analog electrical signal, The control unit controls a signal delay in the analog signal processing based on a pattern of the constellation. 16. The optical repeater device according to claim 15. (Appendix 17) The monitor unit monitors the power of the output optical signal, The control unit controls a signal delay in the analog signal processing based on the amount of fluctuation in the power. 16. The optical repeater device according to claim 15. (Appendix 18) a superimposing means for superimposing a dither signal on the first analog electrical signal output from the coherent optical receiving front-end means, the monitor unit monitors the skew based on the dither signal extracted from the second analog electrical signal. 16. The optical repeater device according to claim 15. (Appendix 19) the frequency of the dither signal is different from the frequencies of the first analog electrical signal and the second analog electrical signal; 19. The optical repeater device of claim 18. (Appendix 20) The frequency of the local light is different from the frequency of the transmission light. 20. An optical repeater device according to any one of claims 1 to 19. (Appendix 21) An optical transmission system including a plurality of optical repeaters, The plurality of optical repeaters include a coherent optical receiving front-end means for coherently detecting an input optical signal input from the optical repeater at a previous stage based on a local light and outputting the coherently detected first analog electrical signal; a coherent optical transmission front-end means for coherently modulating a second analog electric signal obtained by folding back the first analog electric signal based on a transmission light, and outputting the coherently modulated output optical signal to the optical repeater at the next stage; an analog compensation means for performing analog signal processing on the first analog electrical signal so as to compensate for signal quality in accordance with a signal characteristic between an input of the coherent optical receiving front-end means and an output of the coherent optical transmitting front-end means, and generating the second analog electrical signal; An optical transmission system comprising: (Appendix 22) An optical repeating method in an optical repeater having a coherent optical receiving front-end means and a coherent optical transmitting front-end means, comprising: the coherent optical receiving front-end means coherently detects an input optical signal based on a local oscillator light and outputs the coherently detected first analog electrical signal; the coherent optical transmission front-end means coherently modulates a second analog electrical signal obtained by folding back the first analog electrical signal based on a transmission light, and outputs the coherently modulated output optical signal; performing analog signal processing on the first analog electrical signal so as to compensate for signal quality in accordance with signal characteristics between an input of the coherent optical receiving front-end means and an output of the coherent optical transmitting front-end means, thereby generating the second analog electrical signal; Optical relay method. [Explanation of symbols]

[0103] 1. Optical transmission system 2 Optical repeater 3. Optical fiber transmission line 4, 5 Data Center 6 IT service providers 7, 8 Event Venue 100 Transmitter / receiver 101 Optical Transmitter / Receiver 102 Digital coherent optical transceiver 110 Coherent receiving front end 111 Polarization separation section 112 90 degree hybrid circuit 113 O / E conversion unit 114 Amplifier 120 Coherent transmission front-end 121 Amplifier 122 MZ Modulator 123 Polarization synthesis unit 130 Analog compensation section 131 Analog signal processing section 132 Control section 133 Monitor section 134 Rear signal monitor unit 135 Front Signal Monitor Unit 136 Dither signal generator 140 Reference light source 150 Transmitting light source 200 Optical switch section 201 Duplexer 202 Multiplexer 203 Branch Insertion Section 301 Bandwidth Adjustment Circuit 302 Bandwidth Monitor 303a, 303b BPF 304a, 304b Power Monitor 311 AGC Amplifier 312 LPF 313 Delay Circuit 314 Weighting Circuit 315 Addition Circuit 316 Drive circuit 321 Amplifier 322 BPF 323 Variable Gain Amplifier 324 Addition Circuit 401 Frequency Offset Monitor 411 Wideband ADC 412 Complex signal conversion section 413 FFT section 414 Asymmetry Monitor 421 LPF 422 Power Monitor 423 Spectral Restoration Unit 424 Asymmetry Monitor 431a, 431b Tunable Optical BPF 432a, 432b Optical Power Monitor 433a, 433b Spectral restoration section 434 Spectral Comparison Section 501 Amplitude adjustment circuit 502 Skew Adjustment Circuit 503 Amplitude Monitor 504 Skew Monitor 505a, 505b ADCs 506 Constellation Monitor 510 PBS 511 Optical Power Monitor 512 Fluctuation Monitor 521 Dither signal extraction section 522 Dither Signal Monitor

Claims

1. a coherent optical receiving front-end means for coherently detecting an input optical signal based on a local oscillator light and outputting a first analog electrical signal obtained by the coherent detection; a coherent optical transmission front-end means for coherently modulating a second analog electric signal obtained by folding back the first analog electric signal based on a transmission light, and outputting the coherently modulated output optical signal; an analog compensation means for performing analog signal processing on the first analog electrical signal so as to compensate for signal quality in accordance with a signal characteristic between an input of the coherent optical receiving front-end means and an output of the coherent optical transmitting front-end means, and generating the second analog electrical signal; Equipped with the analog compensation means controls an output power of the first band or the second band of the analog signal processing so that a power of the first band and a power of the second band in the second analog electrical signal become equal. Optical repeater.

2. A coherent optical receiving front-end means for coherently detecting an input optical signal based on a local oscillator light and outputting a first analog electrical signal obtained by the coherent detection; a coherent optical transmission front-end means for coherently modulating a second analog electric signal obtained by folding back the first analog electric signal based on a transmission light, and outputting the coherently modulated output optical signal; an analog compensation means for performing analog signal processing on the first analog electrical signal so as to compensate for signal quality in accordance with a signal characteristic between an input of the coherent optical receiving front-end means and an output of the coherent optical transmitting front-end means, and generating the second analog electrical signal; Equipped with the analog compensation means controls a frequency of the local light so that a frequency spectrum of the first analog electrical signal is symmetrical with respect to a center frequency. Optical repeater.

3. A coherent optical receiving front-end means for coherently detecting an input optical signal based on a local oscillator light and outputting a first analog electrical signal obtained by the coherent detection; a coherent optical transmission front-end means for coherently modulating a second analog electric signal obtained by folding back the first analog electric signal based on a transmission light, and outputting the coherently modulated output optical signal; an analog compensation means for performing analog signal processing on the first analog electrical signal so as to compensate for signal quality in accordance with a signal characteristic between an input of the coherent optical receiving front-end means and an output of the coherent optical transmitting front-end means, and generating the second analog electrical signal; Equipped with the analog compensation means controls a frequency of the local light based on a frequency offset of the local light based on a spectrum of the input optical signal and a spectrum of the local light. Optical repeater.

4. A coherent optical receiving front-end means for coherently detecting an input optical signal based on a local oscillator light and outputting a first analog electrical signal obtained by the coherent detection; a coherent optical transmission front-end means for coherently modulating a second analog electric signal obtained by folding back the first analog electric signal based on a transmission light, and outputting the coherently modulated output optical signal; an analog compensation means for performing analog signal processing on the first analog electrical signal so as to compensate for signal quality in accordance with a signal characteristic between an input of the coherent optical receiving front-end means and an output of the coherent optical transmitting front-end means, and generating the second analog electrical signal; Equipped with the analog compensation means controls a signal delay in the analog signal processing based on a constellation pattern of the second analog electrical signal. Optical repeater.

5. A coherent optical receiving front-end means for coherently detecting an input optical signal based on a local oscillator light and outputting a first analog electrical signal obtained by the coherent detection; a coherent optical transmission front-end means for coherently modulating a second analog electric signal obtained by folding back the first analog electric signal based on a transmission light, and outputting the coherently modulated output optical signal; an analog compensation means for performing analog signal processing on the first analog electrical signal so as to compensate for signal quality in accordance with a signal characteristic between an input of the coherent optical receiving front-end means and an output of the coherent optical transmitting front-end means, and generating the second analog electrical signal; Equipped with the analog compensation means controls a signal delay in the analog signal processing based on a fluctuation amount of the power of the output optical signal. Optical repeater.

6. A coherent optical receiving front-end means for coherently detecting an input optical signal based on a local oscillator light and outputting a first analog electrical signal obtained by the coherent detection; a coherent optical transmission front-end means for coherently modulating a second analog electric signal obtained by folding back the first analog electric signal based on a transmission light, and outputting the coherently modulated output optical signal; an analog compensation means for performing analog signal processing on the first analog electrical signal so as to compensate for signal quality in accordance with a signal characteristic between an input of the coherent optical receiving front-end means and an output of the coherent optical transmitting front-end means, and generating the second analog electrical signal; a superimposing means for superimposing a dither signal on the first analog electrical signal output from the coherent optical receiving front-end means; Equipped with the analog compensation means controls a signal delay in the analog signal processing based on a skew based on the dither signal extracted from the second analog electrical signal. Optical repeater.

7. The analog compensation means controls an output amplitude in the analog signal processing based on an amplitude of the first analog electrical signal.

7. An optical repeater device according to claim 1.

8. The signal characteristics include characteristics of a polarized signal or characteristics of a complex signal.

8. An optical repeater device according to claim 1.

9. An optical transmission system including a plurality of optical repeaters, The plurality of optical repeaters include a coherent optical receiving front-end means for coherently detecting an input optical signal input from the optical repeater at a previous stage based on a local light and outputting the coherently detected first analog electrical signal; a coherent optical transmission front-end means for coherently modulating a second analog electric signal obtained by folding back the first analog electric signal based on a transmission light, and outputting the coherently modulated output optical signal to the optical repeater at the next stage; an analog compensation means for performing analog signal processing on the first analog electrical signal so as to compensate for signal quality in accordance with a signal characteristic between an input of the coherent optical receiving front-end means and an output of the coherent optical transmitting front-end means, and generating the second analog electrical signal; Equipped with the analog compensation means controls an output power of the first band or the second band of the analog signal processing so that a power of the first band and a power of the second band in the second analog electrical signal become equal. Optical transmission system.

10. An optical repeating method in an optical repeater having a coherent optical receiving front-end means and a coherent optical transmitting front-end means, comprising: the coherent optical receiving front-end means coherently detects an input optical signal based on a local oscillator light and outputs a first analog electrical signal obtained by the coherent detection; the coherent optical transmission front-end means coherently modulates a second analog electrical signal obtained by folding back the first analog electrical signal based on a transmission light, and outputs the coherently modulated output optical signal; performing analog signal processing on the first analog electrical signal so as to compensate for signal quality in accordance with signal characteristics between an input of the coherent optical receiving front-end means and an output of the coherent optical transmitting front-end means, thereby generating the second analog electrical signal; Including, performing the analog signal processing to generate the second analog electrical signal so as to compensate for the signal quality includes controlling an output power of the first band or the second band of the analog signal processing so that a power of the first band and a power of the second band in the second analog electrical signal are equal to each other; Optical relay method.

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