Optical repeater and optical repeating method

Abstract

To provide an optical repeater, an optical transmission system, and an optical repeating method that are capable of restraining degradation of signal quality.SOLUTION: In a transmitting / receiving unit of an optical repeater, an optical transceiver 101 includes a coherent receiving front-end unit 110 for performing coherent detection on an input optical signal SO1 based on local oscillator emission and outputting a coherently detected first analog electrical signal SA1, a coherent transmitting front-end unit 120 for performing coherent modulation on a second analog electrical signal SA2 obtained by folding back the first analog electrical signal based on the transmitted light and outputting a coherently modulated output optical signal SO2, and an analog compensation unit 130 for performing analog signal processing on the first analog electrical signal so as to compensate for the signal quality according to the signal characteristics between the input of the coherent receiving front-end unit 110 and the output of the coherent transmitting front-end unit 120, thereby generating a second analog electrical signal.SELECTED DRAWING: Figure 7

Description

Technical Field

[0001] The present invention relates to an optical relay device, an optical transmission system, and an optical relay method.

Background Art

[0002] In recent years, the introduction of 5G wireless communication systems has been progressing. For the post-5G era, there is an increasing demand not only for wireless communication but also for optical communication fields to achieve ultra-high speed, further ultra-low latency, and a large number of simultaneous connections. Therefore, regarding optical communication systems, research is being advanced with the expectation of their application to various communication services and industrial uses.

[0003] For example, in backbone optical communication systems, by using a digital coherent method that combines an optical phase modulation method and a polarization multiplexing separation technique, a large capacity exceeding 100 Gbps (Giga bit per second) has been realized. Furthermore, research and development of a transmission method that improves the frequency utilization efficiency and enables a large number of simultaneous connections by narrowing the signal band and performing wavelength division multiplexing (WDM) are also being carried out.

[0004] As a related technology, 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 method.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] In Patent Document 1, in a wavelength converter, a receiving end including a coherent detection front-end module converts an input 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. However, Patent Document 1 does not consider the quality degradation of the optical signal due to passing through a plurality of optical relay devices. That is, in Patent Document 1, since the optical signal is only converted into an analog electrical signal and then the analog electrical signal is converted back into an optical signal, there is a problem that the signal quality may deteriorate.

[0007] In view of such problems, an object of the present disclosure is to provide an optical relay device, an optical transmission system, and an optical relay method capable of suppressing degradation of signal quality.

Means for Solving the Problems

[0008] The optical relay device according to the present disclosure includes coherent optical receiving front-end means for coherently detecting an input optical signal based on local light emission and outputting a first analog electrical signal obtained by the coherent detection, 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, and analog compensation means for performing analog signal processing on the first analog electrical signal so as to compensate for signal quality according to signal characteristics between the input of 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.

[0009] The optical transmission system according to the present disclosure is an optical transmission system including a plurality of optical relay devices. The plurality of optical relay devices perform coherent detection of an input optical signal input from the previous-stage optical relay device based on local light emission, and output a first analog electrical signal obtained by the coherent detection. Coherent optical reception front-end means; a second analog electrical signal obtained by folding back the first analog electrical signal is coherently modulated based on transmission light, and the coherently modulated output optical signal is output to the next-stage optical relay device. Coherent optical transmission front-end means; and analog compensation means for performing analog signal processing on the first analog electrical signal so as to compensate for signal quality according to signal characteristics between the input of the coherent optical reception front-end means and the output of the coherent optical transmission front-end means, and 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 including coherent optical reception front-end means and coherent optical transmission front-end means. The coherent optical reception front-end means performs coherent detection of an input optical signal based on local light emission, 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 transmission light, and outputs the coherently modulated output optical signal. Analog signal processing is performed on the first analog electrical signal so as to compensate for signal quality according to signal characteristics between the input of the coherent optical reception front-end means and the output of the coherent optical transmission front-end means, and the second analog electrical signal is generated.

Effects of the Invention

[0011] According to the present disclosure, it is possible to provide an optical relay device, an optical transmission system, and an optical relay method capable of suppressing deterioration of signal quality.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13A

Figure 13B

Figure 14

Figure 15

Figure 16

Figure 17

Figure 18

Figure 19

Figure 20A

Figure 20B

Figure 21

Figure 22A

Figure 22B

Figure 23

Figure 24

Figure 25

Figure 26

Figure 27

Figure 28A

Figure 28B

Figure 29

Figure 30

Figure 31A

Figure 31B

Figure 32A

Figure 32B

Figure 33

Figure 34

Figure 35

Embodiments for Carrying Out the Invention

[0013] Hereinafter, embodiments will be described with reference to the drawings. In each drawing, the same elements are denoted by the same reference numerals, and redundant descriptions are omitted as necessary. Note that the arrows attached to the configuration diagrams (block diagrams) are for illustrative purposes and do not limit the type or direction of signals.

[0014] (Embodiment 1) Hereinafter, Embodiment 1 will be described with reference to the drawings. FIG. 1 shows a configuration example of an optical transmission system according to this embodiment. The optical transmission system 1 according to this embodiment is, for example, a backbone wavelength division multiplexing optical transmission system, which performs wavelength division multiplexing and digital coherent transmission using optical signals of each wavelength to perform high-capacity communication exceeding 100 Gbps. By wavelength division multiplexing, it is possible to improve the utilization efficiency of the optical frequency and to cope with mobile traffic and wavelength defragmentation. Also, by wavelength division multiplexing, the transmission path (wavelength path) can be flexibly switched while remaining an optical signal. Therefore, by switching the transmission path in case of a failure, the failure can be avoided and the infrastructure can be maintained. Furthermore, in this embodiment, towards the post-5G era, the real-time performance is improved and it becomes possible to cope with ultra-low latency.

[0015] As shown in FIG. 1, the optical transmission system 1 includes a plurality of optical relay devices 2 (for example, 2-1 to 2-10) optically communicably connected via an optical fiber transmission line 3. The optical relay device 2 is a photonic node capable of relaying wavelength-division multiplexed optical signals, and is, for example, a ROADM (Reconfigurable Optical Add / Drop Multiplexer) device. In this example, the optical relay devices 2-1 to 2-10 constitute a ring-type network including three rings, but a network of other topologies may be constituted.

[0016] Each optical relay device 2 is assigned a wavelength path, and transfers the traffic of the local network accommodated via the assigned wavelength path or other optical relay devices 2. For example, the optical relay device 2-1 accommodates the network of the data center 4, the optical relay device 2-2 accommodates the network of the data center 5, and transfers large-capacity traffic such as a video distribution service that distributes high-quality video (4K / 8K). When the optical relay device 2-1 and the optical relay device 2-2 transfer the traffic between the data center 4 and the data center 5 via the wavelength path P1, if a failure occurs in the wavelength path P1, the wavelength path P1 is switched to the wavelength path P2. Thereby, the transfer of the traffic between the data center 4 and the data center 5 can be maintained via a detour path including the optical relay device 2-3 and the optical relay device 2-4.

[0017] For example, the optical relay device 2-5 accommodates the IoT sensor network of the IT service provider 6, and the optical relay device 2-8 accommodates the mobile network of the event venue 7. The traffic of the mobile network is spotty demand traffic by moving users. When the optical relay device 2-5 and the optical relay device 2-8 transfer the traffic between the IT service provider 6 and the event venue 7 via the wavelength path P3 including the optical relay device 2-6 and the optical relay device 2-7, when the user of the event venue 7 moves to the event venue 8, the wavelength path P3 is switched to the wavelength path P4. Thereby, the transfer of the traffic of the user who has moved to the event venue 8 can be maintained via the optical relay device 2-6, the optical relay device 2-4, and the optical relay device 2-10.

[0018] FIG. 2 shows a configuration example of the optical relay device 2 according to the present embodiment. The optical relay device 2 branches / inserts a wavelength-division multiplexed signal and coherently demodulates the signals of the respective wavelengths to be branched / inserted. As shown in FIG. 2, the optical relay device 2 includes a transmission / reception unit 100 and an optical switch unit 200.

[0019] The optical switch unit 200 transfers the optical signal of a predetermined wavelength path received from the preceding optical relay device 2 in the optical transmission system 1 to the succeeding optical relay device 2, and also branches / inserts the received optical signal for each wavelength. For example, the optical switch unit 200 includes a demultiplexer 201, a multiplexer 202, and a branch / insertion unit 203. The demultiplexer 201 separates the optical signal received from the optical fiber transmission line 3 into optical signals of a plurality of wavelengths. The multiplexer 202 multiplexes the optical signals of a plurality of wavelengths into one optical signal and transmits it to the optical fiber transmission line 3. The branch / insertion unit 203 branches / inserts the optical signal of each wavelength between the demultiplexer 201 and the multiplexer 202.

[0020] The transceiver unit (transponder) 100 receives the optical signal of each wavelength branched from the branch / insertion unit 203 of the optical switch unit 200, outputs the received data obtained by coherent demodulation to a local device (network) that accommodates it, and also inputs transmission data from the local device and transmits (inserts) the optical signal of each wavelength obtained by coherent modulation to the branch / insertion 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 transceiver 101 receives an optical signal of a predetermined wavelength and further transmits an optical signal of a predetermined wavelength (the same or different from the received wavelength).

[0021] Here, the problems that occur when using a digital coherent optical transceiver as the optical transceiver 101 will be examined. FIG. 3 shows a configuration example of a related digital coherent optical transceiver. As shown in FIG. 3, the related digital coherent optical transceiver 102 includes a coherent reception front-end unit 110, a coherent transmission front-end unit 120, and a DSP 900.

[0022] The coherent reception front-end unit 110 performs coherent detection on the optical signal received from the preceding optical relay device 2 using local oscillation light (Local oscillator (LO) light) of a predetermined wavelength, and outputs the detected signal to the DSP900. The coherent transmission front-end unit 120 optically modulates (coherent modulation) the signal processed by the DSP900 to a predetermined wavelength, and transmits the generated optical signal to the next-stage optical relay device 2. The DSP900 converts the signal coherently detected by the coherent reception front-end unit 110 into a digital signal, outputs the decoded received data, encodes the input transmission data, and outputs the signal converted for optical modulation to the coherent transmission front-end unit 120.

[0023] When the optical relay device 2 using such a digital coherent optical transceiver 102 relays an optical signal, consider the case where optical signals of the same wavelength collide as shown in FIG. 4. For example, when a wavelength path P5 is set between the optical relay device 2-2 and the optical relay device 2-5 and traffic is being transferred between the IT service provider 6 and the data center 5, a wavelength path P6 is set between the optical relay device 2-2 and the optical relay device 2-8, and it is assumed that traffic is being transferred between the event venue 7 and the data center 5. At this time, when the wavelength slots of the wavelength path P5 and the wavelength path P6 are λ1, at the optical relay device 2-7, the optical signal S1 of the wavelength path P5 and the optical signal S2 of the wavelength path P6 collide.

[0024] In this case, a method of avoiding the collision by switching the wavelength path P5 or the wavelength path P6 to another path can be considered, but the wavelength slot of the other path is not always available. Even if the path is switched, there is a risk that the latency will increase due to the detour path. In addition, in the optical switch of the optical relay device, a method of collectively switching a plurality of wavelengths as they are in units of a wavelength grid including a plurality of wavelength slots (wavelength channels) can be considered, but in this case, switching cannot be performed in units of wavelengths.

[0025] Therefore, as shown in FIG. 5, a method of converting an optical signal into an unused wavelength slot using the optical relay device 2-7 where a collision occurs can be considered. For example, in the digital coherent optical transceiver 102 of the optical relay device 2-7, the wavelength of the optical signal in the wavelength path P6 is converted from λ1 to λ2. As a result, in the path from the optical relay device 2-7 to the optical relay device 2-2, since the wavelengths of the optical signal S1 in the wavelength path P5 and the optical signal S2 in the wavelength path P6 are different, collisions can be avoided.

[0026] However, in the case of FIG. 5, when wavelength conversion and folding-back are performed by the digital coherent optical transceiver 102, there is a problem that the latency increases for regeneration and relaying. That is, in the DSP900 of the digital coherent optical transceiver 102, complex digital signal processing and error correction processing are performed, resulting in a large latency. Also, the circuit size for digital signal processing is large, and the power consumption is high.

[0027] Therefore, in the present embodiment, in the optical transceiver 101, by folding back the optical signal without using a digital coherent optical transceiver, an increase in latency is suppressed. As shown in FIG. 6, in the present embodiment, in the optical transceiver 101, the analog signal output from the coherent reception front-end unit 110 is folded back to the coherent transmission front-end unit 120 and relayed without passing through the DSP (before the DSP). Note that the optical transceiver 101 may not include a DSP, or the DSP may not be used when folding back the 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 the optical transceiver 101 according to the present embodiment. As shown in FIG. 7, the optical transceiver 101 (optical relay device) according to the present embodiment includes a coherent reception front-end unit 110, a coherent transmission front-end unit 120, and an analog compensation unit 130.

[0029] The coherent reception front-end unit 110 and the coherent transmission front-end unit 120 are the same as the above-mentioned digital coherent optical transceiver 102. That is, the coherent reception front-end unit 110 is an optical / electrical conversion unit that converts an optical signal into an electrical signal and is a coherent detection unit that performs coherent detection. The coherent reception front-end unit 110 performs coherent detection on the input optical signal SO1 based on the local oscillation light r1 and outputs the generated analog electrical signal SA1 (the first analog electrical signal).

[0030] The coherent transmission front-end unit 120 is an electrical / optical conversion unit that converts an electrical signal into an optical signal and is a coherent modulation unit that performs coherent modulation. The coherent transmission front-end unit 120 performs coherent modulation on the analog electrical signal SA2 (the second analog electrical signal) obtained by folding back the analog electrical signal SA1 based on the 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 4-lane (4ch) signals including the XI signal of the I component (in-phase component) of the X polarization, the XQ signal of the Q component (orthogonal component) of the X polarization, the YI signal of the I component of the Y polarization, and the YQ signal of the Q component of the Y polarization.

[0032] The frequency of the local oscillation light r1 is the frequency (carrier frequency) of the input optical signal SO1 to be received, and the frequency of the transmission light r2 is the frequency of the output optical signal SO2 to be transmitted. For example, the local oscillation light r1 and the transmission light r2 may have different frequencies, but they may also have the same frequency. By changing the frequencies of the local oscillation light r1 and the transmission light r2, the wavelength of the folded optical signal can be switched. That is, 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 so as to compensate for the signal quality according to 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. Note that either or both of the optical signal and the analog electrical signal may be simply referred to as a "signal". The compensation of the signal quality in the present embodiment includes compensation for the degradation of the optical signal that occurs every time the optical relay device passes through, and compensation for the degradation of the analog electrical signal that occurs within the optical relay device. For example, the degradation of the optical signal to be compensated includes band degradation (PBN: Pass Band Narrowing) that occurs when passing through an optical multiplexer / demultiplexer or an optical filter of each optical relay device, amplitude variation of 4 lanes due to variations in O / E or E / O conversion efficiency, optical frequency offset, and the like. The degradation of the analog electrical signal to be compensated includes band degradation due to characteristic degradation and characteristic variations of the analog electrical circuit of each 4 lanes, amplitude variation of 4 lanes, skew of 4 lanes, and the like. In other words, the compensation of the signal quality includes band compensation for compensating for the band degradation of the signal, frequency offset compensation for compensating for the frequency shift of the local light emission, skew compensation for compensating for the timing variation of each signal component included in the signal, amplitude compensation for compensating for the amplitude variation 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 the present 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. Further, the optical transceiver 101 may include a reference light source 140 that generates local light emission r1 and a transmission light source 150 that generates transmission light r2. The reference light source 140 may be inside the coherent reception front-end unit 110, or the transmission light source 150 may be inside the coherent transmission front-end unit 120.

[0035] The analog signal processing unit 131 is an analog circuit that performs predetermined analog signal processing to compensate 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 only performs analog signal processing and does not perform digital signal processing that causes a large delay. Thereby, the physical delay can be suppressed to, for example, several 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 reception front-end unit 110 and the output of the coherent transmission front-end unit 120. The signal characteristics to be monitored are, for example, the characteristics of the polarization 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, and the like.

[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 the deterioration of the signal quality. Since the signal processing speeds of the control unit 132 and the monitor unit 133 do not affect the latency of the main signal, the time constants of the control unit 132 and the monitor unit 133 may be low. The control unit 132 and the monitor unit 133 may be analog circuits or digital circuits.

[0038] FIG. 9 shows a configuration example of the coherent reception front-end unit 110 according to the present embodiment. As shown in FIG. 9, the coherent reception front-end unit 110 includes a polarization separation unit 111, 90-degree hybrid circuits 112-1 to 112-2, O / E conversion units 113-1 to 113-4, and amplifiers 114-1 to 114-4.

[0039] The polarization beam splitter 111 separates the input optical signal SO1, which is a polarization multiplexed signal, into an X polarization and a Y polarization. The 90-degree hybrid circuits (coherent optical detectors) 112-1 to 112-2 perform coherent detection by interfering the optical signals polarization-separated by the polarization beam splitter 111 with the local light emission r1 of the reference light source 140, and convert the signals detected by the O / E conversion units 113-1 to 113-4, each composed of a Photo Diode or the like, into 4-lane analog electrical signals. The 90-degree hybrid circuit 112-1 separates the X polarization of the input optical signal SO1 into an I component and a Q component, and then performs photoelectric conversion by the O / E conversion units 113-1 to 113-2 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 by the 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 4-lane analog electrical signals SA1 to the analog compensation unit 130. The analog compensation unit 130 performs analog signal processing on all or part (X polarization or Y polarization) of the XI signal, XQ signal, YI signal, and YQ signal.

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

[0041] Amplifiers 121-1 to 121-4 amplify the XI signal, XQ signal, YI signal, and YQ signal of the analog electrical signal SA2 output from the analog compensation unit 130, respectively, 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 of the transmission light source 150 according to the applied XI signal, XQ signal, YI signal, and YQ signal, respectively. The MZ modulators 122-1 to 2 generate an IQ-modulated optical signal of the X polarization based on the XI signal and XQ signal via the amplifiers 121-1 to 121-2. The MZ modulators 122-3 to 4 generate an IQ-modulated optical signal of the Y polarization based on the YI signal and YQ signal via the amplifiers 121-3 to 121-4. The polarization combining unit 123 polarization-combines the generated IQ-modulated optical signal of the X polarization and the IQ-modulated optical signal of the Y polarization, and outputs the combined optical signal as the output optical signal SO2.

[0042] As described above, in an optical transmission system, by performing path switching while keeping the optical signal from the transmission end to the reception end and realizing an optically transparent network that minimizes extra processing, low latency can be achieved. However, in a complex WDM network, due to wavelength conflicts with other signals, it is not possible to establish an end-to-end shortest path with a single wavelength, so it is necessary to establish a path while performing wavelength conversion to an available wavelength slot. In related technologies, wavelength conversion per channel can be realized by once extracting a signal with a ROADM and regenerating and relaying it to another wavelength with a digital coherent optical transceiver. However, there is a problem that the low latency performance is impaired because complex processing involving signal delay such as error correction processing by DSP is involved.

[0043] Therefore, in the present embodiment, the analog signal output from the coherent optical reception front end is subjected to analog signal processing without passing through the digital coherent DSP, and is folded back and relayed to the coherent optical transmission front end. That is, although the optical signal is once converted into an electrical signal, only the minimum necessary analog signal processing is performed, and then it is converted back into an optical signal of a different wavelength and relayed. Physically, an analog delay of the electrical circuit occurs, but it is at most several nsec or less, which is negligibly small. Therefore, a low-latency wavelength conversion function can be realized for each channel. Also, not limited to wavelength conversion, relay can be performed with low latency at the same wavelength. Furthermore, in an optical transmission system, in addition to the quality degradation of the optical signal caused by passing through a plurality of optical relay devices, the signal quality deteriorates due to the imperfection of the electrical circuit in the optical relay device. Therefore, in the present embodiment, these signal degradations are processed while the analog signal is maintained to perform quality compensation. As a result, it is possible to suppress the degradation of the signal quality while suppressing the delay, so that it is possible to perform long-distance communication with low latency.

[0044] (Embodiment 2) Hereinafter, Embodiment 2 will be described with reference to the drawings. The optical signal that has passed through each optical relay device in the optical transmission system undergoes bandwidth narrowing due to the influence of the optical filter included in the optical relay device. In particular, the more stages the configuration has, the more bandwidth narrowing occurs, so the transmission distance is limited. Also, in the optical relay device, since the optical signal is once converted into an electrical signal, it is also affected by the bandwidth of the transceiver. Therefore, in the present embodiment, the analog compensation unit in Embodiment 1 enables bandwidth compensation.

[0045] FIG. 11 shows a configuration example of the optical transceiver 101 according to the present embodiment. As shown in FIG. 11, in the present embodiment, the analog compensation unit 130 includes an analog signal processing unit 131, a control unit 132, and a post-signal monitor 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] BPF303a extracts, for example, the low-frequency component (first band) of the analog electrical signal SA2 (any of the 4 lanes). The power monitor 304a monitors the power of the low-frequency component of the analog electrical signal SA2 extracted by BPF303a. BPF303b extracts, for example, the high-frequency component (second band) of the analog electrical signal SA2 (any of the 4 lanes). The power monitor 304b monitors the power of the high-frequency component of the analog electrical signal SA2 extracted by BPF303b.

[0051] The control unit 132 controls the band adjustment circuit 301 of each lane according to the monitoring results of the low-frequency and high-frequency components of the 4 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 at which the powers are the same in a state where the signal is not deteriorated (for example, a substantially trapezoidal spectrum). Therefore, the difference Δw1 between the power of the low frequency f11 and the power of the high frequency f12 is obtained, and the band adjustment amount is controlled so that Δw1 becomes small. As shown in FIG. 13B, the band lift amount 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 become equal. For example, when the power of the high frequency f12 is Δw1 lower than the power of the low frequency f11, control is performed to increase the power of the high frequency f12 by Δw1.

[0052] FIG. 14 is a configuration example of the band adjustment circuit 301 according to the present embodiment, and shows an example in the case of being configured by an analog FIR filter. As shown in FIG. 14, the band adjustment circuit 301 includes, as an example of an analog FIR filter, 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~gn) 314, an addition circuit (Σ) 315, and a drive circuit (Driver) 316. Note that the configuration of FIG. 14 is an example, and an analog FIR filter may be configured by other circuits.

[0053] The AGC amplifier 311 has its gain adjusted via the LPF 312 connected between the output terminal and the control terminal, and amplifies the input signal with the adjusted gain. The amplified signal is gradually delayed in units of a predetermined time by a plurality of delay circuits 313 connected in series. Weighting is performed on each of the delayed signals by the weighting circuit 314, and the weighted signals are added together by the addition circuit 315. The addition result by the addition circuit 315 is output via the 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 configuration example of the band adjustment circuit 301 according to the present embodiment, and shows an example in the case of being configured by a peaking variable amplifier. As shown in FIG. 15, the band adjustment circuit 301 includes, as an example of a peaking variable amplifier, an amplifier 321, a BPF 322, a variable gain amplifier 323, and an addition circuit 324. Note that the configuration of FIG. 15 is an example, and a peaking variable amplifier may be configured by other circuits. The amplifier 321 amplifies the input signal and outputs the amplified signal G1 to the addition circuit 324. The BPF 322 extracts a predetermined band component (for example, a high-frequency component) of the input signal. The variable gain amplifier 323 amplifies the signal of the extracted band and outputs the amplified signal G2 to the addition circuit 324. The addition circuit 324 adds the signal G1 from the amplifier 321 and the signal G2 from the 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) Next, Embodiment 3 will be described with reference to the drawings. In Embodiment 2, band compensation was performed. However, if there is a frequency offset in the local light emission of the reference light source, it will affect the frequency characteristics of the analog electrical signal obtained by coherent detection due to the local light emission, making it difficult to correctly monitor and compensate for the band. Therefore, in this embodiment, in the analog compensation unit of Embodiment 2, frequency offset compensation and band compensation are performed. Note that only the frequency offset may be compensated without performing band compensation.

[0059] FIG. 18 shows a configuration example of the optical transceiver 101 according to this embodiment. As shown in FIG. 18, in this embodiment, the analog compensation unit 130 includes a pre-signal monitor unit 135 in addition to the configuration of Embodiment 2. Other configurations are the same as those in Embodiment 2.

[0060] The pre-signal monitor unit 135 is an example of the monitor unit 133 and monitors the analog electrical signal SA1 (pre-signal) input to the analog signal processing unit 131. It can also be said that the pre-signal monitor unit 135 monitors the signal output from the coherent reception front-end unit 110. The pre-signal monitor unit 135 includes a frequency offset monitor 401 that monitors the frequency offset of the local light emission r1 based on the analog electrical signal SA1. The frequency offset monitor 401 may monitor the frequency offset using the 4-lane signal of the analog electrical signal SA1, or may monitor the frequency offset using the 2-lane signal (X polarization or Y polarization).

[0061] The control unit 132 controls the frequency of the local light emission 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 shift amount of the output frequency of the reference light source 140 according to the monitoring result of the 4-lane or 2-lane frequency offset. For example, based on the monitoring result of the analog electrical signal SA1, the control unit 132 shifts the frequency of the local light emission r1 so as to return the deviation from the desired spectrum.

[0062] FIG. 19 shows a configuration example of the frequency offset monitor 401 according to the present embodiment. In the example of FIG. 19, an analog electrical signal SA1 is captured by a full-band ADC (Analog / Digital Converter) to restore the spectrum. In this example, although a wide-band ADC or the like is required and thus it is costly, it is possible to monitor while conducting the main signal.

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

[0064] Figures 20A and 20B show the monitoring results by the frequency offset monitor 401 in FIG. 19, and show examples of spectra in the case of no frequency offset and in the case of having a frequency offset, respectively. As shown in FIG. 20A, in the case of no frequency offset, the spectrum of the input optical signal SO1 is symmetric about the center frequency f0. As shown in FIG. 20B, in the case of having a frequency offset, the spectrum of the input optical signal SO1 is asymmetric about the center frequency f0. The control unit 132 controls the shift amount of the local light emission r1 so that the spectrum of the input optical signal SO1 is symmetric about 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 emission r1 is shifted so that Δf21 and Δf22 become equal. The shift amount of the frequency of the local light emission r1 is controlled according to the difference between Δf21 and Δf22.

[0065] FIG. 21 shows another configuration example of the frequency offset monitor 401 according to the present embodiment. In the example of FIG. 21, while sweeping the frequency of the local light emission r1 which is a reference light source, the power of the analog electrical signal SA1 is monitored to restore the spectrum. In this example, since it is monitored while controlling the local light emission, although it is not possible to monitor while conducting the main signal, it can be realized at low cost. Note that a monitoring light source correlated with the local light emission may be prepared, and the frequency offset may be monitored by the monitoring 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 turned on, but it may also be performed at any timing when the optical signal is not being transmitted or received. When performing the monitoring, the frequency of the local emission r1 of the reference light source 140 is swept. That is, the frequency of the local emission r1 is gradually increased from the lowest frequency to the highest frequency within the range of the generated spectrum. The power monitor 422 monitors the power of the analog electrical signal SA1 generated by mixing the locally emitted light r1 with variable frequency and the input optical signal SO1 via the LPF 421. That is, the analog electrical signal SA1 is a signal obtained by extracting the input optical signal SO1 in the band of the LPF 421 centered on the frequency of the local emission r1. In other words, by varying the frequency of the local emission r1, which is the reference light, with respect to the input optical signal SO1 and mixing them, and extracting the signal via the LPF 421, it is possible to perform a role equivalent to that of a variable optical band-pass filter. Therefore, the power monitor 422 can monitor the power of each band of the input optical signal SO1 by varying the local emission r1. The signals of the 4 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 emission r1, and restores (generates) the spectrum. The asymmetry monitor 424 monitors the asymmetry of the restored spectrum in the same manner as in FIG. 19.

[0067] For example, in the configuration of FIG. 21, the frequency offset monitor 401 may monitor the entire spectrum or only some frequency components. The frequency offset monitor 401 monitors 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 emission r1 so that the power of the first band and the power of the second band become equal.

[0068] For example, as shown in FIGS. 22A and 22B, the frequency of the local light emission r1 is varied between -Δf3 and +Δf3, and the frequency components of -Δf3 and +Δf3 are monitored. As shown in FIG. 22A, when there is no frequency offset, since the spectrum of the input optical signal SO1 is symmetric about the vertical axis, the power of -Δf3 is equal to the power of +Δf3. -Δf3 and +Δf3 are the frequencies near both ends in the frequency axis direction in the spectrum without a frequency offset, and are the frequencies near the attenuation points where the power starts to decay from the maximum level. In other words, in a substantially trapezoidal spectrum, they are the frequencies near both ends of the upper base (upper side).

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

[0070] As described above, in the present embodiment, in addition to the band compensation of the second embodiment, the frequency of the local light emission of the reference light source is controlled according to the amount of the frequency offset of the local light emission r1 estimated from the pre-signal (analog electrical signal SA1) of the analog signal processing unit, thereby compensating for the frequency offset. As a result, after suppressing the frequency offset of the local light emission, the band compensation of the second embodiment can be appropriately performed. 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 Embodiment 3) FIG. 23 shows the configuration of a modification of Embodiment 3. In Embodiment 3, the analog electrical signal SA1 between the coherent reception front-end unit 110 and the analog signal processing unit 131 was monitored. However, as shown in FIG. 23, the input optical signal SO1, which is the input of the coherent reception 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 emission r1. That is, based on the spectrum of the input optical signal SO1 and the spectrum of the local light emission r1, the frequency offset of the local light emission r1 is monitored.

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

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

[0074] The variable optical BPF 431a, the optical power monitor 432a, and the spectrum restoration unit 433a monitor the spectrum of the input optical signal SO1. The variable optical BPF 431b, the optical power monitor 432b, and the spectrum restoration unit 433b monitor the spectrum of the local light emission r1. Each one variable optical BPF, optical power monitor, and spectrum restoration unit may switch the input signal to monitor the spectra of the input optical signal SO1 and the local light emission r1.

[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 section) 432a monitors the power of each frequency (band) of the extracted input optical signal SO1. The spectrum restoration section (input optical spectrum generation section) 433a restores the spectrum based on the power of each frequency of the monitored input optical signal SO1. Similarly, the variable optical BPF (local emission BPF) 431b extracts each frequency component of the local emission r1 by varying the center frequency of the passband. The optical power monitor (local emission power monitor section) 432b monitors the power of each frequency (band) of the extracted local emission r1. The spectrum restoration section (local emission spectrum generation section) 433b restores the spectrum based on the power of each frequency of the monitored local emission r1. The spectrum comparison section 434 compares the spectrum of the restored input optical signal SO1 with the spectrum of the local emission r1, and monitors the frequency offset based on the comparison result.

[0076] Figure 25 shows an example of the monitoring result by the frequency offset monitor 401 in Figure 24. When there is a frequency offset, the center frequency f41 of the spectrum of the local emission r1 is shifted with respect to 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 emission r1 is obtained, and the frequency of the local emission r1 is shifted so that Δf5 becomes smaller. Note that, due to the principle of this spectrum monitor, it is impossible to perform a spectrum monitor with a resolution below the bandwidth of the variable optical BPF. Therefore, the spectrum monitor result of the local emission r1 has a broader spectrum compared to the spectrum of the original single frequency (single wavelength), but it can still be monitored with sufficient accuracy for the purpose of monitoring the center frequency. In this example, it is not necessary to sweep the local emission r1 serving as the reference light source. Since the branched input optical signal SO1 and the local emission r1 can be monitored for their respective spectra by varying the center frequency of the variable optical BPF, control is possible even during signal conduction.

[0077] (Embodiment 4) Next, Embodiment 4 will be described with reference to the drawings. Ideally, it is desirable that a signal with skew and amplitude compensated for 4 lanes (XI, XQ, YI, YQ) at the transmitting end reaches the receiving end. However, when an optical signal is once converted into an analog electrical signal in an optical transceiver, if there are skew and amplitude errors between the 4 lanes, the signal quality deteriorates and the transmission distance may be restricted. Therefore, in this embodiment, amplitude compensation and skew compensation are performed in the analog compensation units of Embodiments 1 to 3. Note that only either amplitude compensation or 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, the analog compensation unit 130 includes an analog signal processing unit 131, a control unit 132, a pre-signal monitor unit 135, and a post-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 skew adjustment circuits 502 respectively. Each amplitude adjustment circuit 501 adjusts the amplitude of the signals of the 4 lanes of the analog electrical signal SA1, and each skew adjustment circuit 502 adjusts the skew of the signals of the 4 lanes of the analog electrical signal SA1. In this example, the skew adjustment circuit 502 adjusts the skew for the signal whose amplitude is adjusted by the amplitude adjustment circuit 501. The amplitude adjustment circuit 501 is, for example, an amplifier or the like, but may also be other analog circuits capable of adjusting the amplitude. The skew adjustment circuit 502 is, for example, a delay adjustment device, a phase shifter (phase adjuster), etc., but may also be other analog circuits capable of adjusting the skew.

[0080] The pre-signal monitor section 135 includes an amplitude monitor 503 that monitors the amplitude of the analog electrical signal SA1. The amplitude monitor 503 monitors the amplitudes of the 4-lane signals of the analog electrical signal SA1. The amplitudes of the 4 lanes may be monitored by four amplitude monitors 503, or the signal to be input may be switched by one amplitude monitor 503 to monitor the amplitudes of the 4 lanes. For example, the amplitude monitor 503 is constituted by a power monitor or the like.

[0081] The control section 132 controls the output amplitude (power) of the amplitude adjustment circuit 501 based on the amplitude monitored by the amplitude monitor 503. The control section 132 controls the amplitude adjustment amount of the amplitude adjustment circuit 501 corresponding to each signal according to the monitoring result of the amplitudes of the 4 lanes. For example, the control section 132 increases the amplitude of the attenuated lane as a result of monitoring the 4 lanes of the analog electrical signal SA1 to equalize the amplitudes of the respective lanes.

[0082] The post-signal monitor section 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 4-lane signals of the analog electrical signal SA2, or may monitor the skew of the 2-lane (X polarization or Y polarization) signals. At least the skew between the I component and the Q component of the X polarization or Y polarization included in the analog electrical signal SA2 is monitored.

[0083] The control section 132 controls the skew adjustment amount of the skew adjustment circuit 502 based on the skew monitored by the skew monitor 504. The control section 132 controls the delay of the I component and the Q component in the skew adjustment circuit 502 according to the monitoring result of the skew of the I component and the Q component (4 lanes or 2 lanes). For example, the control section 132 advances the phase of the delayed lane as a result of monitoring the I component and the Q component of the analog electrical signal SA2 to match the phases (timings) of the respective lanes.

[0084] FIG. 27 shows a configuration example of the skew monitor 504 according to the present embodiment. In this example, the constellation of the analog electrical signal SA2 is monitored, and the skew is adjusted according to the pattern of the constellation. 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 based on 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 in the case of having skew, and FIG. 28B shows the constellation in the case of having no skew. As shown in FIG. 28A, in the case of having skew, the shape (pattern) of the constellation becomes an ellipse. The constellation becomes an ellipse when there is a difference in the amplitudes of the I signal and the Q signal or when there is skew. However, in this example, since the amplitude difference is compensated in advance by the amplitude monitor and the amplitude adjustment circuit, the presence or absence of skew can be determined based on whether the constellation of the analog electrical signal SA2 is an ellipse.

[0087] Therefore, as shown in FIG. 28A, when the constellation is an ellipse, the delay amount of the skew adjustment circuit 502 is controlled so that the constellation becomes a perfect circle as shown in FIG. 28B. The degree of ellipticity (for example, the difference between the major axis and the minor axis) is monitored, and the delay of the I component and the Q component is controlled so that it becomes a perfect circle. For example, the length ΔQ in the Q-axis direction and the length ΔI in the I-axis direction of the constellation are compared, 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) so that ΔQ and ΔI become equal.

[0088] As described above, in this embodiment, the variation in the amplitude of the pre-signal of the analog signal processing unit is monitored, and amplitude compensation is performed by the analog signal processing unit according to the monitoring result. Also, the skew of the post-signal of the analog signal processing unit is monitored, and skew compensation is performed by the analog signal processing unit according to the monitoring result. As a result, an optical signal in which amplitude compensation and skew compensation are appropriately performed by the analog signal processing unit can be transmitted, and signal quality degradation due to skew and amplitude variation inside the optical relay device can be suppressed.

[0089] (Modification Example of Embodiment 4) FIG. 29 shows the configuration of a modification example of Embodiment 4. In Embodiment 4, the analog electrical signal SA2 between the analog signal processing unit 131 and the coherent transmission front-end unit 120 is monitored. However, as shown in FIG. 29, the output optical signal SO2, which is the output of the coherent transmission front-end unit 120, may be branched and monitored. For example, the skew monitor 504 monitors an optical signal of the X polarization or Y polarization separated by a PBS (polarization beam splitter) 510.

[0090] FIG. 30 shows a configuration example of the skew monitor 504 according to the modification example of Embodiment 4. In this example, the intensity of the output optical signal is monitored, and the skew is adjusted according to the amount of variation in the intensity of the output optical signal.

[0091] As shown in FIG. 30, the skew monitor 504 includes an optical power monitor 511 and a variation monitor 512. The optical power monitor 511 monitors the power of the X polarization or Y polarization of the output optical signal SO2. The variation monitor 512 monitors the amount of variation in the power of the X polarization or Y polarization 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 variation in the power of the output optical signal SO2. FIGS. 31A and 31B show constellations in the case of having skew and the case of not having skew, respectively. FIGS. 32A and 32B show the intensities of the optical signals in the case of having skew and the case of not having skew, respectively.

[0093] In the case of skew, as shown in FIG. 31A, since the constellation becomes an ellipse, as shown in FIG. 32A, the intensity of the optical signal fluctuates. In the case of no skew, as shown in FIG. 31B, since the constellation becomes a perfect circle, as shown in FIG. 32B, the intensity of the optical signal is constant. Therefore, 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 amounts of the I component and the Q component of the skew adjustment circuit 502 are controlled so that the fluctuation amount Δw3 of the intensity becomes minimum and the intensity becomes constant.

[0094] (Embodiment 5) Hereinafter, Embodiment 5 will be described with reference to the drawings. In this embodiment, an example in which skew is monitored using a dither signal in the analog compensation unit of Embodiment 4 will be described.

[0095] FIG. 33 shows a configuration example of the optical transceiver 101 according to this embodiment. As shown in FIG. 33, the optical transceiver 101 includes a dither signal generation unit 136 in addition to the configuration of Embodiment 4. The dither signal generation unit (dither signal superposition unit) 136 generates a dither signal Sd for skew detection and applies (superposes) the dither signal to the analog electrical signal SA1 output from the coherent reception front-end unit 110.

[0096] The dither signal is simultaneously applied to the four lanes of the main signal (analog electrical signal SA1) in the coherent reception front-end unit 110 and detected by the skew monitor 504 of the post-signal monitor unit 134. The dither signal may be applied to at least two lanes of the I component and the Q component for skew detection. The dither signal is a signal having 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, such as 100 KHz or 50 KHz (separated by a predetermined frequency).

[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 can be modified as appropriate without departing from the spirit and scope of the present disclosure.

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

[0102] Some or all of the above embodiments can be described as follows, but are not limited thereto. (Appendix 1) Coherent optical reception front-end means for coherently detecting an input optical signal to be input based on local light emission and outputting a first analog electrical signal that has been coherently detected; 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 transmission light and outputting the coherently modulated output optical signal; Analog compensation means for performing analog signal processing on the first analog electrical signal so as to compensate for signal quality according to signal characteristics between the input of the coherent optical reception front-end means and the output of the coherent optical transmission front-end means and generating the second analog electrical signal; An optical relay device comprising: (Appendix 2) The signal characteristics include characteristics of a polarization signal or characteristics of a complex signal. The optical relay device according to Appendix 1. (Appendix 3) The compensation of the signal quality includes any one of band compensation, frequency offset compensation, skew compensation, and amplitude compensation. The optical relay device according to Appendix 1 or 2. (Appendix 4) The analog compensation means An analog signal processing unit that performs the analog signal processing; A monitor unit that monitors signal characteristics 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; The optical relay device according to any one of Appendices 1 to 3, comprising: (Appendix 5) The monitor unit monitors the frequency characteristics of the second analog electrical signal, and the control unit controls the output power for each band in the analog signal processing based on the monitored frequency characteristics. The optical relay device according to Appendix 4. (Appendix 6) The monitor unit monitors the power of a first band and the power of a second band in the second analog electrical signal, and the control unit controls the output power of the first band or the second band in the analog signal processing so that the power of the first band is equal to the power of the second band. The optical relay device according to Appendix 5. (Appendix 7) The monitor unit monitors the frequency offset of the local light emission by the first analog electrical signal or the input optical signal, and the control unit controls the frequency of the local light emission based on the monitored frequency offset. The optical relay device according to any one of Appendices 4 to 6. (Appendix 8) The monitor unit monitors the left - right asymmetry of the frequency spectrum of the first analog electrical signal with respect to the center frequency, and the control unit controls the frequency of the local light emission so that the frequency spectrum is symmetric about the center frequency. The optical relay device according to Appendix 7. (Appendix 9) The monitor unit includes an AD conversion means for converting the first analog electrical signal into a digital signal, a complex signal generation means for generating a complex signal from the converted digital signal, an FFT processing means for performing FFT processing on the generated complex signal, and an asymmetry monitor unit for monitoring the asymmetry of the frequency spectrum obtained by the FFT processing. The optical relay device according to Appendix 8, comprising the above. (Appendix 10) The monitor unit power monitor means for monitoring the power for each band of the first analog electrical signal; spectrum generation means for generating a frequency spectrum based on the monitored power; an asymmetry monitor unit for monitoring the asymmetry of the generated frequency spectrum; The optical relay device according to appended note 8, comprising: (Appended note 11) The monitor unit monitors the asymmetry of the frequency spectrum based on the power of the first band and the power of the second band in the first analog electrical signal, and the control unit controls the frequency of the local light emission so that the power of the first band and the power of the second band become equal. The optical relay device according to appended note 10. (Appended note 12) The monitor unit monitors the frequency offset of the local light emission based on the spectrum of the input optical signal and the spectrum of the local light emission. The optical relay device according to appended note 7. (Appended note 13) The monitor unit input optical power monitor means for monitoring the power for each band of the input optical signal; input optical spectrum generation means for generating a frequency spectrum based on the monitored power of the input optical signal; local light emission power monitor means for monitoring the power for each band of the local light emission; local light emission spectrum generation means for generating a frequency spectrum based on the monitored power of the local light emission; a spectrum comparison unit that compares the generated frequency spectrum of the input optical signal and the frequency spectrum of the local light emission and monitors the frequency offset; The optical relay device according to appended note 12, comprising: (Appended note 14) The monitor unit monitors the amplitude of the first analog electrical signal, and the control unit controls the output amplitude in the analog signal processing based on the monitored amplitude. The optical relay device according to any one of Supplementary Notes 4 to 13. (Supplementary Note 15) The monitor unit monitors the skew of the second analog electrical signal or the output optical signal. The control unit controls the delay of the signal in the analog signal processing based on the monitored skew. The optical relay device according to any one of Supplementary Notes 4 to 14. (Supplementary Note 16) The monitor unit monitors the constellation of the second analog electrical signal. The control unit controls the delay of the signal in the analog signal processing based on the constellation pattern. The optical relay device according to Supplementary Note 15. (Supplementary Note 17) The monitor unit monitors the power of the output optical signal. The control unit controls the delay of the signal in the analog signal processing based on the amount of change in the power. The optical relay device according to Supplementary Note 15. (Supplementary Note 18) It includes a superimposing means for superimposing a dither signal on the first analog electrical signal output from the coherent optical reception front-end means. The monitor unit monitors the skew based on the dither signal extracted from the second analog electrical signal. The optical relay device according to Supplementary Note 15. (Supplementary Note 19) The frequency of the dither signal is different from the frequencies of the first analog electrical signal and the second analog electrical signal. The optical relay device according to Supplementary Note 18. (Supplementary Note 20) The frequency of the local light emission is different from the frequency of the transmitted light. The optical relay device according to any one of Supplementary Notes 1 to 19. (Supplementary Note 21) An optical transmission system including a plurality of optical relay devices, wherein the plurality of optical relay devices Coherent optical reception front-end means for coherently detecting an input optical signal input from the optical relay device in the previous stage based on local light emission and outputting a first analog electrical signal that has been coherently detected; 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 transmission light and outputting the coherently modulated output optical signal to the optical relay device in the next stage; Analog compensation means for performing analog signal processing on the first analog electrical signal so as to compensate for signal quality according to signal characteristics between the input of the coherent optical reception front-end means and the output of the coherent optical transmission front-end means and generating the second analog electrical signal; An optical transmission system comprising: (Appendix 22) An optical relay method in an optical relay device including coherent optical reception front-end means and coherent optical transmission front-end means, wherein the coherent optical reception front-end means coherently detects an input optical signal input based on local light emission and outputs a first analog electrical signal that has been coherently detected; 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 transmission light and outputs the coherently modulated output optical signal; and analog signal processing is performed on the first analog electrical signal so as to compensate for signal quality according to signal characteristics between the input of the coherent optical reception front-end means and the output of the coherent optical transmission front-end means, and the second analog electrical signal is generated. An optical relay method.

Explanation of Reference Numerals

[0103] 1 Optical transmission system 2 Optical relay device 3 Optical fiber transmission line 4, 5 Data center 6 IT service provider 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 Unit 414 Asymmetry Monitor 421 LPF 422 Power Monitor 423 Spectrum Restoration Unit 424 Asymmetry Monitor 431a, 431b Variable Optical BPF 432a, 432b Optical Power Monitor 433a, 433b Spectrum Restoration Unit 434 Spectrum Comparison Unit 501 Amplitude Adjustment Circuit 502 Skew Adjustment Circuit 503 Amplitude Monitor 504 Skew Monitor 505a, 505b ADC 506 Constellation Monitor 510 PBS 511 Optical Power Monitor 512 Variation Monitor 521 Dither Signal Extraction Unit 522 Dither Signal Monitor

Claims

1. a coherent receiver that coherently detects an input optical signal based on a local oscillator light and converts the optical signal into an electric signal to output a first electric signal; an IQ modulator that receives a second electrical signal obtained by propagating the first electrical signal as an analog signal, and coherently modulates a transmission light from a light source based on the second electrical signal to output the modulated light; a compensation unit that performs analog signal processing on the first electrical signal so as to compensate for signal quality in accordance with a propagation characteristic of the analog signal, and generates the second electrical signal; Equipped with The compensation means performs the analog signal processing based on the frequency characteristics of at least one of the input optical signal, the modulated optical output, the first electrical signal, and the second electrical signal. Optical repeater.

2. The compensation includes compensation for degradation of an optical signal that occurs when passing through the optical repeater, or compensation for degradation of an analog signal that occurs within the optical repeater.

2. The optical repeater according to claim 1.

3. The light source outputs the transmission light by changing the wavelength of the transmission light from the wavelength of the input optical signal.

3. An optical repeater according to claim 1 or 2.

4. The compensation means comprises: It is an analog circuit or a digital circuit, No digital signal processing is performed on the analog signal, The analog signal is subjected to analog signal processing while the analog signal is still in the form of an analog signal. The optical repeater according to claim 1 .

5. The propagation characteristics are At least one of the characteristics of each polarization multiplexed, the characteristics of the phase-modulated in-phase component and the quadrature component is included. The optical repeater according to claim 1 .

6. The first electrical signal and the second electrical signal are A four-lane signal having an in-phase component of one polarization, a quadrature component of the one polarization, an in-phase component of the other polarization, and a quadrature component of the other polarization. The optical repeater according to claim 1 .

7. The compensation is: Bandwidth compensation for compensating for bandwidth degradation of the signal; a frequency offset compensation for compensating for a shift in the frequency of the local oscillator light; A skew compensation method for compensating for phase variations of a plurality of signal components included in the signal; At least one of the amplitude compensations for compensating for variations in amplitude of a plurality of signal components included in the signal is included. The optical repeater according to claim 1 .

8. The compensation means adjusts the frequency component of the first electrical signal using a band adjustment circuit; The band adjustment circuit includes at least one of an analog FIR filter, a peaking variable amplifier, and an LCR variable filter. The optical repeater according to claim 1 .

9. The compensation means comprises: so that a power of one frequency component in the second electrical signal is equal to a power of another frequency component in the second electrical signal. Adjusting the power of the one frequency component or the other frequency component in the analog signal processing The optical repeater according to any one of claims 1 to 8.

10. coherently detecting the input optical signal based on a local oscillator light, and outputting a first electrical signal obtained by converting the optical signal into an electrical signal; a second electrical signal obtained by propagating the first electrical signal as an analog signal is input, and a transmission light from a light source is coherently modulated based on the second electrical signal to produce an optical output; performing analog signal processing on the first electrical signal to compensate for signal quality in accordance with a propagation characteristic of the analog signal, thereby generating the second electrical signal; The analog signal processing is performed based on the frequency characteristics of at least one of the input optical signal, the modulated optical output, the first electrical signal, and the second electrical signal. Optical relay method.

Images