Spatial multiplexing optical receiver, spatial multiplexing optical transmission system, spatial multiplexing optical receiving method, and program

Abstract

To make it possible to reduce the cost of a receiver in a spatial multiplexing optical transmission system.SOLUTION: A spatial multiplexing optical receiver includes: a plurality of coherent receivers that coherently receive each of signals of a plurality of modes that have been spatially multiplexed and transmitted using continuous wave light that is independent for each mode as local oscillator light; a plurality of frequency offset compensators that perform frequency offset compensation for each of the coherently received signals of the plurality of modes, independently for each mode, based on the correlation between a known training signal and the signal of each mode; and a MIMO signal processing unit that performs MIMO signal processing on the signals of the plurality of modes subjected to the frequency offset compensation in the frequency offset compensators.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a spatial multiplexing optical receiver, a spatial multiplexing optical transmission system, a spatial multiplexing optical receiving method, and a program. [Background technology]

[0002] Optical fiber communications using single-mode fiber (SMF) cannot increase the signal-to-noise ratio (SNR) indefinitely due to nonlinear effects and fiber fuse phenomena. This has led to known limitations on the capacity of optical fiber communications. Spatial multiplexing transmission technology, which multiplexes signals by utilizing the spatial freedom within optical fiber, has been actively studied in recent years as one technology that could overcome this capacity limitation.

[0003] Spatial multiplexing optical transmission systems using spatial multiplexing transmission technology can be broadly divided into two types depending on the degree of coupling between spatial channels. One is a non-coupling type system in which the coupling between spatial channels is small, and the other is a coupling type system. In a non-coupling type system, existing optical transceivers for SMF optical transmission systems can be used as is. However, there is generally a trade-off between the density of spatial channels and the degree of coupling between spatial channels. For this reason, there is a limit to the number of spatial multiplexes that can be achieved in a non-coupling type system.

[0004] On the other hand, in a combined system, coupling between spatial channels is allowed. Typically, the coupling between spatial channels is compensated for by multi-input multi-output (MIMO) signal processing on the receiving side. Although a combined system requires an optical receiver using MIMO signal processing, it achieves a greater spatial channel density, i.e., a larger transmission capacity per optical fiber. This article focuses on a combined-type spatial multiplexing optical transmission system.

[0005] FIG. 11 shows a simplified configuration of the combined spatial multiplexing optical transmission system described in Non-Patent Document 1. Here, as an example, a spatial multiplexing optical transmission system using a 2-core combined multi-core fiber is taken up. In such a spatial multiplexing optical transmission system, a total of four modes are used to transmit optical signals, combining two polarization modes and two spatial channels (cores).

[0006] The encoding unit 211 encodes the transmission data and converts the transmission data into a signal in a predetermined modulation format. The pre-equalization unit 212 performs pre-equalization to compensate for the distortion in the transmitter on the converted transmission data. The Digital-analog converter (DAC) 213 converts the pre-equalized data from a digital signal into an analog electrical signal. In a general coherent optical transmission system, a modulation format having an in-phase (I) component and a quadrature (Q) component is adopted. Therefore, the DCA 213 outputs two analog electrical signals of the I component and the Q component for each mode. In FIG. 11, for simplicity, the signals of the I component and the Q component of each mode are represented by a single line.

[0007] The Laser Diode (LD) 215, which is a laser light source, outputs continuous-wave (CW) light. The optical amplifier 216 amplifies the CW light. The optical coupler 217 outputs the amplified CW light to the optical modulator 214. The optical modulator 214 modulates the CW light input from the optical coupler 217 for each mode with the analog electrical signal output from the DAC 213 to generate an optical signal. The optical signals generated for each mode are sent to the multi-core fiber transmission line 230 using a fan-in fan-out (FIFO) device 218. The multi-core fiber transmission line 230 includes, for example, a multi-core optical fiber 231 that is a combined 2-core fiber and a 2-core optical amplifier 232.

[0008] The optical receiver 250 receives the optical signal transmitted from the optical transmitter 210 via the multicore fiber transmission line 230. In the optical receiver 250, the FIFO device 251 separates the received optical signal for each mode. The FIFO device 251 outputs the separated optical signals for each mode to coherent receivers 252 arranged for each mode. The optical signal is generally separated for each spatial channel, i.e., for each core. For simplicity, in FIG. 11, it is assumed that the optical signal is separated for each core and for each polarization.

[0009] The LD 253 outputs CW light. The optical amplifier 254 amplifies the CW light. The optical coupler 255 outputs the amplified CW light to the coherent receiver 252. The coherent receiver 252 coherently receives the optical signals separated by the FIFO device 251 for each mode using the CW light input from the optical coupler 217. The ADC 256 converts the coherently received signals into digital signals.

[0010] The optical receiver 250 performs common static distortion compensation for each mode on the signals converted into the digital domain. Here, chromatic dispersion compensation (CDC) is considered as static distortion compensation. The CDC filter 257 performs chromatic dispersion compensation on the coherently received signals.

[0011] The MIMO filter 258 performs MIMO signal processing on the signal, for which static distortion has been compensated for for each mode, to compensate for inter-mode coupling and inter-mode dispersion that occurs in the transmission path. Inter-mode coupling in the transmission path varies over time. For this reason, inter-mode coupling in the transmission path is compensated for by a MIMO filter whose coefficients are adaptively controlled. The decoder 259 performs decoding processing, including symbol decision and error correction, on the output of the MIMO filter 258, and outputs received data that has been restored from the transmitted data. [Prior art documents] [Non-patent literature]

[0012] [Non-Patent Document 1] S. Randel et al., “6×56-Gb / s mode-division multiplexed transmission over 33-km few-mode fiber enabled by 6×6 MIMO equalization,” Opt. Express 19(17), 16697 (2011). [Non-Patent Document 2] K. Shibahara et al., “MIMO carrier phase recovery for carrier-asynchronous SDM-MIMO reception based on the extended Kalman filter,” Opt. Express 29(11), 17111 (2021). [Summary of the Invention] [Problems to be Solved by the Invention]

[0013] Here, the optical modulator 214 on the transmission side and the coherent receiver 252 on the reception side each require an input of CW light in a specific single polarization state as a light source for modulation and local oscillator (LO) light at the time of coherent reception. Further, in a combined spatial multiplexing optical transmission system that performs MIMO signal processing on the reception side, in order to obtain good reception characteristics, as described in Non-Patent Document 1, both the optical modulator 214 on the transmission side and the coherent receiver 252 on the reception side require CW light common to all modes. Furthermore, the intensity of the CW light input to the optical modulator 214 and the coherent receiver 252 is required to be a light intensity of a certain level or more, respectively.

[0014] To meet the above requirements, in the combined spatial multiplexing optical transmission system 200, as shown in FIG. 11, on the transmission side, the CW light output from one LD 215 is amplified by an optical amplifier 216, and the amplified CW light is branched using an optical coupler 217. The branched CW light is supplied to each optical modulator 214. The optical amplifier 216 and the optical coupler 217 need to be polarization-maintaining devices in order to meet the requirements for the polarization state of the input to the optical modulator 214.

[0015] The same situation as described above also applies to the LO light on the receiving side. That is, in the optical receiver 250, the optical amplifier 254 used for amplifying the CW light output from the LD 253 and the optical coupler 255 used for branching the CW light need to be polarization-maintaining devices. Generally, polarization-maintaining optical amplifiers and optical couplers are expensive devices.

[0016] Also, the branching loss in the optical couplers 217 and 255 increases as the spatial multiplexing number increases. For this reason, when increasing the spatial multiplexing number, additional amplification of the optical intensity by a polarization-maintaining optical amplifier may be required. Thus, in the combined spatial multiplexing optical transmission system, on both the transmission side and the receiving side, expensive polarization-maintaining optical devices are required to supply common CW light in all modes, and the optical system becomes complicated.

[0017] Regarding the above problem, Non-Patent Document 2 describes a configuration of a spatial multiplexing optical receiver in which CW light independent for each mode is used as the LO light on the receiving side. In Patent Document 2, an extended Kalman filter type carrier phase compensation is used in combination with MIMO signal processing in order to enable the use of CW light independent for each mode as the LO light. However, this method has a problem that the computational amount of signal processing increases due to the extended Kalman filter type carrier phase compensation.

[0018] One object of the present disclosure is to provide a spatial multiplexed optical receiver, a spatial multiplexed optical transmission system, a spatial multiplexed optical reception method, and a program that can reduce the cost of a receiver in a spatial multiplexed optical transmission system.

Means for Solving the Problems

[0019] The spatial multiplexed optical receiver according to the first aspect of the present disclosure includes a plurality of coherent receivers that coherently receive each of the signals of a plurality of modes that are spatially multiplexed and transmitted, using a continuous wave light independent for each mode as local oscillator light, and for each of the signals of the plurality of modes that have been coherently received, a plurality of frequency offset compensators that independently perform frequency offset compensation for each mode based on the correlation between a known training signal and the signal of each mode, and a MIMO signal processing unit that performs MIMO signal processing on the signals of the plurality of modes for which the frequency offset compensation has been performed in the frequency offset compensator.

[0020] The spatial multiplexed optical transmission system according to the second aspect of the present disclosure includes a receiver including the above spatial multiplexed optical receiver, and a transmitter that transmits the signals of the plurality of modes to the receiver via a transmission path.

[0021] The spatial multiplexed optical reception method according to the third aspect of the present disclosure includes coherently receiving each of the signals of a plurality of modes that are spatially multiplexed and transmitted, using a continuous wave light independent for each mode as local oscillator light, independently performing frequency offset compensation for each mode based on the correlation between a known training signal and the signal of each mode for each of the signals of the plurality of modes that have been coherently received, and performing MIMO signal processing on the signals of the plurality of modes for which the frequency offset compensation has been performed.

[0022] The program according to the fourth aspect of the present disclosure causes a processor to execute a process including performing frequency offset compensation for each mode independently based on the correlation between a known training signal and the signal of each mode for each of the signals of a plurality of modes that are spatially multiplexed and coherently received using continuous light waves independent for each mode as local oscillator light, and performing MIMO signal processing on the signals of the plurality of modes for which the frequency offset compensation has been performed.

Advantages of the Invention

[0023] The spatial multiplexing optical receiver, spatial multiplexing optical transmission system, spatial multiplexing optical reception method, and program according to the present disclosure can reduce the cost of a receiver in a spatial multiplexing optical transmission system.

Brief Description of the Drawings

[0024]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Embodiments for Carrying Out the Invention

[0025] Prior to the description of the embodiments of the present disclosure, an overview of the present disclosure will be described. FIG. 1 shows an example of a schematic configuration of a spatial multiplexing optical transmission system according to the present disclosure. The spatial multiplexing optical transmission system 10 includes a transmitter 11 and a receiver 15. In the spatial multiplexing optical transmission system 10, the transmitter 11 and the receiver 15 are connected via a transmission line 13. The transmission line 13 includes, for example, a coupled multi-core fiber.

[0026] The transmitter 11 transmits signals of a plurality of modes to the receiver 15 via the transmission line 13. The receiver 15 includes a spatial multiplexing optical receiver 20. The spatial multiplexing optical receiver 20 receives signals of a plurality of modes that are spatially multiplexed and transmitted via the transmission line 13.

[0027] FIG. 2 shows an example of a schematic configuration of the spatial multiplexing optical receiver 20. The spatial multiplexing optical receiver 20 includes a plurality of coherent receivers 21, a frequency offset compensator 22, and a MIMO signal processing unit 23.

[0028] The coherent receiver 21 coherently receives each of the signals of a plurality of modes that are spatially multiplexed and transmitted, using independently generated CW light for each mode as the LO light. The frequency offset compensator 22 performs frequency offset compensation for each of the coherently received signals of a plurality of modes, independently for each mode, based on the correlation between a known training signal and the signal of each mode. The MIMO signal processing unit 23 performs MIMO signal processing on the signals of a plurality of modes for which frequency offset compensation has been performed.

[0029] In the present disclosure, the coherent receiver 21 uses independent CW light for each mode as LO light to coherently receive each of the signals in multiple modes that are spatially multiplexed and transmitted through the transmission line. The frequency offset compensator 22 performs frequency offset compensation for each of the coherently received signals in multiple modes based on the correlation between a known training signal and the signal in each mode. In this manner, the frequency offset compensator 22 can remove frequency offsets that may differ for each mode and are contained in the coherently received signals. Therefore, in the present disclosure, the spatial multiplexing optical receiver 20 does not require an expensive polarization-maintaining device, thereby reducing the cost of receivers used in spatial multiplexing optical transmission systems.

[0030] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Note that the following description and drawings have been omitted and simplified as appropriate for clarity of explanation. In addition, in each drawing, the same or similar elements are designated by the same reference numerals, and duplicate explanations are omitted as necessary.

[0031] FIG. 3 shows an example configuration of a communication system according to the present disclosure. An embodiment of the present disclosure will be described with reference to FIG. 3. In this embodiment, a communication system 100 is a coupled spatial multiplexing optical transmission system. The communication system 100 includes a transmitter 110, a transmission path 130, and a receiver 150. The communication system 100 corresponds to the spatial multiplexing optical transmission system 10 shown in FIG. 1. The transmitter 110 corresponds to the transmitter 11 shown in FIG. 1. The transmission path 130 corresponds to the transmission path 13 shown in FIG. 1. The receiver 150 corresponds to the receiver 15 shown in FIG. 1.

[0032] In this embodiment, it is assumed that the communication system 100 is a spatial multiplexing optical transmission system using a 2-core combined multi-core fiber. Also, it is assumed that the communication system 100 is a spatial multiplexing optical transmission system that employs the Quadrature amplitude modulation (QAM) scheme for the modulation method of the signals to be transmitted and received and performs coherent reception. In this case, in the communication system 100, a total of four modes are used to transmit optical signals by combining two polarization waves and two spatial channels (cores). The spatial multiplexing technology can be used in combination with the wavelength multiplexing technology. However, hereinafter, for simplicity of explanation, an optical transmission system with one wavelength channel will be described.

[0033] The transmitter 110 includes an encoding unit 111, a pre-equalization unit 112, a plurality of DACs 113, a plurality of optical modulators 114, an LD 115, an optical amplifier 116, an optical coupler 117, and a FIFO device 118. The encoding unit 111 encodes the transmission data and maps the encoded transmission data to a QAM signal. The pre-equalization unit 112 compensates for the distortion that occurs in the optical signal within the transmitter 110.

[0034] Each DAC 113 converts the signal of each mode output from the pre-equalization unit 112 from an electrical signal in the digital domain to an electrical signal in the analog domain. In this embodiment, in the transmitter 110, signals of the I component and the Q component are generated for each of the four modes. That is, a total of eight signals are generated in the transmitter 110. In FIG. 3, the signals of the I component and the Q component of each mode are represented as one line together.

[0035] The LD 115 outputs CW light. The optical amplifier 116 amplifies the CW light output from the LD 115 to a predetermined level. The optical coupler 117 branches the CW light amplified by the optical amplifier 116 and inputs the branched CW light to each of the plurality of optical modulators 114. A polarization-maintaining optical amplifier is used for the optical amplifier 116. Also, a polarization-maintaining optical coupler is used for the optical coupler 117.

[0036] Each optical modulator 114 modulates the CW light input from the optical coupler 117 using two electrical signals of the I component and the Q component output from the DAC 113, and generates optical signals of each mode. When a polarization multiplexed optical modulator is used for the optical modulator 114, four electrical signals, namely, the I component and the Q component of the X polarization and the I component and the Q component of the Y polarization, are input to the optical modulator 114 for each spatial channel. In that case, the optical modulator 114 generates a polarization multiplexed optical signal.

[0037] The FIFO device 118 is a FIFO device used as a fan-in. Optical signals corresponding to spatial channels, here optical signals corresponding to each core, are input to the FIFO device 118. The FIFO device 118 outputs the optical signals corresponding to each core to the corresponding core of the transmission line 130 which is a multi-core optical fiber transmission line. The configuration and operation of the transmitter 110 may be the same as the configuration and operation of the optical transmitter 210 shown in FIG. 11.

[0038] The transmission line 130 is a multi-core optical fiber transmission line and includes a coupled two-core fiber 131 and a two-core optical amplifier 132. The transmission line 130 has, for example, a set of a coupled two-core fiber 131 and a two-core optical amplifier 132 connected in series with each other for each span.

[0039] The receiver 150 has a FIFO device 151 and a space-division multiplexed optical receiver 152. In the receiver 150, the FIFO device 151 is a FIFO device used as a fan-out. The FIFO device 151 separates the optical signals transmitted through the transmission line 130 for each mode. In the present embodiment, the FIFO device 151 separates the optical signals for each spatial channel, that is, for each core. The FIFO device 151 outputs the separated optical signals to the space-division multiplexed optical receiver 152. The space-division multiplexed optical receiver 152 corresponds to the space-division multiplexed optical receiver 20 shown in FIG. 1.

[0040] FIG. 4 shows a configuration example of the spatial multiplexing optical receiver 152. The spatial multiplexing optical receiver 152 includes a plurality of LD 161, a plurality of coherent receivers 162, a plurality of ADCs 163, a plurality of CDC filters 164, a plurality of frequency offset compensators 165, a MIMO filter 166, and a decoding unit 167.

[0041] The plurality of LD 161 supply CW light independent for each mode to the coherent receiver 162. Each of the plurality of LD 161 is arranged independently for each mode. CW light output from an LD 161 independent for each mode or for each spatial channel is supplied as LO light to each of the plurality of coherent receivers 162. Each coherent receiver 162 coherently receives a signal separated for each mode or for each spatial channel by the FIFO device 151 using the CW light output from the corresponding LD 161. In the present embodiment, in the spatial multiplexing optical receiver 152, an LD 161 independent for each mode or for each spatial channel is used, and different from the transmitter 110, a polarization-maintaining optical amplifier and a polarization-maintaining optical coupler are not necessary. The coherent receiver 162 corresponds to the coherent receiver 21 shown in FIG. 2.

[0042] Each ADC 163 converts a signal coherently received for each mode or for each spatial channel from an analog signal to a digital signal. Each of the plurality of CDC filters 164 is configured as a filter that performs static distortion compensation for each mode. Each CDC filter 164 performs wavelength dispersion compensation, which is static distortion compensation, for the signal converted to a digital signal for each mode or for each spatial channel. The spatial multiplexing optical receiver 152 may have a matched filter or a filter for compensating for the incompleteness of the receiver device instead of or in addition to the CDC filter 164.

[0043] The frequency offset compensator 165 performs frequency offset compensation for each mode or spatial channel on the signal that has undergone static distortion compensation for each mode or spatial channel. In this embodiment, the frequency offset compensator 165 performs frequency offset compensation for the received signal before MIMO signal processing, independently for each mode or spatial channel, based on correlation with a known training signal.

[0044] For example, the frequency offset compensator 165 applies a phase rotation corresponding to each of a plurality of frequency offset amounts to the coherently received signal. For example, the frequency offset compensator 165 applies a phase rotation corresponding to each frequency offset amount to the coherently received signal while changing the frequency offset amount within a predetermined sweep range. The frequency offset compensator 165 calculates the cross-correlation between the signal to which the phase rotation has been applied and a known training signal, and detects the strength of the cross-correlation peak.

[0045] The frequency offset compensator 165 determines the amount of frequency offset compensation based on the strength of the peak of cross-correlation detected for each of the multiple frequency offset amounts. For example, the frequency offset compensator 165 compares the strength of the peak detected for each of the multiple frequency offset amounts and determines the amount of frequency offset compensation based on the comparison result. For example, the frequency offset compensator 165 determines the frequency offset amount with the largest peak strength among the multiple frequency offset amounts as the amount of frequency offset compensation. The frequency offset compensator 165 corresponds to the frequency offset compensator 22 shown in FIG. 2.

[0046] The MIMO filter 166 performs MIMO signal processing for compensating for inter-mode coupling and inter-mode dispersion that occur in the transmission line with respect to signals for which frequency offset compensation has been performed for each mode or each spatial channel. The MIMO filter 166 is an adaptive filter whose coefficients are adaptively controlled. The MIMO signal processing performed by the MIMO filter 166 includes, for example, carrier phase compensation of a phase-locked loop type. The MIMO filter 166 corresponds to the MIMO signal processing unit 23 shown in FIG. 2.

[0047] The decoding unit 167 performs decoding processing on the signal output from the MIMO filter 166. The decoding processing includes, for example, symbol determination processing and error correction processing. The decoding unit 167 generates received data in which the transmitted data is restored by the decoding processing.

[0048] In the combined spatial multiplexing optical transmission system shown in FIG. 3, CW light common to all modes is used as a light source for modulation on the transmission side. On the other hand, on the reception side, independent CW light is used as LO light for each mode or each spatial channel. Therefore, the signals received coherently for each mode include a frequency offset reflecting the instantaneous frequency of each independent CW light on the reception side. In the present embodiment, the frequency offset compensator 165 performs frequency offset compensation so as to remove different frequency offsets for each mode or each spatial channel in the stage before the MIMO signal processing. By doing so, the influence of using the independent LD 161 for each mode on the reception side is mitigated.

[0049] Note that the influence of inter-mode coupling and inter-mode dispersion that occur in the transmission line remains in the signal before the MIMO signal processing. For this reason, the frequency offset compensation performed before the MIMO signal processing is required to be robust against the influence of inter-mode coupling and inter-mode dispersion.

[0050] FIG. 5 shows a configuration example of the frequency offset compensator 165. The frequency offset compensator 165 includes a phase rotator 171, a resampling unit 172, a cross-correlation calculation unit 173, a peak intensity detection unit 174, a frequency offset control unit 175, and a peak position detection unit 176. The configuration shown in FIG. 5 corresponds to the configuration of the frequency offset compensator 165 corresponding to one mode or spatial channel.

[0051] The phase rotator 171 receives the signal subjected to static distortion compensation output from the CDC filter 164. The phase rotator 171 rotates the phase of the input signal corresponding to the frequency offset amount output from the frequency offset control unit 175. The resampling unit 172 resamples the output signal of the phase rotator 171 into a signal having the same sampling rate as the known training signal. Usually, the training signal is a transmission symbol sequence, and the sampling rate of the training signal matches the symbol rate. The cross-correlation calculation unit 173 calculates the cross-correlation between the resampled signal and the known training signal. The peak intensity detection unit 174 detects the intensity of the peak for the calculated cross-correlation.

[0052] The frequency offset control unit 175 changes the frequency offset amount with an appropriate resolution over a predetermined range. In other words, the frequency offset control unit 175 sweeps the frequency offset amount within a predetermined range of the frequency offset amount. The peak intensity detection unit 174 calculates the intensity of the peak of the cross-correlation for each of the swept frequency offset amounts. When the frequency offset amount is swept, the frequency offset amount at which the peak of the cross-correlation with the maximum magnitude is obtained is considered to be the most likely frequency offset compensation amount for the signal of the corresponding mode. The frequency offset control unit 175 outputs the frequency offset amount at which the peak of the cross-correlation with the maximum magnitude is obtained to the phase rotator 171, and causes the phase rotator 171 to perform frequency offset compensation.

[0053] After the implementation of frequency offset compensation, the peak position detector 176 detects the time position of the peak, i.e., the peak position, for the cross-correlation calculated by the cross-correlation calculation unit 173. The detected peak position includes information on the time shift between the received signal and the transmitted training signal. The information on the detected peak position is used for frame synchronization. The peak position detector 176 inputs the information on the detected peak position to a frame synchronization unit (not shown in FIG. 3). Thereby, frame synchronization required for data-aided type MIMO signal processing is implemented. Frame synchronization is realized as adjustment of the delay for the signal after frequency offset compensation or adjustment of the delay for the training signal in data-aided type MIMO signal processing.

[0054] FIG. 6 shows another example of the configuration of the frequency offset compensator. In this example, the frequency offset compensator 165a includes a phase rotator 171, a resampling unit 172, a cross-correlation calculation unit 173, a peak intensity detector 174, a frequency offset control unit 175, and a peak position detector 176 for each mode. The frequency offset compensator 165a also includes a peak position detector 176 and an averaging unit 177 common to a plurality of modes. In the frequency offset compensator 165a shown in FIG. 6, the operation of compensating the frequency offset in each mode may be the same as the operation of the frequency offset compensator 165 shown in FIG. 5.

[0055] In the frequency offset compensator 165a, after the frequency offset compensation amount is determined for each mode, the averaging unit 177 averages the cross-correlations calculated by the cross-correlation calculation units 173 of each mode. The peak position detector 176 detects the position of the peak in the cross-correlation averaged by the averaging unit 177. By doing so, the peak position detector 176 can reduce the influence of noise during peak position detection and detect the position of the peak with high accuracy.

[0056] Next, the operation procedure will be described. FIG. 7 shows the operation procedure of the frequency offset compensator 165. The operation procedure of the frequency offset compensator 165 constitutes a part of the space division multiplexed optical reception method. Each of the plurality of coherent receivers 162 coherently receives the signals of each mode using independent CW light for each mode. The signals of each mode coherently received by the coherent receiver 162 are input to the frequency offset compensator 165 via the ADC 163 and the CDC filter 164.

[0057] In the frequency offset compensator 165, the frequency offset control unit 175 outputs a predetermined frequency offset amount to the phase rotator 171 (step S1). The phase rotator 171 rotates the phase of the input signal corresponding to the frequency offset amount output by the frequency offset control unit 175 (step S2).

[0058] The resampling unit 172 resamples the output signal of the phase rotator 171 to a signal having the same sampling rate as the known training signal. The cross-correlation calculation unit 173 calculates the cross-correlation between the resampled signal and the known training signal (step S3). The peak intensity detection unit 174 detects the intensity of the peak with respect to the calculated cross-correlation (step S4).

[0059] The frequency offset control unit 175 determines whether or not the sweep of the frequency offset amount has ended (step S5). If the frequency offset control unit 175 determines that the sweep of the frequency offset amount has not ended, it changes the frequency offset amount output to the phase rotator 171 (step S6). Thereafter, the process returns to step S2, and the phase rotator 171 rotates the phase of the input signal corresponding to the frequency offset amount output by the frequency offset control unit 175.

[0060] Steps S2 to S6 are repeatedly executed until it is determined in step S5 that the sweep of the frequency offset amount has ended. In other words, the frequency offset compensator 165 repeatedly performs phase rotation, calculation of cross-correlation, and detection of peak intensity within a sweep range of a predetermined frequency offset amount.

[0061] When it is determined in step S5 that the sweep of the frequency offset amount has ended, the frequency offset control unit 175 determines a frequency offset compensation amount based on the intensity of the peak detected by the peak intensity detection unit 174 (step S7). In step S7, the frequency offset control unit 175 determines, for example, the frequency offset amount at which a peak of the maximum magnitude of cross-correlation is obtained as the frequency offset compensation amount. The frequency offset control unit 175 outputs a frequency offset amount indicating the determined frequency offset compensation amount to the phase rotator 171. The phase rotator 171 outputs a signal with the frequency offset compensated to the MIMO filter 166.

[0062] The cross-correlation calculation unit 173 calculates the cross-correlation between the signal with the frequency offset compensated and the training signal (step S8). The peak position detection unit 176 detects the position of the peak with respect to the cross-correlation calculated by the cross-correlation calculation unit 173 after the frequency offset compensation is performed (step S9). The peak position detection unit 176 outputs the detected peak position to a frame synchronization unit (not shown) (step S10).

[0063] In this embodiment, in the spatial multiplexing optical receiver 152, the coherent receiver 162 coherently receives signals of a plurality of spatially multiplexed modes, using independent CW light for each mode as the LO light. In this case, the signals coherently received for each mode include a frequency offset reflecting the instantaneous frequency of the CW light independent for each mode. The frequency offset compensator 165 compensates for the frequency offset for each mode independently, based on the correlation between the coherently received signal and a known training signal, before MIMO signal processing is performed in the MIMO filter 166. By doing so, the frequency offset compensator 165 can remove the frequency offsets that are different for each mode or each spatial channel included in the coherently received signal, and good reception characteristics can be obtained.

[0064] In this embodiment, since the LD 161 independent for each mode can be used, it is not necessary to amplify the CW light output from one laser light source with an optical amplifier and distribute it to all the coherent receivers 162 using an optical coupler. When the CW light output from one laser light source is distributed to a plurality of coherent receivers 162, polarization-maintaining devices are used for the optical amplifier and the optical coupler, respectively. Generally, polarization-maintaining optical amplifiers and optical couplers are expensive. In this embodiment, good reception characteristics can be obtained without requiring an expensive polarization-maintaining device. Therefore, this embodiment can reduce the cost of the receiver used in the spatial multiplexing optical transmission system.

[0065] The inventor performed simulations to verify the operation of the spatial multiplexing optical receiver 152. In the simulations, a 100 km single-span transmission of a coupled 2-core fiber was assumed. The transmitted signal was a signal obtained by polarization multiplexing and spatial multiplexing a 32 Gbaud Quadrature phase shift keying (QPSK) signal. The wavelength dispersion of the coupled 2-core fiber was set to 17 ps / nm / km, and the mode dispersion was set to 5 ps / √km. Nonlinear effects in the fiber were ignored. After fiber transmission, additive white Gaussian noise (AWGN) was added to set the received OSNR to 30 dB / 0.1 nm.

[0066] Regarding the light source on the transmission side, the linewidth was set to 100 kHz, and a common light source was used for all modes. Regarding the light source on the reception side, the linewidth was 100 kHz, and a light source having phase noise and frequency offset independent for each mode was used. The frequency offset between the light source on the transmitter side and the light source on the receiver side was set to 100 MHz, and an additional 10 MHz frequency offset was given between the light sources corresponding to the two cores on the reception side. The sampling rate of the ADC on the reception side was 2-fold oversampling. The MIMO signal processing was performed using an adaptive MIMO finite impulse response (FIR) filter, and the filter length of the FIR filter was set to T / 2 101 taps. The filter coefficients were first converged by a data-aided least-mean square (LMS) algorithm, and then switched to a decision-directed LMS for evaluation.

[0067] First, to confirm the problem of simply using an independent light source on the receiving side as the LO light, the received signals of each mode after MIMO signal processing were evaluated when frequency offset compensation was not performed by the frequency offset compensator 165. FIG. 8 shows the constellations of the received signals of each mode after MIMO signal processing when frequency offset compensation is not performed by the frequency offset compensator 165. In FIG. 8, mode 1 corresponds to the X polarization of core 1, and mode 2 corresponds to the Y polarization of core 1. Also, mode 3 corresponds to the X polarization of core 2, and mode 4 corresponds to the Y polarization of core 2. In the constellations shown in FIG. 8, it can be seen that the transmitted QPSK signals cannot be confirmed at all, indicating that demodulation has failed.

[0068] Next, the received signals of each mode after MIMO signal processing were evaluated when signal processing including frequency offset compensation for each mode was performed. FIG. 9 shows the constellations of the received signals of each mode after MIMO signal processing when frequency offset compensation was performed by the frequency offset compensator 165. Referring to FIG. 9, although the phase noise of the light source and the influence of AWGN remain in the received signals, the symbol points of the transmitted QPSK can be clearly confirmed, indicating that the spatial multiplexing optical receiver is functioning.

[0069] In the above embodiment, the CDC filter 164, the frequency offset compensator 165, the MIMO filter 166, and the decoding unit 167 can each be configured using any digital signal processing circuit. FIG. 10 shows a configuration example of a digital signal processing circuit. The digital signal processing circuit 400 is configured as a circuit including one or more processors 410 and one or more memories 420. In the digital signal processing circuit 400, one or more processors 410 read a program stored in one or more memories 420 and execute processing on the read program. Thereby, at least a part of the functions of the CDC filter 164, the frequency offset compensator 165, the MIMO filter 166, or the decoding unit 167 can be realized.

[0070] When the above program is loaded into a computer or a processor, it includes a set of instructions (or software code) for causing the computer to perform one or more functions described in the embodiments. The program may be stored in a non-transitory computer-readable medium or a tangible storage medium. By way of example and not limitation, the computer-readable medium or tangible storage medium includes random-access memory (RAM), read-only memory (ROM), flash memory, solid-state drive (SSD) or other memory technologies, Compact Disc (CD), digital versatile disc (DVD), Blu-ray (registered trademark) disc or other optical disc storage, magnetic cassette, magnetic tape, magnetic disk storage or other magnetic storage devices. The program may also be transmitted on a transitory computer-readable medium or a communication medium. By way of example and not limitation, the transitory computer-readable medium or communication medium includes electrical, optical, acoustic, or other forms of propagated signals.

[0071] In the above embodiment, an example in which the space-division multiplexing optical receiver 152 is used in a space-division multiplexing optical transmission system using a coupled multi-core fiber has been described. However, the present disclosure is not limited thereto. The space-division multiplexing optical receiver 152 can be used as a receiver that performs space-division multiplexing or diversity reception not only in an optical fiber transmission system but also in an optical space communication system such as between satellites or between a satellite and the ground.

[0072] The present disclosure has been described with reference to the embodiments, but the present disclosure is not limited to the above-described embodiments. Various changes 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. And each embodiment can be combined with other embodiments as appropriate.

[0073] The drawings are merely illustrative for explaining one or more embodiments. Each drawing may be associated with not only one specific embodiment but also one or more other embodiments. As can be understood by those skilled in the art, various features or steps described with reference to any one drawing can be combined with features or steps shown in one or more other drawings to create, for example, embodiments not explicitly illustrated or described. Not all of the features or steps shown in any one drawing for explaining exemplary embodiments are necessarily essential, and some features or steps may be omitted. The order of the steps described in any drawing may be changed as appropriate.

[0074] Some or all of the above embodiments may be described as follows, but are not limited thereto.

[0075] [Appendix 1] A plurality of coherent receivers that coherently receive each of a plurality of modes of signals transmitted through spatial multiplexing, using a continuous-wave light independent for each mode as local oscillator light; For each of the coherently received signals of the plurality of modes, a plurality of frequency offset compensators that independently perform frequency offset compensation for each mode based on the correlation between a known training signal and the signal of each mode; A spatial multiplexing optical receiver comprising a MIMO signal processing unit that performs multi-input multi-output (MIMO) signal processing on the signals of the plurality of modes for which the frequency offset compensation has been performed in the frequency offset compensator.

[0076] [Appendix 2] The frequency offset compensator according to Appendix 1, which for each of a plurality of frequency offset amounts, gives a phase rotation corresponding to each frequency offset amount to the coherently received signal, calculates the cross-correlation between the signal given the phase rotation and the training signal, and determines the frequency offset compensation amount based on the intensity of the peak of the cross-correlation.

[0077] [Appendix 3] The frequency offset compensator is the space-division multiplexed optical receiver according to Appendix 2, which gives a phase rotation corresponding to each frequency offset amount to the coherently received signal while changing the frequency offset amount within a predetermined sweep range.

[0078] [Appendix 4] The frequency offset compensator compares the intensities of the peaks detected for each of the plurality of frequency offset amounts, and determines the frequency offset compensation amount based on the comparison result of the intensities of the detected peaks, which is the space-division multiplexed optical receiver according to Appendix 2 or 3.

[0079] [Appendix 5] The frequency offset compensator determines, as the frequency offset compensation amount, the frequency offset amount having the largest peak intensity among the plurality of frequency offset amounts, which is the space-division multiplexed optical receiver according to any one of Appendices 2 to 4.

[0080] [Appendix 6] The frequency offset compensator corresponds to each of the plurality of modes, a frequency offset control unit that controls the frequency offset amount, a phase rotator that gives a phase rotation corresponding to the frequency offset amount output from the frequency offset control unit to the coherently received signal, a cross-correlation calculation unit that calculates the cross-correlation between the signal output from the phase rotator and the training signal, and a peak intensity detection unit that detects the intensity of the peak in the calculated cross-correlation, which is the space-division multiplexed optical receiver according to any one of Appendices 2 to 5.

[0081] [Appendix 7] The frequency offset control unit outputs a plurality of frequency offset amounts to the phase rotator, The peak intensity detector detects the intensity of the peak in the calculated cross-correlation for each of the plurality of frequency offset amounts. The frequency offset controller determines a frequency offset compensation amount based on the intensity of the peak detected for each of the plurality of frequency offset amounts, and outputs the determined frequency offset amount as the frequency offset compensation amount to the phase rotator. The spatial multiplexing optical receiver according to supplementary note 6.

[0082] [Supplementary Note 8] The frequency offset compensator further includes a peak position detector that detects a peak position with respect to the cross-correlation calculated by the cross-correlation calculation unit after the frequency offset compensation amount is determined. The spatial multiplexing optical receiver according to supplementary note 6 or 7.

[0083] [Supplementary Note 9] The frequency offset compensator further includes an averaging unit that averages the cross-correlations calculated for each of the plurality of modes. The peak position detector detects the position of the peak in the averaged cross-correlation. The spatial multiplexing optical receiver according to supplementary note 8.

[0084] [Supplementary Note 10] The position of the peak detected by the peak position detector is used for frame synchronization. The spatial multiplexing optical receiver according to supplementary note 8 or 9.

[0085] [Supplementary Note 11] The continuous wave lights independent for each mode are supplied to the coherent receiver from laser light sources arranged independently for each mode. The spatial multiplexing optical receiver according to any one of supplementary notes 1 to 10.

[0086] [Supplementary Note 12] A filter for compensating for static distortion included in the coherently received signal is further provided in front of the frequency offset compensator for each of the signals of the plurality of modes received coherently. The spatial multiplexing optical receiver according to any one of supplementary notes 1 to 11.

[0087] [Appendix 13] 13. The spatial multiplexing optical receiver according to claim 12, wherein the filter that compensates for static distortion includes a filter that performs chromatic dispersion compensation.

[0088] [Appendix 14] A receiver including the spatial multiplexing optical receiver according to any one of Supplementary Notes 1 to 13; a transmitter that transmits the signals of the plurality of modes to the receiver via a transmission path.

[0089] [Appendix 15] Each of the spatially multiplexed and transmitted signals in multiple modes is coherently received using independent continuous wave light for each mode as local oscillator light, performing frequency offset compensation for each of the coherently received signals in the plurality of modes independently for each mode based on a correlation between a known training signal and the signal in each mode; A spatial multiplexing optical receiving method comprising: performing multi-input multi-output (MIMO) signal processing on the signals of the plurality of modes that have been frequency offset compensated.

[0090] [Appendix 16] performing frequency offset compensation for each of the signals in a plurality of modes that are spatially multiplexed and transmitted and that are coherently received using continuous wave light that is independent for each mode as local oscillator light, based on the correlation between a known training signal and the signal in each mode, independently for each mode; A program for causing a processor to execute processing including performing multi-input multi-output (MIMO) signal processing on signals of multiple modes that have undergone frequency offset compensation.

[0091] Some or all of the elements (e.g., configuration and function) described in Supplementary Notes 2 to 13 that are subordinate to Supplementary Note 1 may be subordinate to Supplementary Notes 15 and 16 in the same subordinate relationship as Supplementary Notes 2 to 13. Some or all of the elements described in any supplementary note may be applied to various hardware, software, recording means for recording software, systems, and methods.

Explanation of Signs

[0092] 10: Spatial multiplexing optical transmission system 11: Transmitter 13: Transmission path 15: Receiver 20: Spatial multiplexing optical receiver 21: Coherent receiver 22: Frequency offset compensator 23: MIMO signal processing unit 100: Communication system 110: Transmitter 111: Encoding unit 112: Equalization unit 113: DAC 114: Optical modulator 115: LD 116: Optical amplifier 117: Optical coupler 118: FIFO device 130: Transmission path 131: Coupled two-core fiber 132: Two-core optical amplifier 150: Receiver 151: FIFO device 152: Spatial multiplexing optical receiver 161: LD 162: Coherent receiver 163: ADC 164: CDC filter 165: Frequency offset compensator 166: MIMO filter 167: Decoding unit 171: Phase rotator 172: Resampling unit 173: Cross-correlation calculation unit 174: Peak intensity detector 175: Frequency offset control section 176: Peak position detector 177: Averaging section

Claims

1. A plurality of coherent receivers that coherently receive each of the signals of a plurality of modes transmitted with spatial multiplexing, using a continuous light wave independent for each mode as local oscillator light; A plurality of frequency offset compensators that independently perform frequency offset compensation for each mode based on the correlation between a known training signal and the signal of each mode, for each of the signals of the plurality of modes coherently received; A spatial multiplexed optical receiver comprising a MIMO signal processing unit that performs multi-input multi-output (MIMO) signal processing on the signals of the plurality of modes for which the frequency offset compensation has been performed in the frequency offset compensator.

2. The frequency offset compensator gives a phase rotation corresponding to each frequency offset amount to the coherently received signal for each of the plurality of frequency offset amounts, calculates the cross-correlation between the signal given the phase rotation and the training signal, and determines the frequency offset compensation amount based on the intensity of the peak of the cross-correlation. The spatial multiplexed optical receiver according to claim 1.

3. The frequency offset compensator gives a phase rotation corresponding to each frequency offset amount to the coherently received signal while changing the frequency offset amount within a predetermined sweep range. The spatial multiplexed optical receiver according to claim 2.

4. The frequency offset compensator corresponds to each of the plurality of modes, A frequency offset control unit that controls the frequency offset amount; A phase rotator that gives a phase rotation corresponding to the frequency offset amount output from the frequency offset control unit to the coherently received signal; A cross-correlation calculation unit that calculates the cross-correlation between the signal output from the phase rotator and the training signal; The spatial multiplexed optical receiver according to claim 2 or 3, comprising a peak intensity detection unit that detects the intensity of the peak in the calculated cross-correlation.

5. The frequency offset control unit outputs a plurality of frequency offset amounts to the phase rotator, The peak intensity detection unit detects the intensity of the peak in the calculated cross-correlation for each of the plurality of frequency offset amounts, The frequency offset control unit determines a frequency offset compensation amount based on the intensity of the peak detected for each of the plurality of frequency offset amounts, and outputs the determined frequency offset amount as the frequency offset compensation amount to the phase rotator. The spatial multiplexing optical receiver according to claim 4.

6. After the frequency offset compensation amount is determined, the frequency offset compensator further includes a peak position detector that detects a peak position with respect to the cross-correlation calculated by the cross-correlation calculation unit. The frequency offset compensator further includes an averaging unit that averages the cross-correlations calculated in each of the plurality of modes. The peak position detector detects the position of a peak in the averaged cross-correlation. The spatial multiplexing optical receiver according to claim 5.

7. The continuous wave lights independent for each mode are supplied from laser light sources arranged independently for each mode to the coherent receiver. The spatial multiplexing optical receiver according to any one of claims 1 to 3.

8. A receiver including the spatial multiplexing optical receiver according to any one of claims 1 to 3, And a transmitter that transmits signals of the plurality of modes to the receiver via a transmission line. A spatial multiplexing optical transmission system.

9. Each of the signals of the plurality of modes multiplexed and transmitted is coherently received using continuous wave lights independent for each mode as local oscillator lights, For each of the coherently received signals of the plurality of modes, frequency offset compensation is independently performed for each mode based on the correlation between a known training signal and the signal of each mode. A spatial multiplexing optical reception method including performing multi-input multi-output (MIMO) signal processing on the signals of the plurality of modes for which the frequency offset compensation has been performed.

10. For each of the signals of the plurality of modes multiplexed and transmitted, which are coherently received using continuous wave lights independent for each mode as local oscillator lights, frequency offset compensation is independently performed for each mode based on the correlation between a known training signal and the signal of each mode. A program for causing a processor to execute a process including performing multi-input multi-output (MIMO) signal processing on the signals of the plurality of modes for which the frequency offset compensation has been performed.

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