Optical receiver, transmission system, and compensation method

Abstract

This optical receiver comprises a signal equalization unit that compensates for signal waveform distortion using a filter in which a filter coefficient is set so as to compensate for crosstalk in which the amount of crosstalk between IQ lanes is equal to or greater than a threshold value, with respect to a multiplexed signal transmitted from an optical transmitter.　

Description

Optical receiver, transmission system and compensation method

[0001] The present invention relates to an optical receiver, a transmission system, and a compensation method.

[0002] With the development of information technology, the demand for communications that supports it continues to expand. Among these, optical communication using optical fibers enables long-distance, high-capacity communication due to its wide available frequency band and low signal attenuation, and is widely used for modern fixed lines. In recent years, research has been conducted on space division multiplexing (SDM) systems, which transmit and receive signals using the core modes of multicore fiber (MCF), multimode fiber (MMF), and multicore multimode fiber (MCMMF) as independent channels.

[0003] Light propagating through an optical fiber is subject to random birefringence within the fiber, crosstalk between spatial channels, polarization-dependent loss, core-dependent loss, mode-dependent loss, etc. These transmission distortions within the fiber can be reduced on the receiving side by a DxD MIMO (Multiple Input Multiple Output) adaptive filter provided in the digital signal processing unit on the receiving side of the optical transceiver (D: polarization / spatial multiplexing number).

[0004] However, crosstalk between IQ lanes across polarization and spatial channels can occur in the optical front end of an optical transceiver that performs spatial multiplexing. It is impossible to completely compensate for this crosstalk between IQ lanes across polarization and spatial channels using general D×D MIMO processing. Therefore, a technology has been proposed that compensates for crosstalk between any lanes by using an IQ characteristic equalization configuration (receiving-side signal processing) in digital signal processing in the signal equalization section on the receiving side (see, for example, Non-Patent Document 2).

[0005] As an IQ characteristic equalization configuration, the signal equalization unit includes a receiver characteristic compensator, a transmission path characteristic compensator, and a transmitter characteristic compensator. The receiver characteristic compensator compensates for waveform distortion occurring in the receiver. The transmission path characteristic compensator compensates for composite distortions occurring in the transmission path other than chromatic dispersion, such as polarization rotation, polarization mode dispersion, and polarization dependent loss. The transmitter characteristic compensator compensates for waveform distortion occurring in the transmitter. The receiver characteristic compensator and transmitter characteristic compensator perform convolution operations on digital filters (e.g., 2D x 2D MIMO configurations) having matrix-form impulse responses.

[0006] T. Morioka, “New Generation Optical Infrastructure Technologies: “EXAT Initiative” Towards 2020 and Beyond”, in Proc. 2009 14th OptoElectronics and Communications Conference, 2009, pp. 1-2. A. Kawai, et al., “Low-Complexity 4D×D MIMO EqualizerEnabling 2.6-Tb / s / λ SDM Signal Reception over Dynamic 34, NO. 8, APRIL 15, 2016, pp. 1754-1761

[0007] However, in the conventional digital signal processing in the signal equalization unit on the receiving side described above, performing a calculation to compensate for crosstalk between all channels results in an increase in the amount of calculation. Since the amount of calculation increases in proportion to the square of the number of channels D, this becomes a more significant problem in spatial multiplexing transmission with a large number of channels D. As a conventional technique, a technique such as sparse processing has been proposed for D×D MIMO processing (see, for example, Non-Patent Document 3), but no method has been established to deal with IQ characteristic compensation. This problem is not limited to spatial multiplexing systems, but also occurs in other systems equipped with an IQ characteristic equalization configuration.

[0008] In view of the above circumstances, an object of the present invention is to provide a technique that can reduce the amount of calculations in an optical receiver.

[0009] One aspect of the present invention is an optical receiver that includes a signal equalization unit that compensates for signal waveform distortion in a multiplexed signal transmitted from an optical transmitter using a filter whose filter coefficients are set to compensate for crosstalk between IQ lanes when the amount of crosstalk between IQ lanes is equal to or greater than a threshold value.

[0010] One aspect of the present invention is a transmission system including an optical transmitter for transmitting a multiplexed signal and the above-described optical receiver.

[0011] One aspect of the present invention is a compensation method for compensating for signal waveform distortion in a multiplexed signal transmitted from an optical transmitter using a filter having a filter coefficient set to compensate for crosstalk between I and Q lanes when the amount of crosstalk between I and Q lanes is equal to or greater than a threshold value.

[0012] The present invention makes it possible to reduce the amount of calculations in an optical receiver.

[0013] FIG. 1 is a diagram illustrating an example of the configuration of a conventional digital coherent optical transmission system. FIG. 2 is a diagram illustrating an example of the configuration of a conventional transmitter and receiver. FIG. 3 is a diagram for explaining the detailed configuration of a conventional signal equalizer. FIG. 4 is a diagram for explaining an overview of the present invention. FIG. 5 is a diagram illustrating an example of the configuration of a signal equalizer in a first embodiment. FIG. 6 is a diagram for explaining an overview of a third embodiment. FIG. 7 is a diagram illustrating an example of the hardware configuration of a digital signal processing unit in each embodiment.

[0014] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0015] (Conventional Configuration) Before describing the configuration of the present invention, a conventional system configuration will be described as a premise. FIG. 1 is a diagram showing an example configuration of a conventional digital coherent optical transmission system 1. The digital coherent optical transmission system 1 includes a transmitter 10, a WDM multiplexer 20, an optical fiber transmission line 30, a WDM demultiplexer 40, and a receiver 50. The transmitter 10 transmits a polarization multiplexed signal that has been spatially D-multiplexed (D≧2). The spatially D-multiplexed polarization multiplexed signal is transmitted to the receiver 50 via, for example, a multicore fiber or a multimode fiber. The receiver 50 receives the spatially D-multiplexed polarization multiplexed signal from the transmitter 10. Because polarization multiplexing and spatial multiplexing can be treated equally in terms of signal processing, the following description will be given assuming that polarization multiplexing and spatial multiplexing are performed simultaneously.

[0016] The WDM multiplexer 20 multiplexes the optical signals output from each transmitter 100 and outputs the multiplexed signal to the optical fiber transmission line 30. The optical fiber transmission line 30 is provided with any number of optical amplifiers 31. Each optical amplifier 31 receives an optical signal from the optical fiber transmission line 30 on the transmitter 10 side, amplifies the signal, and outputs the signal to the optical fiber transmission line 30 on the receiver 50 side. The WDM demultiplexer 40 demultiplexes the optical signals transmitted through the optical fiber transmission line 30 according to wavelength.

[0017] First, the configuration of the transmitter 10 will be described. The transmitter 10 has a plurality of transmitting units 100. In this embodiment, the transmitter 10 has the same number of transmitting units 100 as the number of WDM (Wavelength Division Multiplexing) channels. Each transmitting unit 100 outputs an optical signal of a different wavelength.

[0018] The transmitter 100 includes a digital signal processor 110, integrated modules 120 equal in number to the number of spatially multiplexed signals, and a spatial multiplexer 130. The digital signal processor 110 performs digital signal processing on the transmission bit sequence. Each integrated module 120 generates a polarization multiplexed signal of a different mode using the signal after digital signal processing by the digital signal processor 110. Each integrated module 120 outputs the generated polarization multiplexed signal to the spatial multiplexer 130. The spatial multiplexer 130 spatially multiplexes the polarization multiplexed signals of different modes output from each integrated module 120. The spatial multiplexer 130 outputs the spatially multiplexed polarization multiplexed signal to the WDM multiplexer 20.

[0019] Next, the configuration of the receiver 50 will be described. The receiver 50 has a plurality of receiving units 500. In this embodiment, the transmitter 10 has as many receiving units 500 as there are WDM channels. Each receiving unit 500 receives an optical signal of a different wavelength.

[0020] The receiver 500 includes a spatial demultiplexer 510, optical front-end units 520 corresponding to the number of spatially multiplexed signals, and a digital signal processing unit 530. The spatial demultiplexer 510 demultiplexes, by mode, the polarization multiplexed signal demultiplexed by the WDM demultiplexer 40 for each wavelength. Each optical front-end unit 520 receives a polarization multiplexed signal of a different mode. Using the input polarization multiplexed signal, each optical front-end unit 520 generates electrical signals of I and Q components of the X and Y polarizations. In this case, the optical front-end unit 520 converts the optical signal into an electrical signal while maintaining the phase and amplitude of the polarization multiplexed signal. The digital signal processing unit 530 restores the received bit stream by performing digital signal processing on the electrical signals of I and Q components of the X and Y polarizations generated by each optical front-end unit 520.

[0021] Although the above embodiment describes an example of a single optical fiber transmission line, the same applies to a spatially multiplexed transmission system (for example, a multi-core fiber, a multi-mode fiber, and free space transmission).

[0022] The detailed configurations of the transmitter 100 and the receiver 500 will be described using Figure 2. Figure 2 is a diagram showing an example of the configuration of a conventional transmitter 100 and receiver 500. The multiple transmitters 100 included in the transmitter 10 have the same configuration except for the wavelengths used as light sources, so a light source outputting light of wavelength λ1 will be described as an example. The multiple receivers 500 included in the receiver 50 have the same configuration except for the wavelengths used as local oscillation light sources, so a local oscillation light source outputting light of wavelength λ1 will be described as an example. Note that Figure 2 shows only one transmitter 100 and one receiver 500, and the WDM multiplexer 20, optical fiber transmission line 30, and WDM demultiplexer 40 that were provided between the transmitter 10 and the receiver 50 are not shown.

[0023] First, a description will be given of the configuration of the transmission unit 100. The digital signal processing unit 110 includes an encoding unit 111, a mapping unit 112, and a signal pre-processing unit 113.

[0024] The encoding unit 111 performs FEC (forward error correction) encoding on the transmission bit string and outputs the resulting transmission signal. The mapping unit 112 maps the transmission signal output from the encoding unit 111 to symbols. The signal preprocessing unit 113 performs preprocessing on the transmission signal symbol-mapped by the mapping unit 112. Here, the preprocessing includes, for example, inserting a known training signal, upsampling, limiting the band of the transmission signal, and compensating for waveform distortion.

[0025] The signal pre-processing unit 113 performs pre-processing on the transmission signal to generate an I (in-phase) component of the X polarization of the transmission signal, a Q (quadrature) component of the X polarization of the transmission signal, an I component of the Y polarization of the transmission signal, and a Q component of the Y polarization of the transmission signal. The signal pre-processing unit 113 outputs the generated I (in-phase) component of the X polarization of the transmission signal, the Q (quadrature) component of the X polarization of the transmission signal, the I component of the Y polarization of the transmission signal, and the Q component of the Y polarization of the transmission signal to each integrated module 120.

[0026] Each integrated module 120 includes digital-to-analog converters (DACs) 121-1 to 121-4, a modulator driver 131, a light source 140, IQ modulators 150-1 and 150-2, and a polarization combiner 160. DAC 121-1 converts the I (in-phase) component of the X polarization of the transmission signal output from the signal pre-processing unit 113 from a digital signal to an analog signal, and outputs the analog signal to the modulator driver 131. DAC 121-2 converts the Q (quadrature) component of the X polarization of the transmission signal output from the signal pre-processing unit 113 from a digital signal to an analog signal, and outputs the analog signal to the modulator driver 131. DAC 121-3 converts the I component of the Y polarization of the transmission signal output from the signal pre-processing unit 113 from a digital signal to an analog signal, and outputs the analog signal to the modulator driver 131. The DAC 121 - 4 converts the Q component of the Y polarization of the transmission signal output from the signal pre-processing unit 113 from a digital signal to an analog signal, and outputs the analog signal to the modulator driver 131 .

[0027] The modulator driver 131 has amplifiers 131-1 to 131-4. The amplifier 131-i (i is an integer between 1 and 4) amplifies the analog signal output from the DAC 121-i and drives the IQ modulators 150-1 and 150-2 with the amplified analog signal. The light source 140 is, for example, an LD (semiconductor laser). The light source 140 in the j-th transmitter 100 emits a light of wavelength λ ｊ (j is an integer equal to or greater than 1 and equal to or less than the number of WDM channels) of light is output.

[0028] The IQ modulator 150-1 outputs an X-polarized optical signal generated by modulating the optical signal output by the light source 140 with the I component of the X-polarized wave output by the amplifier 131-1 and the Q component of the X-polarized wave output by the amplifier 131-2. The IQ modulator 150-2 outputs a Y-polarized optical signal generated by modulating the optical signal output by the light source 140 with the I component of the Y-polarized wave output by the amplifier 131-3 and the Q component of the Y-polarized wave output by the amplifier 131-4. The polarization combining unit 160 generates a polarization multiplexed signal by polarization multiplexing the X-polarized optical signal output by the IQ modulator 150-1 and the Y-polarized optical signal output by the IQ modulator 150-2. The polarization combining unit 160 outputs the generated polarization multiplexed signal to the spatial multiplexing unit 130.

[0029] Next, we will explain the configuration of the receiving unit 500. The optical front-end unit 520 includes a local oscillation light source 521, a polarization splitter 522, optical 90-degree hybrid couplers 523-1 and 523-2, BPDs (Balanced Photo Diodes) 524-1 to 524-4, amplifiers 525-1 to 525-4, and analog-to-digital converters (ADCs) 526-1 to 526-4.

[0030] The local oscillation light source 521 is, for example, an LD. The local oscillation light source 521 outputs a local oscillation light (LO). The polarization separation unit 522 separates the input polarization multiplexed signal into an X-polarized optical signal and a Y-polarized optical signal. The polarization separation unit 522 outputs the X-polarized optical signal to the optical 90-degree hybrid coupler 523-1 and outputs the Y-polarized optical signal to the optical 90-degree hybrid coupler 523-2.

[0031] The optical 90-degree hybrid coupler 523-1 causes interference between the X-polarized optical signal and the local oscillator light output from the local oscillator light source 521, and extracts an I-component optical signal and a Q-component optical signal from the received optical field. The optical 90-degree hybrid coupler 523-1 outputs the extracted X-polarized I-component optical signal and Q-component optical signal to the BPDs 524-1 and 524-2.

[0032] The optical 90-degree hybrid coupler 523-2 causes interference between the Y-polarized optical signal and the local oscillation light output from the local oscillation light source 521, and extracts the I and Q components of the received optical field. The optical 90-degree hybrid coupler 523-2 outputs the extracted I and Q components of the Y-polarized wave to the BPD 524-3 and the BPD 524-4.

[0033] BPDs 524-1 to 524-4 are differential input photoelectric converters. BPD 524-i outputs the difference between the photocurrents generated in two photodiodes with matching characteristics to amplifier 525-i. BPD 524-1 converts the I component of the received signal of X polarization into an electrical signal and outputs it to amplifier 525-1. BPD 524-2 converts the Q component of the received signal of X polarization into an electrical signal and outputs it to amplifier 525-2. BPD 524-3 converts the I component of the received signal of Y polarization into an electrical signal and outputs it to amplifier 525-3. BPD 524-4 converts the Q component of the received signal of Y polarization into an electrical signal and outputs it to amplifier 525-4. The amplifier 525-i (i is an integer of 1 to 4) amplifies the electrical signal output from the BPD 524-i and outputs it to the ADC 526-i (i is an integer of 1 to 4).

[0034] The ADC 526 - i (i is an integer between 1 and 4) converts the electrical signal output from the amplifier 525 - i from an analog signal to a digital signal, and outputs the digital signal to the digital signal processing unit 530 .

[0035] The digital signal processing unit 530 includes a signal equalization unit 531, a demapping unit 532, and a decoding unit 533. The signal equalization unit 531 receives as input a set of a real part (I component) and an imaginary part (Q component) of each polarization component output from each optical front-end unit 520. The signal equalization unit 531 adaptively performs equalization processing on the input set of a real part (I component) and an imaginary part (Q component) of each polarization component using an adaptive equalization filter.

[0036] The demapping unit 532 determines the symbols of the received signal that has been adaptively equalized by the signal equalization unit 531. The demapping unit 532 converts the determined symbols into binary data. The decoding unit 533 performs error correction decoding (e.g., forward error correction) on the binary data converted by the demapping unit 532. As a result, the decoding unit 533 generates a received bit string.

[0037] 3 is a diagram illustrating the detailed configuration of a conventional signal equalizer 531. The received signal input to the signal equalizer 531 may be, for example, a set of each polarization component of the received signal and its time-domain phase conjugate component, or a set of the real part and imaginary part of each polarization component of the received signal. The signal equalizer 531 includes a receiver characteristic compensator 535, a transmission path characteristic compensator 536, and a transmitter characteristic compensator 537.

[0038] The receiver characteristic compensating unit 535 uses a digital filter (e.g., a 2D×2D MIMO configuration) on the input received signal to compensate for waveform distortion occurring in the receiver 50. Specifically, the receiver characteristic compensating unit 535 performs an operation of convolving the input received signal with the digital filter. The receiver characteristic compensating unit 535 outputs the signal after compensation for waveform distortion occurring in the receiver 50 to the transmission path characteristic compensating unit 536.

[0039] The transmission path characteristic compensator 536 receives the compensated signal output from the receiver characteristic compensator 535 as input. The transmission path characteristic compensator 536 uses a digital filter (e.g., a D×D MIMO configuration) on the compensated signal to compensate for composite distortions such as polarization rotation, polarization mode dispersion, and polarization dependent loss that occur in the transmission path other than chromatic dispersion. Specifically, the transmission path characteristic compensator 536 performs a calculation to convolve the input compensated signal with a digital filter. The transmission path characteristic compensator 536 outputs the signal after the composite distortion compensation.

[0040] The transmitter characteristic compensating unit 537 receives as input the signal after the composite distortion compensation output from the transmission path characteristic compensating unit 536. The transmitter characteristic compensating unit 537 uses a digital filter (e.g., a 2D×2D MIMO configuration) on the input signal after the composite distortion compensation to compensate for waveform distortion occurring in the transmitter 10. Specifically, the transmitter characteristic compensating unit 537 performs an operation of convolving the input signal after the composite distortion compensation with the digital filter. The transmitter characteristic compensating unit 537 outputs the signal after the waveform distortion compensation occurring in the transmitter 10 to the demapping unit 532.

[0041] Note that a frequency offset compensator that compensates for a frequency offset of the local oscillator light may be provided before or after at least one of the receiver characteristic compensator 535, the transmission path characteristic compensator 536, and the transmitter characteristic compensator 537. In the conventional digital coherent optical transmission system 1, crosstalk between any lanes is compensated for by the above configuration.

[0042] (Outline of the present invention) Next, an outline of the present invention will be explained using Fig. 4. Fig. 4 is a diagram for explaining the outline of the present invention. Fig. 4 shows a case where the number of channels D is "6" (three-mode fiber transmission). Furthermore, the coefficients of the filters shown in Fig. 4 are filter coefficients of the receiver characteristic compensation unit, and the filter coefficient "H Ｒ " is used as the filter coefficient "H Ｔ " to become the filter coefficient of the transmitter characteristic compensation unit.

[0043] As explained in the background art, the conventional digital signal processing in the signal equalizer 531 involves performing crosstalk compensation calculations between all channels (corresponding to "full compensation" in FIG. 4 ), which results in a problem of an increased amount of calculations. The greater the physical distance between lanes in the circuit constituting the signal equalizer 531, the smaller the absolute amount of IQ crosstalk. Therefore, the present invention employs a calculation configuration that omits some of the IQ crosstalk compensation calculations between IQ lanes (distant space / polarization) where the absolute amount of IQ crosstalk is less than a threshold value within the receiver 50. This means that some IQ crosstalk compensation is not performed. This allows the number of IQ crosstalk compensations to be actually performed to be reduced, thereby reducing the amount of calculations compared to conventional methods.

[0044] As an example of a calculation configuration that omits some of the IQ crosstalk compensation calculations, Figure 4 shows a configuration in which the receiver characteristic compensation unit and transmitter characteristic compensation unit of the signal equalization unit 531 compensate for crosstalk between spatial / polarized IQ lanes (N hops) that are separated by up to N (N is an integer equal to or greater than 1) lanes depending on the amount of crosstalk. For example, Figure 4 shows a configuration in which crosstalk is compensated for over one hop or two hops. The greater the spatial multiplexing number, the greater the effect of reducing the amount of calculation. When the number of channels D is "24" (equivalent to 12-core multiplexing) and only compensation for one hop is performed, the amount of calculation per receiving side or transmitting side can be reduced by approximately 87% compared to when all crosstalk compensation calculations (full compensation) are performed as in the past.

[0045] In the above example, whether or not to perform compensation is adjusted for each two input channels, but whether or not to perform compensation can also be determined on a channel-by-channel basis. Furthermore, if all lanes do not have equally spaced wiring, the crosstalk compensation target can be determined according to the actual wiring. If the digital filters of the transmitter characteristic compensation unit and receiver characteristic compensation unit have degrees of freedom in terms of time and frequency that are separate from the channel direction, such as time-domain convolution filters and frequency-domain multiplication filters, whether or not to perform crosstalk compensation can also be determined for each time and frequency component.

[0046] (First Embodiment) A first embodiment of the present invention will be described. The system configuration and the functional units of each device in the first embodiment are basically the same as those shown in Figures 1 and 2. The only difference from the configuration shown in Figures 1 and 2 is the configuration of the signal equalization unit 531. The differences will be described below.

[0047] 5 is a diagram showing an example of the configuration of the signal equalizer 531 in the first embodiment. The signal equalizer 531 includes a receiver characteristic compensator 535, a transmission path characteristic compensator 536, a transmitter characteristic compensator 537, and a controller 538.

[0048] As described above, conventionally, filter coefficients capable of compensating for crosstalk are set so as to compensate for crosstalk between all IQ lanes in each of the receiver characteristic compensating unit 535 and the transmitter characteristic compensating unit 537. In contrast, in the first embodiment, when it is expected that crosstalk between some IQ lanes will exceed a certain threshold value at the design stage of the digital signal processing unit 530 based on a pre-measured value or specification value of crosstalk, the receiver characteristic compensating unit 535 and the transmitter characteristic compensating unit 537 are designed so as to compensate only for crosstalk between the IQ lanes that is expected to exceed the threshold value (hereinafter referred to as "crosstalk to be compensated for").

[0049] That is, in the receiver characteristic compensating unit 535 and the transmitter characteristic compensating unit 537 in the first embodiment, the IQ lanes for which crosstalk is to be compensated are determined and fixed (unchanged) based on the crosstalk pre-measured value or the specification value. Therefore, at the design stage, filter coefficients for compensating for crosstalk between the IQ lanes that are the subject of crosstalk compensation are set, and the digital signal processing unit 530 is designed. The receiver 50 is characterized by including the digital signal processing unit 530 designed in this manner.

[0050] Here, in order to compensate only for the crosstalk to be compensated for, in the filters provided in each of the receiver characteristic compensator 535 and the transmitter characteristic compensator 537, a filter coefficient for compensating for crosstalk is set for IQ lanes that are expected to exceed a threshold, and a filter coefficient for omitting calculations for compensating for crosstalk is set for other IQ lanes. The filter coefficient for omitting calculations is, for example, a zero matrix.

[0051] During operation, the control unit 538 acquires the amount of crosstalk between IQ lanes previously determined to be compensated for, and dynamically updates the filter coefficients of the receiver characteristic compensating unit 535 and the transmitter characteristic compensating unit 537 based on the acquired amount of crosstalk. For example, when filter coefficients have already been set between IQ lanes previously determined to be compensated for (hereinafter referred to as "compensation target IQ lanes"), the control unit 538 calculates new filter coefficients capable of compensating for crosstalk and updates the filter coefficients corresponding to the compensation target IQ lanes with the newly calculated filter coefficients. As such, in the receiver characteristic compensating unit 535 and the transmitter characteristic compensating unit 537 in the first embodiment, the compensation target IQ lanes are fixed, and the filter coefficients are variable.

[0052] According to the digital coherent optical transmission system 1 of the first embodiment configured as described above, the receiver characteristic compensator 535 and the transmitter characteristic compensator 537 included in the receiver 50 set filter coefficients for compensating for the crosstalk to be compensated for, and set filter coefficients for omitting calculations for compensating for crosstalk between the other IQ lanes. By adopting such a design, calculations for compensating for crosstalk are performed between some IQ lanes, and calculations for compensating for crosstalk between the other IQ lanes can be omitted. This makes it possible to reduce the amount of calculations in the optical receiver. As a result, it is possible to reduce power consumption and circuit size.

[0053] Second Embodiment In the first embodiment, a configuration was described in which a design for compensating for crosstalk to be compensated for is performed at the digital signal processing unit design stage. Specifically, in the first embodiment, a configuration was described in which a design is performed in which the IQ lanes to be compensated for are fixed. In this case, only the filter coefficients of the receiver characteristic compensating unit 535 and the transmitter characteristic compensating unit 537 in the first embodiment are variable. Therefore, in the second embodiment, a configuration will be described in which a digital signal processing unit is designed in which the IQ lanes to be compensated for can be changed (reconfigurable).

[0054] The system configuration and the functional units of each device in the second embodiment are the same as those in the first embodiment. The second embodiment differs from the configuration shown in the first embodiment in that the control unit 538 can change the IQ lane to be compensated for depending on the amount of crosstalk. The differences will be described below.

[0055] The control unit 538 acquires the amount of crosstalk between each IQ lane during operation and appropriately changes the IQ lane to be compensated based on the acquired amount of crosstalk between each IQ lane. Specifically, the control unit 538 determines the IQ lane whose acquired amount of crosstalk is equal to or greater than a threshold as the IQ lane to be compensated.

[0056] Furthermore, the control unit 538 updates the filter coefficients in the receiver characteristic compensating unit 535 and the transmitter characteristic compensating unit 537. Specifically, the control unit 538 updates the filter coefficients in the receiver characteristic compensating unit 535 and the transmitter characteristic compensating unit 537 to compensate for crosstalk between the IQ lanes to be compensated for.

[0057] For example, when a zero matrix is ​​set as the filter coefficient between the IQ lanes to be compensated, the control unit 538 calculates filter coefficients capable of compensating for crosstalk and updates the filter coefficients corresponding to the IQ lanes to be compensated with the calculated filter coefficients. Also, for example, when filter coefficients have already been set between the IQ lanes to be compensated, the control unit 538 calculates new filter coefficients capable of compensating for crosstalk and updates the filter coefficients corresponding to the IQ lanes to be compensated with the newly calculated filter coefficients.

[0058] Furthermore, the control unit 538 may identify IQ lanes for which the acquired amount of crosstalk has become less than a threshold, and in order to omit compensation for crosstalk between the identified IQ lanes, update the filter coefficients of the receiver characteristic compensating unit 535 and the transmitter characteristic compensating unit 537. For example, the control unit 538 updates the filter coefficients by setting a zero matrix as the filter coefficients between the IQ lanes for which the amount of crosstalk has become less than the threshold.

[0059] The digital coherent optical transmission system 1 of the second embodiment configured as described above makes it possible to dynamically change the filter coefficients of the receiver characteristic compensator 535 and the transmitter characteristic compensator 537 included in the receiver 50. For example, the control unit 538 sets filter coefficients to compensate for crosstalk between IQ lanes where the amount of crosstalk is equal to or greater than a threshold, and sets a zero matrix as the filter coefficient between IQ lanes where the amount of crosstalk is less than the threshold. This allows crosstalk to be compensated between IQ lanes where crosstalk compensation is required, and allows the calculation for crosstalk compensation to be omitted between IQ lanes where crosstalk compensation is not required. In this way, the receiver of the second embodiment can reduce power consumption by omitting calculations in some calculation circuits depending on the amount of crosstalk.

[0060] (Third Embodiment) In the third embodiment, a configuration will be described in which the method of acquiring crosstalk characteristics in the signal equalizer unit is different from that in the second embodiment. The system configuration and the functional units of each device in the third embodiment are the same as those in the second embodiment. In the third embodiment, the difference from the configuration shown in the second embodiment is the processing of the signal equalizer unit 531. The differences will be described below.

[0061] 6 is a diagram illustrating an overview of the third embodiment. In the third embodiment, the signal equalizer 531 acquires crosstalk characteristics by collectively obtaining crosstalk characteristics between all IQ lanes through a specific calculation, and then sets filter coefficients for the filters included in the receiver characteristic compensation unit 535 and the transmitter characteristic compensation unit 537 by replacing crosstalk terms with small absolute values ​​(crosstalk terms between IQ lanes whose absolute values ​​are less than a threshold) with zero.

[0062] As the specific calculation, a partial freezing method may be used in which a receiver characteristic compensation unit 535 and a transmitter characteristic compensation unit 537 are separated from an adaptive filter configuration capable of collective compensation, such as a 4D×D MIMO AEQ (a collective compensation filter shown in the upper part of FIG. 6), as shown in FIG. 6. The partial freezing method may use the technique described in Non-Patent Document 2.

[0063] The signal equalization unit 531 uses a partial freezing method to separate the collective compensation filter shown in the upper part of Fig. 6 into a receiver characteristic compensation unit 535, a transmission path characteristic compensation unit 536, and a transmitter characteristic compensation unit 537 (middle part of Fig. 6). Then, the signal equalization unit 531 replaces crosstalk terms with small absolute values ​​(crosstalk terms between IQ lanes whose absolute values ​​are less than a threshold) with 0 in the receiver characteristic compensation unit 535 and the transmitter characteristic compensation unit 537 (lower part of Fig. 6). This processing may be performed by the control unit 538 shown in Fig. 5.

[0064] According to the digital coherent optical transmission system 1 of the third embodiment configured as described above, the receiver 50 includes a signal equalization unit 531 including a receiver characteristic compensation unit 535 and a transmitter characteristic compensation unit 537, to which the filter coefficients obtained as described above are set. This makes it possible to compensate for crosstalk between IQ lanes that require crosstalk compensation, and to omit calculations for crosstalk compensation between IQ lanes that do not require crosstalk compensation. Therefore, omitting calculations in some calculation circuits makes it possible to reduce power consumption.

[0065] (Modifications common to the first to third embodiments) In the above-described embodiments, the application of the present invention to a system that performs both polarization multiplexing and spatial multiplexing has been described as an example, but the application of the present invention is not limited to this. For example, the configuration of each of the above-described embodiments can be applied to a system including a receiver that compensates for receiver characteristics and transmitter characteristics in receiver 50.

[0066] 7 is a diagram illustrating an example of the hardware configuration of a digital signal processing unit 530 (signal processing device) in each embodiment. The digital signal processing unit 530 is realized as software by a processor 101, such as a CPU (Central Processing Unit), executing a program stored in a storage device 103 having a non-volatile recording medium (non-transitory recording medium) and a memory 102. The program may be recorded on a computer-readable recording medium. Examples of computer-readable recording media include portable media such as a flexible disk, a magneto-optical disk, a ROM (Read Only Memory), and a CD-ROM (Compact Disc Read Only Memory), and non-transitory recording media such as a hard disk or a solid-state drive (SSD) built into a computer system. The communication unit 104 executes predetermined communication processing.

[0067] The digital signal processing unit 530 may be realized using hardware (accelerator) including an electronic circuit (electronic circuit or circuitry) using, for example, an LSI (Large Scale Integrated circuit), an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device), or an FPGA (Field Programmable Gate Array).

[0068] Although an embodiment of the present invention has been described above in detail with reference to the drawings, the specific configuration is not limited to this embodiment, and includes designs within the scope of the gist of the present invention.

[0069] The present invention is applicable to optical transmission systems.

[0070] 10... transmitter, 20... WDM multiplexer 20, 30... optical fiber transmission line, 40... WDM demultiplexer, 50... receiver, 100... transmitting unit, 110... digital signal processing unit, 111... encoding unit, 112... mapping unit, 113... signal pre-processing unit, 120... integrated module, 121-1 to 121-4... digital-to-analog converters (DAC), 130... spatial multiplexing unit, 131... modulator driver, 131-1 to 131-4... amplifier, 140... light source, 150-1, 150-2... IQ modulator, 160... polarization combining unit, 500... receiving unit, 510... spatial demultiplexer, 520... optical front end unit, 521... local oscillation light source, 522... polarization separation unit, 523-1, 523-2...optical 90-degree hybrid couplers, 524-1 to 524-4...BPDs, 525-1 to 525-4...amplifiers, 526-1 to 526-4...analog-to-digital converters, 530...digital signal processing unit, 531...signal equalization unit, 532...demapping unit, 533...decoding unit, 535...receiver characteristic compensation unit, 536...transmission path characteristic compensation unit, 537...transmitter characteristic compensation unit, 538...control unit

Claims

1. An optical receiver equipped with a signal equalization unit that compensates for signal waveform distortion in a multiplexed signal transmitted from an optical transmitter using a filter whose filter coefficient is set to compensate for crosstalk between IQ lanes when the amount of crosstalk between IQ lanes exceeds a threshold.

2. The optical receiver of claim 1, wherein the signal equalization unit is comprised of a receiver characteristic compensation unit that compensates for waveform distortion occurring in the optical receiver, a transmission path characteristic compensation unit that compensates for distortion occurring in a transmission path connecting the optical transmitter and the optical receiver, and a transmitter characteristic compensation unit that compensates for waveform distortion occurring in the optical transmitter, and the receiver characteristic compensation unit and the transmitter characteristic compensation unit compensate for waveform distortion using a filter whose filter coefficient is set to compensate for crosstalk between IQ lanes when the amount of crosstalk between IQ lanes is equal to or greater than a threshold.

3. The optical receiver according to claim 2, wherein the IQ lanes to be compensated for crosstalk are fixed based on a pre-measured value of crosstalk or a specification value for the filters used by the receiver characteristic compensation unit and the transmitter characteristic compensation unit, and the optical receiver further comprises a control unit that updates the filter coefficients of the filters used by the receiver characteristic compensation unit and the transmitter characteristic compensation unit based on the amount of crosstalk between the IQ lanes to be compensated for crosstalk.

4. The optical receiver of claim 2, further comprising a control unit that updates the filter coefficients of the filter used by the receiver characteristic compensation unit and the filter used by the transmitter characteristic compensation unit to compensate for crosstalk at least between IQ lanes where the crosstalk amount is equal to or greater than a threshold, based on the amount of crosstalk between each IQ lanes.

5. The optical receiver according to claim 2, wherein the filter used by the receiver characteristic compensation unit and the filter used by the transmitter characteristic compensation unit have filter coefficients set so as to compensate for crosstalk between IQ lanes whose amount of crosstalk between IQ lanes is equal to or greater than a threshold value by replacing crosstalk terms between IQ lanes whose absolute values ​​are less than a threshold value with zero after obtaining the crosstalk characteristics between all IQ lanes collectively through a specific calculation.

6. An optical receiver according to any one of claims 2 to 5, wherein a zero matrix is ​​set for filter coefficients that make the amount of crosstalk between I and Q lanes less than a threshold.

7. A transmission system comprising an optical transmitter for transmitting a multiplexed signal and an optical receiver according to claim 1.

8. A compensation method for compensating for signal waveform distortion in a multiplexed signal transmitted from an optical transmitter using a filter whose filter coefficient is set to compensate for crosstalk between I and Q lanes when the amount of crosstalk between I and Q lanes is equal to or greater than a threshold.

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