Characteristic measurement device, characteristic measurement method, and computer program

Abstract

This characteristic measuring device that measures lane-to-lane characteristics between a transmitter and a receiver connected through an optical fiber transmission path, the characteristic measuring device comprising: an adaptive equalizer unit that performs an equalization process on an input signal, the input signal being a polarization multiplexed received signal, a phase conjugate signal of the polarization multiplexed received signal, or a plurality of signals mathematically equivalent to the received signal and the phase conjugate signal of the received signal; and a characteristic function derivation unit that calculates a first inverse characteristic representing an inverse characteristic of a transmitter and a second inverse characteristic representing an inverse characteristic of a receiver on the basis of a filter coefficient obtained during the equalization processing performed by the adaptive equalizer unit, and a frequency offset.　

Description

[Technical field]

[0001] The present invention relates to a characteristic measuring device, a characteristic measuring method, and a computer program. This application claims priority to PCT / JP2022 / 000858, filed in Japan on January 13, 2022, the contents of which are incorporated herein by reference. [Background technology]

[0002] Optical communications using optical fibers (hereinafter referred to as "optical transmission systems") have the advantages of a wide usable frequency band and little signal attenuation. As a result, optical transmission systems are capable of long-distance, high-capacity communications, and are widely used in modern fixed-line communications. Optical transmission systems achieve highly reliable optical communications by compensating for signal distortions that occur in optical transceivers and optical fiber transmission paths using digital signal processing.

[0003] In recent years, polarization division multiplexing systems, which transmit and receive signals using two degrees of polarization freedom of light as independent channels, have been put to practical use and are widely used in long-distance, large-capacity systems in combination with wavelength division multiplexing, which multiplexes signals using light of different wavelengths. Furthermore, as a method of improving the degree of multiplexing, research is being conducted on space division multiplexing systems, which transmit and receive signals using the core modes of multicore fibers or multimode fibers as independent channels.

[0004] One of the means to improve the transmission capacity in an optical transmission system is to increase the transmission and reception signal's multi-level and baud rate (modulation speed). However, high-level multi-level, high-baud rate signals are significantly affected by signal waveform distortion caused by skew, imbalance, crosstalk, etc. that occurs relatively between the IQ lanes of each signal. Therefore, it is necessary to compensate for these signal waveform distortions within the transmitter and receiver.

[0005] Therefore, a method has been proposed to compensate for signal distortion occurring inside a transceiver by using an adaptive filter with a special configuration such as a multi-stage configuration in the receiver (see, for example, Non-Patent Document 1). The method described in Non-Patent Document 1 increases the internal degrees of freedom of a MIMO (multiple-input / multiple-output) adaptive filter mounted in a coherent optical receiver to compensate for distortion occurring in the polarization degrees of freedom, thereby making it possible to compensate for signal distortion caused by the IQ characteristics of the transceiver. However, the multi-stage configuration has instability in convergence, and the waveform distortion caused by the IQ characteristics has many static components in time. In contrast, the method described in Non-Patent Document 1 has the problem of poor calculation efficiency because it dynamically compensates for each symbol.

[0006] Therefore, the next promising technology is a method of estimating the transfer functions of the transmitter and receiver in some way, and inputting the inverse functions as fixed values ​​to the (pre)equalization filter circuits of the transmitter and receiver to compensate for signal distortion. To carry out this method, it is necessary to measure the characteristics of the transmitter and receiver in advance and obtain the fixed values ​​to be input to the filters. As one method for measuring the characteristics of the transmitter and receiver, a method using a multistage MIMO adaptive filter has been proposed (see, for example, Non-Patent Document 2). In the method described in Non-Patent Document 2, first, a normal received signal is adaptively compensated using a multistage MIMO configuration, and the transmitter and receiver characteristics are measured by analyzing the values ​​of the coefficients of the adaptive filter obtained at that time. [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] CRS Fludger and T. Kupfer, “Transmitter Impairment Mitigation and Monitoring for High Baud-Rate, High Order Modulation Systems”, 42nd European Conference and Exhibition on Optical Communications, p.256-258(2016) [Non-Patent Document 2] Manabu Arikawa and Kazunori Hayashi, “Transmitter and receiver impairment monitoring using adaptive multi-layer linear and widely linear filter coefficients controlled by stochastic gradient descent”, Opt. Express 29, 11548-11561 (2021) Summary of the Invention [Problem to be solved by the invention]

[0008] However, existing measurement techniques have treated polarization, wavelength, core, and mode independently, and although it was possible to evaluate the IQ characteristics within one polarization, wavelength, core, and mode, it was not possible to derive the characteristics of an operating transceiver when crosstalk between lanes exists beyond them. For example, in optical transmission using a single-mode fiber, crosstalk may occur between the Q lane of X polarization and the I lane of Y polarization in the optical modulator driver circuit of the transmitter or the optical receiving circuit of the receiver, but such crosstalk cannot be expressed as a combination of IQ crosstalk within a polarization and polarization rotation. Therefore, the existing measurement techniques had the problem of being unable to calculate the characteristics of a transceiver including IQ crosstalk across multiple polarizations.

[0009] In view of the above circumstances, an object of the present invention is to provide a technique capable of calculating the characteristics of a transmitter / receiver, including IQ crosstalk across multiple polarized waves. [Means for solving the problem]

[0010] One aspect of the present invention is a characteristic measuring device that measures characteristics between lanes of a transmitter and a receiver connected via an optical fiber transmission path, and includes an adaptive equalization unit that performs equalization processing on an input signal, which is a polarization multiplexed received signal and a phase conjugate signal of the polarization multiplexed received signal, or the received signal and a plurality of signals that are mathematically equivalent to the phase conjugate signals of the received signal, and a characteristic function derivation unit that calculates a first inverse characteristic representing the inverse characteristic of the transmitter and a second inverse characteristic representing the inverse characteristic of the receiver based on a filter coefficient obtained during the equalization processing performed by the adaptive equalization unit and a frequency offset.

[0011] One aspect of the present invention is an optical transmission system comprising the above-mentioned characteristic measuring device, and the transmitter and receiver having a filter function for compensating for signal waveform distortion based on the first inverse characteristic and the second inverse characteristic obtained by the characteristic measuring device.

[0012] One aspect of the present invention is a characteristic measurement method performed by a characteristic measurement device that measures the characteristics between lanes of a transmitter and a receiver connected via an optical fiber transmission path, the characteristic measurement method including, as input signals, a polarization multiplexed received signal and a phase conjugate signal of the polarization multiplexed received signal, or the received signal and a plurality of signals that are mathematically equivalent to the phase conjugate signals of the received signal, performing an equalization process on the input signals, and calculating a first inverse characteristic representing the inverse characteristic of the transmitter and a second inverse characteristic representing the inverse characteristic of the receiver based on a filter coefficient obtained during the equalization process and a frequency offset.

[0013] One aspect of the present invention is a computer program for causing a computer to function as a characteristic measuring device for measuring characteristics between lanes of a transmitter and a receiver connected via an optical fiber transmission path, the computer program executing the following steps: an adaptive equalization step for performing an equalization process on a polarization multiplexed received signal and a phase conjugate signal of the polarization multiplexed received signal, or the received signal and a plurality of signals that are mathematically equivalent to the phase conjugate signal of the received signal, as input signals; and a characteristic function derivation step for calculating a first inverse characteristic representing the inverse characteristic of the transmitter and a second inverse characteristic representing the inverse characteristic of the receiver based on a filter coefficient obtained during the equalization process performed in the adaptive equalization step and a frequency offset. Effect of the Invention

[0014] According to the present invention, it is possible to calculate the characteristics of a transmitter and receiver, including IQ crosstalk across multiple polarized waves. [Brief description of the drawings]

[0015] [Figure 1] 1 is a diagram illustrating an example of the configuration of a digital coherent optical transmission system according to a first embodiment. [Diagram 2] 3 is a diagram for explaining an outline of a process for deriving inverse characteristics of a transmitter and a receiver in the first embodiment. FIG. [Diagram 3] 2 is a diagram illustrating an example of the configuration of a demodulation digital signal processing unit including an adaptive equalization unit according to the first embodiment. [Figure 4] FIG. 4 is a diagram illustrating an example of the configuration of a characteristic function derivation unit in the first embodiment. [Diagram 5] 5 is a flowchart showing a flow of processing in a receiver in the first embodiment. [Figure 6] FIG. 11 is a diagram for explaining an outline of a process for deriving inverse characteristics of a transmitter and a receiver in the second embodiment. [Figure 7] FIG. 11 is a diagram illustrating an example of the configuration of a demodulation digital signal processing unit including an adaptive equalization unit according to a second embodiment. [Figure 8]FIG. 11 is a diagram illustrating an example of the configuration of a characteristic function derivation unit in the second embodiment. [Figure 9] FIG. 11 is a diagram illustrating an example of the configuration of a digital coherent optical transmission system 1 according to a third embodiment. [Figure 10] FIG. 13 is a diagram showing an example (part 1) of a combination of 2×2 type filters in multiple stages according to the third embodiment. [Figure 11] FIG. 13 is a diagram showing an example (part 2) of a combination of 2×2 type filters in multiple stages according to the third embodiment. [Figure 12] FIG. 13 is a diagram illustrating an example of a method for determining a value to be input to a filter in the third embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. (First embodiment) 1 is a diagram showing an example of the configuration of a digital coherent optical transmission system 1 in the first embodiment. The digital coherent optical transmission system 1 includes a transmitter 10 and a receiver 50. The receiver 50 receives a polarization multiplexed signal from the transmitter 10.

[0017] The transmitter 10 has one or more transmitting units 100. The transmitting units 100 output an optical signal of a specified wavelength to an optical fiber transmission line 30. The optical fiber transmission line 30 is provided with any number of optical amplifiers 31. Each optical amplifier 31 inputs 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 receiver 50 has one or more receiving units 500. The receiving units 500 receive the optical signal.

[0018] The transmitting unit 100 includes a digital signal processing unit 110, a modulator driver 120, a light source 130, and an integrated module 140. The digital signal processing unit 110 includes an encoding unit 111, a mapping unit 112, a training signal inserting unit 113, a frequency changing unit 114, a waveform shaping unit 115, a pre-equalization unit 116, and digital-to-analog converters (DACs) 117-1 to 117-4.

[0019] The coding unit 111 performs FEC (forward error correction) coding on the transmission bit string and outputs the resulting transmission signal. The mapping unit 112 maps the transmission signal output from the coding unit 111 to symbols. The training signal insertion unit 113 inserts a known training signal into the transmission signal symbol-mapped by the mapping unit 112. The frequency change unit 114 performs upsampling by changing the sampling frequency of the transmission signal into which the training signal has been inserted. The waveform shaping unit 115 limits the band of the sampled transmission signal.

[0020] The pre-equalization unit 116 compensates for distortion of the waveform of the transmission signal band-limited by the waveform shaping unit 115, and outputs the result to the DACs 117-1 to 117-4. The DAC 117-1 converts the I (in-phase) component of the X-polarized wave of the transmission signal input from the pre-equalization unit 116 from a digital signal to an analog signal, and outputs the result to the modulator driver 120. The DAC 117-2 converts the Q (quadrature) component of the X-polarized wave of the transmission signal input from the pre-equalization unit 116 from a digital signal to an analog signal, and outputs the result to the modulator driver 120. The DAC 117-3 converts the I component of the Y-polarized wave of the transmission signal input from the pre-equalization unit 116 from a digital signal to an analog signal, and outputs the result to the modulator driver 120. The DAC 117-4 converts the Q component of the Y-polarized wave of the transmission signal input from the pre-equalization unit 116 from a digital signal to an analog signal, and outputs the result to the modulator driver 120.

[0021] The modulator driver 120 has amplifiers 121-1 to 121-4. The amplifier 121-i (i is an integer between 1 and 4) amplifies the analog signal output from the DAC 117-i, and drives the modulator of the integrated module 140 with the amplified analog signal. The light source 130 is, for example, an LD (semiconductor laser). The light source 130 outputs light of a specified wavelength.

[0022] The integrated module 140 includes IQ modulators 141-1 and 141-2, and a polarization combining unit 142. The IQ modulator 141-1 outputs an X-polarized optical signal generated by modulating the optical signal output by the light source 130 with the I component of the X-polarized wave output by the amplifier 121-1 and the Q component of the X-polarized wave output by the amplifier 121-2. The IQ modulator 141-2 outputs a Y-polarized optical signal generated by modulating the optical signal output by the light source 130 with the I component of the Y-polarized wave output by the amplifier 121-3 and the Q component of the Y-polarized wave output by the amplifier 121-4. The polarization combining unit 142 polarization-multiplexes the X-polarized optical signal output by the IQ modulator 141-1 and the Y-polarized optical signal output by the IQ modulator 141-2, and outputs the result.

[0023] The receiving unit 500 includes a local oscillation light source 510, an optical front end 520, and a digital signal processing unit 530. The local oscillation light source 510 is, for example, an LD. The local oscillation light source 510 outputs a local oscillation light (LO).

[0024] The optical front end 520 converts the optical signal into an electrical signal while maintaining the phase and amplitude of the polarization multiplexed phase modulated signal. The optical front end 520 includes a polarization splitter 521, optical 90-degree hybrid couplers 522-1 and 522-2, BPDs (Balanced Photo Diodes) 523-1 to 523-4, and amplifiers 524-1 to 524-4.

[0025] The polarization splitter 521 splits the input optical signal into an X-polarized wave and a Y-polarized wave. The polarization splitter 521 outputs the X-polarized optical signal to an optical 90-degree hybrid coupler 522-1, and outputs the Y-polarized optical signal to an optical 90-degree hybrid coupler 522-2.

[0026] The optical 90-degree hybrid coupler 522-1 causes interference between the X-polarized optical signal and the local oscillation light output from the local oscillation light source 510, and extracts the I and Q components of the received optical field. The optical 90-degree hybrid coupler 522-1 outputs the extracted I and Q components of the X-polarized wave to the BPDs 523-1 and 523-2.

[0027] The optical 90-degree hybrid coupler 522-2 causes interference between the Y-polarized optical signal and the local oscillation light output from the local oscillation light source 510, and extracts the I and Q components of the received optical field. The optical 90-degree hybrid coupler 522-2 outputs the extracted I and Q components of the Y-polarized wave to the BPD 523-3 and the BPD 523-4.

[0028] BPDs 523-1 to 523-4 are differential input type photoelectric converters. BPD 523-i outputs the difference value of the photocurrents generated in two photodiodes with uniform characteristics to amplifier 524-i. BPD 523-1 converts the I component of the X-polarized reception signal into an electric signal and outputs it to amplifier 524-1. BPD 523-2 converts the Q component of the X-polarized reception signal into an electric signal and outputs it to amplifier 524-2. BPD 523-3 converts the I component of the Y-polarized reception signal into an electric signal and outputs it to amplifier 524-3. BPD 523-4 converts the Q component of the Y-polarized reception signal into an electric signal and outputs it to amplifier 524-4. Amplifier 524-i (i is an integer between 1 and 4) amplifies the electric signal output from BPD 523-i and outputs it to digital signal processor 530.

[0029] The digital signal processing unit 530 includes analog-to-digital converters (ADC) 531-1 to 531-4, a front-end correction unit 532, a chromatic dispersion compensation unit 533, an adaptive equalization unit 534, a frequency and phase offset compensation unit 535, a demapping unit 536, a decoding unit 537, and a characteristic function derivation unit 538. The ADC 531-i (i is an integer between 1 and 4) converts the electrical signal output from the amplifier 524-i from an analog signal to a digital signal, and outputs the digital signal to the front-end correction unit 532.

[0030] The front-end correction unit 532 receives an I component of the X-polarized received signal from the ADC 531-1, a Q component of the X-polarized received signal from the ADC 531-2, an I component of the Y-polarized received signal from the ADC 531-3, and a Q component of the Y-polarized received signal from the ADC 531-4. The front-end correction unit 532 uses the received signals to generate a received signal in which the frequency characteristics in the optical front-end 520 have been compensated for, and outputs the generated signal to the chromatic dispersion compensation unit 533.

[0031] The chromatic dispersion compensating unit 533 estimates the chromatic dispersion received in the optical fiber transmission line 30, compensates for the estimated chromatic dispersion on the electrical signal output from the front-end correcting unit 532, and outputs the electrical signal to the adaptive equalizing unit 534. The adaptive equalizing unit 534 adaptively performs equalization processing on the received signal output from the chromatic dispersion compensating unit 533. The adaptive equalizing unit 534 outputs a filter coefficient and a frequency offset obtained during equalization processing to a characteristic function derivation unit 538, and outputs the received signal after equalization processing to a frequency and phase offset compensating unit 535. The frequency and phase offset compensating unit 535 performs processing such as compensation for frequency offset and phase noise on the received signal equalized by the adaptive equalizing unit 534.

[0032] The demapping unit 536 determines the symbols of the received signal output by the frequency and phase offset compensation unit 535 and converts the determined symbols into binary data. The decoding unit 537 performs error correction decoding processing such as FEC on the binary data demapped by the demapping unit 536 to obtain a received bit string.

[0033] The characteristic function derivation unit 538 derives the inverse characteristics of the transmitter 10 and the receiver 20 based on the filter coefficients obtained from the adaptive equalization unit 534 and the frequency offset. By inputting the inverse characteristics derived by the characteristic function derivation unit 538 as fixed values ​​in the (pre)equalization filter circuits of the transmitter and receiver, it is possible to compensate for waveform distortion occurring between the IQ lanes of the transmitter 10 and the receiver 20.

[0034] 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).

[0035] FIG. 2 is a diagram for explaining an outline of a process for deriving the inverse characteristics of the transmitter 10 and the receiver 20 in the first embodiment. In the first embodiment, the filter coefficients (h1, ..., h2) obtained by the adaptive equalization unit 534 using the IQ signal and its phase conjugate signal as input signals are 16 ) and the frequency offset (exp(jω x (n / T)),(jω y This utilizes the fact that (n / T)) contains complete information on the IQ characteristics (corresponding to a frequency-dependent 4 × 4 matrix) of the receiving system including crosstalk between lanes. n represents the symbol interval, and T represents the symbol period. This MIMO configuration is not a multi-stage configuration, so the convergence problem that could be a conventional problem is also improved, and once the IQ characteristic function is obtained, there is no need to change the filter coefficients in the adaptive equalization unit 534, so calculation efficiency is improved compared to when dynamic compensation is always performed.

[0036] First, the calculation to compensate for the frequency characteristics of the receiver (h RXI ,h RXQ ,h RYI ,h RYQ ) and the calculation to compensate for the chromatic dispersion of the transmission line (h CD -1), and a phase conjugate signal of the signals (XI, XQ, YI, YQ) are input to an adaptive equalizer 534. The adaptive equalizer 534 performs adaptive equalization. A characteristic function derivation unit 538 calculates the filter coefficients (h1, ..., h 16 ) and the frequency offset (exp(jω x (n / T)),(jω y (n / T)) is input. The inverse characteristic H T -1 (ω) and the inverse characteristic H of the receiver 50 R -1 (ω) is calculated. Note that the inverse characteristic H T -1 (ω) is one aspect of the first inverse characteristic, and the inverse characteristic H R -1 (ω) is one aspect of the second inverse characteristic.

[0037] 3 is a diagram showing an example of the configuration of a demodulation digital signal processing unit including an adaptive equalization unit 534 in the first embodiment. The demodulation digital signal processing unit includes a front-end correction unit 532, a chromatic dispersion compensation unit 533, an adaptive equalization unit 534, and a frequency and phase offset compensation unit 535.

[0038] The demodulation digital signal processor receives the real component XI and imaginary component XQ of the X-polarized received complex signal, which have been converted into digital signals by the ADCs 531-1 to 531-4, and the real component YI and imaginary component YQ of the Y-polarized received complex signal. The demodulation digital signal processor calculates an impulse response (h RXI ,h RXQ ,h RYI ,h RYQ ) and the complex impulse response for chromatic dispersion compensation h CD -1 This outputs two complex signals for the X-polarized component and the Y-polarized component.

[0039] Next, the demodulation digital signal processing unit generates a phase conjugate of each of the two complex signals, and receives eight signals, namely, a real component XI, an imaginary component XQ, a real component YI, and an imaginary component YQ, and their respective phase conjugates, for each of the X-polarized component and the Y-polarized component. This enables the adaptive equalization unit 534 of the receiver 50 to dynamically compensate for the IQ imbalance and IQ lane skew generated in the transmitter 10, the bias deviation of the IQ modulators 141-1 and 141-2, and the like, generated in addition to the impairment generated in the optical fiber transmission line 30 and the receiver 50, thereby improving the quality of the received signal.

[0040] Specifically, the demodulation digital signal processor derives an impulse response h that compensates for the frequency characteristics of the receiver 50 from the real component XI of the received complex signal of the X-polarized component. RXI and the impulse response for chromatic dispersion compensation h CD -1 The impulse response h RXQ and the impulse response for chromatic dispersion compensation h CD -1 The following shall be implemented.

[0041] Similarly, the demodulation digital signal processing unit applies an impulse response h that compensates for the frequency characteristics of the receiver 50 to the real component YI of the received complex signal of the Y polarization component. RYI and the impulse response for chromatic dispersion compensation h CD -1 and an impulse response h that compensates for the frequency characteristics of the receiver 50 is generated for the imaginary component YQ of the received complex signal of the Y polarization component. RYQ and the impulse response for chromatic dispersion compensation h CD -1 The following shall be implemented.

[0042] The demodulation digital signal processing unit branches each of the real component XI, imaginary component XQ, real component YI, and imaginary component YQ, which are convoluted with the impulse response that compensates for the frequency characteristics of the receiver 50 and the impulse response for chromatic dispersion compensation, into four, and inputs two of the four branched signals directly to the adaptive equalization unit 534 and converts the remaining two signals into phase conjugate signals and inputs them to the adaptive equalization unit 534.

[0043] The adaptive equalizer 534 generates a real component XI convolved with an impulse response h1, an imaginary component XQ convolved with an impulse response h5, a real component YI convolved with an impulse response h9, and an imaginary component YI convolved with an impulse response h 13 The frequency offset exp(jω x Furthermore, the adaptive equalizer 534 multiplies the real component phase conjugate XI * and the imaginary component phase conjugate XQ convolved with impulse response h6 * and the impulse response h 10 The real component phase conjugate YI * and the impulse response h 14 The imaginary component YQ is convoluted with * The frequency offset exp(-jω x (n / T) is multiplied.

[0044] The demodulation digital signal processor calculates the frequency offset exp(jω x (n / T)) and the sum signal multiplied by the frequency offset exp(-jω x The demodulation digital signal processing unit adds the sum signal multiplied by the sum signal of the X-polarized component to the received signal of the X-polarized component obtained by the demodulation digital signal processing unit, and adds a transmission data bias correction signal C for canceling the bias shift of the X-polarized component to the received signal of the X-polarized component obtained by the demodulation digital signal processing unit. X The distortion-corrected received signal X of the X-polarized component is obtained by adding (or subtracting) Rsig The demapping unit 536 obtains the received signal X Rsig The received signal X^ obtained by performing symbol decision on (n) Rsig Output (n)

[0045] On the other hand, the adaptive equalization unit 534 outputs a real component XI convolved with the impulse response h3, an imaginary component XQ convolved with the impulse response h7, and an impulse response h 11 The real component YI convolved with and the impulse response h 15The frequency offset exp(jω y Furthermore, the adaptive equalizer 534 multiplies the real component phase conjugate XI * and the impulse response h 12 The imaginary component phase conjugate XQ * and the impulse response h 16 The real component phase conjugate YI * and the impulse response h 14 The imaginary component YQ is convoluted with * The frequency offset exp(-jω y (n / T) is multiplied.

[0046] The demodulation digital signal processor calculates the frequency offset exp(jω y (n / T)) and the sum signal with the frequency offset exp(-jω y The demodulation digital signal processing unit adds the received signal of the Y-polarized component to the demodulated signal, and the sum signal to which the demodulation digital signal processing unit has been applied (n / T)) to obtain a received signal of the Y-polarized component. The demodulation digital signal processing unit applies a transmission data bias correction signal C for canceling the bias shift of the Y-polarized component to the received signal of the Y-polarized component obtained. Y The distortion-corrected received signal Y of the X-polarized component is obtained by adding (or subtracting) Rsig The demapping unit 536 obtains the received signal Y Rsig The received signal Y^ obtained by performing symbol decision on (n) Rsig Output (n).

[0047] The complex impulse response for chromatic dispersion compensation, h CD -1 , impulse response h1~h 16 , and frequency offset exp(jω x (n / T)), exp(-jω x (n / T)), exp(jω y (n / T)), exp(-jω y (n / T) is adaptively and dynamically changed. The receiver 50 may obtain these values ​​in any manner.

[0048] In addition, the impulse response h RXI , h RXQ , h RYI , h RYQ The convolution of the impulse response h CD -1 The convolution of corresponds to the processing of the chromatic dispersion compensation unit 533. The frequency offset exp(jω x (n / T)) and exp(-jω x (n / T)), exp(jω y (n / T)), exp(-jω y The multiplication process of (n / T) corresponds to the function of the frequency and phase offset compensation unit 535 .

[0049] The filter coefficients (h1, . . . , h2) obtained by the processing of the adaptive equalizer 534 in the demodulation digital signal processor described above 16 ) and the frequency offset (exp(jω x (n / T)),(jω y (n / T)) is output to the characteristic function derivation unit 538.

[0050] Next, the detailed principle of the above-mentioned demodulation digital signal processing section will be described. First, FT(s i,in (t))=s i,in (ω), note that the following equation (1) holds, and let the general variable a(x) be (~)a(ω)=a * (-ω). Note that (~) is placed above the a.

[0051]

number

[0052] In order to take into account the skew and crosstalk between the IQ lanes, the signal waveform is expressed in a 4×1 vector representation as shown in the following equation (2).

[0053]

number

[0054] Here, the vectors before and after propagation can be expressed as in the following equation (3).

[0055]

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[0056] The symbols in formula (3) have the following meanings. H R (ω): A 4×4 matrix representing the IQ characteristics of the transmitter 10 and receiver 50, including the IQ skew, imbalance, and crosstalk between lanes on the receiving side. H fR (t): A 4×4 matrix representing the frequency of the local oscillator of each receiver H CD (ω): 4×4 matrix representing the effect of chromatic dispersion on the transmission line H couple (ω): 4×4 matrix representing channel crosstalk in the transmission path H fT (t): A 4 × 4 matrix representing the frequency of the carrier light of each transmitter H T (ω): A 4×4 matrix representing the IQ characteristics of the transmitter 10 and receiver 50, including the IQ skew, imbalance, and crosstalk between lanes on the transmission side.

[0057] By calculating the inverse matrix of each symbol in equation (3), out (ω) to s in (ω) can be calculated. s in (ω) is expressed as follows:

[0058]

number

[0059] H in Equation (4) R -1 (ω),H fR -1 (t),H CD-1 (ω),H couple -1 (ω),H fT -1 (t),H T -1 Each matrix in (t) is expressed as in the following equations (5) to (10).

[0060]

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[0061]

number

[0062]

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[0063]

number

[0064]

number

[0065]

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[0066] Here, it is assumed that the frequency and phase fluctuations of the local light on the receiving side are the same between the polarized waves. x =ω Rx -ω T ,Δω y =ω Ry -ω T Then, A(ω), B(ω), and M are defined as in equations (11) to (13). Rx x is the subscript of R, and ω Ry The y in is a subscript of R.

[0067]

number

[0068]

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[0069]

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[0070] In that case, S in (ω) is expressed as equation (14).

[0071]

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[0072] Then, when equation (14) is converted into the time domain, it is expressed as equation (15).

[0073]

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[0074] According to equation (15), to compensate for distortion that occurs during transmission, after compensating for chromatic dispersion for the real and imaginary parts of the signal for each polarization, the four signals and their complex conjugate signals are convoluted with an appropriate function in 4 × 4 matrix format (e.g., equivalent to A(t)M, B(t)M based on equations (11) to (13) above), and multiplied by a term that corrects the frequency offset of the local oscillator. One adaptive filter that performs equalization based on this principle is the 8 × 2 MIMO configuration shown in Figure 3.

[0075] In the 8×2 MIMO configuration, the odd rows of A(t)M and B(t)M are adaptively obtained. That is, h1, h3, . . . , h 15 h2, h4, . . . , h 16 corresponds to the elements in odd rows of B(t)M.in By the condition that the complex conjugate of the 2i+1th row component of (ω) is equal to the 2ith row component, the even row components of A(ω) and B(ω) are obtained from the odd row components of the other, and A(ω) and B(ω) are the filter coefficients h1 to h 16 From the Fourier transform of the above, equations (16) and (17) can be obtained.

[0076]

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[0077]

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[0078] In the first embodiment, the characteristic function derivation unit 538 shown in FIG. 4 calculates the characteristic H T (ω) and the characteristic H of receiver 50 R (ω) and H T (ω) and H R Regarding (ω), the complex conjugate of the 2i+1th row component of the input / output vector is equal to the 2ith row component, and the transmitter / receiver characteristics can be considered the same even if a matrix representing any polarization rotation and phase rotation is applied to the transmission line side. Therefore, H T (ω) and H R (ω) can be regarded as equations (18) and (19) by reducing the degrees of freedom from 16 complex numbers to 4.

[0079]

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[0080]

number

[0081] Here, from the definition of A, A(ω) is expressed as the following equation (20).

[0082]

number

[0083] As a result, A(ω) is expressed as follows:

[0084]

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[0085] By calculating A(ω) as described above and placing A(ω) as shown in the following equation (22), the relationship in equation (23) can be derived based on equations (21) and (22).

[0086]

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[0087]

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[0088] Furthermore, the relationship in equation (24) is derived based on equations (21) and (22).

[0089]

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[0090] Based on equations (24) and (23), equation (25) is derived.

[0091]

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[0092] Furthermore, the relationship in equation (26) is derived based on equations (21) and (22).

[0093]

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[0094] Based on equation (26) and equation (23), equation (27) is derived.

[0095]

number

[0096] By carrying out the above calculations, each component is calculated. T -1 Each element of Δω x ,Δω y However, since the frequency offset of the local oscillator is obtained during adaptive equalization, this value can be convoluted to correct the factor. The characteristic function derivation unit 538 uses the equation obtained above to calculate the inverse characteristic H T -1 (ω) and inverse characteristic H R -1 Specifically, the characteristic function derivation unit 538 derives the inverse characteristic H T -1 (ω) and by substituting each matrix element of equation (27) into equation (19), the inverse characteristic H R -1 Derive (ω).

[0097] FIG. 5 is a flowchart showing the flow of processing by the receiver 50 in the first embodiment. The optical front end 520 receives an optical signal (polarization multiplexed signal) transmitted through the optical fiber transmission line 30 (step S101). Each functional unit in the optical front end 520 performs polarization separation on the received optical signal by the polarization separation unit 521, extraction of the I and Q components of the X polarization by the optical 90-degree hybrid coupler 522-1, extraction of the I and Q components of the Y polarization by the optical 90-degree hybrid coupler 522-2, conversion to an electrical signal, and amplification of the electrical signal.

[0098] The ADC 531-i converts the electrical signal output from the amplifier 524-i from an analog signal to a digital signal (step S102). The front-end correction unit 532 uses each input signal to generate a received signal in which the frequency characteristics of the optical front-end 520 have been compensated for (step S103). The chromatic dispersion compensation unit 533 performs chromatic dispersion compensation on the electrical signal output from the front-end correction unit 532 (step S104).

[0099] The adaptive equalization unit 534 performs equalization processing on the received signal output from the chromatic dispersion compensation unit 533 (step S105). The adaptive equalization unit 534 outputs the filter coefficient and frequency offset obtained during the equalization processing to the characteristic function derivation unit 538. Note that in FIG. 5, the description of the frequency and phase offset compensation unit 535 and subsequent units will be omitted.

[0100] The characteristic function derivation unit 538 derives the inverse characteristics of the transmitter 10 and the receiver 20 based on the filter coefficients output from the adaptive equalization unit 534 and the frequency offset (step S106).

[0101] According to the receiver 50 configured as described above, the polarization multiplexed received signal and the phase conjugate signal of the polarization multiplexed received signal are used as input signals, and the adaptive equalizer 534 performs equalization processing on the input signal. The inverse characteristic H T -1 (ω) and the inverse characteristic H R -1 (ω) is calculated. In this way, by analyzing the polarization multiplexed received signal in the receiver 50, it becomes possible to estimate the characteristics of the transmitter 10 and the receiver 50, including the IQ crosstalk between lanes beyond the polarization, core, and mode. Therefore, by inputting the inverse characteristic function to the equivalent filter of the transmitter 10 and the receiver 50 to correct the communication distortion, it is possible to realize a highly efficient and highly reliable optical communication system.

[0102] (Modification of the first embodiment) In the above embodiment, the configuration was shown in which the signals input to the adaptive equalization unit 534 are a set of an IQ signal and a phase conjugate signal of the IQ signal. The signals input to the adaptive equalization unit 534 may be a set of signals mathematically equivalent to the set of an IQ signal and a phase conjugate signal of the IQ signal. For example, a set of four signals, a signal obtained by performing chromatic dispersion compensation on a complex signal and its phase conjugate signal, and a phase conjugate signal of these two signals, is linked to the set of an IQ signal and a phase conjugate signal of the IQ signal by an orthogonal linear transformation. Therefore, the set of four signals described above may be input to the adaptive equalization unit 534 as an input signal. Even in such a configuration, it is possible to measure the characteristics of the transmitter 10 and the characteristics of the receiver 50. During the measurement, the filter coefficients may be converted using an inverse transformation of the set used as the input signal and the set of the IQ signal and its phase conjugate signal.

[0103] Second Embodiment In the first embodiment, only polarization division multiplexing was considered, but in the second embodiment, a configuration will be described in which the polarization division multiplexing is expanded to an arbitrary multiplexing number N (N≧2) by combining space division multiplexing and wavelength division multiplexing in addition to polarization division multiplexing. The basic system configuration of the digital coherent optical transmission system of the second embodiment differs from the digital coherent optical transmission system 1 shown in FIG. 1 in the following points.

[0104] The transmitter 10 further includes a transmitting unit 100 equal to the number of WDM (Wavelength Division Multiplexing) channels. Each transmitting unit 100 outputs an optical signal of a different wavelength. A WDM multiplexer, an optical fiber transmission line 30, and a WDM demultiplexer are provided between the transmitter 10 and the receiver 50. The WDM multiplexer multiplexes the optical signals output by each transmitting unit 100 and outputs the multiplexed optical signal to the optical fiber transmission line 30. The WDM demultiplexer demultiplexes the optical signal transmitted through the optical fiber transmission line 30 according to wavelength. The receiver 50 further includes a receiving unit 500 equal to the number of WDM channels. Each receiving unit 500 receives the optical signal demultiplexed by the WDM demultiplexer 40. The wavelengths of the optical signals received by each receiving unit 500 are different. The above is a configuration in which polarization division multiplexing and wavelength division multiplexing are combined.

[0105] Furthermore, when space division multiplexing is combined, the transmitter 10 transmits polarization multiplexed signals that are spatially multiplexed in N numbers, a device for spatial multiplexing / demultiplexing such as a mode multiplexer / demultiplexer is inserted in addition to a WDM multiplexer and a WDM demultiplexer, and the number of optical front ends 520 in the receiver 50 is the same as the number of spatial multiplexing, and the number of inputs and complex impulse responses of the MIMO equalizer (demodulation digital signal processing unit) is increased to 16N 2 and a point where N pairs of polarization multiplexed signals are demodulated is added. The spatially N multiplexed polarization multiplexed signals are transmitted to the receiver 50 by, for example, a multicore fiber or a multimode fiber.

[0106] 6 is a diagram for explaining an outline of a process for deriving the inverse characteristics of the transmitter 10 and the receiver 20 in the second embodiment. N I,X N The adaptive equalization process is performed on 2N inputs of Q. The characteristic function derivation unit 538 calculates the filter coefficients h1, ..., h2 obtained in the process of the adaptive equalization process by the adaptive equalization unit 534. 4(N 2 ) and exp(jω x1 (n / T)),…,exp(jω xN (n / T)) where ω x1 The 1 in is the subscript of x, and ω xN In the above, N is a subscript of x. The inverse characteristic H T -1 (ω) and the inverse characteristic H of the receiver 50 R -1 (ω) is calculated.

[0107] Fig. 7 is a diagram showing a configuration example of a demodulation digital signal processing unit including an adaptive equalization unit 534 in the second embodiment. Fig. 7 shows the configuration of the adaptive equalization unit 534 in the case where 8x2 MIMO is extended to the case where the number of multiplexing is N, resulting in a (4NxN) MIMO configuration.

[0108] The demodulation digital signal processing unit demodulates the I component signal of the X polarization component of the kth (k is an integer between 1 and N) polarization multiplexed received signal output from the optical front end 520 into a real component X k I, Q component signals are converted to imaginary components X k Q, and the I component signal of the Y polarization component is expressed as real component Y k The I and Q components are converted to imaginary components Y k The demodulation digital signal processor demodulates the k-th polarization multiplexed received signal into a real component X k I, the imaginary component X k Q, real component Y k I and real component Y k Each of Q is convoluted with an impulse response for compensating for the frequency characteristics of the receiver and a complex impulse response for chromatic dispersion compensation according to each component.

[0109] The demodulation digital signal processor outputs the convolved real component X k I, the imaginary component X k Q, real component Y k I and the imaginary component Y k Each of Q is branched into 4N signals. The demodulation digital signal processing unit inputs 2N of the branched 4N signals directly to the adaptive equalization unit 534, and converts the remaining 2N signals into phase conjugate signals and inputs them to the adaptive equalization unit 534.

[0110] Real component X k I, the imaginary component X k Q, real component Y k I, the imaginary component Y k The phase conjugate of each Q is the real component phase conjugate X k I * , imaginary component phase conjugate X k Q * , real component phase conjugate Y k I * , imaginary component phase conjugate Y k Q * Let the real component X k I, the imaginary component X k Q, real component Y k I, the imaginary component Y k Q, real component phase conjugate X k I * , imaginary component phase conjugate Xk Q * , real component phase conjugate Y k I * , and the imaginary component phase conjugate Y k Q * Each of the 2N sets of N corresponds to the X-polarized component and the Y-polarized component of the N polarization multiplexed received signals.

[0111] The adaptive equalization unit 534 calculates 2N real components X1I to X N I, imaginary component X1Q~X N Q, real components Y1I~Y N I, imaginary component Y1Q~Y N Q, real component phase conjugate X1I * ~X N I * , imaginary component phase conjugate X1Q * ~X N Q * , real component phase conjugate Y1I * ~Y N I * , imaginary component phase conjugate Y1Q * ~Y N Q * The adaptive equalizer 534 convolves an impulse response into each of the real components X1I to X1I, which are convolved with an impulse response corresponding to the polarization and each component for each polarization of each polarization multiplexed received signal. N I, imaginary component X1Q~X N Q, real components Y1I~Y N I and imaginary components Y1Q~Y N The demodulation digital signal processing unit performs phase rotation for frequency offset compensation on this sum signal to generate a first sum signal.

[0112] Similarly, the adaptive equalizer 534 outputs, for each polarization of each polarization multiplexed received signal, a real component phase conjugate X1I * ~X N I * , imaginary component phase conjugate X1Q * ~X N Q * , real component phase conjugate Y1I * ~Y N I *, and the imaginary component phase conjugate Y1Q * ~Y N Q * The demodulation digital signal processing unit performs a phase rotation on this sum signal that is opposite to the phase rotation for frequency offset compensation, to generate a second sum signal.

[0113] The demodulation digital signal processing unit adds the first sum signal and the second sum signal generated for each polarization of each polarization multiplexed received signal to obtain a received signal, and then adds (or subtracts) the transmit data bias correction signal for that polarization to perform distortion correction.

[0114] The filter coefficients (h1, . . . , h2) obtained by the processing of the adaptive equalizer 534 in the demodulation digital signal processor described above 4(N 2 ) ) and the frequency offset (exp(jω x1 (n / T)),…,(jω xN (n / T)) is output to the characteristic function derivation unit 538.

[0115] In the second embodiment, the characteristic function derivation unit 538 shown in FIG. 8 calculates the characteristic H T (ω) and the characteristic H of receiver 50 R (ω) is derived.

[0116] The characteristic function derivation unit 538 calculates A(ω) based on the following equation (29), using a matrix M defined by the following equation (28).

[0117]

number

[0118]

number

[0119] The characteristic function derivation unit 538 calculates the inverse characteristic H R -1Calculate (ω).

[0120]

number

[0121] The characteristic function derivation unit 538 adds exp(jω x1 (n / T)),…,exp(jω xN (n / T)) is convoluted to obtain the inverse characteristic H T -1 Calculate (ω).

[0122]

number

[0123] According to the receiver 50 of the second embodiment configured as described above, even when the number of multiplexed signals is increased to N, it is possible to calculate the characteristics of the transmitter / receiver, including the IQ crosstalk across multiple polarized waves.

[0124] (Modification of the second embodiment) As in the first embodiment, the signals input to the adaptive equalization unit 534 may be signals equivalent to an IQ signal and a phase conjugate signal of the IQ signal.

[0125] (Modifications common to the first and second embodiments) The adaptive equalization unit 534 and the characteristic function derivation unit 538 may be configured as a characteristic measurement device for measuring a characteristic function between lanes between the transmitter 10 and the receiver 50. In the above example, the characteristic measurement device is provided in the receiver 50, but the characteristic measurement device may be provided in a housing separate from the receiver 50.

[0126] (Third embodiment) In the third embodiment, a configuration will be described in which signal waveform distortion is compensated for based on the inverse characteristic obtained by the characteristic function derivation unit using the method described in each of the above embodiments. The various transformations and calculations described below are performed by the characteristic function derivation unit. T -1 (ω) and the inverse characteristic of the receiver H R -1 To improve the quality of the signal using (ω), the pre-equalization unit in the transmitter and the front-end correction unit in the receiver are equipped with a function to perform (static) filtering of the signal in the corresponding format, and each inverse characteristic is input as a filter coefficient. T -1 (ω), the inverse characteristic of the receiver, H R -1 (ω) is input as the filter coefficient.

[0127] Here, the inverse characteristic obtained in each of the above embodiments is (s x (ω) (~)s x (ω)s y (ω) (~)s y The filter has the form of a 4 × 4 matrix acting on a signal vector of the form (ω), and in order to use this to compensate for signal waveform distortion, a static filter configuration of 4 × 4 form acting on a signal vector of the above form was required.

[0128] However, in some signal processing circuits in transmitters and receivers, the I component (real component) and Q component (imaginary component) of a signal are the basic elements of signal processing, rather than the complex signal vector of the above format. Therefore, the signal vector is not in the above format, but in the (Re(s x (ω)) Im(s x (ω)) Re(s y (ω)) Im(s y In this case, the characteristic function derivation unit 538 uses H -1 T / R Instead of M -1 H -1 T / RM is the characteristic and can be input into the filter.

[0129] Hereinafter, signal processing using this characteristic function is assumed. In the following, a single mode fiber is assumed as a transmission medium, but the same application is possible even for a spatial multiplexing fiber. In order to further reduce the amount of calculation, as shown in the pre-equalization unit 116 and the front-end correction unit 532 in FIG. 9, there are cases where it is desired to perform distortion compensation by combining smaller filters, for example, 2×2 type filters (for example, static filters 151-1 to 151-3, static filters 550-1 to 550-3) in multiple stages. However, in this case, the inverse characteristic obtained by the above method cannot be directly applied to signal processing. FIG. 9 is a diagram showing an example of the configuration of a digital coherent optical transmission system 1 in the third embodiment. In the third embodiment, the configurations of the pre-equalization unit 116 and the front-end correction unit 532 are different from those of the first embodiment. Hereinafter, the differences will be mainly described.

[0130] As an example of using a combination of multiple stages of 2×2 filters (e.g., static filters 151-1 to 151-3, static filters 550-1 to 550-3), consider applying a filter consisting of three 2×2 static filters that can be applied to the XI lane and XQ lane, the XQ lane and YI lane, and the YI lane and YQ lane, respectively, to a signal as shown in Fig. 10. In this case, the action of the above filter on a signal expressed in the form of the following equation (32) is in the form of the following equation (33), and some components are always 0.

[0131]

number

[0132]

number

[0133] However, M -1 H -1 T / RThe transmitter / receiver characteristic represented by M generally has a component that is not 0, and therefore cannot be handled by the filter of this example configuration. More generally, there are cases where only a specific (i, j) component must be 0 for a characteristic matrix input to a filter for characteristic compensation. In the third embodiment, as described above, when there are multiple static filters that compensate for characteristics between some lanes and it is desired to perform compensation by combining these, a value to be input to the filter can be found from the inverse characteristic by an appropriate calculation.

[0134] The method of transformation is described below in detail. First, the transmission side is described below. Regarding the transmitter characteristics, the same transmitter characteristics can be considered even if the transmission line side is multiplied by a matrix representing an arbitrary polarization rotation and phase rotation. Therefore, an arbitrary H c For (ω), the inverse characteristic of the transmitter, H -1 T From left to right: H c (ω) and input to a filter (e.g., the pre-equalization unit 116) as a function M -1 H c (ω)H -1 T Even if you are M, there will be no change in the compensation effect.

[0135]

number

[0136] Therefore, H c (ω) can be determined so as to satisfy the conditions for the desired filter type. To do so, solve the following equation (35) for all (i,j) such that the (i,j) element of the characteristic matrix is ​​0.

[0137]

number

[0138] Here, the subscripts of the matrix represent the matrix elements. In this technology, the characteristic function derivation unit 538 calculates H c (ω) is obtained, and M-1 H c (ω)H -1 T Let M be the coefficient to be input to the filter. Here, as an example, consider the configuration of FIG. 10. In this case, the above equation (35) is solved for (i,j)=(1,3),(1,4),(4,1),(4,2), and the following equation (36) exists as a solution, so based on this, M -1 H c (ω)H -1 T Once M is calculated, compensation can be made using a filter.

[0139]

number

[0140] For the receiving side characteristics, similarly to the transmitting side, the characteristic function derivation unit 538 calculates H c (ω) is obtained, and M -1 H -1 R H c Let (ω)M be the coefficient input to a filter (eg, front-end correction unit 532).

[0141]

number

[0142] In this case, we solve the above equation (37) for (i,j) = (1,3), (1,4), (4,1), (4,2) as before, and find the solution in the following equation (38), so based on this, we can calculate M -1 H -1 R H c Once (ω)M is calculated, compensation can be made using a filter.

[0143]

number

[0144] Next, as an example, consider the configuration shown in Fig. 11. In this case, by solving the same equations for (i,j)=(1,4),(2,4),(3,1),(4,1), we obtain the following equation (39) for the transmitting side and the following equation (40) for the receiving side.

[0145]

number

[0146]

number

[0147] The method of determining the value to be input to the filter may be other than the above method. For example, as shown in FIG. -1 T From the characteristics of transmitter 10 H T It is possible to calculate the above, multiply it by a known signal by signal processing, and then apply a static filter. In this case, the filter coefficient of the static filter may be determined by the least squares method or the like so that the difference between the signal after the filter processing and the initial known signal is minimized.

[0148] In all the above embodiments, when operating the adaptive filter, the characteristics may be calculated by filtering a known signal or a signal demodulated using an error correction code for the purpose of convergence of the received signal. In this case, the transmitter and receiver are reversed in the equation, and forward characteristics are obtained instead of the inverse characteristics of the transmitter and receiver. By appropriately switching the transmitter and receiver and calculating the inverse matrix, it is possible to make the same arguments as in the above embodiments.

[0149] Some of the functional units of the receiver 50 in the above-described embodiment may be realized by a computer. In this case, a program for realizing the functions may be recorded on a computer-readable recording medium, and the program recorded on the recording medium may be read into a computer system and executed. Note that the term "computer system" here includes hardware such as an OS and peripheral devices.

[0150] In addition, the term "computer-readable recording medium" refers to portable media such as flexible disks, optical magnetic disks, ROMs (Read Only Memory), and CD-ROMs, as well as storage devices such as hard disks built into computer systems. Furthermore, the term "computer-readable recording medium" may also include those that dynamically hold a program for a short period of time, such as a communication line when transmitting a program via a network such as the Internet or a communication line such as a telephone line, and those that hold a program for a certain period of time, such as a volatile memory inside a computer system that serves as a server or client in such a case. Furthermore, the above-mentioned program may be one that realizes part of the above-mentioned functions, or may be one that can realize the above-mentioned functions in combination with a program already recorded in the computer system, or may be one that is realized using a programmable logic device such as an FPGA (Field-Programmable Gate Array).

[0151] Although an embodiment of the present invention has been described in detail above with reference to the drawings, the specific configuration is not limited to this embodiment, and designs that do not deviate from the gist of the present invention are also included. [Industrial Applicability]

[0152] The present invention is applicable to techniques for measuring the characteristics of transmitters and receivers. [Explanation of symbols]

[0153] 10...Transmitter, 30...Optical fiber transmission line, 50...Receiver, 100...Transmitter, 110...Digital signal processing unit, 111...Encoder, 112...Mapping unit, 113...Training signal insertion unit, 114...Frequency change unit, 115...Waveform shaping unit, 116...Pre-equalization unit, 117-1 to 117-4...Digital-to-analog converter (DAC), 120...Modulator driver, 121-1 to 121-4...Amplifier, 130...Light source, 140...Integrated module, 141-1, 141-2...IQ modulator, 142...Polarization synthesis unit, 500...Receiver, 510...Local oscillator light source, 520...Optical front end, 521...Polarization separation unit, 522-1, 522-2...Optical 90-degree hybrid coupler, 523-1 to 523-4...BPD, 524-1 to 524-4...amplifier, 530...digital signal processing unit, 531-1 to 531-4...analog-to-digital converter, 532...front-end correction unit, 533...chromatic dispersion compensation unit, 534...adaptive equalization unit, 535...frequency and phase offset compensation unit, 536...demapping unit, 537...decoding unit, 538...characteristic function derivation unit

Claims

1. A characteristic measuring device for measuring characteristics between lanes of a transmitter and a receiver connected via an optical fiber transmission line, comprising: an adaptive equalization unit that receives as input signals a polarization multiplexed received signal and a phase conjugate signal of the polarization multiplexed received signal, or the received signal and a plurality of signals mathematically equivalent to the phase conjugate signals of the received signal, and performs an equalization process on the input signal; a characteristic function derivation unit that calculates a first inverse characteristic representing an inverse characteristic of the transmitter and a second inverse characteristic representing an inverse characteristic of the receiver based on a filter coefficient obtained during the equalization process performed by the adaptive equalization unit and a frequency offset; A characteristic measuring device comprising:

2. The characteristic function derivation unit Calculating the first inverse characteristic and the second inverse characteristic including IQ crosstalk across a plurality of spatially multiplexed signals; The characteristic measuring device according to claim 1 .

3. The characteristic function derivation unit Calculating the first inverse characteristic and the second inverse characteristic including IQ crosstalk across a plurality of wavelength-multiplexed signals; The characteristic measuring device according to claim 1 .

4. The plurality of signals mathematically equivalent to the received signal and the phase conjugate signal of the received signal are a complex signal, a phase conjugate signal of the complex signal, and a phase conjugate signal of the complex signal that has been subjected to chromatic dispersion compensation, and the phase conjugate signal of the signal that has been subjected to chromatic dispersion compensation. The characteristic measuring device according to claim 1 .

5. The first inverse characteristic and the second inverse characteristic are multiplied by a matrix representing the degrees of freedom of polarization rotation and phase rotation to transform them into a form that can be handled by a combination of filters that compensate for characteristics between some lanes of the transmitter and the receiver. The characteristic measuring device according to claim 1 .

6. The forward characteristic obtained from the first inverse characteristic and the second inverse characteristic is applied to a known signal, and a filter coefficient is determined so as to cancel the forward characteristic, thereby transforming the signal into a form that can be handled by a combination of filters that compensate for characteristics between some lanes of the transmitter and the receiver. The characteristic measuring device according to claim 1 .

7. 4. An optical transmission system comprising: the characteristic measuring device according to claim 1; and the transmitter and the receiver, each having a filter function for compensating for signal waveform distortion based on the first inverse characteristic and the second inverse characteristic obtained by the characteristic measuring device.

8. A characteristics measurement method performed by a characteristics measurement device that measures characteristics between lanes of a transmitter and a receiver connected via an optical fiber transmission line, comprising: an input signal is a polarization multiplexed received signal and a phase conjugate signal of the polarization multiplexed received signal, or a plurality of signals mathematically equivalent to the received signal and the phase conjugate signal of the received signal, and an equalization process is performed on the input signal; calculating a first inverse characteristic representing an inverse characteristic of the transmitter and a second inverse characteristic representing an inverse characteristic of the receiver based on a filter coefficient obtained during the equalization process and a frequency offset; Property measurement methods.

9. A computer program for causing a computer to function as a characteristic measuring device for measuring characteristics between lanes of a transmitter and a receiver connected via an optical fiber transmission line, comprising: an adaptive equalization step of performing an equalization process on a polarization multiplexed received signal and a phase conjugate signal of the polarization multiplexed received signal, or a plurality of signals mathematically equivalent to the received signal and the phase conjugate signal of the received signal, as input signals; a characteristic function derivation step of calculating a first inverse characteristic representing an inverse characteristic of the transmitter and a second inverse characteristic representing an inverse characteristic of the receiver based on a filter coefficient obtained during the equalization process performed in the adaptive equalization step and a frequency offset; A computer program for causing a computer to execute the above.

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