Signal processing circuit, optical receiver, detector and method for compensating for waveform distortion

The present invention is signal processing<u style="single">circuit</u>The present invention relates to an optical receiver, a detection device, and a waveform distortion compensation method, and for example, a signal processing device, an optical receiver, a detection device, and a waveform distortion compensation method for compensating for waveform distortion.

In optical communication systems, optical receivers capable of transmitting signals with transmission rates exceeding 40 Gbit / s or 100 Gbitp / s per wavelength are being developed. In recent years, a digital coherent reception method has attracted attention as a reception method for an optical communication system (for example, Non-Patent Document 1). The digital coherent method is a method in which light intensity and phase information are extracted by a coherent reception method, the extracted light intensity and phase information are digitally converted, and demodulation is performed by a digital signal processing circuit.

On the other hand, in the optical communication system, dispersion compensation such as wavelength dispersion is performed. When the digital coherent reception method is used, the wavelength dispersion of the transmission line can be compensated by using the digital signal processing technology. However, the receiving device estimates and compensates for the wavelength dispersion of the transmission line. As a method of dispersion compensation, it is known to set the dispersion compensation amount based on the number of corrections by the error correction circuit after decoding, that is, to minimize the number of corrections (for example, Patent Documents 1 to 3). ). Further, a method of optimizing the dispersion compensation according to the clock reproduced from the analog clock reproduction circuit is known (for example, Patent Document 4).

<p><patcit num="1"><text>Japanese Unexamined Patent Publication No. 2002-208892</text></patcit><patcit num="2"><text>Japanese Unexamined Patent Publication No. 2004-236097</text></patcit><patcit num="3"><text>Japanese Unexamined Patent Publication No. 2008-58610</text></patcit><patcit num="4"><text>Japanese Patent Application Laid-Open No. 2007-60583</text></patcit></p>

<p><nplcit num="1"><text>IEEE JTL, Vol. 24, pp. 12-21, 2006</text></nplcit></p>

<p> However, in the method of setting the dispersion compensation amount of the waveform distortion compensation circuit for dispersion compensation based on the number of corrections, since the receiving device performs decoding and error correction and then resets the dispersion compensation amount, the optimum dispersion compensation is performed. It takes time to set the amount. Further, in the method using the analog clock reproduction circuit, the analog clock reproduction circuit is used. In an optical transmission system using a digital coherent reception method, the residual dispersion tolerance of the receiving device is large due to the waveform distortion compensation of digital signal processing, so that the amount of wavelength dispersion compensation is large in analog, and clock reproduction is difficult. Therefore, it is difficult to set the dispersion compensation amount of the waveform distortion compensation circuit for wavelength dispersion compensation using the analog clock reproduction circuit in the digital coherent reception method. As described above, in the digital coherent optical receiver, it is an issue to appropriately perform dispersion compensation.</p><p> This signal processing<u style="single">circuit</u>, The optical receiver, the detection device and the waveform distortion compensation method aim at appropriately performing dispersion compensation.</p>

<p> For example, a plurality of digital electric signals converted by sampling a plurality of analog electric signals obtained by photoelectric conversion of a plurality of optical signals including the intensity and phase information of the received optical signal are used. The first compensation circuit that compensates for the waveform distortion corresponding to the wavelength dispersion of the received light signal, and<u style="single">By the first compensation circuit</u>Wavelength dispersion compensation that controls the compensation amount of wavelength dispersion in the first compensation circuit based on the phase shift detection output value between the sampling signal and the modulation frequency of the received light signal from the digital electric signal whose wavelength dispersion is compensated. A signal processing circuit including a control circuit is used.</p><p> Further, for example, a plurality of analog electric signals obtained by photoelectric conversion of a plurality of optical signals including the intensity and phase information of the received optical signal are converted into a plurality of digital electric signals by sampling using the sampling signal. An AD conversion circuit, a first compensation circuit that compensates for waveform distortion corresponding to the wavelength dispersion of the received light signal from the plurality of digital electric signals, and a first compensation circuit.<u style="single">By the first compensation circuit</u>Wavelength dispersion compensation control that controls the compensation amount of wavelength dispersion in the first compensation circuit based on the phase shift detection output value of the modulation frequencies of the sampling signal and the received light signal from the digital electric signal whose wavelength dispersion is compensated. An optical receiver comprising a circuit is used.</p><p> Further, for example, a plurality of digital electric signals converted by sampling a plurality of analog electric signals obtained by photoelectric conversion of a plurality of optical signals including the intensity and phase information of the received optical signal by using a sampling signal. The first compensation circuit that compensates for the waveform distortion corresponding to the wavelength dispersion of the received light signal, and<u style="single">By the first compensation circuit</u>A detection circuit that detects the wavelength dispersion amount of the received light signal based on the phase shift detection output value between the sampled signal and the modulation frequency of the received light signal from the digital electric signal whose wavelength dispersion is compensated is provided. Use a detection device.</p><p> Further, for example, a plurality of digital electric signals converted by sampling a plurality of analog electric signals obtained by photoelectric conversion of a plurality of optical signals including the intensity and phase information of the received optical signal by using a sampling signal. The step of compensating for the waveform distortion corresponding to the wavelength dispersion of the received light signal of<u style="single">By the step of compensating for the waveform distortion</u>Waveform distortion including a step of controlling a compensation amount for compensating for the wavelength dispersion based on a phase shift detection output value between the sampling signal and the modulation frequency of the received light signal from the digital electric signal whose wavelength dispersion is compensated. Use the compensation method.</p>

<p> This signal processing<u style="single">circuit</u>According to the optical receiver, the detection device, and the waveform distortion compensation method, dispersion compensation can be appropriately performed.</p>

<figref num="1">FIG. 1 is a block diagram showing an optical receiver.</figref><figref num="2">FIG. 2 is a block diagram of a polarization diversity 90 ° hybrid circuit.</figref><figref num="3">FIG. 3 is a block diagram around the waveform distortion compensation circuit.</figref><figref num="4">FIG. 4 is a block diagram showing an example of a wavelength dispersion compensation circuit.</figref><figref num="5">FIG. 5 is a block diagram showing another example of the wavelength dispersion compensation circuit.</figref><figref num="6">FIG. 6 is a diagram showing an example of an adaptive equalization type waveform distortion compensation circuit.</figref><figref num="7">FIG. 7 is a schematic diagram showing the detection output value of the sampling phase shift detector with respect to the sampling phase shift.</figref><figref num="8">FIG. 8 is a schematic diagram showing the sensitivity of sampling phase shift detection with respect to the shift of wavelength dispersion compensation.</figref><figref num="9">FIG. 9 is a flowchart showing the process of the first embodiment.</figref><figref num="10">FIG. 10 is a block diagram of the vicinity of the waveform distortion compensation circuit of the second embodiment.</figref><figref num="11">FIG. 11 is a block diagram of the vicinity of the waveform distortion compensation circuit of the third embodiment.</figref><figref num="12">FIG. 12 is a flowchart showing the process of the third embodiment.</figref><figref num="13">FIG. 13 is a block diagram of the wavelength dispersion compensation control circuit of the fourth embodiment.</figref><figref num="14">FIG. 14 is a diagram for explaining a method in which the maximum / minimum detection circuit of the fourth embodiment detects the maximum / minimum value.</figref><figref num="15">FIG. 15 is a block diagram of the vicinity of the waveform distortion compensation circuit of the fifth embodiment.</figref><figref num="16">FIG. 16 is a flowchart showing the process of the fifth embodiment.</figref><figref num="17">FIG. 17 is a block diagram of the application equalization type waveform distortion compensation circuit of the sixth embodiment.</figref><figref num="18">FIG. 18 is a flowchart showing the process of step S32 of FIG.</figref><figref num="19">FIG. 19 is a block diagram of the optical receiver of the seventh embodiment.</figref><figref num="20">FIG. 20 is a block diagram of the wavelength dispersion detection device of the eighth embodiment.</figref>

Hereinafter, examples of the present invention will be described with reference to the drawings.

<p> FIG. 1 is a block diagram showing an optical receiver. With reference to FIG. 1, the received optical signal is, for example, a single-wavelength optical signal obtained by demultiplexing a wavelength-multiplexed optical signal. The received optical signal has a transmission rate of, for example, 40 Gbit / s or 100 Gbit / s, and is QPSK (quadrature phase shift key) modulated. The received light signal is mixed with the station emission output from the station emission oscillator 42 in the polarization diversity 90 degree hybrid circuit 40, and two orthogonally polarized real part signals and imaginary part signals of the received light signal are extracted. Hereinafter, the real part signal is referred to as an I phase (In Phase), and the imaginary part signal is referred to as a Q phase (Quadrature phase). As a result, the received optical signal is separated into a plurality of optical signals including intensity and phase information.</p><p> The O / E (photoelectric conversion circuit) 44 converts a plurality of analog optical signals corresponding to the I phase and the Q phase of each polarization output by the polarization diversity 90 ° hybrid circuit 40 into a plurality of analog electric signals. The ADC (Analog Digital converter) 20 converts a plurality of analog electric signals converted by the O / E44 into a plurality of digital electric signals by sampling at a timing synchronized with the sampling signal. The sampling clock generation circuit 22 outputs a sampling signal used by the ADC 20 for AD conversion. The sampling frequency is, for example, twice the modulation frequency of the received optical signal. The output of the ADC 20 is input to the digital signal processing circuit 10.</p><p> The digital signal processing circuit 10 includes a waveform distortion compensation circuit 12, a carrier frequency / phase synchronization circuit 14 that compensates for the phase shift between the received optical signal carrier wave and the station emission, an identification demodulation circuit 16, and a sampling phase shift detection circuit 28. .. The waveform distortion compensation circuit 12 is a circuit that compensates for waveform distortion caused by an optical signal propagating in an optical transmission line. The waveform distortion compensated by the waveform distortion compensation circuit 12 includes wavelength dispersion, polarization variation, polarization mode dispersion, and the like. The carrier frequency / phase-locked loop 14 corrects the phase rotation due to the carrier frequency and the frequency of station emission or the phase shift, and synchronizes with the carrier frequency. The identification demodulation circuit 16 identifies the signal and demodulates it into a digital signal before QPSK modulation.</p><p> FIG. 2 is a block diagram of the polarization diversity 90 ° hybrid circuit 40. As shown in FIG. 2, the polarization diversity 90 ° hybrid circuit 40 includes two polarization beam splitters 46a and 46b and two 90 ° hybrids 48a and 48b. The polarization beam splitters 46a and 46b split the received light signal and the station emission into two polarization direction light signals, respectively. The 90 ° hybrids 48a and 48b extract phase I and phase Q from the optical signal using local emission in their respective polarization directions.</p><p> The waveform distortion compensation circuit will be described in detail with reference to FIG. The waveform distortion compensation circuit 12 includes a wavelength dispersion compensation circuit 24 as a first compensation circuit, an adaptive equalization type waveform distortion compensation circuit 26 and a wavelength dispersion compensation control circuit 30 as a second compensation circuit. The wavelength dispersion compensating circuit 24 compensates for the wavelength dispersion of the waveform distortion. In this way, the wavelength dispersion compensation circuit 24 compensates for the waveform distortion corresponding to the wavelength dispersion of the optical signal in the digital electric signal. The adaptive equalization type waveform distortion compensation circuit 26 adaptively equalizes and compensates for the waveform distortion remaining in the wavelength dispersion compensation circuit 24. For example, the fluctuation of polarization, the polarization dispersion mode, and the wavelength dispersion that the wavelength dispersion compensation circuit 24 cannot compensate are compensated.</p><p> FIG. 4 is a block diagram showing an example of the wavelength dispersion compensation circuit 24. As shown in FIG. 4, the wavelength dispersion compensation circuit 24 is an FIR (Finite Impulse Response) filter and includes a delay device 50, an FIR coefficient, a multiplier 52, and an adder 56. The delayer 50 delays the signal by time τ. The multiplier 52 determines each delayed signal and the FIR coefficient C._kMultiply with (n). Here, k indicates the number of coefficients, and FIG. 4 shows an example in which k is 1 to 5. The number of coefficients can be set arbitrarily. The adder 56 adds each multiplied signal. Wavelength dispersion can be compensated by setting the FIR coefficient appropriately.</p><p> FIG. 5 is a block diagram showing another example of the wavelength dispersion compensation circuit 24. In FIG. 5, the wavelength dispersion compensation circuit 24 includes a time-frequency domain conversion unit 60, a frequency domain linear compensation unit 62, and a frequency-time domain conversion unit 64. The time-frequency domain transforming unit 60 performs FFT (Fast Fourier Transform) on the input signal and converts it into a signal in the frequency domain. The frequency domain linear compensation unit 62 performs wavelength dispersion compensation in the frequency domain. The frequency-time domain converter 64 reverse-FFTs the wavelength dispersion-compensated signal and converts it into a time domain signal. In this way, wavelength dispersion compensation may be performed in the frequency domain.</p><p> FIG. 6 is a diagram showing an example of the adaptive equalization type waveform distortion compensation circuit 26. The adaptive equalization type waveform distortion compensation circuit 26 includes FIR filters 70a to 70d, adders 72a and 72b, and a weighting coefficient calculation unit 74. A parallel polarization component signal Ih + jQh and a vertically polarization component signal Iv + jQv are input to the adaptive equalization type waveform distortion compensation circuit 26. The signal Ih + jQh is input to the FIR filters 70a and 70b, and the signal Iv + jQv is input to the FIR filters 70c and 70d. The adder 72a adds the outputs of the FIR filters 70a and 70c and outputs them as signals Ix + jQx. The adder 72b adds the outputs of the FIR filters 70b and 70d and outputs them as signals Iy + jQy. The weighting coefficient calculation unit 74 calculates the weighting coefficients of the FIR filters 70a to 70d based on the input signals Ih + jQh and Iv + jQv and the output signals Ix + jQx and Iy + jQy. For example, the weighting coefficient calculation unit 74 calculates the weighting coefficient so that the waveform distortion of the output signal is largely compensated (that is, the waveform distortion due to residual dispersion is reduced, for example). With such a configuration, in addition to wavelength dispersion, polarization mode dispersion and the like can be compensated.</p><p> Returning to FIG. 3, the sampling signal control circuit 29 includes a sampling phase shift detection circuit 28. The sampling phase shift detection circuit 28 detects the phase shift between the sampling signal at the time of AD conversion and the analog electric signal input to the ADC 20, that is, the modulation frequency of the received light signal. The sampling signal control circuit 29 controls the sampling clock generation circuit 22 based on the phase shift detection output value to control the phase or frequency of the sampling signal. This makes it possible to synchronize the modulation frequency of the sampling signal and the received light signal.</p><p> The wavelength dispersion compensation control circuit 30 includes a sensitivity monitor 34 and a wavelength dispersion compensation amount setting unit 32. The sensitivity monitor 34 monitors the sensitivity of sampling phase shift detection. The wavelength dispersion compensation amount setting unit 32 sets the compensation amount of the wavelength dispersion compensation circuit 24 based on the monitored sensitivity. For example, in the example where the wavelength dispersion compensation circuit 24 is shown in FIG. 4, the FIR coefficient C_kSet the value of (n).</p><p> FIG. 7 is a schematic diagram showing the detection output value of the sampling phase shift detector with respect to the sampling phase shift. In FIG. 7, the sampling phase shift (the phase shift between the sampling signal and the analog electric signal input to the ADC 20) is defined as 1 when the sampling signal is shifted by one cycle. When the sampling phase shift is 0, it indicates that the sampling signal and the analog electric signal input to the ADC 20 are in phase. When the sampling phase shift deviates from 0, it indicates that a phase difference is generated between the sampling signal and the analog electric signal input to the ADC 20. The solid line indicates the case where the wavelength dispersion is almost compensated in the wavelength dispersion compensation circuit 24, and the broken line indicates the case where the wavelength dispersion remains. With reference to the solid line in FIG. 7, when the phase shift is 0, the sampling phase is not shifted and the detected output value is 0. If the sampling phase deviates from 0, the detected output value deviates from 0. The absolute value of the slope (Δ detection output value / Δ phase shift) whose phase shift is near 0 is used as the sensitivity for sampling phase shift detection. When the sampling phase shifts by half a wavelength, the detected output value returns to 0. When the wavelength dispersion remains as shown by the broken line, the waveform is distorted due to the wavelength dispersion, so that the phase shift detection sensitivity becomes small.</p><p> FIG. 8 is a schematic diagram showing the sensitivity of sampling phase shift detection with respect to the shift of wavelength dispersion compensation. In FIG. 8, the wavelength dispersion compensation deviation indicates the deviation of the compensation amount of the wavelength dispersion compensation circuit 24 from the wavelength dispersion amount of the received optical signal. When the wavelength dispersion compensation deviation is 0, the wavelength dispersion compensation circuit 24 indicates that the wavelength dispersion can be substantially compensated. When the wavelength dispersion compensation deviation deviates from 0, it indicates that the residual wavelength dispersion that cannot be compensated by the wavelength dispersion compensation circuit 24 is large. As shown in FIG. 8, when the wavelength dispersion compensation deviation is 0, the sensitivity of sampling phase shift detection is the highest, and when the wavelength dispersion compensation deviation deviates from 0, the sensitivity decreases. FM Gardener, ABPSK / QPSK Timing-Error Detector for Sampled Receiver, IEEE Trans. Commun., VOL. COM-34, No. 5, May 1986 and T. Tanimura et. The method described in al, Digital clock recovery algorithm for optical coherent receivers operating independent of laser frequency offset, ECOC2008, Mo.3.D.2 can be used.</p><p> FIG. 9 is a flowchart showing the process of the first embodiment. As shown in FIG. 9, the sampling signal control circuit 29 stops the feedback of the sampling phase shift to the sampling clock generation circuit 22. As a result, the phase-locked loop of the sampling signal becomes an open loop (step S10). As a result, the frequency of the sampling signal deviates from the modulation frequency of the signal input to the ADC22. Therefore, the sampling phase shift is swept. The wavelength dispersion compensation amount setting unit 32 sets the compensation amount (for example, the FIR coefficient in FIG. 4) to the initial value (step S12). The sensitivity monitor 34 monitors the sensitivity of sampling phase shift detection (step S14). The wavelength dispersion compensation amount setting unit 32 determines whether the sensitivity is maximum (step S16). If No, the wavelength dispersion compensation amount setting unit 32 resets the compensation amount (step S18).</p><p> Then, the process returns to step S14. If Yes in step S16, the sampling signal control circuit 29 starts feeding back the sampling phase shift to the sampling clock generation circuit 22. As a result, the phase-locked loop of the sampling signal becomes a closed loop (step S20). As a result, the sampling signal can be synchronized with the modulated signal of the signal input to the ADC22. Next, the adaptive equalization type waveform distortion compensation circuit 26 performs adaptive equalization distortion compensation. After that, the carrier frequency / phase synchronization circuit 14 and the identification demodulation circuit 16 operate.</p><p> An example of a method of setting the compensation amount of the wavelength dispersion compensation amount setting unit 32 in steps S12 to S18 will be described. First, the maximum dispersion compensation amount is set as the initial value in step S12. In step S18, the compensation amount is reduced each time. Sensitivity increases every time. In step S16, when the sensitivity becomes smaller than the sensitivity one step before, the sensitivity one step before is set as the maximum sensitivity. As another example, in step S16, an initial value as a guide is set. In step S18, the compensation amount can be alternately changed from the initial value to the larger side and the smaller side, and the compensation amount can be set so that the compensation amount moves away from the initial value each time the number of times is increased.</p><p> According to the first embodiment, as shown in steps S14 to 18 of FIG. 9, the wavelength dispersion compensation control circuit 30 modulates the sampling signal and the received optical signal from the digital electric signal in which the wavelength dispersion compensation circuit 24 compensates for the wavelength dispersion. The compensation amount of wavelength dispersion in the wavelength dispersion compensation circuit 24 is controlled based on the phase shift detection output value with respect to the frequency. In this way, by controlling the compensation amount of the wavelength dispersion based on the phase shift detection output value of the sampling, the compensation of the wavelength dispersion can be appropriately performed.</p><p> Further, the wavelength dispersion compensation control circuit 30 controls the compensation amount of the wavelength dispersion so that the phase shift detection detection value becomes large. For example, as shown in step S16 of FIG. 9, the compensation amount is controlled so that the detection sensitivity of the sampling phase shift is maximized. As a result, as shown in FIG. 8, the residual wavelength dispersion of the wavelength dispersion compensation circuit 24 can be suppressed.</p><p> In the first embodiment, as the waveform distortion compensation, the wavelength dispersion compensation circuit 24 compensates for the wavelength dispersion, and the waveform distortion remaining in the wavelength dispersion compensation circuit is adaptively equalized by using the adaptive equalization type waveform distortion compensation circuit 26. Compensate. As a result, the circuit scale of the adaptive equalization type waveform distortion compensation circuit 26 can be reduced as compared with the case where the waveform distortion is performed only by the adaptive equalization type waveform distortion compensation circuit. Therefore, the followability of adaptive control is improved.</p><p> When the circuit scale of the adaptive equalization type waveform distortion compensation circuit 26 is reduced, the dispersion range to which the adaptive equalization type waveform distortion compensation circuit 26 can be applied becomes smaller. Therefore, it is required to reduce the residual dispersion in the wavelength dispersion compensation circuit 24. For example, when the setting of the wavelength dispersion compensation circuit 24 is estimated from the type and distance of the optical transmission line, if the estimation is different from the actual wavelength dispersion, the residual dispersion becomes large. Further, when the wavelength dispersion is controlled based on the number of corrections of the error correction circuit after decoding as in Patent Documents 1 to 3, the compensation amount is controlled because the compensation amount is controlled after the correction number is calculated. It takes time. According to the first embodiment, since the compensation amount of the wavelength dispersion compensation circuit 24 is appropriately set based on the sampling phase shift detection sensitivity, the residual dispersion in the wavelength dispersion compensation circuit 24 can be reduced. Therefore, the circuit scale of the adaptive equalization type waveform distortion compensation circuit 26 can be reduced. Further, the compensation amount can be controlled more accurately than the case of estimating from the type and distance of the optical transmission line. Compared to the case where the wavelength dispersion is controlled based on the number of corrections as in Patent Documents 1 to 3, high-speed control is possible because the sampling clock and the modulation frequency are not synchronized or demodulated.</p><p> Further, according to the first embodiment, as shown in step S10 of FIG. 9, the sampling signal control circuit 29 controls the phase or frequency of the sampling signal when the wavelength compensation compensation control circuit 30 controls the compensation amount of the wavelength dispersion. To stop. As a result, the phase of the sampling signal is swept, and as shown in FIG. 7, the sensitivity of sampling phase shift detection can be monitored.</p>

<p> The second embodiment is an example in which the wavelength dispersion compensation is controlled based on the maximum value or the minimum value of the sampling phase shift detection output value. FIG. 10 is a block diagram of the vicinity of the waveform distortion compensation circuit 12 of the second embodiment. With reference to FIG. 10, the wavelength dispersion compensation control circuit 30 includes a wavelength dispersion compensation amount setting unit 32, a sampling phase shift detection circuit 37, and a maximum / minimum detection circuit 38. The sampling phase shift detection circuit 37 detects the sampling phase shift in the same manner as the sampling phase shift detection circuit 28. For example, in FIG. 7, the maximum / minimum detection circuit 38 detects the maximum value and the minimum value of the sampling phase shift detection output value with respect to the sampling phase shift. In step S16 of FIG. 9, the wavelength dispersion compensation amount setting unit 32 determines whether or not the sensitivity is maximum depending on whether or not the maximum value of the sampling phase shift detection output value is the maximum value or whether or not the minimum value is the minimum value. Can be done. Other configurations are the same as those in FIG. 3 of the first embodiment, and the description thereof will be omitted.</p><p> As a method of controlling the compensation amount of the wavelength dispersion so that the wavelength dispersion compensation control circuit 30 increases the detection output amount of the phase shift, it can be based on the sampling phase shift detection sensitivity as in the first embodiment. Further, as in the second embodiment, the maximum value or the minimum value of the sampling phase shift detection output value can be used as the sampling phase shift detection sensitivity. Further, as the sampling phase shift detection sensitivity, for example, the effective value of the detection output value of the sampling phase shift detection circuit 37 can be used. In FIG. 10, the sampling phase shift detection circuit 28 and the sampling phase shift detection circuit 37 have different configurations, but may be shared.</p>

<p> The third embodiment is an example in which the wavelength dispersion compensation control circuit uses a signal having a frequency different from the natural frequency of the modulation frequency of the optical signal as a sampling signal when controlling the compensation amount of the wavelength dispersion. FIG. 11 is a block diagram of the vicinity of the waveform distortion compensation circuit 12 of the third embodiment. With reference to FIG. 11, for example, a fixed frequency oscillation circuit 21 and a switch 23 are provided as signal generation circuits. The fixed frequency oscillation circuit 21 generates a signal having a frequency other than a natural number multiple of the modulation frequency of the optical signal. The switch 23 selects either the output signal of the sampling clock generation circuit 22 or the fixed frequency oscillation circuit 21 and supplies it to the ADC 20 as a sampling signal. Other configurations are the same as those in FIG. 3 of the first embodiment, and the description thereof will be omitted.</p><p> FIG. 12 is a flowchart showing the process of the third embodiment. As shown in FIG. 12, the switch 23 uses the output signal of the fixed frequency oscillation circuit 21 as a sampling signal (step S30). After that, steps 12 to S18 in FIG. 9 are performed. The switch 23 uses the output signal of the sampling clock generation circuit 22 as the sampling signal (step S32). Then, the process proceeds to step S22. Other processes are the same as those in FIG. 9 of the first embodiment, and the description thereof will be omitted.</p><p> According to the third embodiment, as shown in step S30 of FIG. 12, the fixed frequency oscillation circuit 21 has a natural multiple of the modulation frequency of the optical signal when the wavelength dispersion compensation control circuit 30 controls the compensation amount of the wavelength dispersion. Outputs signals of different frequencies as sampling signals. Since the sampling frequency is different from the modulation frequency of the signal, the sampling phase shift in FIG. 7 can be swept quickly. Therefore, the measurement speed of the detected value sensitivity and the like can be increased. Further, in the third embodiment, when the wavelength dispersion compensation control circuit 30 controls the compensation amount of the wavelength dispersion, the sampling signal control circuit 29 does not have to stop the feedback of the sampling phase shift to the sampling clock generation circuit 22.</p>

<p> The fourth embodiment is an example in which the output of the wavelength dispersion compensation circuit is delayed and the phase shift is detected. FIG. 13 is a block diagram of the wavelength dispersion compensation control circuit 30 of the fourth embodiment. The delay circuits 39a to 39n delay the output of the wavelength dispersion compensation circuit 24 and output the output to the sampling phase shift detection circuits 36a to 36n, respectively. For example, an FIR filter can be used as the delay circuits 39a to 39n. The maximum / minimum detection circuit 38 detects the maximum value or the minimum value of the sampling phase shift detection value based on the outputs of the sampling phase shift detection circuits 36a to 36n.</p><p> FIG. 14 is a diagram for explaining a method in which the maximum / minimum detection circuit of the fourth embodiment detects the maximum / minimum value. As shown in FIG. 14, the delay time τ0 to τn of the delay circuits 39a to 39n corresponds to the phase shift between the output signal of the delay circuits 39a to 39n and the sampling signal. Delay circuits 39a-39n delay digital electrical signals for different times. As a result, a phase shift can be caused, and the sensitivity or the maximum / minimum value of the sampling phase shift detection value can be detected.</p><p> According to the fourth embodiment, the wavelength dispersion compensation control circuit 30 sets the phase shift detection output value between the sampling signal and the modulation frequency of the received optical signal calculated by using the digital electric signals delayed by the delay circuits 39a to 39n. Based on this, the compensation amount of the wavelength dispersion of the wavelength dispersion compensation circuit 24 can be controlled. As a result, the wavelength dispersion compensation control circuit 30 can control the amount of compensation for the wavelength dispersion of the wavelength dispersion compensation circuit 24 even when the sampling signal is fixed in a state where the phase shift is small.</p>

<p> Example 5 is an example of controlling a wavelength dispersion compensation circuit using residual dispersion and signal quality. FIG. 15 is a block diagram of the vicinity of the waveform distortion compensation circuit 12 of the fifth embodiment. A residual dispersion monitor 76 and a signal quality monitor 80 are provided as compared with the first embodiment. The residual variance monitor 76 monitors the residual variance of the adaptive equalization type waveform distortion compensation circuit 26 based on the weighting coefficient calculated by the weighting coefficient calculation unit 74 of FIG. The signal quality monitor 80 monitors the signal quality obtained by demodulating the digital electric signal by the identification demodulation circuit 16. The signal quality is, for example, the noise distribution of the signal.</p><p> FIG. 16 is a flowchart showing the process of the fifth embodiment. As shown in FIG. 16, after step S22, the wavelength dispersion compensation control circuit 30 controls the wavelength dispersion compensation circuit 24 based on the signal quality (step S34). For example, the wavelength dispersion compensation circuit 24 is controlled so that the signal quality is improved. Next, the wavelength dispersion compensation control circuit 30 controls the wavelength dispersion compensation circuit 24 based on the residual dispersion (step S36). For example, the wavelength dispersion compensation circuit 24 is controlled so that the residual dispersion becomes small. Other processing is the same as in FIG. 9, and the description thereof will be omitted.</p><p> If the wavelength dispersion cannot be sufficiently compensated up to step S20 in FIG. 16, the wavelength dispersion of the signal input to the applied equalization type waveform distortion compensation circuit 26 is the range in which the applied equalization type waveform distortion compensation circuit 26 can compensate the dispersion. It may be outside. In this range, the signal demodulated by the identification demodulation circuit 16 has poor signal quality. Therefore, as shown in step S34 of FIG. 16, the wavelength dispersion compensation control circuit 30 controls the wavelength dispersion compensation circuit 24 based on the signal quality. For example, the wavelength dispersion compensation control circuit 30 sets the compensation amount until the signal quality becomes constant. In this way, the signal quality is used so that the wavelength dispersion of the output of the wavelength dispersion compensation circuit 24 is within the dispersion compensation range of the equalized waveform distortion compensation circuit 26. Next, as in step S36, the wavelength dispersion compensation control circuit 30 finely adjusts the wavelength dispersion compensation amount based on the residual dispersion of the applied equalization type waveform distortion compensation circuit 26. Here, a method for fine-tuning the amount of chromatic dispersion compensation based on the residual dispersion is described in, for example, Liu et al., OFC2009, JWA36.</p><p> As described above, the wavelength dispersion compensation control circuit 30 controls the compensation amount of wavelength dispersion based on the sampling phase shift detection output value as in Examples 1 to 4. After that, the wavelength dispersion compensation amount is controlled based on the signal quality of the demodulated signal obtained by demodulating a plurality of digital electric signals. After that, it is preferable to control the wavelength dispersion compensation amount based on the dispersion amount compensated by the applied equalization type waveform distortion compensation circuit 26.</p><p> Further, in addition to the wavelength dispersion control using sampling phase shift as in Examples 1 to 4, the wavelength dispersion compensation circuit 24 may be controlled by using at least one of signal quality and residual dispersion. As a result, waveform distortion can be suppressed. Further, when the wavelength dispersion of the transmission line fluctuates during operation, it is possible to finely adjust the compensation amount of the wavelength dispersion compensation circuit based on the residual division monitor.</p>

<p> The sixth embodiment is an example of monitoring the residual dispersion amount of the applied equalization type waveform distortion compensation circuit 26. FIG. 17 is a block diagram of the application equalization type waveform distortion compensation circuit 26 of the sixth embodiment. The digital signal processing circuit 10 may develop the digital electric signal AD-converted by the ADC 22 in parallel and perform pipeline processing. For example, when processing a 25 GHz signal at 500 MHz, 50 parallel polarized signals and vertically polarized signals are processed, respectively. In FIG. 17, a plurality of parallel polarization signals are shown by Ih1 + jQh1 to Ihn + jQhn, and a plurality of vertically polarization signals are shown by Iv1 + jQv1 to Ivn + jQvn. The FIR filters 82a to 82n are butterfly type FIR filters, and correspond to the FIR filters 70a to 70d and the adders 72a and 72b in FIG.</p><p> Parallel polarization signals Ih1 + jQh1 to Ihn + jQhn and vertical polarization signals Iv1 + jQv1 to Ivn + jQvn are input to the FIR filters 82a to 82n, respectively. The FIR filters 82a to 82n output the output signals Ix1 + jQx1 to Ixn + jQxn and the signals Iy1 + jQy1 to Iyn + jQyn, respectively. The weighting coefficient calculation unit 84 calculates the weighting coefficient of the FIR filter so that the waveform distortion of the signal becomes small. The second residual variance monitor 86 calculates the second residual variance of the applied equalization type waveform distortion compensation circuit 26.</p><p> The FIR filter 92 is a butterfly type FIR filter, and Ih1 + jQh1 and Iv1 + jQv1 are input. The number of taps of the FIR filter 92 is larger than that of the FIR filters 82a to 82n. The weighting coefficient calculation unit 94 calculates the weighting coefficient of the FIR filter 92. The first residual variance monitor 96 calculates the first residual variance of Ih1 + jQh1 and Iv1 + jQv1. The switch 88 outputs the output of either the first residual dispersion monitor 96 or the second residual dispersion monitor 86 as the residual dispersion amount to the wavelength dispersion compensation amount setting unit 32.</p><p> FIG. 18 is a flowchart showing the details of the process of step S32 of FIG. 16 in the sixth embodiment. First, the switch 88 outputs the first residual dispersion amount output by the first residual dispersion monitor 96 to the wavelength dispersion compensation amount setting unit 32. The wavelength dispersion compensation amount setting unit 32 controls the wavelength dispersion compensation amount based on the first residual dispersion amount (step S34). Next, the switch 88 outputs the second residual dispersion amount output by the second residual dispersion monitor 86 to the wavelength dispersion compensation amount setting unit 32. The wavelength dispersion compensation amount setting unit 32 finely adjusts the wavelength dispersion compensation amount based on the second residual dispersion amount (step S36).</p><p> With reference to FIG. 17, in the applied equalization type waveform distortion compensation circuit 26, the range in which dispersion compensation is possible can be increased by increasing the number of taps of the FIR filters 82a to 82n. However, if the number of taps of the FIR filters 82a to 82n is increased, the circuit scale of the FIR filters 82a to 82n becomes larger. In addition, the followability of adaptation equalization deteriorates. Therefore, in the initial step in which a signal having a relatively large wavelength dispersion is input, the wavelength dispersion compensation circuit 24 is controlled by using the residual dispersion amount using the FIR filter 92 having a large number of taps. Since the FIR filter 92 has a large number of taps, the dispersion compensation range is wide. Therefore, even if a signal having a relatively large wavelength dispersion is input, the residual dispersion amount can be calculated more accurately. Further, the FIR filter 92 is input with a signal of a part (for example, one) of the signals expanded in parallel. Therefore, even if the number of taps is large, it does not significantly affect the circuit scale of the entire applied equalization type waveform distortion compensation circuit 26. In addition, the followability of adaptation equalization is also good.</p><p> On the other hand, in the fine adjustment of the compensation amount of the wavelength dispersion compensation circuit 24 after step S36 in FIG. 18, the wavelength dispersion of the output signal of the wavelength dispersion compensation circuit 24 is not large. Therefore, even if FIR filters 82a to 82n having a small number of laps are used, control within a range in which dispersion compensation is possible is possible.</p><p> As described above, according to the sixth embodiment, the wavelength dispersion compensation control circuit 30 controls the wavelength dispersion compensation amount based on the first residual dispersion amount calculated by using a part of the signals obtained by developing the digital electric signals in parallel. After that, the wavelength dispersion compensation amount is controlled based on the second residual dispersion amount calculated using all the signals obtained by developing the digital electric signals in parallel. The number of taps of the FIR filter 92 for calculating the first residual dispersion amount is larger than the number of taps of the FIR filters 82a to 82n for calculating the second residual dispersion amount. This makes it possible to control the wavelength dispersion in the initial step even if the wavelength dispersion is relatively large.</p><p> In the sixth embodiment, the compensation amount of the wavelength dispersion based on the sampling phase shift detection is controlled, the compensation amount of the wavelength dispersion based on the signal quality is controlled, and the compensation amount of the wavelength dispersion based on the residual dispersion is controlled. However, only the compensation amount of the wavelength dispersion based on the residual dispersion of Example 6 may be controlled.</p>

<p> Example 7 is an example in which polarization diversity is not used. FIG. 19 is a block diagram of the optical receiver of the seventh embodiment. With reference to FIG. 19, the 90 ° hybrid circuit 40 of the optical receiver 100a of Example 7 does not include a polarizing beam splitter and separates the received optical signal into phase I and phase Q. Therefore, two O / E44s and two ADC20s are provided. Other configurations are the same as those in FIG. 1 of the first embodiment, and the description thereof will be omitted. As described above, the light receiving devices of Examples 1 to 6 do not have to use the polarization diversity method.</p><p> Although QPSK has been described as an example of the optical signal modulation method in Examples 1 to 7, mPSK (M-ary PSK), QAM (Quadrature amplitude modulation) or the like may be used as the modulation method. Further, the optical signal may be multiplexed by using a multiplexing method such as OFDM (Othogonal frequency division multiplexing) or FDM, or may use a polarization multiplexing method.</p>

<p> Example 8 is an example of a wavelength dispersion amount detection device. FIG. 20 is a block diagram of the wavelength dispersion detection circuit of the eighth embodiment. As shown in FIG. 20, the wavelength dispersion detection device 110 of the eighth embodiment includes a wavelength dispersion compensation circuit 24 and a detection circuit 30a. The operation of the wavelength dispersion compensation circuit 24 is the same as that in the first embodiment. The detection circuit 30a has the same configuration as the wavelength dispersion compensation control circuit 30 of the first embodiment, and outputs the compensation amount of the wavelength dispersion of the wavelength dispersion compensation circuit 24 as the wavelength dispersion amount of the optical signal. That is, the detection circuit 30a detects the wavelength dispersion amount of the optical signal from the digital electric signal based on the phase shift detection output value of the modulation frequencies of the sampling signal and the received optical signal. The frequency of the sampling signal output by the sampling clock generation circuit 22 may be set so that the sampling phase shift is swept.</p><p> As described above, the wavelength dispersion compensation control circuit 30 of Examples 1 to 7 is the optical transmission path in which the received optical signal is transmitted from the compensation amount set by the wavelength dispersion compensation circuit 24 to appropriately compensate for the wavelength dispersion. The amount of wavelength dispersion can be detected.</p><p> Although the examples of the present invention have been described in detail above, the present invention is not limited to the specific examples, and various modifications and modifications are made within the scope of the gist of the present invention described in the claims. It can be changed.</p><p> The following additional notes will be further disclosed with respect to the embodiments including the first to eighth embodiments. Appendix 1: Using a plurality of digital electric signals converted by sampling a plurality of analog electric signals obtained by photoelectric conversion of a plurality of optical signals including the intensity and phase information of the received optical signal using a sampling signal, A first compensation circuit that compensates for waveform distortion corresponding to the wavelength dispersion of the received light signal, and Wavelength dispersion compensation that controls the compensation amount of wavelength dispersion in the first compensation circuit based on the phase shift detection output value between the sampling signal and the modulation frequency of the received light signal from the digital electric signal whose wavelength dispersion is compensated. Control circuit and A signal processing circuit comprising. Appendix 2: The signal processing device according to Appendix 1, wherein the wavelength dispersion compensation control circuit controls the compensation amount of the wavelength dispersion in the first compensation circuit so that the phase shift detection output value becomes large. Appendix 3: The signal processing circuit according to Appendix 1 or 2, further comprising a second compensation circuit for adaptively equalizing and equalizing the waveform distortion remaining in the first compensation circuit. Appendix 4: A sampling signal control circuit for controlling the phase or frequency of the sampling signal is provided. The sampling signal control circuit is characterized in that when the wavelength dispersion compensation control circuit controls the compensation amount of the wavelength dispersion in the first compensation circuit, the control of the phase or frequency of the sampling signal is stopped. The signal processing circuit according to any one of 3 to 3. Appendix 5: When the wavelength dispersion compensation control circuit controls the compensation amount of wavelength dispersion in the first compensation circuit, a signal generation that outputs a signal having a frequency different from the natural number multiple of the modulation frequency of the received optical signal as the sampling signal. The signal processing circuit according to any one of Appendix 1 to 3, wherein the signal processing circuit comprises the circuit. Appendix 6: A delay circuit for delaying the digital electric signal compensated for the wavelength dispersion for different times is provided. The wavelength dispersion compensation control circuit determines the wavelength dispersion in the first compensation circuit based on the phase shift detection output value between the sampling signal and the modulation frequency of the received light signal from the digital electric signal delayed by the delay circuit. The signal processing circuit according to any one of Appendix 1 to 5, wherein the compensation amount is controlled. Appendix 7: The wavelength dispersion compensation control circuit controls the compensation amount of the wavelength dispersion in the first compensation circuit based on the phase shift detection output value. Then, based on the signal quality of the demodulated signal obtained by demodulating the plurality of digital electric signals output from the second compensation circuit, the compensation amount of the wavelength dispersion in the first compensation circuit is controlled. The signal processing circuit according to Appendix 3, further comprising controlling the compensation amount of wavelength dispersion in the first compensation circuit based on the dispersion amount compensated by the second compensation circuit. Appendix 8: The wavelength dispersion compensation control circuit controls the compensation amount of wavelength dispersion in the first compensation circuit based on the first residual dispersion amount calculated by using a part of the signals obtained by developing the plurality of digital electric signals in parallel. After that, the compensation amount of the wavelength dispersion in the first compensation circuit is controlled based on the second residual dispersion amount calculated by using all the signals obtained by developing the plurality of digital electric signals in parallel. The signal processing circuit according to Appendix 3, wherein the number of taps of the FIR filter for calculating the first residual dispersion amount is larger than the number of taps of the FIR filter for calculating the second residual dispersion amount. Appendix 9: An AD conversion circuit that converts a plurality of analog electric signals obtained by photoelectric conversion of a plurality of optical signals including the intensity and phase information of the received optical signal into a plurality of digital electric signals by sampling using the sampling signal. , A first compensation circuit that compensates for waveform distortion corresponding to the wavelength dispersion of the received light signal from the plurality of digital electric signals. Wavelength dispersion compensation control that controls the compensation amount of wavelength dispersion in the first compensation circuit based on the phase shift detection output value of the modulation frequencies of the sampling signal and the received light signal from the digital electric signal whose wavelength dispersion is compensated. Circuit and An optical receiver comprising. Appendix 10: The optical receiver according to Appendix 9, wherein the wavelength dispersion compensation control circuit controls the compensation amount of the wavelength dispersion in the first compensation circuit so that the phase shift detection output value becomes large. Appendix 11: The optical receiver according to Appendix 9 or 10, further comprising a second compensation circuit for adaptively equalizing and equalizing the waveform distortion remaining in the first compensation circuit. Appendix 12: A sampling signal control circuit for controlling the phase or frequency of the sampling signal is provided. The sampling signal control circuit is characterized in that when the wavelength dispersion compensation control circuit controls the compensation amount of the wavelength dispersion in the first compensation circuit, the control of the phase or frequency of the sampling signal is stopped. The optical receiver according to any one of 1 to 11. Appendix 13: When the wavelength dispersion compensation control circuit controls the compensation amount of wavelength dispersion in the first compensation circuit, a signal generation that outputs a signal having a frequency different from the natural number multiple of the modulation frequency of the received optical signal as the sampling signal. The optical receiver according to any one of Appendix 9 to 11, wherein the optical receiver comprises a circuit. Appendix 14: A delay circuit for delaying the digital electric signal compensated for the wavelength dispersion for different times is provided. The wavelength dispersion compensation control circuit determines the wavelength dispersion in the first compensation circuit based on the phase shift detection output value between the sampling signal and the modulation frequency of the received light signal from the digital electric signal delayed by the delay circuit. The optical receiver according to any one of Supplementary note 9 to 13, wherein the compensation amount is controlled. Appendix 15: The wavelength dispersion compensation control circuit controls the compensation amount of the wavelength dispersion in the first compensation circuit based on the phase shift detection output value. Then, based on the signal quality of the demodulated signal obtained by demodulating the plurality of digital electric signals output from the second compensation circuit, the compensation amount of the wavelength dispersion in the first compensation circuit is controlled. The optical receiver according to Appendix 11, further comprising controlling the compensation amount of the wavelength dispersion in the first compensation circuit based on the dispersion amount compensated by the second compensation circuit. Appendix 16: The wavelength dispersion compensation control circuit controls the compensation amount of wavelength dispersion in the first compensation circuit based on the first residual dispersion amount calculated by using a part of the signals obtained by developing the plurality of digital electric signals in parallel. After that, the compensation amount of the wavelength dispersion in the first compensation circuit is controlled based on the second residual dispersion amount calculated by using all the signals obtained by developing the plurality of digital electric signals in parallel. The Shinko receiver according to Appendix 11, wherein the number of taps of the FIR filter for calculating the first residual dispersion amount is larger than the number of taps of the FIR filter for calculating the second residual dispersion amount. Appendix 17: Using a plurality of digital electric signals converted by sampling a plurality of analog electric signals obtained by photoelectric conversion of a plurality of optical signals including the intensity and phase information of the received optical signal using a sampling signal, A first compensation circuit that compensates for waveform distortion corresponding to the wavelength dispersion of the received light signal, and A detection circuit that detects the wavelength dispersion amount of the received light signal from the digital electric signal compensated for the wavelength dispersion based on the phase shift detection output value between the sampling signal and the modulation frequency of the received light signal. A detection device comprising. Appendix 18: The received light of a plurality of digital electric signals converted by sampling a plurality of analog electric signals obtained by photoelectric conversion of a plurality of optical signals including the intensity and phase information of the received optical signal using a sampling signal. Steps to compensate for waveform distortion corresponding to signal wavelength dispersion, A step of controlling the compensation amount for compensating for the wavelength dispersion based on the phase shift detection output value between the sampling signal and the modulation frequency of the received light signal from the digital electric signal whose wavelength dispersion is compensated. A waveform distortion compensation method comprising.</p>

10 Digital signal processing circuit 12 Waveform distortion compensation circuit 20 ADC 22 Sampling clock generator circuit 24 Wavelength dispersion compensation circuit 26 Adaptive equalization type waveform distortion compensation circuit 28 Sampling phase shift detection circuit 29 Sampling signal control circuit 29 30 Wavelength dispersion compensation control circuit 32 Wavelength dispersion compensation amount setting unit 34 Sensitivity monitor