JP2014195149A - Light receiving device and light receiving method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve accuracy of phase compensation in a light transmission and reception system.SOLUTION: A light receiving device 2 receives signal light obtained by overlapping a pilot tone with a wavelength different from a main signal on the main signal, detects phase variation from the pilot tone, and estimates the varied phase. The light receiving device 2 compensates for the phase variation at a position of the main signal overlapped on a position of the phase variation of the pilot tone while slightly adjusting time difference between the main signal and the pilot tone according to wavelength dispersion. Therefore, the light receiving device 2 has such a constitution for coupling N-stages of sets of wavelength dispersion compensating parts 241 and phase noise compensating parts 242. The wavelength dispersion compensating part 241 allocates a dispersion compensating amount in each stage to satisfy a total of dispersion amounts accompanied with the wavelength dispersion, so as to compensate for displacement corresponding to the dispersion compensating amount. Also, the phase noise compensating part 242 extracts the pilot tone, estimates an amount of the varied phase, applies the phase compensating amount to the main signal, and compensates for the phase variation.

Description

  The present invention relates to a technique for compensating for phase noise generated in an optical fiber transmission line.

  In the field of backbone optical transmission systems, studies are underway to accommodate high-speed signals economically and perform large-capacity transmission. For example, from the viewpoint of improving frequency utilization efficiency, a digital coherent transmission method combining coherent detection and digital signal processing has been studied.

  As a technique for improving the frequency utilization efficiency, there is a multi-level modulation method. In the multi-level modulation method, as the multi-level number (number of bits per symbol) increases, it is greatly affected by phase noise generated in the optical transmitter, optical receiver, and optical fiber transmission line, causing transmission degradation. It has been known. In Non-Patent Document 1 and Non-Patent Document 2, a pilot tone is superimposed as a reference signal on the transmitting side at a different frequency position with respect to the main signal, and phase noise generated in the optical fiber transmission line is estimated on the receiving side. A technique for compensating for the phase variation of the main signal is disclosed.

T. Kobayashi, et al., “Nonlinear tolerant long-haul WDM transmission over 1200km using 538Gb / s / ch PDM-64QAM SC-FDM signals with pilot tone”, OM2A.5, OFC / NFOEC (2012) S. L. Jansen, et al., “20-Gb / s OFDM Transmission over 4, 160km SSMF Enabled by RF-Pilot Tone Phase Noise Compensation”, PDP 15, OFC / NFOEC (2007)

However, since the techniques described in Non-Patent Document 1 and Non-Patent Document 2 do not sufficiently consider the influence of chromatic dispersion that occurs in the optical fiber transmission line, the position of the phase fluctuation estimated from the pilot tone and the main signal There is a problem that the correlation with the position of the phase fluctuation is lowered, and a compensation effect for the phase fluctuation cannot be obtained sufficiently.
Therefore, an object of the present invention is to provide a technique for improving the accuracy of compensation for phase fluctuation in an optical receiver.

  The optical receiver according to the first aspect of the present invention receives signal light in which a pilot tone having a frequency component different from that of the main signal is superimposed on the main signal via an optical fiber transmission line. A receiving unit that converts the signal into a signal, a chromatic dispersion compensation unit that compensates the chromatic dispersion for the input electrical signal, and outputs an electrical signal that compensates the chromatic dispersion, and a pilot tone included in the input electrical signal. A phase noise compensator that estimates the phase fluctuation, compensates for the phase fluctuation of the main signal included in the input electric signal based on the estimated phase fluctuation, and outputs the electric signal compensated for the phase fluctuation; It is characterized by providing.

  According to such a configuration, the optical receiving apparatus receives the signal light in which the pilot tone having a frequency component different from that of the main signal is superimposed on the main signal via the optical fiber transmission line and converts it into an electric signal. After the conversion, chromatic dispersion is compensated for the electrical signal, phase fluctuation is estimated from the pilot tone, and phase fluctuation of the main signal can be compensated based on the estimated phase fluctuation.

  The invention according to claim 2 that cites claim 1 is characterized by comprising a configuration in which a plurality of the chromatic dispersion compensation units and the phase noise compensation units are alternately connected.

  The phase noise compensator of the invention according to claim 3 quoting claim 1 or claim 2 includes: a pilot tone extractor that extracts the pilot tone from the input electrical signal; and A phase estimation unit that detects phase fluctuation and estimates the amount of the phase that has fluctuated, and a compensation that calculates a phase compensation amount by multiplying the phase amount estimated by the phase estimation unit by a predetermined weighting factor An amount setting unit, and a phase compensation application unit that applies the phase compensation amount to the electrical signal to compensate for a changed phase are provided.

  According to such a configuration, the optical receiver includes a configuration in which a plurality of chromatic dispersion compensation units and phase noise compensation units are alternately connected, and the chromatic dispersion compensation unit is configured to distort the electrical signal due to chromatic dispersion. By compensating the dispersion compensation amount indicating the amount of compensation, the phase noise compensation unit estimates the amount of phase variation from the pilot tone while reducing the deviation between the main signal and the pilot tone, and Compensates for phase fluctuations. In other words, the optical receiving apparatus can improve the accuracy of phase compensation by repeatedly compensating for the phase fluctuation while reducing the deviation between the main signal and the pilot tone accompanying chromatic dispersion.

  According to a fourth aspect of the present invention, the phase noise compensation unit includes a pilot tone extraction unit that extracts the pilot tone from the input electrical signal, and a phase variation of the pilot tone. A phase estimation unit that detects and estimates the amount of the phase that has changed, and a compensation amount setting unit that calculates a phase compensation amount by multiplying the phase amount estimated by the phase estimation unit by a predetermined weighting factor A phase compensation applying unit that applies the phase compensation amount to the electrical signal to compensate for a changed phase, and a dispersion reduction filter that compensates chromatic dispersion for the pilot tone, and For the electrical signal, the apparatus alternately connects a plurality of the chromatic dispersion compensation units and the phase compensation application unit to compensate for chromatic dispersion and phase fluctuations, and for the pilot tone. The pilot tone extractor, after processing in the order of the phase estimator, the plurality connecting the variance reduction filter on the output side of the phase estimator, and for compensating for the wavelength dispersion.

  According to such a configuration, the optical receiving device is configured to connect the chromatic dispersion compensation unit and the phase compensation application unit alternately to the pilot signal while compensating for the chromatic dispersion and the phase variation with respect to the electrical signal. Tones are processed in the order of the pilot tone extraction unit and the phase estimation unit, and then a plurality of dispersion reduction filters are connected to the output side of the phase estimation unit to compensate for chromatic dispersion. That is, the optical receiving apparatus needs to extract the pilot tone and calculate the changed phase only once. The optical receiving apparatus can improve the accuracy of phase compensation by repeatedly compensating for the phase fluctuation while reducing the deviation between the main signal and the pilot tone. Further, the circuit scale of the dispersion reducing filter that compensates the dispersion compensation amount for the pilot tone can be realized with a circuit scale smaller than that of the chromatic dispersion compensation unit for the electric signal.

  In addition, the dispersion reducing filter or the chromatic dispersion compensation unit according to the invention described in claim 5 quoting claim 4 compensates for a delay due to chromatic dispersion.

  According to such a configuration, the optical receiving apparatus can improve the compensation accuracy with respect to the phase variation simply by causing the dispersion low-pass filter to act as a delay element. In addition, the circuit scale can be reduced.

  The chromatic dispersion compensator of the invention described in claim 6 changes a chromatic dispersion compensation amount corresponding to a signal power level diagram of the optical fiber transmission line, and the compensation amount setting unit The weighting factor is changed corresponding to a signal power level diagram of a fiber transmission line.

  According to such a configuration, the chromatic dispersion compensation unit changes the compensation dispersion amount finely, and the compensation amount setting unit improves the transmission characteristics by changing the weight compensation coefficient and changing the phase compensation amount. can do. That is, it is possible to cope with a phenomenon that the magnitude of the phase variation differs according to the signal power level. And the optical receiver can improve the accuracy of phase compensation.

  The invention relating to the optical receiving method has the same technical features as the optical receiving apparatus according to claim 1 and has the same effects as the optical receiving apparatus, and therefore the description thereof is omitted. .

  ADVANTAGE OF THE INVENTION According to this invention, the compensation precision with respect to a phase variation can be improved in an optical receiver.

It is a figure which shows the structural example of an optical transmission / reception system, an optical transmitter, and an optical receiver. It is a figure which shows the function example of the digital signal processing part of the optical receiver of 1st Embodiment. It is a figure which shows the function example of the phase noise compensation part of 1st Embodiment. It is a figure which shows an example of the condition where phase fluctuation generate | occur | produced, (a) represents the case where a transmission distance is short, (b) represents the case where a transmission distance is long. It is a figure which shows the outline | summary of the process which performs chromatic dispersion compensation and phase noise compensation repeatedly, (a) represents the result of compensation by the 1st step, (b) represents the result of compensation by the Nth step. . It is a figure which shows an example of the simulation result of 1st Embodiment. It is a figure which shows an example of the simulation result at the time of changing a weighting coefficient in the compensation amount setting part of the phase noise compensation part of 1st Embodiment. It is a figure which shows the function example of the digital signal processing part of 2nd Embodiment. It is a figure which shows the function example of the phase noise compensation part of 2nd Embodiment. It is a figure which shows the function example of the phase noise compensation part of 3rd Embodiment. It is a figure which shows the process example which compensates an incorrect position.

  A mode for carrying out the present invention (hereinafter referred to as “the present embodiment”) will be described in detail with reference to the drawings as appropriate.

(Optical transmission / reception system)
The optical transmission / reception system 100 according to the present embodiment includes an optical transmission device 1 and an optical reception device 2 as shown in FIG. The optical transmission device 1 has a function of generating modulated light 111 including data and sending the modulated light 111 to the optical fiber transmission line 5. The optical receiver 2 has a function of receiving the modulated light 111 that has propagated through the optical fiber transmission line 5 as the received signal light 211 and demodulating the received signal light 211 to acquire data.

As illustrated in FIG. 1, the optical transmission device 1 includes a signal light source unit 10, an optical modulation unit 11, and a modulation signal generation unit 12.
The signal light source unit 10 has a function of generating an optical signal using a laser and outputting the optical signal to the optical modulation unit 11.
The optical modulation unit 11 modulates the output signal of the signal light source unit 10 using the modulation signal 123 generated by the modulation signal generation unit 12 to generate modulated light (signal light) 111, and transmits the modulated light to the optical fiber transmission line 5. Has a function to output.

The modulation signal generation unit 12 includes a symbol mapping unit 121 and a pilot tone multiplexing unit 122.
Symbol mapping section 121 receives a binary sequence of 0 and 1 as data, generates a main signal by symbol mapping the data to corresponding complex points according to the modulation scheme, and outputs the main signal to pilot tone multiplexing section 122 It has a function.
The pilot tone multiplexing unit 122 has a function of generating a modulation signal 123 by superimposing one or more pilot tones on the main signal and outputting the modulation signal 123 to the optical modulation unit 11. The frequency component of the pilot tone is different from the frequency component of the main signal.

  The modulated light 111 transmitted from the optical transmitter 1 is received by the optical receiver 2 as the received signal light 211 via the optical fiber transmission path 5.

  As shown in FIG. 1, the optical receiver 2 includes a local light source unit 20, a polarization multiplexed optical hybrid unit (reception unit) 21, a photoelectric conversion unit (reception unit) 22, an AD (analog / digital) conversion unit 23, and A digital signal processing unit 24 is provided.

The local light source unit 20 has a function of generating an optical signal using a local laser for performing coherent reception.
The polarization multiplexed optical hybrid unit 21 receives the received signal light 211, combines it with the output light of the local light source unit 20, and converts it into four component optical signals of the in-phase component and the quadrature component of the X polarization and the Y polarization. It has a function of separating (performing coherent detection).

The photoelectric conversion unit 22 includes a photoelectric conversion element and has a function of converting the output signal of the polarization multiplexed optical hybrid unit 21 into an analog electric signal.
The AD conversion unit 23 has a function of converting the analog electric signal into a received digital signal (electric signal) 231. The received digital signal 231 includes a main signal and a pilot tone.
The digital signal processing unit 24 has a function of performing chromatic dispersion compensation and phase noise compensation on the received digital signal 231, demodulating the main signal, and outputting the original data. Details of the function of the digital signal processing unit 24 will be described later as first to fourth embodiments.

(First embodiment)
Next, a functional example of the digital signal processing unit 24 of the first embodiment will be described with reference to FIG. 2 (see FIG. 1 as appropriate).
The digital signal processing unit 24 includes a chromatic dispersion compensation unit 241, a phase noise compensation unit 242, a pilot tone removal unit 243, an adaptive equalization unit 244, a frequency offset compensation unit 245, a carrier phase synchronization unit 246, and a symbol identification unit 247.

The chromatic dispersion compensator 241 has a function of compensating the distortion associated with chromatic dispersion for the received digital signal 231.
The phase noise compensation unit 242 extracts a pilot tone, detects a phase variation from the pilot tone, estimates the phase that has changed, and applies a phase amount opposite to the estimated phase to the received digital signal 231. And has a function of compensating for phase fluctuations.

In the first embodiment, the digital signal processing unit 24 has a configuration in which the chromatic dispersion compensation unit 241 and the phase noise compensation unit 242 are repeatedly connected in N stages (N ≧ 2). That is, the output of the chromatic dispersion compensator 241 is input to the phase noise compensator 242 and the output of the phase noise compensator 242 is input to the chromatic dispersion compensator 241 again to repeat N stages. In the following description, the first stage is referred to as the first stage, and the Nth repetition is referred to as the Nth stage.
The chromatic dispersion compensation unit 241 divides the total dispersion amount accompanying the chromatic dispersion of the received digital signal 231 into N, and compensates the divided dispersion amount (dispersion compensation amount) for each stage. For example, each stage may be executed so as to evenly compensate for the total 1 / N of the amount of dispersion. Each stage may be executed so as to compensate for different dispersion compensation amounts determined in advance within a range that satisfies the total dispersion amount.

  Next, the outline of the processing in the N-stage connection configuration will be described. Before that, in the optical fiber transmission line 5, what will happen to the synchronization between the main signal and the pilot tone due to the influence of chromatic dispersion? This will be described with reference to FIG. 4 together with the influence of phase fluctuation.

4A shows a case where the transmission distance is short and the influence of chromatic dispersion is small, and FIG. 4B shows a case where the transmission distance is long and the influence of chromatic dispersion is large. Yes.
4A and 4B, for each of the “main signal” and the “pilot tone”, a section signal when an arbitrary position of the modulation signal 123 generated by the modulation signal generation unit 12 on the transmission side is cut out. Is schematically represented. Moreover, the double arrow represents the position where the phase fluctuation has occurred.

In the “main signal”, a small vertically long rectangle represents one symbol, and a plurality of the symbols represent a section signal. Further, the position subjected to the phase fluctuation is hatched.
The “pilot tone” is schematically represented by a waveform, and the phase is changed at the position where the phase fluctuation is received.

  As shown in FIG. 4A, when the transmission distance is short, even if the main signal and the pilot tone have different wavelengths, the influence of chromatic dispersion is small. For this reason, the main signal and the pilot tone are in a substantially synchronized positional relationship. The main signal and the pilot tone are subjected to the same phase fluctuation at the position of the double arrow. In FIG. 4A, the position subjected to the phase fluctuation is substantially the same in the section signals of the main signal and the pilot tone, and is shown near the right end of the section signal.

  Next, when the transmission distance is long, as shown in FIG. 4B, synchronization between the main signal and the pilot tone is shifted due to wavelength dispersion. As an example, FIG. 4B shows a case where the pilot tone is delayed from the main signal. Both the main signal and the pilot tone are subjected to phase fluctuations at the positions of the double arrows. For this reason, the main signal is subjected to phase fluctuation at a position on the right side near the center of the section signal. On the other hand, in the pilot tone, the phase fluctuation is received at a position on the left side of the section signal near the center. That is, from the state shown in FIG. 4B, when the influence of the chromatic dispersion is compensated and the synchronization of the “main signal” and the “pilot tone” is matched, the correlation between the positions subjected to the phase fluctuation is lowered. become.

  Therefore, in order to cope with the influence of chromatic dispersion, it is possible to compensate for phase fluctuation while gradually compensating for the influence of chromatic dispersion. Specifically, the optical receiver 2 has a configuration in which the chromatic dispersion compensation unit 241 and the phase noise compensation unit 242 are repeatedly connected in N stages (N times). Next, an outline of processing in the N-stage connected configuration will be described with reference to FIG. FIG. 5A shows the result of compensation by the first stage, and FIG. 5B shows the result of compensation by the Nth stage. However, it is assumed that both the main signal and the pilot tone are subjected to substantially the same phase fluctuation due to the phase fluctuation.

In FIG. 5A, the received digital signal 231 input to the first stage is in a state in which the synchronization between the main signal and the pilot tone is shifted (not matched) as in the case of FIG. 4B. It has become.
In this state, the phase noise compensation unit 242 causes the phase of the main signal corresponding to the position of the phase fluctuation in the pilot tone (phase inversion in FIG. 5A) (in FIG. 5A, “phase compensation”). Phase compensation). When the phase compensation process is performed, the phase noise compensation unit 242 stores the position of the phase variation of the pilot tone that has been subjected to the phase compensation as already handled, and does not use it again for phase compensation. For example, the pilot tone phase inversion portion may be returned to the original state in order to deal with it.

  Then, processing is performed so that the synchronization shift (delay difference) due to chromatic dispersion decreases as the number of stages increases. In the N-th stage shown in FIG. 5B, the main signal and the pilot tone are almost synchronized as in the case of FIG. That is, the shift between the main signal and the pilot tone is reduced, and the phase fluctuation position in the pilot tone is newly overlapped with the section signal of the main signal. Therefore, the phase noise compensation unit 242 performs phase compensation on a portion where the position of the phase fluctuation in the pilot tone newly overlaps the main signal (denoted as “phase compensation position” in FIG. 5B).

If N = 1 and a single-stage configuration is used, as shown in FIG. 11, the chromatic dispersion compensator 241 compensates for the effects of chromatic dispersion at one time. It will be in a synchronized state. Next, the phase noise compensation unit 242 performs a process of compensating for the phase variation at the location of the main signal corresponding to the position of the phase variation of the pilot tone. For this reason, in FIG. 11, the phase fluctuation at the right end of the main signal is compensated, but erroneous phase compensation is executed for the wrong position of the main signal.
That is, it can be expected that the compensation accuracy is improved by setting N to 2 or more.

Here, a function example of the phase noise compensation unit 242 will be described with reference to FIG. 3 (see FIG. 2 as appropriate).
The phase noise compensation unit 242 includes a pilot tone extraction unit 31, a phase estimation unit 32, a compensation amount setting unit 33, and a phase compensation application unit 34.
The pilot tone extraction unit 31 has a function of extracting a pilot tone from the received digital signal 231. Specifically, the pilot tone extraction unit 31 extracts a pilot tone using a passband filter (not shown) that passes the frequency and phase fluctuation components of the pilot tone. Note that the frequency width of the phase fluctuation component is, for example, several MHz to several GHz. The pilot tone extraction unit 31 receives a frequency domain signal calculated by a fast Fourier transform operation for dispersion compensation, applies a passband filter to the frequency where the pilot tone and the phase fluctuation component are located, and produces a pilot tone. Can be extracted. The pilot tone extraction unit 31 may extract pilot tones using a digital filter based on time domain FIR (Finite Impulse Response) or IIR (Infinite Impulse Response).

  The phase estimator 32 has a function of detecting the phase variation of the pilot tone and estimating the varied phase. Specifically, the phase estimation unit 32 uses the fact that the pilot tone superimposed on the main signal on the transmission side is known, and shifts the pilot tone by the opposite amount by the shifted frequency of the detected pilot tone. Into the baseband, and the amount of phase fluctuation can be estimated.

The compensation amount setting unit 33 has a function of calculating a phase compensation amount by multiplying the amount of the changed phase estimated by the phase estimation unit 32 by a weighting factor.
The phase compensation application unit 34 has a function of applying the phase compensation amount calculated by the compensation amount setting unit 33 to the received digital signal 231 to compensate for phase fluctuation.

Here, two methods will be described with respect to a method for compensating for the phase fluctuation of the received digital signal 231.
The first method performs phase compensation after converting the received digital signal 231 from a signal represented as a complex number to a signal represented by a phase angle. In this case, the phase estimation unit 32 uses the real part and imaginary part values of the complex signal to refer to the memory table in which the relationship between the real part and the imaginary part value and the relationship between the phase angles is stored in advance. Find the phase angle. The memory table may be generated by, for example, calculating the value of (imaginary part / real part) and the phase angle in advance using Arctan (imaginary part / real part). Further, the phase estimation unit 32 may calculate, for example, Arctan (imaginary part / real part) to obtain the phase angle. Then, the phase compensation application unit 34 converts the phase angle opposite to the estimated phase angle again into a phase compensation amount represented by a complex number, and applies it to the received digital signal 231. The memory table can also be used for the reverse conversion.

  The second method uses phase information represented by complex numbers as it is. The compensation amount setting unit 33 calculates a phase compensation amount by multiplying a complex signal indicating the amount of phase opposite to the changed phase estimated by the phase estimation unit 32 by a weighting factor. The phase compensation application unit 34 performs a complex multiplication process of the complex signal of the phase compensation amount and the complex signal of the received digital signal 231. At this time, the phase amount in the reverse direction can be expressed as a complex conjugate with respect to the amount of the phase that has changed.

Returning to FIG. 2, the pilot tone removing unit 243 has a function of removing the pilot tone from the output signal of the N-th phase noise compensating unit 242 and extracting the main signal.
The adaptive equalization unit 244 has a function of executing polarization mode dispersion compensation.
The frequency offset compensation unit 245 has a function of estimating a frequency offset amount and executing frequency offset compensation.

The carrier phase synchronization unit 246 has a function of synchronizing with the carrier wave.
The symbol identification unit 247 has a function of demodulating a symbol series from the main signal and outputting original data transmitted from the transmission side.

  Next, simulation results of N-stage processing of the chromatic dispersion compensation unit 241 and the phase noise compensation unit 242 of the digital signal processing unit 24 will be described with reference to FIG. 6 (see FIGS. 1 to 3 as appropriate). The simulation is performed by constructing an experimental system having the same configuration as that of the optical transmission / reception system 100 shown in FIG. 1, capturing the received digital signal 231 in the computer in the optical receiver 2, and simulating the processing of the digital signal processing unit 24 offline. It was. The dispersion compensation amount per stage of the chromatic dispersion compensation unit 241 is set to be 1 / N of the total dispersion compensation amount.

  In FIG. 6, the horizontal axis represents the transmission distance (km), and the vertical axis represents the Q value (dB). The number N of repeated stages represents a case where N = 4 and a case where N = 1. When the transmission distance was 200 km, the Q value was almost the same in both cases of N = 4 and N = 1. This is because the transmission distance is short when the transmission distance is 200 km, and even when N = 4, the main signal and the pilot tone are almost synchronized as in the case of N = 1. . On the other hand, as the transmission distance increases, the difference between the Q value when N = 4 and the Q value when N = 1 increases. The reason for this is that, as shown in FIGS. 5 (a) and 5 (b), the number of stages is set to N = 4, and the amount of deviation due to chromatic dispersion is gradually changed, thereby overlapping the pilot tone and the main signal. This is probably because the part can be changed.

  The Q value tends to improve as the number of N is increased, but N is determined to be a predetermined numerical value in consideration of the circuit scale and real-time characteristics (computation processing is terminated within a predetermined time). Is preferred. According to the simulation result, N = 3 to 5 is preferable.

Further, in the compensation amount setting unit 33 of the phase noise compensation unit 242 shown in FIG. 3, excessive compensation is suppressed by multiplying the amount of the changed phase estimated by the phase estimation unit 32 by a weighting factor (1 or less). The Q value can be improved as shown in FIG. In FIG. 7, the horizontal axis represents the weighting factor, and the vertical axis represents the relative Q value (when the weighting factor = 1, 0 dB). The parameters are for transmission distances of 800 km and 1000 km.
From FIG. 7, it can be seen that the Q value is improved when the weighting coefficient is smaller than 1 regardless of whether the transmission distance is 800 km or 1000 km.

(Second Embodiment)
In the second embodiment, as shown in FIG. 8, the N-stage configuration of the chromatic dispersion compensation unit 241 and the phase noise compensation unit 242 in the digital signal processing unit 24 (see FIG. 2) of the first embodiment is as shown in FIG. The chromatic dispersion compensator 241 and the phase noise compensator 250 are combined. Note that, in the second embodiment, the reference numerals are given as the digital signal processing unit 24a and the phase noise compensation unit 250, and the same functions as those in the first embodiment are given the same reference numerals and the description thereof is omitted. To do.

  FIG. 8 shows an example of the function of the digital signal processing unit 24a in the second embodiment. In the digital signal processing unit 24a, the function of the phase noise compensation unit 250 is different from the function of the phase noise compensation unit 242 of the first embodiment. Therefore, the function of the phase noise compensation unit 250 will be described with reference to FIG. 9 (see FIG. 2 as appropriate).

  As shown in FIG. 9, in the phase noise compensation unit 250, for the received digital signal 231, the phase compensation application unit 34 and the chromatic dispersion compensation unit 241 are alternately repeated, and in the Nth stage, the phase compensation application unit Only (N) 34 is configured. The phase noise compensation unit 250 processes the pilot tone in the order of the pilot tone extraction unit 31 and the phase estimation unit 32, and then applies a dispersion reduction filter 35 to the output side of the phase estimation unit 32 (N-1). ) It has a configuration to connect in stages. Further, the phase noise compensation unit 250 inputs the output signal of the phase estimation unit 32 to the phase compensation application unit (1) 34 via the compensation amount setting unit (1) 33, and outputs the output signal of the dispersion reduction filter (M) 35. Is input to the phase compensation application unit (M + 1) 34 via the compensation amount setting unit (M + 1) 33. However, N> M ≧ 1. In this way, the phase noise compensation unit 250 alternately connects a plurality of chromatic dispersion compensation units 241 and phase compensation application units 34 to the received digital signal 231 to compensate for at least one of chromatic dispersion or phase fluctuation. can do.

In the first embodiment, the pilot tone is extracted every time N stages are repeated in the pilot tone extraction unit 31 of the phase noise compensation unit 242. However, when the ratio of the phase variation bandwidth of the pilot tone at the frequency ω _PT to the bandwidth of the chromatic dispersion compensated by the chromatic dispersion compensation unit 241 is smaller than a predetermined value, the bandwidth of the phase variation of the pilot tone is Even if the processing of the dispersion compensator 241 is executed, it hardly changes. Therefore, as shown in FIG. 9, the pilot tone extraction unit 31 performs pilot tone extraction only once. Each of the dispersion reduction filters (1) 35 to (N-1) 35 has a function of reducing the chromatic dispersion amount equal to the chromatic dispersion amounts of the chromatic dispersion compensation units (2) 241 to (N) 241 from the pilot tone. Have. By providing such a configuration, an effect equivalent to the function of the N-stage configuration of the chromatic dispersion compensation unit 241 (see FIG. 2) of the first embodiment can be obtained. However, it is assumed that the chromatic dispersion amount set in the dispersion reduction filter (M) 35 is equal to the chromatic dispersion amount set in the chromatic dispersion compensation unit (M + 1) 241. The compensation amount setting unit 33 is used for fine adjustment of the phase compensation amount. Further, since the dispersion reduction filter 35 only acts on the pilot tone, the circuit scale may be smaller than the circuit scale for the received digital signal 231. Further, as in the second embodiment, when the ratio of the bandwidth of the chromatic dispersion compensated by the chromatic dispersion compensation unit 241 to the bandwidth of the phase fluctuation of the pilot tone is smaller than a predetermined value, it is shown in FIG. A simulation result is obtained that the Q value is higher than that of the first embodiment even if the value of N of the circuit is smaller than the value of the number of stages N in the case of the first embodiment.

(Third embodiment)
The third embodiment differs from the phase noise compensation unit 250 of the second embodiment shown in FIG. 9 in that a delay amount reduction unit 36 is provided instead of the dispersion reduction filter 35. An example of the function of the phase noise compensation unit 251 of the third embodiment is shown in FIG.
When the bandwidth of phase fluctuation is smaller than that of the second embodiment, that is, when the bandwidth of phase fluctuation is much smaller than the bandwidth of chromatic dispersion applied by the dispersion reduction filter 35, the dispersion reduction filter 35 is In the third embodiment, since it acts as a mere delay in the distortion of the received digital signal due to chromatic dispersion, it is not necessary to have a function of a band pass filter, and the circuit can be simplified as the delay amount reducing unit 36. Therefore, the circuit scale of the third embodiment can be made smaller than the circuit scale of the first and second embodiments. The compensation amount setting unit 33 is used for fine adjustment of the phase compensation amount. Further, the dispersion reduction filter 35 may be used to substantially compensate for the delay due to chromatic dispersion.
The delay amount τ reduced by the delay amount reducing unit 36 can be obtained using the following equation (1).

ただし、Ｄは波長分散パラメータ、Ｌ_ｎは遅延に対する伝送距離、λ_Ｃは主信号の中心波長、ｃ_０は光速、ｎは図１０中のカッコ（）内の数字である。

Here, D is a chromatic dispersion parameter, L _n is a transmission distance with respect to delay, λ _C is the center wavelength of the main signal, c ₀ is the speed of light, and n is a number in parentheses () in FIG.

(Fourth embodiment)
Since the fourth embodiment has the same function as any of the first to third embodiments, illustration is omitted (see FIGS. 2, 3, 9, and 10 as appropriate). Note that the fourth embodiment is characterized in that the fineness of setting the weighting coefficient added to the phase compensation amount and the compensation dispersion amount varies depending on the magnitude (level diagram) of the signal power.

  The signal power of the signal transmitted from the optical transmitter 1 propagates through the optical fiber transmission line 5 and decreases due to the loss of the optical fiber. In addition, since the amount of phase rotation due to the nonlinear optical effect generated in the optical fiber transmission line 5 is proportional to the signal power, it undergoes a large phase rotation in a region where the signal power is high. That is, if the amount of phase compensation in the transmission region subjected to a large phase rotation is increased, the transmission characteristics can be improved.

  For example, in the transmission region where the signal power is high in accordance with the signal power level diagram of the optical fiber transmission line 5, the chromatic dispersion compensation unit 241 finely sets the compensation dispersion amount, and the compensation amount setting unit 33 of the phase noise compensation unit 242. In this case, it is preferable to increase the phase compensation amount by increasing the weight coefficient.

  As described above, the optical receiver 2 of the present embodiment receives the signal light in which the pilot tone having a wavelength different from that of the main signal is superimposed on the main signal via the optical fiber transmission line 5, and outputs the phase from the pilot tone. Change is detected and the changed phase is estimated. The optical receiver 2 compensates for the phase fluctuation of the position of the main signal that overlaps the position of the phase fluctuation of the pilot tone, while gradually adjusting the temporal deviation between the main signal and the pilot tone according to the chromatic dispersion. Therefore, the optical receiver 2 repeatedly executes chromatic dispersion compensation by the chromatic dispersion compensation unit 241 and phase fluctuation compensation by the phase noise compensation unit 242. The chromatic dispersion compensator 241 allocates a dispersion compensation amount at each stage so as to satisfy the total dispersion amount accompanying the chromatic dispersion, and compensates for the deviation corresponding to the dispersion compensation amount. Further, the phase noise compensation unit 242 extracts the pilot tone, estimates the amount of the fluctuating phase, applies a phase compensation amount having a phase opposite to the estimated phase to the main signal, and performs the phase variation. To compensate.

  Further, when the compensation amount setting unit 33 calculates the phase compensation amount by multiplying the time waveform of the estimated phase angle by a weighting factor, among the estimated phase angles calculated in each of the N stages, A large weighting factor may be applied to the estimated phase angle to be emphasized.

  Further, the digital signal processing unit 24 illustrated in FIG. 2 may be configured without the frequency offset compensation unit 245.

  In FIG. 2, the chromatic dispersion compensation unit 241 and the phase noise compensation unit 242 are described as having a configuration of N stages (N ≧ 2). The configuration may be omitted.

  In the present embodiment, the case where there is one pilot tone has been described. However, when there are a plurality of pilot tones, a plurality of configurations described in the present embodiment may be provided in parallel for each pilot tone.

Further, in the present embodiment, the digital signal processing unit 24 has been described as executing processing with the received digital signal 231 as an input of an electrical signal, but the input is not necessarily limited to a digital signal.
Further, the chromatic dispersion compensation unit 241 may have the same function as the dispersion reduction filter 35.

DESCRIPTION OF SYMBOLS 1 Optical transmitter 2 Optical receiver 5 Optical fiber transmission line 21 Polarization multiplexing optical hybrid part (receiver)
22 Photoelectric converter (receiver)
24, 24a Digital signal processing unit 31 Pilot tone extraction unit 32 Phase estimation unit 33 Compensation amount setting unit 34 Phase compensation application unit 35 Dispersion reduction filter 36 Delay amount reduction unit 111 Modulated light (signal light)
231 Received digital signal (electrical signal)
241 Wavelength dispersion compensation unit 242, 250 Phase noise compensation unit 243 Pilot tone removal unit 244 Adaptive equalization unit 245 Frequency offset compensation unit 246 Carrier phase synchronization unit 247 Symbol identification unit

The optical receiver according to the first aspect of the present invention receives signal light in which a pilot tone having a frequency component different from that of the main signal is superimposed on the main signal via an optical fiber transmission line. a receiving section for converting the signal to compensate for the chromatic dispersion with respect to the electric signal output from the receiving unit, the first wavelength dispersion compensating unit for outputting an electrical signal to compensate for the wavelength dispersion, first the A phase noise compensation unit that receives an electrical signal output from the chromatic dispersion compensation unit and outputs an electrical signal compensated for chromatic dispersion and phase variation, and the phase noise compensation unit includes the first chromatic dispersion compensation. A first phase compensation application unit that outputs an electric signal in which a phase variation corresponding to the phase compensation amount output from the first compensation amount setting unit is output with respect to the output of the first compensation unit, and the first phase compensation application unit Compensates for chromatic dispersion for the output of A phase compensation amount output from the second compensation amount setting unit with respect to the output of the second chromatic dispersion compensation unit; A second phase compensation applying unit that outputs an electric signal that compensates for a corresponding phase variation; and a pilot tone extracting unit that extracts the pilot tone from the output of the first chromatic dispersion compensating unit, connected in series; A phase estimation unit that detects a phase variation of the pilot tone and estimates an amount of the phase that has varied, and the phase amount estimated by the phase estimation unit is the same as the second chromatic dispersion compensation unit. A first low-pass filter that compensates for the amount of chromatic dispersion, and the first compensation amount setting unit uses a predetermined weighting factor for the phase amount estimated by the phase estimation unit. Multiplied by the phase Compensation amount is calculated and output to the first phase compensation application unit, and the second compensation amount setting unit multiplies the output of the first dispersion low-pass filter by a predetermined weighting factor to obtain the phase compensation amount. Is calculated and output to the second phase compensation application unit .

  The phase noise compensation unit of the optical receiver according to claim 4 further compensates chromatic dispersion with respect to an output of the nth (n ≧ 2) phase compensation application unit, and compensates the chromatic dispersion. Phase variation corresponding to the phase compensation amount output from the (n + 1) th compensation amount setting unit with respect to the output of the (n + 1) th wavelength dispersion compensation unit and the (n + 1) th wavelength dispersion compensation unit. An n + 1 th phase compensation application unit that outputs a compensated electrical signal, and is connected in series to the output side of the n−1 th dispersion low-pass filter in the same amount as the n + 1 th chromatic dispersion compensation unit. The nth dispersion low-pass filter that compensates for chromatic dispersion is connected, and the (n + 1) th compensation amount setting unit multiplies the output of the nth dispersion low-pass filter by a predetermined weighting factor to obtain the phase compensation amount. And output to the (n + 1) th phase compensation application unit. To, characterized in that.

  According to such a configuration, the optical receiving apparatus receives the signal light in which the pilot tone having a frequency component different from that of the main signal is superimposed on the main signal via the optical fiber transmission line and converts it into an electric signal. After conversion, the chromatic dispersion is compensated for the electric signal, and the phase is changed from the pilot tone.
 The fluctuation can be estimated, and the phase fluctuation of the main signal can be compensated based on the estimated phase fluctuation.
  The optical receiver includes a configuration in which a plurality of chromatic dispersion compensators and phase noise compensators are alternately connected, and the chromatic dispersion compensator indicates an amount of compensation for distortion caused by chromatic dispersion for an electrical signal. By compensating for the dispersion compensation amount, the phase noise compensation unit estimates the amount of phase variation from the pilot tone and compensates for the phase variation of the electric signal while reducing the deviation between the main signal and the pilot tone. In other words, the optical receiving apparatus can improve the accuracy of phase compensation by repeatedly compensating for the phase fluctuation while reducing the deviation between the main signal and the pilot tone accompanying chromatic dispersion.
  Further, the optical receiving device compensates for chromatic dispersion and phase variation by alternately connecting a plurality of chromatic dispersion compensation units and phase compensation application units for electrical signals, while for pilot tones, After processing in the order of the pilot tone extraction unit and the phase estimation unit, a plurality of dispersion reduction filters are connected to the output side of the phase estimation unit to compensate for chromatic dispersion. That is, the optical receiving apparatus needs to extract the pilot tone and calculate the changed phase only once. The optical receiving apparatus can improve the accuracy of phase compensation by repeatedly compensating for the phase fluctuation while reducing the deviation between the main signal and the pilot tone. Further, the circuit scale of the dispersion reducing filter that compensates the dispersion compensation amount for the pilot tone can be realized with a circuit scale smaller than that of the chromatic dispersion compensation unit for the electric signal.

Further, the light receiving device according to claim 2 quoting Claim 1, the first of the variance reduction filter, to compensate for the delay caused by the wavelength dispersion of the second of the wavelength dispersion compensator and the same amount It is characterized by.
Further, in the optical receiving device according to claim 5, wherein the nth dispersion reduction filter compensates for a delay due to the same amount of chromatic dispersion as the n + 1th chromatic dispersion compensation unit. It is characterized by.

The second chromatic dispersion compensator according to claim 3 changes a compensation amount of chromatic dispersion corresponding to a level diagram of signal power of the optical fiber transmission line, and sets the first and second compensation amounts. part is, in correspondence with the level diagram of the signal power of the optical fiber transmission line, characterized in that changing the weighting factors.
The n + 1 th chromatic dispersion compensation unit according to claim 6 changes a compensation amount of chromatic dispersion corresponding to a signal power level diagram of the optical fiber transmission line, and the n + 1 th compensation amount setting unit. Is characterized in that the weighting factor is changed corresponding to a signal power level diagram of the optical fiber transmission line.

The invention relating to the optical receiving method has the same technical features as the optical receiving device according to claims 1 and 4 and has the same effect as the optical receiving device. Omitted.

Claims (7)

A receiving unit that receives a signal light in which a pilot tone having a frequency component different from that of the main signal is superimposed on the main signal via an optical fiber transmission line, and converts the signal light into an electric signal;
A chromatic dispersion compensator that compensates chromatic dispersion for the input electrical signal and outputs an electrical signal compensated for the chromatic dispersion;
Phase fluctuation was estimated from the pilot tone included in the input electric signal, and based on the estimated phase fluctuation, the phase fluctuation of the main signal included in the input electric signal was compensated, and the phase fluctuation was compensated. A phase noise compensator for outputting electrical signals;
An optical receiving device comprising:   The optical receiver according to claim 1, comprising a configuration in which a plurality of the chromatic dispersion compensation units and the phase noise compensation units are alternately connected. The phase noise compensation unit is
A pilot tone extractor for extracting the pilot tone from the input electrical signal;
Detecting a phase variation of the pilot tone and estimating an amount of the varied phase;
A compensation amount setting unit that calculates a phase compensation amount by multiplying the amount of the phase estimated by the phase estimation unit by a predetermined weighting factor;
Applying the phase compensation amount to the electrical signal to compensate for the changed phase; and
The optical receiver according to claim 1, further comprising: The phase noise compensation unit is
A pilot tone extractor for extracting the pilot tone from the input electrical signal;
Detecting a phase variation of the pilot tone and estimating an amount of the varied phase;
A compensation amount setting unit that calculates a phase compensation amount by multiplying the amount of the phase estimated by the phase estimation unit by a predetermined weighting factor;
Applying the phase compensation amount to the electrical signal to compensate for the changed phase; and
A dispersion reducing filter for compensating chromatic dispersion for the pilot tone;
With
The optical receiver is
For the electrical signal, a plurality of the chromatic dispersion compensation unit and the phase compensation application unit are alternately connected to compensate for at least one of chromatic dispersion or phase fluctuation,
The pilot tone is processed in the order of the pilot tone extraction unit and the phase estimation unit, and then a plurality of dispersion reduction filters are connected to the output side of the phase estimation unit to compensate for chromatic dispersion. The optical receiver according to claim 1. The optical receiver according to claim 4, wherein at least one of the dispersion reduction filter and the chromatic dispersion compensation unit compensates for a delay due to chromatic dispersion. The chromatic dispersion compensation unit changes a compensation amount of chromatic dispersion corresponding to a level diagram of signal power of the optical fiber transmission line,
6. The optical reception according to claim 3, wherein the compensation amount setting unit changes the weighting coefficient in accordance with a level diagram of signal power of the optical fiber transmission line. apparatus. An optical reception method of an optical receiver for receiving a signal light in which a pilot tone having a frequency component different from that of the main signal is superimposed on a main signal via an optical fiber transmission line,
The optical receiver is
A receiving step of converting the received signal light into an electrical signal;
A chromatic dispersion compensation step of compensating the chromatic dispersion for the input electrical signal and outputting the electrical signal compensated for the chromatic dispersion;
Phase fluctuation was estimated from the pilot tone included in the input electric signal, and based on the estimated phase fluctuation, the phase fluctuation of the main signal included in the input electric signal was compensated, and the phase fluctuation was compensated. A phase noise compensation step for outputting an electrical signal;
An optical receiving method comprising:

Images