JP2015185974A - Optical receiver, optical communication system, and inter-polarization crosstalk compensation method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To compensate inter-polarization crosstalk deriving from an optical Kerr effect varying in high speed.SOLUTION: An optical receiver 2 comprises: an analog-digital conversion unit 203 that generates a polarization multiplexed main signal and receives signal light in which a pilot tone is multiplexed on the main signal in a frequency domain via an optical fiber transmission path 5, and separates the received signal light into a first polarization channel and a second polarization channel; a pilot tone separation unit 205 that separates the pilot tone and the main signal from each of the polarization channels; and inter-polarization crosstalk compensation units (206-209) that performs inter-polarization crosstalk compensation of an amount proportional to absolute value amplitude difference to the main signals on the first and second polarization channels by using difference between absolute value amplitude of the pilot tone on the first polarization channel and absolute value amplitude of the pilot tone on the second polarization channel.

Description

  The present invention relates to a technique for compensating for crosstalk between polarizations derived from the optical Kerr effect.

  In order to meet the transmission demand of the core network that is increasing year by year, an optical transmission device that realizes a transmission capacity of 100 Gb / s class per wavelength is being put into practical use. In long-distance optical transmission of 100 Gb / s class, a coherent reception method is mainly used. Furthermore, by using a phase modulation technique and a polarization multiplexing technique, information transmission of 100 Gb / s is realized at a symbol rate of about 28 to 35 GHz.

  In long-distance optical transmission of 100 Gb / s class, signal distortion due to nonlinear optical effects occurs. The signal distortion becomes a factor that limits the transmittable distance. The nonlinear optical effect increases the deterioration of the signal quality as the intensity of the optical signal input to the transmission optical fiber is increased. Also, the nonlinear optical effect increases the signal quality degradation as the transmission distance increases. When the signal quality is greatly deteriorated, a bit error occurs on the receiving side.

  Therefore, the nonlinear optical effect directly limits the transmission distance in that the transmission distance must be selected so that the bit error does not exceed the allowable value of the forward error correction code. In addition, in order to prevent signal quality degradation due to the nonlinear optical effect from becoming significantly large, it is necessary to limit the intensity of the optical signal input to the transmission optical fiber. For this reason, the signal-to-noise ratio on the receiving side cannot be increased, and the nonlinear optical effect indirectly limits the transmittable distance.

The signal quality degradation caused by the nonlinear optical effect is roughly classified into three types: self-phase modulation, cross-phase modulation, and four-wave mixing.
Self-phase modulation refers to a nonlinear optical effect that is completed within a single transmission channel, and is a phenomenon in which signal distortion occurs due to the electric field of the signal itself. Since this phenomenon is closed in the channel, it is known that signal distortion due to self-phase modulation can be compensated or reduced by using the back propagation method.
Four-wave mixing is a big problem when the chromatic dispersion of the transmission optical fiber is very small, but it is not a big problem if operation near the zero dispersion wavelength is avoided.

  Cross phase modulation becomes a problem when wavelength multiplexing technology is employed. Since nonlinear optical effects occur across channels, it is difficult to compensate with only signal processing within a single channel. However, it is not practical to perform signal processing across all wavelength-multiplexed channels.

In an optical communication system using wavelength multiplexing technology and employing polarization multiplexing means, polarization rotation derived from the optical Kerr effect, which is a kind of cross-phase modulation, becomes a problem.
The optical Kerr effect is a phenomenon in which the refractive index of a medium varies depending on the intensity of a photoelectric field. Specifically, the optical Kerr effect is a phenomenon that causes birefringence in a medium by causing separate refractive index fluctuations in two directions perpendicular to the traveling direction of light. When the optical Kerr effect occurs, the optical signal undergoes polarization rotation in the transmission optical fiber. That is, when a polarization multiplexed optical signal is used, crosstalk between polarizations derived from the optical Kerr effect occurs.

  Both polarizations of the polarization multiplexed optical signal have the same intensity. However, when wavelength multiplexing is performed, the intensities of both polarizations are not necessarily constant because they happen to locally interfere with each other or interfere with each other. Therefore, crosstalk between polarizations derived from the optical Kerr effect occurs. Further, the amplitude imbalance between the polarizations fluctuates as fast as the symbol rate, so the crosstalk amount fluctuates as fast as the symbol rate. However, when the chromatic dispersion is not zero, the amplitude imbalance between the polarizations is temporally averaged due to the chromatic dispersion, and the fluctuation rate of the crosstalk amount is about one digit slower than the symbol rate.

  An optical receiver that receives a polarization-multiplexed optical signal generally has a function of discriminating two orthogonal polarized waves and removing crosstalk. Such a function is realized by, for example, an adaptive MIMO (Multiple Input Multiple Output) filter controlled using an LMS (Least Mean Square) algorithm (see Non-Patent Documents 1 and 2). When the time fluctuation of the polarization rotation is gentle compared with the tracking speed of the adaptive MIMO filter, the crosstalk between the polarizations can be compensated.

S. Haykin, “Adaptive Filter Theory”, Prentice-Hall, 1996 P. Johannisson, et al., “Convergence Comparison of CMA and ICA for Blind Polarization Demultiplexing of QPSK and 16-QAM Signalsn”, ECOC 2010, Th.9.A.3, (2010.9)

  However, the tracking speed of an LMS (eg, CMA; Constant Modulus Algorithm) algorithm is determined by a step size parameter, which takes into account the trade-off between tracking speed and noise suppression, and is a symbol rate of 1. / 1000 to 1/10000. Therefore, when the cross-polarization crosstalk change rate is about 1/1000 of the symbol rate, the cross-polarization crosstalk can be removed by the adaptive MIMO filter controlled using the LMS algorithm. Crosstalk between polarizations that fluctuates at high speed cannot be removed and remains.

  Therefore, an object of the present invention is to provide a technique for compensating for inter-wave crosstalk derived from the optical Kerr effect that fluctuates at high speed.

  The optical receiving apparatus of the optical communication system of the present invention receives a signal light obtained by multiplexing a polarization-multiplexed main signal with a pilot tone in the frequency domain via an optical fiber transmission line, and receives the received signal light as a first signal. A converter that separates the polarization channel into a second polarization channel; a pilot tone separator that separates the pilot tone and the main signal from each polarization channel separated by the converter; An inter-polarization crosstalk compensation unit that performs phase difference compensation on the main signal of the second polarization channel by an amount corresponding to the phase difference between pilot tones of the second polarization channel. To do.

  According to such a configuration, the optical receiver of the optical communication system compensates for the phase difference of the crosstalk between polarizations remaining in the main signal based on the phase difference between the polarization channels of the pilot tone. be able to. Therefore, the optical receiver can compensate the crosstalk between polarizations derived from the optical Kerr effect that fluctuates at high speed with respect to the main signal.

  Further, the inter-polarization crosstalk compensating unit of the optical receiving device includes an absolute value amplitude of a pilot tone of the first polarization channel and an absolute value of a pilot tone of the second polarization channel before the phase difference compensation. Based on the difference from the amplitude, polarization crosstalk compensation in an amount proportional to the difference is performed on the main signal of the second polarization channel after the first and the phase difference compensation. .

  According to such a configuration, the optical receiver can compensate for the amplitude imbalance among the crosstalk between polarizations remaining in the main signal based on the absolute value amplitude of the pilot tone. Therefore, the optical receiver can compensate the crosstalk between polarizations derived from the optical Kerr effect that fluctuates at high speed with respect to the main signal.

  The optical receiver performs phase difference compensation for the pilot tone of the second polarization channel in an amount corresponding to the phase difference between the pilot tones of the first and second polarization channels. And generating a pilot tone of the second polarization channel after the phase difference compensation, and an absolute value amplitude of the pilot tone of the first polarization channel and a pilot of the second polarization channel before the phase difference compensation Based on the difference from the absolute value of the tone amplitude, the cross-polarization crosstalk compensation in an amount proportional to the difference is applied to the pilot tone of the first polarization channel and the pilot of the second polarization channel after the phase difference compensation. Generating tones of the first and second polarization channels after compensation for cross-polarization crosstalk, and performing the first and second after compensation for cross-polarization crosstalk. And executes a main signal phase compensation of the first and second polarization channels after the polarization crosstalk compensation based on pilot tones of the wave channel.

  According to such a configuration, the optical receiving apparatus performs cross-polarization compensation for the pilot tone between the polarizations, and performs phase compensation for the phase noise on the main signal based on the pilot tone after the compensation for the cross-polarization crosstalk. be able to.

  An optical receiving apparatus of an optical communication system according to the present invention receives a signal light in which a pilot tone is multiplexed in a frequency domain on a polarization-multiplexed main signal via an optical fiber transmission line, and the received signal light is a first polarization. A converter that separates the channel into a second polarization channel, a pilot tone separator that separates the pilot tone and the main signal from each polarization channel separated by the converter, and the first polarization channel Based on the difference between the absolute value of the pilot tone and the absolute value of the pilot tone of the second polarization channel, the cross-polarization compensation between the polarizations is proportional to the difference. And a polarization crosstalk compensation unit that executes the main signal of the polarization channel.

  According to such a configuration, the optical receiver of the optical communication system can compensate for the amplitude imbalance of the crosstalk between polarizations remaining in the main signal based on the absolute value amplitude of the pilot tone. . Therefore, the optical receiver can compensate the crosstalk between polarizations derived from the optical Kerr effect that fluctuates at high speed with respect to the main signal.

  In addition, the inter-polarization crosstalk compensation unit of the optical receiver performs phase difference compensation of an amount matching the phase difference between pilot tones of the first and second polarization channels after the inter-polarization crosstalk compensation. This is performed on the main signal of the second polarization channel.

  According to such a configuration, the optical receiving apparatus can compensate for the phase difference of the crosstalk between polarizations remaining in the main signal based on the phase difference between the polarization channels of the pilot tone. Therefore, the optical receiver can compensate the crosstalk between polarizations derived from the optical Kerr effect that fluctuates at high speed with respect to the main signal.

  The optical receiver is proportional to the difference based on a difference between an absolute value amplitude of a pilot tone of the first polarization channel and an absolute value amplitude of a pilot tone of the second polarization channel. An amount of inter-polarization crosstalk compensation is performed on the pilot tones of the first and second polarization channels to generate first and second polarization channel pilot tones after the inter-polarization crosstalk compensation The phase difference compensation of an amount that matches the phase difference between the pilot tones of the first and second polarization channels before the inter-polarization crosstalk compensation is applied to the second polarization channel before the inter-polarization crosstalk compensation. A pilot tone of the second polarization channel after the phase difference compensation is generated by executing the pilot tone, and the pilot polarization of the first polarization channel after the inter-polarization crosstalk compensation is generated. The main signal of the first polarization channel after compensation of crosstalk between the polarizations and the second polarization channel after compensation of the phase difference based on the tone and the pilot tone of the second polarization channel after compensation of the phase difference The phase compensation of the main signal is performed.

  According to such a configuration, the optical receiving apparatus performs cross-polarization compensation for the pilot tone between the polarizations, and performs phase compensation for the phase noise on the main signal based on the pilot tone after the compensation for the cross-polarization crosstalk. be able to.

  The invention related to the optical communication system having the optical receiver and the invention related to the method of compensating for crosstalk between polarizations have the same technical features as the optical receiver described above, and have the same operation as the optical receiver. Since it has an effect, description is abbreviate | omitted in the means for solving this subject.

  According to the present invention, it is possible to compensate for crosstalk between polarizations derived from the optical Kerr effect that fluctuates at high speed in an optical receiver.

It is a figure which shows the structural example of an optical communication system. It is a figure which shows the function example of an optical transmitter. It is a figure which shows the function example of an optical receiver. It is a figure which shows the function example of a signal processing part. It is a figure which shows another example of a function of a signal processing part. It is a figure which shows an example of the experimental result of Q value in the case with and without crosstalk compensation between polarization | polarized-light.

  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 communication system)
As shown in FIG. 1, the optical communication system 100 includes an optical transmission device 1 and an optical reception device 2. The optical transmission device 1 has a function of generating a signal signal by generating a main signal obtained by polarization-multiplexing transmission data and performing optical modulation of an electric signal obtained by multiplexing a pilot tone in the frequency domain. Then, the optical transmission device 1 outputs the signal light to the optical fiber transmission line 5.
The optical receiver 2 receives the signal light transmitted through the optical fiber transmission line 5 as received light, separates the received light into two polarization channels, and converts the received light into pilot tones included in each polarization channel. Based on this, it has a function of executing crosstalk compensation between polarizations on the main signal. That is, the optical receiver 2 reduces cross-polarization crosstalk.
The optical fiber transmission line 5 is constituted by an optical fiber or an optical amplifier (not shown).

(Optical transmitter)
Next, an example of functions of the optical transmission device 1 will be described with reference to FIG.
As shown in FIG. 2, the optical transmission device 1 includes a symbol mapping unit 101, a pilot tone insertion unit 102, a digital / analog conversion unit 103, an optical modulation unit 104, and a laser oscillation unit 105.

  The symbol mapping unit 101 receives the transmission data 10 by receiving it from the outside or reading it from the memory. The symbol mapping unit 101 maps the received transmission data 10 to a polarization multiplexed QPSK (Quadrature Phase Shift Keying) signal, and digitally represents time-series signal waveform data (hereinafter referred to as a main signal). It has a function to generate. Symbol mapping section 101 outputs the generated main signal to pilot tone insertion section 102.

The pilot tone insertion unit 102 has a function of receiving a main signal, frequency-multiplexing a pilot tone with the received main signal, and generating a modulated signal. It is assumed that the pilot tone and the main signal are phase-synchronized. Further, it is preferable that the frequency of the pilot tone is close to the center frequency of the occupied band of the main signal in a range that does not overlap with the occupied band of the main signal. The main signal is composed of an X polarization channel and a Y polarization channel superimposed, and the X polarization and the Y polarization are orthogonal to each other. The X polarization component and the Y polarization component of the pilot tone are designed or adjusted so as to have the same amplitude and phase.
The pilot tone insertion unit 102 outputs the generated modulation signal to the digital / analog conversion unit 103.

  The digital-analog conversion unit 103 has a function of receiving a modulation signal and converting it into an analog electric signal. The analog electric signal is a four-lane electric signal and corresponds to an X polarization in-phase component, an X polarization quadrature component, a Y polarization in-phase component, and a Y polarization quadrature component, respectively. The digital / analog conversion unit 103 outputs the four-lane electrical signal to the light modulation unit 104.

  The optical modulation unit 104 has a function of receiving an electric signal of four lanes, modulating the laser light of the laser oscillation unit 105 by the received electric signal, and generating a polarization multiplexed QPSK optical signal (signal light 11). Then, the optical modulation unit 104 outputs the generated signal light 11 to the optical fiber transmission line 5.

  The laser oscillation unit 105 has a function of generating an optical signal using a laser and outputting the optical signal to the optical modulation unit 104.

  In this embodiment, the modulation scheme is polarization multiplexed QPSK, but other modulation schemes such as polarization multiplexed BPSK (Binary Phase Shift Keying) and polarization multiplexed QAM (Quadrature Amplitude Modulation) may be employed. Is possible. Further, the pilot tone multiplexing is described as being performed at the stage of the digital signal that is the output of the symbol mapping unit 101. However, the pilot tone multiplexing may be performed at the stage of the analog signal that is the output of the digital / analog conversion unit 103.

(Optical receiver)
Next, a function example of the optical receiver 2 will be described with reference to FIG.
As shown in FIG. 3, the optical receiver 2 includes a coherent photoelectric conversion unit (conversion unit) 201, a local oscillation laser unit 202, an analog-digital conversion unit (conversion unit) 203, a polarization separation unit 204, and a pilot tone separation unit 205. , XY phase difference detection unit (cross-polarization crosstalk compensation unit) 206, XY phase difference compensation unit (cross-polarization crosstalk compensation unit) 207, polarization rotation detection unit (cross-polarization crosstalk compensation unit) 208, A polarization rotation compensation unit (cross-polarization crosstalk compensation unit) 209, a phase compensation unit 210, and a signal processing unit 211 are provided.

  The coherent photoelectric conversion unit 201 receives the signal light 11 that has passed through the optical fiber transmission line 5. Here, the received signal is referred to as received light 21. The coherent photoelectric conversion unit 201 has a function of mixing the received light 21 with the local light 22 output from the local oscillation laser unit 202 and converting the received light 21 into the received analog electrical signal 23 while maintaining the phase information between the polarizations. Then, the coherent photoelectric conversion unit 201 outputs the received analog electrical signal 23 to the analog / digital conversion unit 203.

  The local oscillation laser unit 202 has a function of generating an optical signal using a local oscillation laser in order to perform coherent reception.

  The analog-digital conversion unit 203 has a function of receiving the received analog electrical signal 23 and converting the received received analog electrical signal 23 into a digital signal. The digital signal is referred to as a received digital signal 24, and the original 4-lane signal is expressed as a 2-lane complex signal. That is, the received digital signal 24 is represented by the first polarization channel and the second polarization channel that are the X polarization component and the Y polarization component. The analog / digital conversion unit 203 outputs the received digital signal 24 to the polarization separation unit 204.

  The polarization separation unit 204 has a function of receiving the received digital signal 24 and applying an adaptive MIMO filter to the received received digital signal 24 to remove static or quasi-static polarization crosstalk. Polarization separator 204 outputs received digital signal 24 from which crosstalk between polarizations has been removed to pilot tone separator 205. The adaptive MIMO filter receives a 2-lane complex signal as input, compensates for cross-polarization crosstalk between signals in both lanes, and outputs a compensated 2-lane complex signal. The adaptive MIMO filter can remove the crosstalk by the adaptive MIMO filter when the change rate of the crosstalk between the polarizations is about 1 / 10,000 of the symbol rate. However, cross-polarization crosstalk that fluctuates at a speed higher than 1/10000 of the symbol rate cannot be removed and remains.

  The pilot tone separation unit 205 has a function of receiving the output signal of the polarization separation unit 204 and applying a digital filter to the output signal to separate the output signal into the pilot tone 26 and the main signal 25. Pilot tone separation section 205 then outputs main signal 25 to XY phase difference compensation section 207. Pilot tone separation section 205 outputs pilot tone 26 to XY phase difference detection section 206. In FIG. 3, the path of the main signal 25 is represented by a one-dot chain arrow, and the path of the pilot tone 26 is represented by a dotted arrow.

  An XY phase difference detection unit (cross-polarization crosstalk compensation unit) 206 receives the pilot tone 26, obtains a phase difference between the X polarization and the Y polarization of the pilot tone 26, and determines the phase difference. The phase difference information 27 is output to the XY phase difference compensation unit 207. Then, the XY phase difference detection unit 206 outputs the received pilot tone 26 to the XY phase difference compensation unit 207. The pilot tone 26 is linearly polarized when the phase difference between the X polarization and the Y polarization is zero, and the pilot tone 26 is elliptically polarized when the phase difference is not zero. Yes.

  The XY phase difference compensation unit (cross-polarization crosstalk compensation unit) 207 receives the main signal 25, the pilot tone 26, and the phase difference information 27. The XY phase difference compensation unit 207 adjusts the phase of either the X polarized wave or the Y polarized wave of the pilot tone 26 so that the pilot tone 26 becomes linearly polarized light based on the phase difference information 27. Have Further, the XY phase difference compensation unit 207 adjusts the phase of either the X polarization or the Y polarization of the main signal 25 based on the phase difference information 27 so that the main signal 25 becomes linearly polarized light. It has the function to do. Then, the XY phase difference compensation unit 207 outputs the pilot tone before phase difference compensation and the pilot tone after phase difference compensation to the polarization rotation detection unit 208. Further, the XY phase difference compensation unit 207 outputs the main signal after phase difference compensation to the polarization rotation compensation unit 209.

  A polarization rotation detection unit (cross-polarization crosstalk compensation unit) 208 receives a pilot tone before phase difference compensation and a pilot tone after phase difference compensation. The polarization rotation detection unit 208 has a function of obtaining the absolute value amplitudes of the X polarization and Y polarization of the received pilot tone before phase difference compensation, and further obtaining a value proportional to the difference between the absolute value amplitudes of both. The proportional value is amplitude imbalance data (polarization rotation information 28) indicating polarization rotation that cancels the amplitude imbalance of the pilot tone 26. Then, the polarization rotation detection unit 208 outputs the pilot tone and main signal after phase difference compensation, and amplitude imbalance data (polarization rotation information 28) to the polarization rotation compensation unit 209.

  A polarization rotation compensation unit (cross-polarization crosstalk compensation unit) 209 receives the pilot tone and main signal after phase difference compensation, and amplitude imbalance data (polarization rotation information 28). The polarization rotation compensation unit 209 has a function of executing cross-polarization crosstalk compensation processing for the pilot tone after phase difference compensation based on the received polarization rotation information 28. In addition, the polarization rotation compensation unit 209 has a function of executing cross-polarization crosstalk compensation processing on the main signal after phase difference compensation based on the received polarization rotation information 28. The polarization rotation compensation unit 209 outputs the pilot tone and main signal after the crosstalk compensation between polarizations to the phase compensation unit 210.

  In FIG. 3, it has been described that the processing of the polarization rotation detection unit 208 and the polarization rotation compensation unit 209 is performed after the processing of the XY phase difference detection unit 206 and the XY phase difference compensation unit 207. In this order, the processing of the XY phase difference detection unit 206 and the XY phase difference compensation unit 207 is executed after the processing of the polarization rotation detection unit 208 and the polarization rotation compensation unit 209. It doesn't matter.

  The phase compensation unit 210 receives the pilot tone and the main signal after the crosstalk compensation between polarizations. The phase compensator 210 has a function of obtaining a complex conjugate value of the received pilot tone after cross-polarization crosstalk compensation and multiplying the main signal by the complex conjugate value to compensate for phase fluctuation (phase noise) of the main signal. Have. Then, the phase compensation unit 210 outputs the main signal compensated for the phase fluctuation (phase noise) to the signal processing unit 211.

As illustrated in FIG. 4, the signal processing unit 211 includes, for example, a frequency offset compensation unit 212, a carrier phase synchronization unit 213, and a symbol determination unit 214.
The frequency offset compensation unit 212 has a function of estimating a frequency offset amount and executing frequency offset compensation.
The carrier phase synchronization unit 213 has a function of synchronizing the received main signal with the carrier wave.
The symbol determination unit 214 has a function of demodulating a symbol series from the main signal that is an output signal of the carrier phase synchronization unit 213 and outputting the original transmission data 10 transmitted from the transmission side.
Note that the signal processing unit 211 may include only the carrier phase synchronization unit 213 and the symbol determination unit 214 as shown in FIG.

(Example of processing for crosstalk compensation between polarizations)
Here, cross-polarization crosstalk compensation processing in the cross-polarization crosstalk compensation unit (XY phase difference detection unit 206, XY phase difference compensation unit 207, polarization rotation detection unit 208, and polarization rotation compensation unit 209). An example and a processing example of phase compensation in the phase compensation unit 210 will be described below.

Here, each of the X polarization component and the Y polarization component (the main signal of the first polarization channel and the main signal of the second polarization channel) of the main signal 25 output from the pilot tone separation unit 205 is represented by S _x0. , S _y0 . Also, the X polarization component and the Y polarization component (pilot tone of the first polarization channel and pilot tone of the second polarization channel) of the pilot tone 26 output from the pilot tone separation unit 205 are respectively represented by P _x0 , _{Let Py0} . Each of S _x0 , S _y0 , P _x0 , and P _y0 represents a one-dimensional time series and is a function of time. The main signals S _x0 and S _y0 have a crosstalk component between polarizations caused by dynamic polarization rotation derived from the optical Kerr effect. Polarization rotation also occurs for pilot tones P _x0 and P _y0 as in the case of the main signal.

Both pilot tones P _x0 and P _y0 have the same amplitude and phase in an ideal state. However, due to the polarization rotation, the pilot tones P _x0 and P _y0 include the phase difference d and the amplitude imbalance δ. The phase difference d and the amplitude imbalance δ can be expressed by the following equation (1) using the pilot tones P _x0 and P _y0 .

ただし、＊は複素共役を表す。

However, * represents a complex conjugate.

Next, using the phase difference d and the amplitude imbalance δ, the main signals S _x1 and S _y1 after polarization crosstalk compensation can be expressed as the following equation (2).

First, with respect to the phase difference, the amount of phase difference compensation corresponding to the phase difference d between the pilot tone P _x0 of the X polarization component and the pilot tone P _y0 of the Y polarization component is compensated for using the equation (2). This is performed on the main signal S _{y0 of the} wave component. In addition, the amplitude imbalance is proportional to the difference between the absolute value amplitude of the pilot tone of the X polarization component and the absolute value amplitude of the pilot tone of the Y polarization component from Equation (1). A value of the amplitude imbalance δ is obtained. Then, the amplitude imbalance δ is applied to the equation (2) to perform polarization crosstalk compensation on the main signal in an amount proportional to the absolute value amplitude difference. As a result, compensation for crosstalk between polarizations of the main signal can be performed.

Note that the signal quality of the main signal can be further improved by performing phase compensation using the pilot tone. First, compensation processing for cross-polarization crosstalk similar to that of the main signal is performed for pilot tones P _x0 and P _y0 . The pilot tones P _x1 and P _y1 after compensation of crosstalk between polarizations can be expressed as the following equation (3).

By the equation (3), the phase difference compensation of the amount corresponding to the phase difference d between the pilot tone P _x0 of the X polarization component and the pilot tone P _y0 of the Y polarization component is applied to the pilot tone P _{y0 of the} Y polarization component. And execute. Next, the amplitude imbalance δ is applied to Equation (3) to perform cross polarization compensation between the pilot tones. This makes it possible to compensate for the crosstalk between the pilot tones of polarization. The pilot tones P _x1 and P _y1 after compensation of crosstalk between polarizations are equal except for the influence of noise.
Further, the phase compensation coefficient is obtained using the following equation (4).

ただし、ｆはパイロットトーン周波数（ｆ_ＰＴ）と主信号の中心周波数(ｆｃ)との差を示す周波数差であり、ｆ＝ｆｃ−ｆ_ＰＴで与えられる。ｔは時間である。ｉは虚数単位である。

Here, f is a frequency difference indicating a difference between the pilot tone frequency (f _PT ) and the center frequency (fc) of the main signal, and is given by f = fc−f _PT . t is time. i is an imaginary unit.

P _c shown in equation (4) can be expected to correspond to the phase noise of the main signal, and the phase-compensated main signals S _x2 and S _y2 can be calculated by the following equation (5).

In the above description, the phase difference compensation is executed first and then the cross-polarization crosstalk compensation is executed, but conversely, as is clear from the equations (2) and (3), the cross-polarization cross-talk is performed. The talk compensation may be performed first and the phase difference compensation may be performed later.
Note that the X polarization component and the Y polarization component in the above description may be interchanged.

Next, an example of a Q value experimental result with and without cross-polarization crosstalk compensation will be described with reference to FIG.
In FIG. 6, the horizontal axis represents OSNR (Optical Signal to Noise Ratio), and the vertical axis represents the Q value. The circles indicate the case where cross-polarization crosstalk compensation of the present invention is performed, that is, the case of “with cross-polarization crosstalk compensation”. In addition, a solid line is displayed between the plots marked with ○. Further, Δ marks indicate a case where the cross-polarization crosstalk compensation of the present invention is not performed, that is, the case of “no cross-polarization crosstalk compensation”. In addition, a dotted line is displayed between plots marked with Δ.

  From FIG. 6, it can be seen that the Q value when the OSNR is 29 dB in the case of “without polarization crosstalk compensation” and the Q value when the OSNR is 26 dB in the case of “with polarization crosstalk compensation” are equal. Recognize. Therefore, when the cross-polarization crosstalk compensation of the present invention is applied, even when the OSNR deteriorates by 3 dB as the transmission distance increases, it is equivalent to the case of “no cross-polarization crosstalk compensation”. Quality transmission is possible. That is, there is an effect that the transmission distance can be extended by applying the crosstalk compensation between polarizations of the present invention.

As described above, the optical receiver 2 of the present embodiment receives the received light 21 in which the main signal and the pilot tone are multiplexed in the frequency domain in the optical transmitter 1 and uses the pilot tone as a reference signal to cross-polarize crosstalk of the main signal. Perform compensation. Specifically, the optical receiver 2 receives the signal light 11 obtained by multiplexing the polarization-multiplexed main signal with the pilot tone in the frequency domain via the optical fiber transmission line 5, and receives the received signal light (received light 21). Are converted into the first polarization channel and the second polarization channel (coherent photoelectric conversion unit 201, analog-digital conversion unit 203), and the pilot tone and the pilot tone from each polarization channel separated by the conversion unit A pilot tone separation unit 205 that separates the main signal, based on a difference between an absolute value amplitude of a pilot tone of the first polarization channel and an absolute value amplitude of a pilot tone of the second polarization channel, An inter-polarization crosstalk compensation unit (XY) that performs cross-polarization cross-polarization compensation in an amount proportional to the difference with respect to the main signals of the first and second polarization channels. Phase difference detection unit 206, X-Y phase difference compensation unit 207 includes a polarization rotation detector 208, the polarization rotation compensation part 209), the.
Thus, the optical receiver 2 (optical communication system 100) of the present invention can compensate for cross-polarization crosstalk due to the optical Kerr effect that fluctuates at high speed.

DESCRIPTION OF SYMBOLS 1 Optical transmitter 2 Optical receiver 5 Optical fiber transmission line 10 Transmission data 11 Signal light 21 Reception light 22 Local light 23 Reception analog electrical signal 24 Reception digital signal 25 Main signal 26 Pilot tone 27 Phase difference information 28 Polarization rotation information 100 Optical communication system 101 Symbol mapping unit 102 Pilot tone insertion unit 103 Digital analog conversion unit 104 Optical modulation unit 105 Laser oscillation unit 201 Coherent photoelectric conversion unit (conversion unit)
202 Local oscillation laser unit 203 Analog-digital conversion unit (conversion unit)
204 Polarization Separation Unit 205 Pilot Tone Separation Unit 206 XY Phase Difference Detection Unit (Inter-Polarization Crosstalk Compensation Unit)
207 XY phase difference compensation unit (cross-polarization crosstalk compensation unit)
208 Polarization rotation detection unit (cross-polarization crosstalk compensation unit)
209 Polarization rotation compensation unit (cross-polarization crosstalk compensation unit)
210 Phase compensation unit 211 Signal processing unit 212 Frequency offset compensation unit 213 Carrier phase synchronization unit 214 Symbol determination unit

The optical receiver of the optical communication system according to the present invention receives a signal light obtained by multiplexing a polarization-multiplexed main signal with a pilot tone in the frequency domain via an optical fiber transmission line, and the received signal light is an electric digital signal. was converted into a conversion unit which is separated into a first polarization channel and the second polarization channel, the first polarization channel and adaptive MIMO said second polarization channels (Multiple Input Multiple Output) filter By applying the polarization separation unit, the polarization separation unit that generates an output signal obtained by removing static or quasi-static cross-polarization crosstalk from the first polarization channel and the second polarization channel from each polarization channel of the generated the output signal, a first pilot tone and a second pilot tones, and the pilot tone separation unit for separating the first main signal and second main signal, Run the retardation compensation amount that matches the phase difference between the serial first pilot tone and said second path for the pilot tone to the second main signal, the second main signal after the phase difference compensation A cross-talk compensation unit for generating polarization between polarizations;

Further, the polarization crosstalk compensation portion of the light receiving device, and the absolute value amplitude of the first pilot tone, before SL on the basis of the difference between the absolute value amplitude of the second pilot tone, proportional to the difference To compensate for the first main signal and the second main signal after the phase difference compensation, and the first main signal and the cross-polarization crosstalk after the polarization crosstalk compensation. A second main signal after compensation is generated .

Further, the light receiving device, the first pilot tone and the second after phase difference compensation running phase difference compensation amount that matches the phase difference with respect to the second pilot tone between pilot tones the second generates a pilot tone, polarizations running polarization crosstalk compensation amount proportional before Symbol difference with respect to the first pilot tone and a second pilot tone of the phase-difference compensation generating a first pilot tone and a second pilot tone after polarization crosstalk compensation after crosstalk compensation, first after the first pilot tone and the polarization crosstalk compensation after the polarization crosstalk compensation after the polarization crosstalk compensation based on the second pilot tones first main signal and the polarization crosstalk compensated second main signals of the phase And executes the amortization.

The optical receiver of the optical communication system of the present invention receives the signal light obtained by multiplexing the polarization-multiplexed main signal with the pilot tone in the frequency domain via the optical fiber transmission line, and converts the received signal light into an electric digital signal. The conversion unit separating the first polarization channel and the second polarization channel, an adaptive MIMO filter applied to the first polarization channel and the second polarization channel, Each of the output signal generated by the polarization separation unit that generates an output signal obtained by removing static or quasi-static cross-polarization crosstalk from the polarization channel and the second polarization channel, and each of the output signals generated by the polarization separation unit from the polarization channel, the first pilot tone and a second pilot tones, and the pilot tone separation unit for separating the first main signal and second main signal, absolute of the first pilot tone Value amplitude, based on the difference between the absolute value amplitude of the second pilot tone, the polarization crosstalk compensation amount proportional to the difference to the first main signal and the second main signal And an inter-polarization crosstalk compensation unit configured to generate a compensated first main signal corresponding to the difference and a compensated second main signal corresponding to the difference .

Further, the inter-polarization crosstalk compensation unit of the optical receiver performs phase difference compensation of an amount matching the phase difference between the first pilot tone and the second pilot tone after compensation corresponding to the difference . It is performed on the second main signal, and a first main signal after polarization crosstalk compensation and a second main signal after polarization crosstalk compensation are generated .

Further, the light receiving device, the inter-polarization crosstalk compensation amount proportional before Symbol difference running against the first pilot tone and the second pilot tone after compensation corresponding to the difference generates one of the second pilot tone after compensation corresponding to the pilot tone and the difference, before Symbol said retardation compensation amount that matches the phase difference between the first pilot tone and the second pilot tone running against the second pilot tone after compensation corresponding to the difference, to generate a first pilot tone and polarization crosstalk second pilot tone after compensation after polarization crosstalk compensation, the polarization the polarization crosstalk compensation based on the first pilot tone and a second pilot tone after the polarization crosstalk compensation after waves crosstalk compensation And executes a first phase compensation of the main signal and second main signal after the polarization crosstalk compensation.

Claims (8)

A signal light obtained by multiplexing a pilot tone with a polarization multiplexed main signal in a frequency domain is received via an optical fiber transmission line, and the received signal light is separated into a first polarization channel and a second polarization channel. Conversion part,
A pilot tone separation unit for separating the pilot tone and the main signal from each polarization channel separated by the conversion unit;
An inter-polarization crosstalk compensation unit that performs phase difference compensation on the main signal of the second polarization channel in an amount corresponding to the phase difference between pilot tones of the first and second polarization channels;
An optical receiving device comprising: The polarization crosstalk compensation unit is
Based on the difference between the absolute value amplitude of the pilot tone of the first polarization channel and the absolute value amplitude of the pilot tone of the second polarization channel before the phase difference compensation, a deviation of an amount proportional to the difference is made. 2. The optical receiver according to claim 1, wherein inter-wave crosstalk compensation is performed on the main signal of the second polarization channel after the first and the phase difference compensation. The optical receiver performs phase difference compensation for the pilot tone of the second polarization channel by performing phase difference compensation in an amount that matches the phase difference between the pilot tones of the first and second polarization channels. A pilot tone of the second polarization channel after phase difference compensation is generated, and the absolute value of the pilot tone of the first polarization channel and the pilot tone of the second polarization channel before phase difference compensation are generated. Based on the difference from the absolute amplitude, an amount of cross-polarization crosstalk compensation proportional to the difference is applied to the pilot tone of the first polarization channel and the pilot tone of the second polarization channel after the phase difference compensation. The first and second polarization channels after the cross-polarization crosstalk compensation is performed, and the first and second polarization channels after the cross-polarization crosstalk compensation are generated. Channel optical receiving apparatus according to claim 2, characterized in that to perform phase compensation of the main signals of the first and second polarization channels after the polarization crosstalk compensation based on pilot tones. A signal light obtained by multiplexing a pilot tone with a polarization multiplexed main signal in a frequency domain is received via an optical fiber transmission line, and the received signal light is separated into a first polarization channel and a second polarization channel. Conversion part,
A pilot tone separation unit for separating the pilot tone and the main signal from each polarization channel separated by the conversion unit;
Based on the difference between the absolute value amplitude of the pilot tone of the first polarization channel and the absolute value amplitude of the pilot tone of the second polarization channel, an inter-polarization crosstalk compensation in an amount proportional to the difference is performed. An inter-polarization crosstalk compensation unit to be executed with respect to a main signal of the first and second polarization channels;
An optical receiving device comprising: The polarization crosstalk compensation unit is
A phase difference compensation of an amount matching the phase difference between the pilot tones of the first and second polarization channels is performed on the main signal of the second polarization channel after the inter-polarization crosstalk compensation. The optical communication system according to claim 4. The optical receiver is
Based on the difference between the absolute value amplitude of the pilot tone of the first polarization channel and the absolute value amplitude of the pilot tone of the second polarization channel, an inter-polarization crosstalk compensation in an amount proportional to the difference is performed. Performing on the pilot tones of the first and second polarization channels to generate the pilot tones of the first and second polarization channels after compensating for the cross-polarization crosstalk, and compensating for the cross-polarization crosstalk A phase difference compensation of an amount matching the phase difference between the pilot tones of the previous first and second polarization channels is performed on the pilot tone of the second polarization channel before the inter-polarization crosstalk compensation. A pilot tone of the second polarization channel after the phase difference compensation is generated, the pilot tone of the first polarization channel after the cross-polarization crosstalk compensation, and the phase difference Based on the pilot tone of the compensated second polarization channel, the phase of the main signal of the first polarization channel after the crosstalk compensation between the polarizations and the phase of the main signal of the second polarization channel after the phase difference compensation 6. The optical receiver according to claim 5, wherein compensation is performed. An optical transmitter that generates a polarization-multiplexed main signal, generates and outputs a signal light in which a pilot tone is multiplexed on the main signal in a frequency domain, and
A converter that receives the signal light via an optical fiber transmission line and separates the received signal light into a first polarization channel and a second polarization channel;
A pilot tone separation unit for separating the pilot tone and the main signal from each polarization channel separated by the conversion unit;
Based on the difference between the absolute value amplitude of the pilot tone of the first polarization channel and the absolute value amplitude of the pilot tone of the second polarization channel, an inter-polarization crosstalk compensation in an amount proportional to the difference is performed. An inter-polarization crosstalk compensation unit to be executed with respect to a main signal of the first and second polarization channels;
An optical receiver having
An optical communication system comprising: A crosstalk compensation method between polarizations of an optical receiver that receives a signal light obtained by multiplexing a pilot tone with a polarization multiplexed main signal in a frequency domain via an optical fiber transmission line,
The optical receiver is
A conversion step of separating the received signal light into a first polarization channel and a second polarization channel;
A pilot tone separation step of separating the pilot tone and the main signal from each polarization channel separated in the conversion step;
Based on the difference between the absolute value amplitude of the pilot tone of the first polarization channel and the absolute value amplitude of the pilot tone of the second polarization channel, an inter-polarization crosstalk compensation in an amount proportional to the difference is performed. An inter-polarization crosstalk compensation step performed on the main signals of the first and second polarization channels;
A method for compensating for crosstalk between polarizations, characterized in that:

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