JP2016100815A - Wavelength multiplex optical transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve crosstalk compensation, which is applicable to long-haul transmission, by estimating a propagation delay difference from a wavelength dispersion in a transmission line to compensate the propagation delay difference by digital signal processing.SOLUTION: A dispersion compensation amount compensated by a dispersion compensation circuit is a wavelength dispersion amount in a carrier optical frequency of a channel to be demodulated (an attention channel), and is configured to be applied to also a signal in an adjacent channel to be used to compensate a crosstalk component. By compensating a propagation delay difference in an optical signal of another channel at the same timing as a crosstalk component from the other channel superposed on the optical signal of the attention channel and removing a crosstalk component, crosstalk compensation by MIMO processing is also applicable to a wavelength multiplex optical signal which is transmitted through an optical fiber transmission line having a wavelength dispersion.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a wavelength division multiplexing optical transmission system for coherently detecting a wavelength division multiplexed optical signal using a plurality of receivers and demodulating it by digital signal processing.

  As the demand for data communication increases, optical transmission networks using optical signal modulation technology and optical signal multiplexing technology that enable transmission of large-capacity traffic are becoming widespread. In particular, digital coherent technology combining coherent detection and digital signal processing technology has been widely used in ultra-high-speed transmission systems with a transmission rate per wave of 100 Gbit / s or higher.

  The DP-QPSK (Dual Polarization Quadrature Phase Shift Keying) method, which is widely used as a modulation / demodulation method in 100 Gbit / s class long-distance optical transmission systems, generates 64 Gbit / s optical signals by using quaternary phase modulation. Furthermore, a 128 Gbit / s optical signal is generated by multiplexing two polarizations. On the receiving side, the signal coherently detected using local light having the same wavelength as that of the signal light is digitized using an analog-digital converter (A / D converter) and subjected to digital signal processing, so that the transmission path Excellent transmission characteristics are realized by performing chromatic dispersion compensation, polarization dispersion compensation, polarization separation, phase estimation, and the like. By wavelength multiplexing these DP-QPSK optical signals, an optical transmission system having a transmission capacity of several Tbit / s per fiber can be realized.

  As an approach to further increase capacity, crosstalk components between wavelength multiplexed optical signals, that is, crosstalk components between channels, are estimated and compensated by digital signal processing using MIMO (Multi-input multi-output) processing. Therefore, a MIMO processing crosstalk compensation technique that enables higher-density wavelength multiplexing and provides a large-capacity wavelength multiplexing optical transmission system has been proposed (Non-Patent Document 1).

F. Hamaoka, et.al., ‘Super High Density Multi-carrier Transmission Systems by MIMO Processing,‘ ECOC 2014, Mo.3.5.4

  The conventional MIMO processing crosstalk compensation technology does not take into account the propagation delay difference between the wavelength multiplexed optical signals generated due to the chromatic dispersion of the optical fiber transmission line. There is a problem that it cannot be applied to.

  The present invention provides a wavelength division multiplexing optical transmission system that realizes crosstalk compensation applicable to long-distance transmission by estimating the propagation delay difference from the chromatic dispersion of the transmission line and compensating the propagation delay difference by digital signal processing. The purpose is to provide.

The present invention provides a plurality N of carrier frequencies f ₁ , f ₂ ,..., F _N (f ₁ <f ₂ <... <f _N ) between a transmitter and a receiver connected via an optical fiber transmission line. In a wavelength division multiplexing optical transmission system that transmits wavelength-multiplexed optical signals obtained by wavelength-multiplexing optical signals of channels, the transmitter has a frequency interval of each channel smaller than the modulation frequency of each optical signal, and the optical signal of an adjacent channel is higher in frequency. a configuration for transmitting the optical fiber transmission line wavelength-division-multiplexed optical signals that overlap, the receiving unit, an optical frequency f ₁ different by the optical coupler N branched WDM optical signals, respectively, f _2, ..., stations f _N Crosstalk between adjacent channels due to optical spectrum overlap in the transmitted signal by executing digital signal processing on the coherent receiver that performs coherent detection by light emission and the electrical signal output from the coherent receiver A digital signal processing unit that compensates for the A / D converter that converts the electrical signal output from the coherent receiver into a digital signal, and the chromatic dispersion generated in the optical fiber transmission line. Adaptively changes the tap coefficient of the dispersion compensation circuit that compensates for the degraded optical waveform, the phase rotation compensation circuit that compensates for the rotation component caused by the difference in the optical frequency of the local light, and the filter that inputs the output of the phase rotation compensation circuit A waveform equalization circuit that compensates for crosstalk using a plurality of adaptive filters, a phase estimation circuit that estimates a carrier phase of an optical signal, and an identification circuit that identifies a transmission signal superimposed on the optical signal The dispersion compensation amount compensated by the dispersion compensation circuit is the chromatic dispersion amount at the carrier optical frequency of the channel to be demodulated, and is used to compensate for the crosstalk component. The phase rotation compensation amount in the phase rotation compensation circuit is the optical frequency interval Δf _k (k = 1, 2,..., N) with the adjacent channel.

  The present invention compensates for the propagation delay difference of the optical signal of the other channel at the same timing as the crosstalk component from the other channel superimposed on the optical signal of the channel (target channel) to be demodulated, and With the configuration in which the crosstalk component is removed from the signal, crosstalk compensation by MIMO processing can be applied to a wavelength multiplexed optical signal transmitted through an optical fiber transmission line having chromatic dispersion.

It is a figure which shows the structure of Example 1 of the wavelength division multiplexing optical transmission system of this invention. FIG. 3 is a diagram illustrating a configuration example of a digital signal processing unit 24 in the first embodiment. FIG. 6 is a diagram illustrating an operation example of the dispersion compensation circuit according to the first embodiment. It is a figure which shows the example of a demodulated signal by the structure of the conventional structure and Example 1 of this invention. 6 is a diagram illustrating a configuration example of a digital signal processing unit 24 in Embodiment 2. FIG. It is a figure which shows the electrical spectrum before and behind phase rotation compensation in Example 2. FIG. It is a figure which shows the image of the propagation delay difference in the case of implementing phase rotation compensation before dispersion compensation. It is a figure which shows the example of a demodulation signal by the structure of the conventional structure and Example 2 of this invention. 10 is a diagram illustrating a configuration example of a digital signal processing unit 24 in Embodiment 3. FIG. It is a figure which shows the electrical spectrum before and behind phase rotation compensation in Example 3. FIG. FIG. 10 is a diagram illustrating an example of dispersion compensation after phase rotation compensation in the third embodiment.

FIG. 1 shows the configuration of Embodiment 1 of the wavelength division multiplexing optical transmission system of the present invention.
In FIG. 1, in the wavelength division multiplexing optical transmission system according to the first embodiment, a transmission unit 10 and a reception unit 20 are connected via an optical fiber transmission line 50. The transmitter 10 includes signal light sources 11-1 to 11-3, polarization multiplexed vector modulators 12-1 to 12-3, and an optical coupler 13.

Signal source 11-1 to 11-3, respectively and outputs the optical carrier of the optical frequency f ₁ ~f _3. Here, it is assumed that f ₁ <f ₂ <f ₃ . Polarization multiplexing vector modulator 12-1, transmits the optical carrier of the optical frequency f ₁ output from the signal source 11-1 signals Data1x, generates an optical signal of the channel 1 with polarization multiplexing modulation in Data1y. Polarization multiplexing vector modulator 12-2 transmits the optical carrier of the optical frequency f ₂ output from the signal source 11-2 signals Data2x, generates an optical signal of the channel 2 which is polarization multiplexing modulation in Data2y. Polarization multiplexing vector modulator 12-3, transmission signal optical carrier of the optical frequency f ₃ output from the signal source 11-3 Data3x, generates an optical signal of the channel 3 with polarization multiplexing modulation in Data3y. The optical coupler 13 outputs a wavelength multiplexed optical signal obtained by combining the optical signals of channels 1 to 3 output from the polarization multiplexed vector modulators 12-1 to 12-3 to the optical fiber transmission line 50.

  At this time, the frequency interval between the optical signals of each channel may be smaller than the modulation frequency. In general, when the frequency interval is smaller than the modulation frequency, the optical spectrum of each optical signal overlaps the optical spectrum of an optical signal adjacent on the frequency axis. That is, crosstalk occurs between adjacent optical signals. In addition, in the wavelength multiplexed optical signal, due to the chromatic dispersion of the optical fiber transmission line 50, a waveform spread occurs in each channel and a propagation delay difference occurs between each optical signal.

  The receiving unit 20 includes an optical coupler 21, coherent receivers 22-1 to 22-3, local light sources 23-1 to 23-3, and a digital signal processing unit 24.

The wavelength-multiplexed optical signal transmitted through the optical fiber transmission line 50 is branched into three by the optical coupler 21 and input to the coherent receivers 22-1 to 22-3 at the same timing. Local light source 23-1～23-3 outputs equal the local light with each carrier frequency f ₁ ~f ₃ wavelength-multiplexed optical signal. The coherent receivers 22-1 to 22-3 receive the wavelength multiplexed optical signal and the local light of the optical frequencies f _{1 to} f ₃ , respectively perform coherent detection, and output a baseband signal based on the optical frequency of the local light To do. The digital signal processing unit 24 performs digital signal processing on the electrical signals input from the coherent receivers 22-1 to 22-3, and demodulates the transmission signals Data1x and Data1y, the transmission signals Data2x and Data2y, and the transmission signals Data3x and Data3y.

Here, the wavelength multiplexed optical signals received by the coherent receivers 22-1 to 22-3 have propagation delay differences T ₁ and T ₂ corresponding to the optical carrier frequency between the optical signals of the respective channels as shown in FIG. Has occurred. F _1, f ₂ the optical carrier frequency of each optical signal, ..., f _N (unit Hz), Δf _1, Δf ₂ the frequency spacing between optical signals which are adjacent in the frequency domain, ..., Δf _N-1 (unit (Hz), and the chromatic dispersion of the optical fiber transmission line 50 is D (unit: ps / nm), the propagation delay difference T _k (unit: ps) after transmission between optical signals adjacent in frequency is the communication wavelength. In the band (approximately 1550 nm to 1610 nm), it can be expressed by the following formula. c is the speed of light (unit: m / s).

  In the MIMO processing crosstalk compensation technique of the digital signal processing unit 24 in the present invention, in order to remove the crosstalk component from the other channel superimposed on the optical signal of the channel of interest, the same as the superimposed crosstalk component. Information on the optical signal of the other channel at the timing is required. For this purpose, it is necessary to compensate for the propagation delay difference between the optical signals of the respective channels caused by chromatic dispersion. As a result, crosstalk compensation by MIMO processing can be applied to a wavelength multiplexed optical signal transmitted through the optical fiber transmission line 50 having chromatic dispersion.

FIG. 2 shows a configuration example of the digital signal processing unit 24 in the first embodiment.
In FIG. 2, the digital signal processing unit 24 includes A / D converters 1-1 to 1-3, dispersion compensation circuits 2-1A to 2-1C, 2-2A to 2-2C, and 2-3A to 2-3C. , Propagation delay difference compensation circuits 3-1B, 3-1C, 3-2A, 3-2C, 3-3A, 3-3B, phase rotation compensation circuits 4-1B, 4-1C, 4-2A, 4-2C, 4-3A, 4-3B, waveform equalization circuits 5-1 to 5-3, phase estimation circuits 6-1 to 6-3, and identification circuits 7-1 to 7-3.

  The signals output from the coherent receivers 22-1 to 22-3 are composed of complex signals corresponding to orthogonal polarization states, and sampling frequencies fs are respectively obtained by the A / D converters 1-1 to 1-3. Is converted into a digital signal. Here, based on the sampling theorem, the sampling frequency fs is assumed to be larger than twice the modulation frequency of each channel.

The output of the A / D converter 1-1 is input to the dispersion compensation circuits 2-1A, 2-2A, 2-3A, and the output of the A / D converter 1-2 is the dispersion compensation circuits 2-1B, 2-2B. , 2-3B, and the output of the A / D converter 1-3 is input to the dispersion compensation circuits 2-1C, 2-2C, 2-3C.
The dispersion compensation circuits 2-1A, 2-1B, and 2-1C perform dispersion compensation around the optical frequencies f1, f2, and f3 of the local light used for coherent detection. The amount of dispersion compensation is the channel 1 of interest. The total chromatic dispersion amount at the optical frequency f1. In contrast to the output of the dispersion compensation circuit 2-1A of channel 1, the outputs of the dispersion compensation circuits 2-1B and 2-1C cause propagation delay differences T ₁ and T ₁ + T ₂ as shown in FIG. The propagation delay difference compensation circuits 3-1B and 3-1C in the subsequent stage compensate for the propagation delay difference.

The dispersion compensation circuits 2-2A, 2-2B, and 2-2C perform dispersion compensation around the optical frequencies f1, f2, and f3 of local light used for coherent detection. The amount of dispersion compensation is the channel 2 of interest. The total chromatic dispersion amount at the optical frequency f2. With respect to the output of the dispersion compensation circuit 2-2B of the channel 2, the outputs of the dispersion compensation circuits 2-2A and 2-2C have propagation delay differences −T ₁ and T _{2 as} shown in FIG. The propagation delay difference compensation circuits 3-2A and 3-2C in the subsequent stage compensate for the propagation delay difference.

The dispersion compensation circuits 2-1A, 2-1B, and 2-1C perform dispersion compensation around the optical frequencies f1, f2, and f3 of local light used for coherent detection. The amount of dispersion compensation is the channel 3 of interest. The total chromatic dispersion amount at the optical frequency f3. In contrast to the output of the dispersion compensation circuit 2-3C of the channel 3, the outputs of the dispersion compensation circuits 2-3A and 2-3B have propagation delay differences -T ₁ -T ₂ and -T as shown in FIG. ₂ occurs, and the propagation delay difference compensation circuits 3-3A and 3-3B in the subsequent stage compensate the propagation delay difference.

Here, a complex signal E _1x output from the dispersion compensator 2-2A, E _1y, complex signal E _2x outputted from the dispersion compensation circuit 2-2B, the E _2y, is output from the dispersion compensator 2-2C Complex signals E _3x and E _3y . Since crosstalk components from adjacent channels are superimposed on each complex signal, if the transmission signals of each channel are S _1x , S _1y , S _2x , S _2y , S _3x , S _3y , for example, channel 2 The complex signals E _2x and E _2y are expressed by the following equations.

R _i (i = 1, 2, 3) is a transfer function matrix of each channel, and r _ij (i, j = 1, 2, 3) is an attenuation determined by the frequency characteristic of the coherent receiver. The first and third terms on the right side indicate crosstalk components from adjacent channels, and the waveform equalization circuit 5-2 at the subsequent stage compensates for the crosstalk components.

Propagation delay difference compensation circuits 3-1B, 3-1C, 3-2A, 3-2C, 3-3A, and 3-3B cancel the delay difference given by equation (1) by buffering the digital data. By shifting the data timing, the propagation delay difference compensation for each complex signal is realized. For example, for complex signals E _1x (t) and E _1y (t) and complex signals E _3x (t) and E _3y (t) used to realize crosstalk compensation for the channel 2 signal, FIG. As shown in (2), since timing shifts of T ₁ , T ₁ , -T ₂ , and -T ₂ are given, the output signals from the propagation delay compensation circuits 3-2A and 3-2C are E _1x ( t + T ₁ ), E _1y (t + T ₁ ), E _3x (t−T ₂ ), E _3y (t−T ₃ ).

In the phase rotation compensation circuits 4-1 B, 4-1 C, 4-2 A, 4-2 C, 4-3 A, and 4-3 B, phase rotation due to the difference in the carrier frequency between each channel is compensated. For example, in the phase rotation compensation circuits 4-2A and 4-2C, the complex signals E _1x (t + T ₁ ), E _1y (t + T ₁ ), E _3x (t−T ₂ ), E _3y (t−T ₃ ) On the other hand, since phase rotations of −Δf ₁ , −Δf ₁ , Δf ₂ , and Δf ₂ are given, the output signal from each phase rotation compensation circuit is exp [−j2πΔf ₁ t] E _1x (t + T ₁ ), exp [-j2πΔf ₁ t] E _1y (t + T ₁ ), exp [j2πΔf ₂ t] E _3x (t−T ₂ ), exp [j2πΔf ₂ t] E _3y (t−T ₂ ).

The waveform equalization circuits 5-1 to 5-3 are configured by FIR filters. By performing adaptive signal processing of the FIR filters by maximum likelihood estimation for each polarization component, crosstalk compensation, polarization separation, Realizes residual dispersion compensation and polarization dispersion compensation. For example, in the waveform equalization circuit 5-2, the outputs E _2x and E _2y of the dispersion compensation circuit 2-2B and the output exp [−j2πΔf ₁ t] E _1x (t + T ₁ ) of the phase rotation compensation circuit 4-2A are input signals. ), exp [-j2πΔf ₁ t] E _1y (t + T ₁ ) and the output exp [j2πΔf ₂ t] E _3x (t−T ₂ ), exp [j2πΔf ₂ t] E _3y ( When t−T ₂ ) is used, optical signals E ′ _2x (t) and E ′ _2y (t) of the channel 2 subjected to the above-described compensation are obtained.

Here, a well-known CMA (Constant Modulus Algorithm) can be used as the tap update algorithm of the FIR filter accompanying the adaptive signal processing. Assuming that the tap coefficient of each FIR filter is h _ij (i, j = 1, 2, 3), E ′ _2x (t) and E ′ _2y (t) are expressed by the following equations. By optimizing h _ij by tap update based on CMA, polarization separation, residual dispersion compensation, and polarization dispersion compensation are performed, and the crosstalk component in Equation (2) is compensated. E ′ _2x (t) and E ′ _2y (t) are as shown in the following equations.

Outputs of the waveform equalization circuits 5-1 to 5-3 are processed by the phase estimation circuits 6-1 to 6-3 and the identification circuits 7-1 to 7-3, thereby restoring the transmission signals of the respective channels. . For example, transmission signals S _2x (t) and S _2y (t) of channel 2 are obtained from E ′ _2x (t) and E ′ _2y (t) of channel 2.

  The first embodiment has a configuration in which a three-channel transmission signal is wavelength-multiplexed with a three-wave optical signal. In general, an N-channel transmission signal is wavelength-division-multiplexed with an N-wave optical signal. Can be applied.

FIG. 4 shows an example of a demodulated signal according to the conventional configuration and the configuration of the first embodiment of the present invention.
Here, N = 2, that is, the number of channels constituting the wavelength multiplexed optical signal is 2, each channel is configured by a 128 Gbit / s polarization multiplexed QPSK signal, and the carrier frequency interval is 25 GHz. The Q value representing the signal quality is the value after 480 km transmission of the optical fiber transmission line, and the chromatic dispersion of the optical fiber transmission line is 9329 ps / nm. By applying the present invention, it can be confirmed that the crosstalk compensation operates normally even in a chromatic dispersion environment, and the Q value is improved from 8.5 dB to 11.2 dB. The value of the delay difference compensation amount T ₁ used for propagation delay difference compensation is 1865.8 ps based on the equation (1).

  In addition, the propagation delay difference compensation circuit in this embodiment is not necessarily arranged between the dispersion compensation circuit and the phase rotation compensation circuit, and is placed anywhere between the A / D converter and the waveform equalization circuit. However, the same effect can be obtained. Therefore, for example, it may be arranged before the dispersion compensation circuit or after the phase rotation compensation circuit.

FIG. 5 shows a configuration example of the digital signal processing unit 24 in the second embodiment.
The configuration of the transmission unit and the reception unit in the second embodiment is the same as that of the first embodiment shown in FIG. The characteristic of the second embodiment is that in the digital signal processing unit 24 of the receiving unit, the phase rotation compensation circuits 4-1 B, 4-1 C, 4-2 A, 4-2 C, 4-3 A, 4-3 B are replaced by the dispersion compensation circuit 2. -1A to 2-1C, 2-2A to 2-2C, and 2-3A to 2-3C. The phase rotation compensation circuit and the propagation delay difference compensation circuit, which are functional parts unique to the present invention, which are different from the conventional system, and the dispersion compensation circuit, waveform equalization circuit, phase estimation circuit, identification circuit, which are functional parts used in the conventional system, It can be separated.

  In the second embodiment, after the phase rotation compensation is performed, the dispersion compensation circuit executes the dispersion compensation. Therefore, the propagation delay difference between the channels after the dispersion compensation is different from the propagation delay difference in the first embodiment.

FIG. 6 shows electrical spectra before and after phase rotation compensation of signals received by the coherent receivers 22-1 and 22-2 when the number of channels in the second embodiment is 2 (N = 2). Here, fs is a sampling frequency. The electrical spectrum is band limited at the sampling frequency of the A / D converter. Since the signal after phase rotation compensation is subjected to a frequency shift of Δf ₁ , for example, in the phase rotation compensation circuit 4-2A that processes the signal received by the coherent receiver 22-1, the phase rotation compensation of −Δf ₁ is performed. The low frequency component of the channel 1 signal appears on the high frequency side. In the phase rotation compensation circuit 4-1B that processes the signal received by the coherent receiver 22-2, an event occurs in which the high frequency component of the channel 2 signal appears on the low frequency side due to the phase rotation compensation of Δf ₁ .

  In the dispersion compensation circuit, in order to realize dispersion compensation based on the electrical spectrum information, the channel between channels after dispersion compensation depends on whether phase rotation compensation is performed before dispersion compensation or phase rotation compensation is performed after dispersion compensation. The difference in the propagation delay of is different. The propagation delay difference when phase rotation compensation is performed after dispersion compensation is as shown in Equation (1).

FIG. 7 shows an image of a propagation delay difference when phase rotation compensation is performed before dispersion compensation. For example, when the phase rotation compensation is performed on the signal received by the coherent receiver 22-2 with the compensation amount Δf ₁ and the dispersion compensation is performed, the channel 1 component and the low frequency component of the channel 2 of the received signal are obtained. Although the timing is the same as the signal received by the coherent receiver 22-1, the high frequency component of the channel 2 is shifted in timing. In the crosstalk compensation for the channel 1 signal, the channel 2 information is required, and the channel 2 high frequency component information is required. The low frequency component of channel 2 can also be used for crosstalk compensation, but since the information of channel 1 is superimposed on this low frequency component, channel 2 has an important role in crosstalk compensation. Of high frequency components.

This propagation delay difference T ₁ is generated as a result of dispersion compensation being performed on the electrical spectrum shown in FIG. 6, and for example, the signal received by the coherent receiver 22-2 is phase rotated by a compensation amount Δf _1. If attention is paid to the case of compensation, the behavior of the high-frequency component of the channel 2 signal should be taken into consideration at this time. Originally, dispersion compensation should have been performed for the high frequency component of channel 2 with the compensation amount given to the component of electrical frequency fs. However, due to phase rotation compensation, the electrical frequency Δf ₁ -fs / 2 As a result, dispersion compensation is performed with a compensation amount given to a component having an electrical frequency of −fs. From the above consideration, the propagation delay difference T ₁ in the second embodiment is expressed by the following equation.

FIG. 8 shows an example of a demodulated signal according to the conventional configuration and the configuration of the second embodiment of the present invention.
Here, N = 2, that is, the number of channels constituting the wavelength multiplexed optical signal is 2, each channel is configured by a 128 Gbit / s polarization multiplexed QPSK signal, and the carrier frequency interval is 25 GHz. The Q value representing the signal quality is the value after 480 km transmission of the optical fiber transmission line, and the chromatic dispersion of the optical fiber transmission line is 9329 ps / nm. By applying the present invention, it can be confirmed that crosstalk compensation is operating normally even in a chromatic dispersion environment, and the Q value is improved from 8.5 dB to 11.0 dB. The value of the delay difference compensation amount T ₁ used in the propagation delay difference compensation is 4776.4 ps based on the equation (4).

  In addition, the propagation delay difference compensation circuit in this embodiment is not necessarily arranged between the phase rotation compensation circuit and the dispersion compensation circuit, and is placed anywhere between the A / D converter and the waveform equalization circuit. However, the same effect can be obtained. Therefore, for example, it may be arranged after the dispersion compensation circuit or before the phase rotation compensation circuit.

FIG. 9 shows a configuration example of the digital signal processing unit 24 in the third embodiment.
The configuration of the transmission unit and the reception unit in the third embodiment is the same as that of the first embodiment shown in FIG. The feature of the third embodiment is that the sampling frequency fs in the A / D converters 1-1 to 1-3 is made larger than the signal band including the adjacent channels, so that the digital signal processing of the second embodiment shown in FIG. The propagation delay difference compensation circuits 3-1B, 3-1C, 3-2A, 3-2C, 3-3A, and 3-3B are removed from the unit 24. More precisely, when the modulation frequency of the channel k is f _mk (k = 1, 2,..., N), the sampling frequency fs satisfies the following relationship.

When the sampling frequency fs is larger than the signal band including the adjacent channel, the electrical spectrum of the signal received by each coherent receiver is as shown in FIG. Here, for the sake of simplicity, the spectrum of three channels, channel 1, channel 2, and channel 3, is shown from the left. Further, coherent receivers 22-1, coherent receiver 22-2, the optical frequency of the local light to be input to the coherent receiver 22-3, and f _1, f _2, f _3, respectively.

Here, for example, considering the case where the crosstalk component superimposed on the channel 2 signal is compensated, the signal received by the coherent receiver 22-1 and the coherent receiver 22-3 is processed by the phase rotation compensation circuit. Therefore, it is necessary to perform phase rotation compensation of compensation amounts −Δf ₁ and Δf ₂ , respectively. The signal spectrum after the phase rotation compensation is as shown in FIG. 10. When the sampling frequency is larger than the signal band including the adjacent channel, that is, when Expression (5) is satisfied, Is the same as the electrical spectrum of the signal received by the coherent receiver 22-2. Therefore, the same dispersion compensation is applied to the signal after phase rotation compensation received by the coherent receiver 22-1, the coherent receiver 22-2, and the coherent receiver 22-3. As shown in FIG. 11, the occurrence of a propagation delay difference can be suppressed even after dispersion compensation. This eliminates the need to incorporate a propagation delay difference compensation circuit in the digital signal processing unit 24. At this time, the output from the waveform equalization circuit in the channel k is as follows.

DESCRIPTION OF SYMBOLS 1 A / D converter 2 Dispersion compensation circuit 3 Propagation delay difference compensation circuit 4 Phase rotation compensation circuit 5 Waveform equalization circuit 6 Phase estimation circuit 7 Identification circuit 11 Signal light source 12 Polarization multiplexing vector modulator 13 Optical coupler 21 Optical coupler 22 Coherent receiver 23 Local light source 24 Digital signal processor

Claims (5)

A plurality of N-channel optical signals having carrier frequencies f ₁ , f ₂ ,..., F _N (f ₁ <f ₂ <... <f _N ) between a transmitter and a receiver connected via an optical fiber transmission line In a wavelength division multiplexing optical transmission system that transmits a wavelength division multiplexed optical signal,
The transmission unit is configured to send a wavelength-multiplexed optical signal in which the frequency interval of each channel is smaller than the modulation frequency of each optical signal and the optical signals of adjacent channels overlap in frequency to the optical fiber transmission line,
The receiver is
Optical frequency f ₁ different said wavelength division multiplexed optical signal N branched by the optical coupler, respectively, f _2, ..., a coherent receiver for coherent detection by local light f _N,
A digital signal processing unit that compensates for crosstalk between adjacent channels caused by optical spectrum overlap in the transmission signal by performing digital signal processing on the electrical signal output from the coherent receiver;
The digital signal processor is
An A / D converter that converts an electrical signal output from the coherent receiver into a digital signal;
A dispersion compensation circuit for compensating for optical waveform degradation caused by chromatic dispersion generated in the optical fiber transmission line;
The cross rotation using a phase rotation compensation circuit that compensates for a rotation component caused by a difference in optical frequency of the local light, and a plurality of adaptive filters that adaptively change a tap coefficient of a filter that inputs an output of the phase rotation compensation circuit. A waveform equalization circuit that compensates for talk;
A phase estimation circuit for estimating a carrier phase of the optical signal;
An identification circuit for identifying a transmission signal superimposed on the optical signal,
The dispersion compensation amount compensated by the dispersion compensation circuit is the chromatic dispersion amount at the carrier optical frequency of the channel to be demodulated, and is also applied to the signal of the adjacent channel used for compensating the crosstalk component. ,
The phase rotation compensation amount in the phase rotation compensation circuit is an optical frequency interval Δf _k (k = 1, 2,..., N) between adjacent channels. The wavelength division multiplexing optical transmission system according to claim 1,
The digital signal processing unit includes a propagation delay difference compensation circuit that compensates for a propagation delay difference between channels that is generated when the dispersion compensation circuit is disposed before the phase rotation compensation circuit and further transmits the optical fiber transmission line. Is provided before the waveform equalization circuit,
The propagation delay difference compensation amount T _k (unit: ps) given to the digital signal by the propagation delay difference compensation circuit is D (unit: ps / nm) of wavelength dispersion of the optical fiber transmission line, and c (unit: light speed). m / s), expressed as

A wavelength division multiplexing optical transmission system. The wavelength division multiplexing optical transmission system according to claim 1,
The digital signal processing unit includes a propagation delay difference compensation circuit that compensates for a propagation delay difference between channels that is generated when the phase rotation compensation circuit is disposed in front of the dispersion compensation circuit and is further transmitted through the optical fiber transmission line. Is arranged before the waveform equalization circuit, and the sampling frequency fs of the A / D converter is set to a value larger than twice the modulation frequency,
The propagation delay difference compensation amount T _k (unit: ps) given to the digital signal by the propagation delay difference compensation circuit is D (unit: ps / nm) of wavelength dispersion of the optical fiber transmission line, and c (unit: light speed). m / s), expressed as

A wavelength division multiplexing optical transmission system. The wavelength division multiplexing optical transmission system according to claim 1,
In the digital signal processing unit, the phase rotation compensation circuit is disposed in a stage before the dispersion compensation circuit, and the sampling frequency fs in the A / D converter is larger than a signal band including an adjacent channel, When the modulation frequency of the optical signal of k is f _mk (k = 1, 2,…, N), the following relationship is satisfied.

A wavelength division multiplexing optical transmission system. In the wavelength division multiplexing optical transmission system according to any one of claims 1 to 4,
A wavelength division multiplexing optical transmission system using a CMA (Constant Modulus Algorithm) as a tap update algorithm of the waveform equalization circuit.

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