CN111355536B - Nonlinear damage compensation system and method for CO-OFDM transmission system - Google Patents

Nonlinear damage compensation system and method for CO-OFDM transmission system Download PDF

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CN111355536B
CN111355536B CN202010084455.1A CN202010084455A CN111355536B CN 111355536 B CN111355536 B CN 111355536B CN 202010084455 A CN202010084455 A CN 202010084455A CN 111355536 B CN111355536 B CN 111355536B
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ofdm
radio frequency
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CN111355536A (en
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王晶晶
于宏光
张立人
杜勇涛
孔晓艺
颜正凯
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Shandong Normal University
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/60—Receivers
    • H04B10/61—Coherent receivers
    • H04B10/616—Details of the electronic signal processing in coherent optical receivers
    • H04B10/6163—Compensation of non-linear effects in the fiber optic link, e.g. self-phase modulation [SPM], cross-phase modulation [XPM], four wave mixing [FWM]
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/25—Arrangements specific to fibre transmission
    • H04B10/2507—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion
    • H04B10/2543—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion due to fibre non-linearities, e.g. Kerr effect
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/50—Transmitters
    • H04B10/516—Details of coding or modulation
    • H04B10/54—Intensity modulation
    • H04B10/541—Digital intensity or amplitude modulation

Abstract

The utility model discloses a nonlinear damage compensation system and method of a CO-OFDM transmission system, comprising: the radio frequency OFDM transmitter modulates and maps signals to be transmitted and transmits the modulated and mapped signals to the optical I/O modulator; the optical I/O modulator converts the received signal from an electric signal into an OFDM optical signal and transmits the OFDM optical signal to an optical fiber link; each OPC optical phase conjugation device in the optical fiber link couples signals and generates opposite signals through a high nonlinear optical fiber HNLF, so that nonlinear damage of the optical fiber link is compensated; the optical fiber link sends the output signal to an optical coherent detector; the optical coherence detector demodulates the received signal, demodulates the optical signal into an electric signal and sends the demodulated electric signal to the radio frequency OFDM receiver; the radio frequency OFDM receiver restores the received electric signals.

Description

Nonlinear damage compensation system and method for CO-OFDM transmission system
Technical Field
The disclosure relates to the technical field of optical communication, in particular to a nonlinear damage compensation system and method for a CO-OFDM transmission system.
Background
The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
An Optical communication CO-OFDM (Coherent Optical Orthogonal Frequency division Multiplexing) transmission system perfectly integrates an OFDM modulation technology and a Coherent receiving technology, but corresponding problems and challenges also come along while combining the advantages of the two.
In the course of implementing the present disclosure, the inventors found that the following technical problems exist in the prior art:
since the OFDM technology divides the channel into a plurality of sub-channels orthogonal and parallel to each other, it results in that the system is very easy to satisfy the phase matching condition of the nonlinear four-wave mixing effect, thereby causing serious inter-carrier interference. Meanwhile, the CO-OFDM transmission signal has a high peak-to-average power ratio, so that the system can reach the threshold of Kerr effect more easily, and is further influenced by self-phase modulation and cross-phase modulation. How to effectively compensate the nonlinear damage generated by the system in the optical fiber transmission process is a key problem in the current research of the CO-OFDM system.
Disclosure of Invention
The present disclosure is directed to solve the above problems and to provide a system and a method for compensating for nonlinear impairments in a CO-OFDM transmission system.
In order to achieve the purpose, the following technical scheme is adopted in the disclosure:
in a first aspect, the present disclosure provides a nonlinear impairment compensation system for a CO-OFDM transmission system;
a CO-OFDM transmission system nonlinear impairment compensation system comprising:
the system comprises a radio frequency OFDM transmitter, an optical I/O modulator, an optical fiber link, an optical coherent detector and a radio frequency OFDM receiver which are connected in sequence; a plurality of OPC optical phase conjugation devices are arranged on the optical fiber link at intervals;
the radio frequency OFDM transmitter modulates and maps signals to be transmitted and transmits the modulated and mapped signals to the optical I/O modulator; the optical I/O modulator converts the received signal from an electric signal into an OFDM optical signal and transmits the OFDM optical signal to an optical fiber link;
each OPC optical phase conjugation device in the optical fiber link couples signals and generates opposite signals through a high nonlinear optical fiber HNLF, so that nonlinear damage of the optical fiber link is compensated;
the optical fiber link sends the output signal to an optical coherent detector; the optical coherence detector demodulates the received signal, demodulates the optical signal into an electric signal and sends the demodulated electric signal to the radio frequency OFDM receiver; the radio frequency OFDM receiver restores the received electric signals.
In a second aspect, the present disclosure provides a method for compensating for nonlinear impairments of a CO-OFDM transmission system;
a nonlinear impairment compensation method for a CO-OFDM transmission system comprises the following steps:
the radio frequency OFDM transmitter modulates and maps signals to be transmitted and transmits the modulated and mapped signals to the optical I/O modulator; the optical I/O modulator converts the received signal from an electric signal into an OFDM optical signal and transmits the OFDM optical signal to an optical fiber link;
each OPC optical phase conjugation device in the optical fiber link couples signals and generates opposite signals through a high nonlinear optical fiber HNLF, so that nonlinear damage of the optical fiber link is compensated;
the optical fiber link sends the output signal to an optical coherent detector; the optical coherence detector demodulates the received signal, demodulates the optical signal into an electric signal and sends the demodulated electric signal to the radio frequency OFDM receiver; the radio frequency OFDM receiver restores the received electric signals.
The beneficial effect of this disclosure:
the optical wave whose output signal light and input signal light are time-reversed in amplitude, phase and polarization is realized by OPC (optical phase conjugation), that is, by spectrum inversion, and the transmission direction and phase of any light beam plane wave component are reversed compared with the original one after the signal passes through OPC. Therefore, the link damage accumulated before the OPC is compensated by the nonlinear damage in the link experienced by the reverse signal after the OPC, so that the damage caused by the nonlinear effect of the CO-OFDM transmission system is effectively, comprehensively and timely compensated.
The nonlinear damage compensation method of the CO-OFDM transmission system provided by the disclosure is implemented by adding an OPC device from the angle of an optical domain to realize the inversion of a signal spectrum, thereby achieving the effect of compensating the nonlinear damage. The method is different from the conventional common electric domain method, but the compensation effect of the method is much better than that of other methods after experimental verification.
Drawings
Fig. 1 is a schematic diagram of a CO-OFDM transmission system according to a first embodiment of the disclosure;
FIG. 2 is a schematic diagram illustrating an OPC operation principle according to a first embodiment of the present disclosure;
fig. 3 is an internal schematic diagram of OPC according to an embodiment of the present disclosure.
Detailed Description
The present disclosure is further described with reference to the following drawings and examples.
Chinese and English explanation:
ofdm (orthogonal Frequency Division multiplexing), an orthogonal Frequency Division multiplexing technique;
in a first embodiment, the present embodiment provides a nonlinear impairment compensation system for a CO-OFDM transmission system;
as shown in fig. 1, a nonlinear impairment compensation system for a CO-OFDM transmission system includes:
the system comprises a radio frequency OFDM transmitter, an optical I/O modulator, an optical fiber link, an optical coherent detector and a radio frequency OFDM receiver which are connected in sequence; a plurality of OPC optical phase conjugation devices are arranged on the optical fiber link at intervals;
the radio frequency OFDM transmitter modulates and maps signals to be transmitted and transmits the modulated and mapped signals to the optical I/O modulator; the optical I/O modulator converts the received signal from an electric signal into an OFDM optical signal and transmits the OFDM optical signal to an optical fiber link;
each OPC optical phase conjugation device in the optical fiber link couples signals and generates opposite signals through a high nonlinear optical fiber HNLF, so that nonlinear damage of the optical fiber link is compensated;
the optical fiber link sends the output signal to an optical coherent detector; the optical coherence detector demodulates the received signal, demodulates the optical signal into an electric signal and sends the demodulated electric signal to the radio frequency OFDM receiver; the radio frequency OFDM receiver restores the received electric signals.
As one or more embodiments, the radio frequency OFDM transmitter modulates and maps a signal to be transmitted, and transmits the modulated and mapped signal to an optical I/O modulator; the method specifically comprises the following steps:
the radio frequency OFDM transmitter carries out serial-parallel conversion processing on a signal to be transmitted and realizes M-QAM modulation on the signal after the serial-parallel conversion processing to form a modulation information sequence; then, performing inverse Fourier transform on the modulation information sequence modulated by the M-QAM, performing parallel-serial conversion on signals subjected to inverse Fourier transform, adding a cyclic prefix to the signals subjected to parallel-serial conversion, and finally performing first digital-to-analog conversion on the signals added with the cyclic prefix to obtain first electric signals; and performing second digital-to-analog conversion on the signal with the added cyclic prefix to obtain a second electric signal.
It should be understood that the radio frequency OFDM transmitter is used at a transmitting end of a system, and is used for performing serial-to-parallel conversion on a data bit stream to be input; then according to the adopted modulation mode, completing corresponding modulation mapping, then carrying out IFFT transformation on X (k), and calculating a sampling sequence x (n) of the OFDM modulated signal time domain; then the CP is added to obtain x' (n). Then, D/A conversion is performed to obtain the time domain x' (n) of the OFDM signal.
As one or more embodiments, the optical I/O modulator converts the received signal from an electrical signal to an OFDM optical signal; the method specifically comprises the following steps:
the optical I/O modulator processes the received first electric signal by using a first Mach-Zehnder modulator to obtain a first OFDM optical signal;
the optical I/O modulator processes the received second electric signal by using a second Mach-Zehnder modulator to obtain a second OFDM optical signal;
and the subcarriers of the first OFDM optical signal and the second OFDM optical signal are orthogonal to each other.
It should be understood that the optical I/Q modulator is used to convert the electrical signal modulation into OFDM optical signals, i.e. each OFDM electrical signal output by the transmitter is converted into OFDM optical signals orthogonal to each other and transmitted out to the optical fiber link.
As one or more embodiments, a plurality of OPC optical phase conjugation devices are arranged on the optical fiber link at intervals; for example: three OPC optical phase conjugation devices are arranged on the optical fiber link at intervals; wherein the first OPC optical phase conjugation device spans 8 from the start of the optical fiber link; the second OPC optical phase conjugation means is spaced from the first OPC optical phase conjugation means by a spacing of 16 spans; the third OPC optical phase conjugation means is also spaced from the second OPC optical phase conjugation means by a span of 16; the third OPC optical phase conjugate device spans 8 from the fiber link terminus.
It should be understood that the fiber optic link, including the optical fiber and various optical amplifiers. Some of the damage faced by optical fiber transmission, such as fiber loss, fiber dispersion, and fiber nonlinear effects, are all caused by this.
In one or more embodiments, the optical coherence detector demodulates the received signal from an optical signal into an electrical signal; the method specifically comprises the following steps:
a local oscillator laser in the optical coherence detector processes the received signals, and a balance photoelectric receiver in the optical coherence detector processes the signals processed by the local oscillator laser; a coupler in the optical coherent detector is used for coupling and post-processing signals processed by the balanced photoelectric receiver; demodulation of the optical signal into an electrical signal is achieved.
It should be understood that the number of couplers is two and the number of balanced photoreceivers is four.
As one or more embodiments, the radio frequency OFDM receiver recovers a received electrical signal; the method specifically comprises the following steps:
the radio frequency OFDM receiver performs analog-to-digital conversion on the received electric signal; performing Cyclic Prefix (CP) (cyclic prefix) removal processing on the data after analog-to-digital conversion, performing serial-to-parallel conversion processing on the signals after the cyclic prefix removal processing, performing Fourier transform on the signals after the serial-to-parallel conversion, performing M-QAM demodulation processing on the data after the Fourier transform, and performing parallel-to-serial processing on the signals after the M-QAM demodulation processing to obtain the restored signals.
It will be appreciated that the radio frequency OFDM receiver corresponds to a radio frequency OFDM transmitter and is arranged to perform an a/D conversion to remove the CP and obtain a sample sequence x of the OFDM modulated signalRx(n), and then FFT-transformed. When x is obtainedRxAfter (n), finally restoring the modulation information sequence X of the input endk。
According to the method, the optical phase conjugation device OPC is added in the middle of an optical fiber link of the whole system, original signals in the optical phase conjugation device OPC are coupled with pump light, and opposite signals are generated through the high nonlinear optical fiber HNLF, so that the effect of compensating nonlinear damage of the whole system is achieved.
The method and the device can effectively, comprehensively and real-timely compensate the damage caused by the nonlinear effect in the CO-OFDM transmission system.
OPC (optical domain phase conjugation) working principle:
the optical domain phase conjugation, also known as Spectral Inversion (SI), is based on the principle that the propagation direction of each plane wave component in an arbitrary beam and its phase factor at any one point are inverted in time. Therefore, the link damage accumulated before the OPC is compensated by the nonlinear damage in the link experienced by the reverse signal after the OPC, so that the damage caused by the nonlinear effect of the CO-OFDM transmission system is effectively, comprehensively and timely compensated.
OPC (optical domain phase conjugation) implementation mechanism:
to implement optical phase conjugation, a nonlinear system must be used, and generally, the method for generating optical phase conjugation is divided into two methods: one is a parametric process, also called elastic scattering process, such as three-wave mixing, four-wave mixing, etc.; the other is a nonparametric process, also known as the inelastic scattering process of photons, such as stimulated brillouin scattering, stimulated rayleigh scattering, etc. But the wavelength conversion transparent to bit rate and modulation format should be full wavelength conversion, and FWM is the only technology that can satisfy the strict transparency condition. HNLF (high nonlinear fiber) has the advantages of low price, easy generation of nonlinear effect and the like, and has wide application in optical communication systems.
The working process of the OPC optical phase conjugation device comprises the following steps:
first, a signal E transmitted over half the total fiber lengthTx(wavelength is. lambda.)11550nm) is amplified by a first erbium-doped fiber amplifier EDFA and filtered by a first band-pass filter BPF;
then, the process of the present invention is carried out,output signal E of the first band-pass filter BPFsigAnd the wavelength after being amplified by a second EDFA21552nm Pump Signal EpumpCoupling is carried out;
then, inputting the coupled signal into a high nonlinear fiber HNLF to generate FWM effect;
then, the signal output by the high nonlinear fiber HNLF filters a pumping signal and an original signal through a second band-pass filter BPF;
finally, the signal output by the second band-pass filter BPF is amplified by the first EDFA to obtain OPC signal EOPCAnd (6) outputting.
Therefore, the inversion of the signal spectrum is realized, so that the nonlinearity opposite to the nonlinearity of the first half section can be generated in the second half section of the optical fiber link, and the nonlinearity are mutually offset, thereby achieving the compensation effect. This process is illustrated in fig. 3.
In a CO-OFDM transmission system, as shown in fig. 1, which is a system block diagram of CO-OFDM-16QAM, a CO-OFDM-16QAM signal with a rate of 40Gb/s is generated through serial-to-parallel conversion, 16-QAM modulation, inverse fourier transform, parallel-to-serial conversion, cyclic prefix addition, digital-to-analog conversion, and other steps, where the FFT length of the CO-OFDM system is 1024, the number of subcarriers is 768, and the cyclic prefix length is 154.
A continuous wave laser with 193.1THz of central frequency, 5dBm of power and 0.3MHz of line width is adopted, an electric signal of CO-OFDM-16QAM is converted into an optical signal through a Mach-Zehnder modulator, the modulated optical signal is coupled through a 3dB coupler and then enters an optical fiber link part with an optical phase conjugation device.
The optical fiber has a total of 480KM, each span (span) is 10KM, and OPC and optical fiber deployment are shown in fig. 1. And after being transmitted by the optical fiber link, the optical fiber enters a local oscillator laser for coherent detection.
And then, the OFDM signal is restored at the receiving end of the radio frequency OFDM receiver through the steps of photoelectric detection, digital-to-analog conversion, cyclic prefix removal, serial-to-parallel conversion, Fourier transform (FFT), 16-QAM demodulation, parallel-to-serial conversion and the like.
Figure 2 is a schematic diagram of the principle in actual operation,the transmitting end transmits a signal, the signal pulse E before passing through OPC (optical phase conjugation)s(t) has an optical carrier frequency of ωsThe first half section of the optical fiber has a length L1(ii) a After passing through the phase conjugation device, the optical fiber length of the second half is L2Output pulse Ec(t) having an optical carrier frequency of ωcWherein the dispersion of the channel after phase conjugation and the link length satisfy the following formula:
β21L1＝β22L2
Figure BDA0002381551370000081
wherein
Figure BDA0002381551370000082
Which refers to the average optical power in fiber j, is expressed as follows:
Figure BDA0002381551370000083
wherein beta issj(j ═ 1,2) is the dispersion coefficient in the link, γj(j ═ 1,2) represents a nonlinear coefficient.
If the residual dispersion in the fiber link before and after the phase conjugation is equal, then the effect of compensating for the non-linearity can be achieved.
Fig. 3 is an internal schematic diagram of an OPC apparatus, in which opposite non-linearity is generated by using a four-wave mixing effect, in which a pump light is added to a coupler, and after coupling with an original signal, an OPC signal opposite to the original signal, i.e., an opposite non-linear effect, is generated through a HNLF highly non-linear fiber, thereby achieving compensation. The method comprises the following specific steps:
first, a signal E transmitted over half the total fiber lengthTx(wavelength is. lambda.)11550nm) is amplified by a first amplifier EDFA and filtered by a first band pass filter BPF;
then, the first band-pass filter BPF outputs a signal EsigThe same wavelength is lambda21552nm Pump Signal EpumpCoupling is carried out;
then, inputting the coupled signal into a high nonlinear fiber HNLF to generate FWM effect;
finally, the pump signal and the original signal are filtered by a second band-pass filter BPF, and an OPC signal E is obtainedOPCAnd outputting to further realize the inversion of the signal frequency spectrum and play a role in compensating the nonlinear damage.
Second, the present embodiment provides a method for compensating for nonlinear impairments of a CO-OFDM transmission system;
the radio frequency OFDM transmitter modulates and maps signals to be transmitted and transmits the modulated and mapped signals to the optical I/O modulator; the optical I/O modulator converts the received signal from an electric signal into an OFDM optical signal and transmits the OFDM optical signal to an optical fiber link;
each OPC optical phase conjugation device in the optical fiber link couples signals and generates opposite signals through a high nonlinear optical fiber HNLF, so that nonlinear damage of the optical fiber link is compensated;
the optical fiber link sends the output signal to an optical coherent detector; the optical coherence detector demodulates the received signal, demodulates the optical signal into an electric signal and sends the demodulated electric signal to the radio frequency OFDM receiver; the radio frequency OFDM receiver restores the received electric signals.
As one or more embodiments, the radio frequency OFDM transmitter modulates and maps a signal to be transmitted, and transmits the modulated and mapped signal to an optical I/O modulator; the method specifically comprises the following steps:
the radio frequency OFDM transmitter carries out serial-parallel conversion processing on a signal to be transmitted and realizes M-QAM modulation on the signal after the serial-parallel conversion processing to form a modulation information sequence; then, performing inverse Fourier transform on the modulation information sequence modulated by the M-QAM, performing parallel-serial conversion on signals subjected to inverse Fourier transform, adding a cyclic prefix to the signals subjected to parallel-serial conversion, and finally performing first digital-to-analog conversion on the signals added with the cyclic prefix to obtain first electric signals; and performing second digital-to-analog conversion on the signal with the added cyclic prefix to obtain a second electric signal.
It should be understood that the radio frequency OFDM transmitter is used at a transmitting end of a system, and is used for performing serial-to-parallel conversion on a data bit stream to be input; then according to the adopted modulation mode, completing corresponding modulation mapping, then carrying out IFFT transformation on X (k), and calculating a sampling sequence x (n) of the OFDM modulated signal time domain; then the CP is added to obtain x' (n). Then, D/A conversion is performed to obtain the time domain x' (n) of the OFDM signal.
As one or more embodiments, the optical I/O modulator converts the received signal from an electrical signal to an OFDM optical signal; the method specifically comprises the following steps:
the optical I/O modulator processes the received first electric signal by using a first Mach-Zehnder modulator to obtain a first OFDM optical signal;
the optical I/O modulator processes the received second electric signal by using a second Mach-Zehnder modulator to obtain a second OFDM optical signal;
and the subcarriers of the first OFDM optical signal and the second OFDM optical signal are orthogonal to each other.
It should be understood that the optical I/Q modulator is used to convert the electrical signal modulation into OFDM optical signals, i.e. each OFDM electrical signal output by the transmitter is converted into OFDM optical signals orthogonal to each other and transmitted out to the optical fiber link.
As one or more embodiments, a plurality of OPC optical phase conjugation devices are arranged on the optical fiber link at intervals; for example: three OPC optical phase conjugation devices are arranged on the optical fiber link at intervals; wherein the first OPC optical phase conjugation device spans 8 from the start of the optical fiber link; the second OPC optical phase conjugation means is spaced from the first OPC optical phase conjugation means by a spacing of 16 spans; the third OPC optical phase conjugation means is also spaced from the second OPC optical phase conjugation means by a span of 16; the third OPC optical phase conjugate device spans 8 from the fiber link terminus.
It should be understood that the fiber optic link, including the optical fiber and various optical amplifiers. Some of the damage faced by optical fiber transmission, such as fiber loss, fiber dispersion, and fiber nonlinear effects, are all caused by this.
In one or more embodiments, the optical coherence detector demodulates the received signal from an optical signal into an electrical signal; the method specifically comprises the following steps:
a local oscillator laser in the optical coherence detector processes the received signals, and a balance photoelectric receiver in the optical coherence detector processes the signals processed by the local oscillator laser; a coupler in the optical coherent detector is used for coupling and post-processing signals processed by the balanced photoelectric receiver; demodulation of the optical signal into an electrical signal is achieved.
It should be understood that the number of couplers is two and the number of balanced photoreceivers is four.
As one or more embodiments, the radio frequency OFDM receiver recovers a received electrical signal; the method specifically comprises the following steps:
the radio frequency OFDM receiver performs analog-to-digital conversion on the received electric signal; performing Cyclic Prefix (CP) (cyclic prefix) removal processing on the data after analog-to-digital conversion, performing serial-to-parallel conversion processing on the signals after the cyclic prefix removal processing, performing Fourier transform on the signals after the serial-to-parallel conversion, performing M-QAM demodulation processing on the data after the Fourier transform, and performing parallel-to-serial processing on the signals after the M-QAM demodulation processing to obtain the restored signals.
The working process of the OPC optical phase conjugation device comprises the following steps:
first, a signal E transmitted over half the total fiber lengthTx(wavelength is. lambda.)11550nm) is amplified by a first erbium-doped fiber amplifier EDFA and filtered by a first band-pass filter BPF;
then, the output signal E of the first band-pass filter BPFsigAnd the wavelength after being amplified by a second EDFA21552nm Pump Signal EpumpCoupling is carried out;
then, inputting the coupled signal into a high nonlinear fiber HNLF to generate FWM effect;
then, the signal output by the high nonlinear fiber HNLF filters a pumping signal and an original signal through a second band-pass filter BPF;
finally, the signal output by the second band-pass filter BPF is amplified by the first EDFA to obtain OPC signal EOPCAnd (6) outputting.
Although the present disclosure has been described with reference to specific embodiments, it should be understood that the scope of the present disclosure is not limited thereto, and those skilled in the art will appreciate that various modifications and changes can be made without departing from the spirit and scope of the present disclosure.

Claims (10)

1. A nonlinear damage compensation system of a CO-OFDM transmission system is characterized by comprising:
the system comprises a radio frequency OFDM transmitter, an optical I/O modulator, an optical fiber link, an optical coherent detector and a radio frequency OFDM receiver which are connected in sequence; a plurality of OPC optical phase conjugation devices are arranged on the optical fiber link at intervals;
the radio frequency OFDM transmitter modulates and maps signals to be transmitted and transmits the modulated and mapped signals to the optical I/O modulator; the optical I/O modulator converts the received signal from an electric signal into an OFDM optical signal and transmits the OFDM optical signal to an optical fiber link;
each OPC optical phase conjugation device in the optical fiber link couples signals and generates opposite signals through a high nonlinear optical fiber HNLF, so that nonlinear damage of the optical fiber link is compensated;
the optical fiber link sends the output signal to an optical coherent detector; the optical coherence detector demodulates the received signal, demodulates the optical signal into an electric signal and sends the demodulated electric signal to the radio frequency OFDM receiver; the radio frequency OFDM receiver restores the received electric signals;
OPC optical phase conjugation means for transmitting signals E through half the total fibre lengthTxAfter being amplified by a first erbium-doped fiber amplifier EDFA, filtering by a first band-pass filter BPF; first band-pass filter BOutput signal E of PFsigAnd the wavelength after being amplified by a second EDFA21552nm Pump Signal EpumpCoupling is carried out; inputting the coupled signal into a high nonlinear fiber HNLF to generate FWM effect;
signal pulses E before passing through OPCs(t) has an optical carrier frequency of ωsWherein the dispersion of the channel and the link length after phase conjugation satisfy: beta is a21L1＝β22L2，
Figure FDA0003053773220000011
Wherein
Figure FDA0003053773220000012
Which refers to the average optical power in fiber j, is expressed as follows:
Figure FDA0003053773220000013
i is 1, 2; wherein beta iss，jIs the dispersion coefficient in the link; gamma rayjRepresents a nonlinear coefficient, j is 1, 2; if the residual dispersion in the fiber link before and after the phase conjugation is equal, then the effect of compensating for the non-linearity can be achieved.
2. The system of claim 1, wherein the radio frequency OFDM transmitter modulates and maps a signal to be transmitted, and transmits the modulated and mapped signal to the optical I/O modulator; the method specifically comprises the following steps:
the radio frequency OFDM transmitter carries out serial-parallel conversion processing on a signal to be transmitted and realizes M-QAM modulation on the signal after the serial-parallel conversion processing to form a modulation information sequence; then, performing inverse Fourier transform on the modulation information sequence modulated by the M-QAM, performing parallel-serial conversion on signals subjected to inverse Fourier transform, adding a cyclic prefix to the signals subjected to parallel-serial conversion, and finally performing first digital-to-analog conversion on the signals added with the cyclic prefix to obtain first electric signals; and performing second digital-to-analog conversion on the signal with the added cyclic prefix to obtain a second electric signal.
3. The system of claim 1, wherein the optical I/O modulator converts the received signal from an electrical signal to an OFDM optical signal; the method specifically comprises the following steps:
the optical I/O modulator processes the received first electric signal by using a first Mach-Zehnder modulator to obtain a first OFDM optical signal;
the optical I/O modulator processes the received second electric signal by using a second Mach-Zehnder modulator to obtain a second OFDM optical signal;
and the subcarriers of the first OFDM optical signal and the second OFDM optical signal are orthogonal to each other.
4. The system of claim 1, wherein the optical coherent detector demodulates the received signal from the optical signal into an electrical signal; the method specifically comprises the following steps:
a local oscillator laser in the optical coherence detector processes the received signals, and a balance photoelectric receiver in the optical coherence detector processes the signals processed by the local oscillator laser; a coupler in the optical coherent detector is used for coupling and post-processing signals processed by the balanced photoelectric receiver; demodulation of the optical signal into an electrical signal is achieved.
5. The system of claim 1, wherein the radio frequency OFDM receiver recovers the received electrical signal; the method specifically comprises the following steps:
the radio frequency OFDM receiver performs analog-to-digital conversion on the received electric signal; and performing cyclic prefix CP (code stream) removal processing on the data subjected to the analog-to-digital conversion, performing serial-to-parallel conversion processing on the signals subjected to the cyclic prefix removal processing, performing Fourier transform on the signals subjected to the serial-to-parallel conversion, performing M-QAM (M-ary quadrature amplitude modulation) demodulation processing on the data subjected to the Fourier transform, and performing parallel-to-serial processing on the signals subjected to the M-QAM demodulation processing to obtain the restored signals.
6. The system of claim 1, wherein the operation of the OPC optical phase conjugation mechanism further comprises:
then, the signal output by the high nonlinear fiber HNLF filters a pumping signal and an original signal through a second band-pass filter BPF;
finally, the signal output by the second band-pass filter BPF is amplified by the first EDFA to obtain OPC signal EOPCAnd (6) outputting.
7. A method for compensating for nonlinear impairments of a CO-OFDM transmission system according to claim 1, characterized in that it comprises:
the radio frequency OFDM transmitter modulates and maps signals to be transmitted and transmits the modulated and mapped signals to the optical I/O modulator; the optical I/O modulator converts the received signal from an electric signal into an OFDM optical signal and transmits the OFDM optical signal to an optical fiber link;
each OPC optical phase conjugation device in the optical fiber link couples signals and generates opposite signals through a high nonlinear optical fiber HNLF, so that nonlinear damage of the optical fiber link is compensated;
the optical fiber link sends the output signal to an optical coherent detector; the optical coherence detector demodulates the received signal, demodulates the optical signal into an electric signal and sends the demodulated electric signal to the radio frequency OFDM receiver; the radio frequency OFDM receiver restores the received electric signals.
8. The method of claim 7, wherein the radio frequency OFDM transmitter modulates and maps a signal to be transmitted, and transmits the modulated and mapped signal to an optical I/O modulator; the method specifically comprises the following steps:
the radio frequency OFDM transmitter carries out serial-parallel conversion processing on a signal to be transmitted and realizes M-QAM modulation on the signal after the serial-parallel conversion processing to form a modulation information sequence; then, performing inverse Fourier transform on the modulation information sequence modulated by the M-QAM, performing parallel-serial conversion on signals subjected to inverse Fourier transform, adding a cyclic prefix to the signals subjected to parallel-serial conversion, and finally performing first digital-to-analog conversion on the signals added with the cyclic prefix to obtain first electric signals; and performing second digital-to-analog conversion on the signal with the added cyclic prefix to obtain a second electric signal.
9. The method of claim 7, wherein the optical I/O modulator converts the received signal from an electrical signal to an OFDM optical signal; the method specifically comprises the following steps:
the optical I/O modulator processes the received first electric signal by using a first Mach-Zehnder modulator to obtain a first OFDM optical signal;
the optical I/O modulator processes the received second electric signal by using a second Mach-Zehnder modulator to obtain a second OFDM optical signal;
and the subcarriers of the first OFDM optical signal and the second OFDM optical signal are orthogonal to each other.
10. The method of claim 7, wherein the operation of the OPC optical phase conjugation mechanism comprises:
first, a signal E transmitted over half the total fiber lengthTxAfter being amplified by a first erbium-doped fiber amplifier EDFA, filtering by a first band-pass filter BPF;
then, the output signal E of the first band-pass filter BPFsigAnd the wavelength after being amplified by a second EDFA21552nm Pump Signal EpumpCoupling is carried out;
then, inputting the coupled signal into a high nonlinear fiber HNLF to generate FWM effect;
then, the signal output by the high nonlinear fiber HNLF filters a pumping signal and an original signal through a second band-pass filter BPF;
finally, the signal output by the second band-pass filter BPF is amplified by the first EDFA to obtain OPC signal EOPCAnd (6) outputting.
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