The present disclosure relates to fiber communications, and in particular, to a dispersion compensation method and a fiber transmission system.
Dispersion means that the waveform of a transmit signal in a fiber is distorted due to different frequency components or different transmission rates of signal components in different modes. Dispersion generates inter-symbol interferences between data pulses in the optical transmission. The impact of dispersion on the system performance cannot be ignored. An optical transmission system with the transmission rate of more than 10 Gbit/s needs a dispersion compensation technology to ensure the system transmission. Currently, dispersion compensation fiber (DCF) is a popular dispersion compensation technology to implement dispersion compensation, the dispersion characteristics of which are opposite to those of the transmission fiber. This dispersion compensation mode is easy to use, but has the following unavoidable weaknesses: large volume, signal delay, need of an amplifier for additional loss compensation, and high cost. This compensation mode cannot provide flexible dispersion compensation. Though the DCF mode has practical utility in a point-to-point optical transmission system, it is difficult to meet application requirements in a complicated network with wavelength add/drop, especially in a flexible network that may be dynamically rebuilt. The reason is that dispersion varies with transmission paths through which the fiber compensation passes. However, as the network traffic continues to converge to dynamic IP traffic, a flexible and dynamic fiber network infrastructure is indispensable. A flexible optical network layer needs flexible network nodes to perform dynamic and simple network rebuilding and respond to any requirements for wavelength grooming and dynamic routing.
In recent years, electrical dispersion compensation has attracted attention from technicians. Electronic dispersion compensation means partially or completely compensating the transmit signal for losses incurred due to dispersion through electrical domain signal processing in a transmitter or receiver of an optical transmission system. The compensation mode in which the signal processing is performed in the transmitter is called pre-processing mode, and the compensation mode in which the signal processing is performed in the receiver is called post-compensation mode. The electrical dispersion compensation mode overcomes all the weaknesses of the preceding DCF compensation mode. Besides the merit of low cost, the electrical domain compensation mode can further provide adaptive dispersion compensation, that is, it can adjust the amount of dispersion compensation. This function serves as the basis for dynamic network configuration.
The electrical domain compensation mode, however, has limitations. In post-compensation mode, the dispersion compensation is limited to the scope of 2,000 ps/nm, that is, the post-compensation mode can only compensate a single-mode fiber within a transmission distance of 200 km only; in pre-compensation mode, the compensation may be provided over a transmission distance of more than 1,000 km, but must be received in a distance near a preset compensation distance. Therefore, it is difficult to apply the two electrical domain compensation modes in building a long-distance transmission network without online dispersion compensation or a network with dynamic configuration.
As shown in
The signal has a certain tolerance of dispersion (the tolerance depends on the transmission rate. The higher the transmission rate is, the smaller the tolerance will be). A system that adopts the pre-compensation mode has receiving limitations, as shown in
The prior art provides a tunable dispersion compensation method. This method is based on the combination of optical tunable dispersion compensator and receiving end electrical dispersion compensator (EDC), thus expanding the tunable dispersion compensation scope. The optical tunable dispersion compensator achieves a dispersion compensation scope of less than 3,000 ps/nm, and may support the transmission of a 10 Gbit/s signal on a single-mode fiber for less than 200 km. The EDC based on maximum likelihood sequence estimate (MLSE) may also achieve a compensation scope of less than 3,000 ps/nm. Thus, this compensation solution needs an additional online DCF compensation technology to implement long-distance transmission.
Besides dispersion, a non-linear effect may damage the optical transmission system. Through emulation, it is found that the dispersion compensation at either end of the line cannot well suppress the non-linear effect.
To sum up, the dispersion compensation method in the prior art cannot meet actual requirements with respect to long-distance dispersion compensation without online DCF and non-linear effect suppression.
Embodiments of the present disclosure provide a dispersion compensation method and a fiber transmission system to overcome the weaknesses of the long-distance dispersion compensation without online DCF and to suppress the non-linear effect of the dispersion compensation in the prior art.
A dispersion compensation method includes: performing, by a transmitting end, electrical pre-compensation processing on a transmit signal to obtain a distorted electrical signal, and converting an optical carrier signal into a distorted optical signal through modulation according to the distorted electrical signal; and after recovering the distorted optical signal to a recovered optical signal through a transmission line, sending the signal to a receiving end; upon receipt of the recovered optical signal, the receiving end, performs post-compensation processing after converting the recovered optical signal into a pre-compensation electrical signal, or performs post-compensation processing before converting the recovered optical signal into a post-compensation electrical signal.
A fiber transmission system provided in an embodiment of the present disclosure includes a transmitting end, a fiber transmission line and a receiving end.
The transmitting end includes: a pre-compensation signal processing module, adapted to perform electrical pre-compensation processing on a transmit signal to obtain a distorted electrical signal; and an electrical/optical converting module, adapted to convert an optical carrier signal into a distorted optical signal through modulation according to the distorted electrical signal sent from the pre-compensation signal processing module.
The receiving end includes: an optical/electrical converting module, adapted to convert the received optical signal into an electrical signal, where the optical signal is recovered from the distorted optical signal through the fiber transmission line; and a post-compensation processing module, adapted to perform dispersion compensation on the optical signal before the optical signal is converted by the optical/electrical converting module, or the electrical signal after the optical signal is converted by the optical/electrical converting module.
The present disclosure is hereinafter described in detail with reference to the accompanying drawings and preferred embodiments, and is not limited to these embodiments.
Embodiments of the present disclosure achieve the objects of suppressing the non-linear effect and improving the system transmission performance through laying out the dispersion distribution on a transmission line reasonably, i.e., through the combination of pre-compensation and post-compensation, and by configuring the dispersion compensation of an optical network dynamically.
As shown in
The transmitting end includes: a pre-compensation signal processing module 1, an optical source 2, and an electrical/optical converting module 3. The pre-compensation signal processing module 1 is adapted to perform electrical pre-compensation processing on a digital transmit signal to obtain a distorted electrical signal of an electrical/optical converting module 3 The optical source 2 is adapted to provide the electrical/optical converting module 3 with an optical carrier signal. The electrical/optical converting module 3 is adapted to convert the optical carrier signal into a distorted optical signal through modulation according to the distorted electrical signal sent from the pre-compensation signal processing module 1, and transmit the distorted optical signal to a fiber transmission line 5. The fiber transmission line 5, adapted to transmit the distorted optical signal sent from the electrical/optical converting module 3, where the distorted optical signal which passes through the fiber transmission line 5 is recovered to a recovered optical signal, and the recovered optical signal is transmitted to an optical/electrical converting module 6 of the receiving end.
The receiving end includes: the optical/electrical converting module 6 and a post-compensation processing module 7. The optical/electrical converting module 6 is adapted to convert the received recovered optical signal into an electrical signal, and transmit the electrical signal to the post-compensation processing module 7. The post-compensation processing module 7 is adapted to perform dispersion compensation on the received electrical signal.
To adjust the compensation amount dynamically, the system further includes: a detection and feedback module 8, adapted to detect the quality of the received electrical signal, and feed back the detection result to the post-compensation processing module 7 by the receiving end.
When the system is used for dispersion compensation in a wavelength division multiplexing (WDM) system, an optical multiplexer 41 needs to be set between the electrical/optical converting module 3 and the fiber transmission line 5 and an optical demultiplexer 42 needs to be set between the fiber transmission line 5 and the optical/electrical converting module 6.
The pre-compensation signal processing module 1 may adjust the optimal receiving point of the whole system through configuration. As shown in
In the embodiment shown in
As shown in
The network configuration information changes only when the network is rebuilt.
If the optical signal is modulated in such special modes as optical duobinary (ODB) and differential phase shift keying (DPSK), a pre-encoding processing module 14 needs to be added to the pre-compensation signal processing module 1. The pre-encoding processing module 14 is adapted to pre-encode the transmit signal, and send the pre-encoded transmit signal to the digital pre-processing module 12.
As shown in
where β2 and L values are controlled by the control signal). The frequency time transforming module 124 is adapted to perform inverse fast Fourier transform (IFFT) on the compensated signal, and send the transformed signal to the modulator I/O converting module 125. The modulator I/O converting module 125 is adapted to convert the transformed signal into a drive signal of the electrical/optical converting module.
The digital pre-processing module 12 may be implemented through a digital signal processor (DSP), a field programmable gate array (FPGA) or an application specific integrated circuit (ASIC). In this embodiment, the FPGA is used to implement the digital pre-processing module 12.
The post-compensation processing module 7 may perform dispersion compensation in real time dynamically, and expand the dispersion tolerance scope of the system from AB to A1B1, as shown in
(1) an adaptive forward equalizer (FFE), using an eye pattern detection circuit or a decision feedback circuit to detect the quality of a signal;
(2) a multi-threshold equalizer, using a FEC error correction circuit to detect the quality of a signal; and
(3) a maximum likelihood equalizer (MLSE).
When the transmission distance is long, the post-compensation processing mode may compensate a small amount of dispersion only, usually within the distance of 250 km. Thus, when the system transmission distance is longer than 1,000 km, the amount of dispersion that needs to be compensated at the transmitting end exceeds 75% of the total amount of dispersion of the system, as shown in
If a tunable optical dispersion compensating module is used, a post-compensation adjustment control signal may be obtained through the feedback signal provided by the detection and feedback module 8, as shown in
The detection and feedback module 8 shown in
(1) detecting the error rate of the signal from the EDC;
(2) detecting the eye pattern openness;
(3) detecting the mean square error of the electrical signal;
(4) by the FEC error correction circuit, detecting the quality of the signal.
The first detection and feedback module 81 and the second detection and feedback module 82 shown in
As shown in
The second detection and feedback module 82 includes the filter 102 and the processing unit 103 only.
As shown in
Step 101: The transmitting end performs electrical pre-compensation processing on a digital transmit signal by using the pre-compensation signal processing module 1 to obtain a distorted electrical signal.
The electrical pre-compensation process is described as follows: transmitting the network configuration information to the pre-compensation control module 11 to obtain the amount of dispersion when the transmit signal passes through the transmission line, and to obtain a control signal; pre-distorting the transmit signal through the digital pre-processing module 12 according to the control signal to obtain a distorted electrical signal, and to compensate the amount of dispersion; and the distorted electrical signal is converted into an analog distorted electrical signal by the digital/analog converter 13. The analog distorted electrical signal controls the electrical/optical converting module 3 to modulate an optical carrier signal of a DC optical source 2 to generate a pre-compensated distorted optical signal.
If the optical signal is modulated in such special modes as ODB and DPSK, the pre-encoding processing module 14 is needed to pre-encode the transmit signal. The pre-encoding process is as follows: inputting the transmit signal to the pre-encoding processing module 14, encoding the transmit signal to obtain a pre-encoded signal, and sending the pre-encoded signal to the digital pre-processing module 12.
The digital pre-processing module 12 performs digital pre-processing through the following steps: the sampling module 121 performs time frequency transform on the pre-encoded signal; then the time frequency transforming module 122 performs FFT, and sends the transformed signal to the compensating module 123; the compensating module 123 performs dispersion compensation on the transformed signal according to a pre-compensation control signal upon the time frequency transform; the sampling function H(ω) of the compensating module 123 performs dispersion compensation, where the H(ω) function is the conjugation of link dispersion transmission functions, that is, H(ω)=exp(−jβ2ω2L/2), and β2 and L values are controlled by the control signal; and the frequency time transforming module 124 performs IFFT on the compensated signal to transform from the frequency domain to the time domain; the modulator I/O converting module 125 converts the transformed signal into the drive signal of the modulator.
Step 102: The distorted electrical signal is used to control the electrical/optical converting module 3 to output a distorted optical signal.
Step 103: After the distorted optical signal is transmitted through the fiber transmission line 5, the distorted optical signal is recovered to a normal optical signal, and the recovered optical signal is sent to the receiving end. The receiving end converts the optical signal into an electrical signal by using the optical/electrical converting module 6, and outputs the electrical signal.
Step 104: The post-compensating module 7 performs post-compensation processing on the electrical signal output by the receiving end. The post-compensating module 7 may perform real-time adjustment by detecting the post-compensation adjustment control signal output by the detection and feedback module 8.
In addition, there is another method for post-compensation processing, that is, performing post-compensation processing on an optical signal before the optical/electrical converting module 6, and converting the compensated optical signal into an electrical signal.
The following emulation has been performed on the present disclosure: transmission rate of 10 Gbit/s, channel central wavelength of 1550 nm, standard single-mode fiber, 80 km span, total transmission distance of 640 km; power generation is performed in each segment through an erbium doped fiber amplifier (EDFA); dispersion compensation is performed at the transmitting end and the receiving end, that is, dispersion compensation is not performed on the line, and the total amount of dispersion compensated at the transmitting end and the receiving end is equal to the total amount of dispersion on the line; the dispersion allocation proportion of the transmitting end and the receiving end is changed to check the system transmission performance.
Although the present disclosure has been described through some exemplary embodiments, the present disclosure is not limited to such embodiments. It is apparent that those skilled in the art can make various modifications and variations to the present disclosure without departing from the spirit and scope of the present disclosure. The disclosure is intended to cover the modifications and variations provided that they fall in the scope of protection defined by the following claims or their equivalents.
Number | Date | Country | Kind |
---|---|---|---|
200610167786.1 | Dec 2006 | CN | national |
This application is a continuation application of PCT/CN2007/002587, filed on Aug. 28, 2007, which claims the priority benefit of Chinese Patent Application No. 200610167786.1, filed on Dec. 21, 2006. The contents of the above identified applications are incorporated herein by reference in their entirety.
Number | Date | Country | |
---|---|---|---|
Parent | PCT/CN2007/002587 | Aug 2007 | US |
Child | 12403865 | US |