CN115842595A

CN115842595A - Optical comb coherent transmission system master-slave phase compensation method based on optical phase conjugation

Info

Publication number: CN115842595A
Application number: CN202211458306.2A
Authority: CN
Inventors: 王宏祥; 郑瑜; 纪越峰
Original assignee: Beijing University of Posts and Telecommunications
Current assignee: Beijing University of Posts and Telecommunications
Priority date: 2022-11-17
Filing date: 2022-11-17
Publication date: 2023-03-24

Abstract

The invention discloses an optical comb coherent transmission system master-slave phase compensation method based on Optical Phase Conjugation (OPC). In a coherent light WDM system with an optical comb as a light source and a local oscillator, a transmitting end performs signal modulation by using different wavelength comb teeth of an optical comb signal as carrier waves; an OPC module is arranged in the middle of the transmission link and used for converting the WDM signal into a conjugate signal of another wavelength; and carrying out coherent reception on N paths of signals at the receiving end by using a local optical frequency comb, and carrying out phase estimation and compensation on the received signals by adopting a master-slave phase recovery algorithm after the received signals are subjected to digital signal preprocessing. The invention compensates the dispersion damage of the signal through the OPC device, thereby adjusting the relative transmission time delay between N paths of multiplexing signals, effectively relieving the problem of decoherence of the phase of the received signal caused by dispersion walk-off, and realizing master-slave phase compensation in an optical comb coherent transmission system.

Description

Optical comb coherent transmission system master-slave phase compensation method based on optical phase conjugation

Technical Field

The invention relates to the field of optical communication, in particular to a master-slave phase compensation method based on optical phase conjugation in an optical comb coherent transmission system.

Background

With the continuous development of new services such as big data and internet of things, the global data traffic is continuously increasing. This has prompted network operators to continually seek to achieve greater transmission capacity over existing networks. In the past decades, the transmission capacity of optical fiber communication systems has been greatly increased due to the development of Erbium Doped Fiber Amplifiers (EDFAs), WDM, and other technologies. To further increase the transmission capacity of the optical fiber, multiplexing of more wavelengths needs to be achieved. However, in existing networks, the carriers used in WDM optical communication systems come from separate lasers. The addition of WDM channels means that more lasers are required, which increases the overall cost and power consumption of the system.

When the optical-frequency comb is used as a light source of a WDM system instead of a laser array, a schematic diagram of the optical-comb WDM system is shown in fig. 2. The system can effectively solve the problems, and the advantages can be mainly summarized into three points: first, the optical-frequency comb is usually generated by a seed laser, and can provide multiple carriers for signal modulation, so that the use of the optical-frequency comb in a WDM system can reduce the number of independent lasers and the system cost. Secondly, the frequency drifts of the independent lasers are mutually independent, the frequency intervals among a plurality of independent lasers are not fixed, and the frequency intervals among the comb teeth of the optical frequency comb are very stable, so that the channel protection interval can be reduced by using the optical frequency comb as a signal light source, and the channel utilization rate is improved. Thirdly, in a normal WDM system, each path of signal needs to go through the DSP flow shown in fig. 3, including the steps of orthogonalization, dispersion compensation, clock recovery, equalization, frequency offset compensation, phase compensation, and the like; in the optical comb WDM system, since the optical frequency comb has phase coherence, and phase noise between the respective comb teeth is substantially consistent, when the optical source at the transmitting end and the local oscillator at the receiving end are both optical frequency combs, the phase noise caused by the line widths of the carrier and the local oscillator received by the received signal of each channel is the same, and the phase compensation step in the DSP process can be jointly processed, as shown in fig. 4. After the received signal of each channel is subjected to the steps of the DSP such as orthogonalization, dispersion compensation, clock recovery, equalization and frequency offset compensation, the phase noise estimation and compensation are carried out on all the channels by using a phase compensation algorithm of a master-slave structure, the calculation complexity of phase estimation is greatly reduced, the calculation force requirement on a DSP chip can be reduced, and the energy consumption of a receiver is reduced. FIG. 5 shows the structure of the master-slave phase recovery (MS-CPR) algorithm, and for the detailed description of the master-slave phase recovery algorithm, see the references "Lars L, magnus K, abel L R, et al. Frequency Comb-Based WDM Transmission Systems Enabling Signal Processing [ J ]. Applied Sciences,2018,8 (5): 718.".

The MS-CPR algorithm works well in short-distance transmission systems, but in long-distance transmission systems based on optical frequency combs, due to the existence of fiber dispersion, the group velocities of signals of different wavelengths are not the same, and the time required for passing through the same length of fiber channel, i.e., the transmission delay, is not uniform, which results in the reduced coherence of the received signal. If the center frequency interval between the master channel CH 1 and the slave channel CH i is Δ f _i Corresponding to a central wavelength difference of Δ λ _i Neglecting the influence of dispersion slope, after the transmission of the optical fiber with length L, the transmission delay difference of the two signals at the receiving end is Delta T _i ＝DLΔλ _i . Since the phase noise of the laser for generating the optical frequency comb varies with time, the phase noise of each channel is not uniform at the receiving end, and the phase coherence between the channels is weakened. In fact, the phase noise variation of the laser can be regarded as a wiener process due to the propagation delay difference Δ T _i The resulting phase noise difference between the master and slave channels follows a Gaussian distribution with a mean of 0 and a variance of

Wherein Δ ν is the linewidth of the seed laser. Therefore, in the long-distance optical comb system, after the received signal is subjected to digital signal preprocessing and master-slave phase estimation and compensation, residual phase noise exists in the slave channel, and the residual phase noise is the phase noise difference between the master channel and the slave channel when the received signal is received. The magnitude of the residual phase noise difference is positively correlated with the line width of the seed laser and the transmission delay difference of the master channel and the slave channel. When the residual phase noise difference is large, the quality of the signal finally recovered from the channel will be affected, so that the BER from the channel increases.

Fig. 6 is a graph of the phase noise variation due to laser linewidth for three different channels after transmission over a distance of 1000 km. As shown in fig. 6, after three different channels are transmitted, their phase noise changes are correlated, but not completely consistent, and the phase noise estimated by one channel cannot directly compensate for the other channels; otherwise, other channels will have residual phase noise, thereby affecting the quality of the recovered signal constellation; the middle channel is used as the main channel, and the other channels are used as the auxiliary channels, and the recovered constellation diagram is shown in fig. 7, wherein the farther the channel is from the main channel, the worse the quality of the constellation diagram is.

There are currently two main solutions to the above problem. One is to add a second-stage slave phase tracker to compensate the residual phase noise after the slave channel performs the first-stage phase compensation by using the estimated phase noise of the master channel through a two-stage algorithm, but this scheme increases the complexity of the master-slave phase compensation algorithm; the other is to add a delay module on each channel at the receiving end through improvement on a hardware system to compensate the influence of dispersion walk-off, so as to improve the performance of the algorithm.

Disclosure of Invention

The invention solves the problem of weakening the coherence of a long-distance optical comb system caused by dispersion walk-off, and compensates the dispersion and nonlinear effect in a link by using an optical phase conjugation module on the system hardware level, so that the signal still has strong phase coherence when being received, and the system still can use a master-slave phase compensation algorithm in the DSP processing stage.

In order to achieve the above object, the method for master-slave phase compensation of an optical comb coherent transmission system based on optical phase conjugation comprises the following steps:

step 1, transmitting end signal modulation: the optical frequency comb used as light source filters out N channels of comb teeth used for signal modulation as modulation carrier waves through a wavelength division demultiplexer, and the center frequency of the carrier waves is f _k ＝f ₀ K · Δ f, k =1,2, \ 8230, N, Δ f being the frequency spacing between the comb teeth of the optical comb; each filtered comb tooth passes through an I/Q modulator to obtain a signal x _k (m), m represents an mth symbol; all the N paths of modulation signals are input into the optical fiber link for transmission after passing through the wavelength division multiplexing module to obtain WDM signals.

Step 2, signal transmission and OPC module configuration: the WDM signal passes through 2M section of fixed span optical fiber in the whole transmission process, the optical amplifier is used for compensating signal attenuation after passing through each section of optical fiber, and the wavelength conversion device is positioned in the middle of the whole link; in the OPC signal generation process, the center frequency is set to f _P1 And f _P2 Is coupled with the transmitted WDM signal and obtains the conjugate signal of the input WDM signal through a highly nonlinear fiber HNLF, wherein f _P1 +f _P2 ＝2·f ₀ And f is _P1 <f _N So that the center frequency of the obtained N-path conjugate signal is

The conjugate signal is filtered by a filter and then is input into the following M sections of optical fibers for transmission, and the dispersion and nonlinear damage received by the signal in the preceding M sections of links can be effectively compensated in the transmission process.

Step 3, multi-channel coherent reception: in an optical comb system based on an OPC module, N paths of conjugate signals are separated from each other at a receiving end through a wavelength division demultiplexer, corresponding center frequency comb teeth obtained after each path of signals and a local optical comb are demultiplexed are accessed into a coherent receiver to realize coherent reception of the N paths of signals, and the obtained N paths of digital signals are expressed as N paths of digital signals

n is the sampling instant. />

Step 4, preprocessing the digital signal: the sampled signal compensates signal damage caused by timing error through a clock recovery algorithm, and then the equalizer is used for compensating residual chromatic dispersion and polarization mode chromatic dispersion to obtain a preprocessed signal

And 5, master-slave phase compensation: and taking the central channel of the N paths of conjugate signals as a main channel, and performing phase estimation and compensation on the N paths of conjugate signals by using a main-slave phase compensation algorithm.

The OPC module obtains the conjugate signal of the WDM signal by using a double-pump structure, and the step enables the original signal with relatively high group speed in the WDM to be changed into the signal with relatively low group speed in the conjugate signal, so that the transmission time of each signal is approximately the same, the transmission delay difference of each signal at a receiving end is greatly reduced, the problem of reduced correlation in a long-distance optical comb system caused by dispersion walk-off is solved, and the master-slave phase compensation with low complexity can be realized in the optical comb coherent transmission system.

Drawings

FIG. 1 is a schematic diagram of a coherent optical comb transmission system based on optical phase conjugation

FIG. 2 is a schematic diagram of a WDM system structure with an optical comb as a signal light source and a local oscillator

FIG. 3 is a flow chart of a conventional DSP coherent transmission system

FIG. 4 is a schematic diagram of DSP flow of WDM system based on optical comb

FIG. 5 is a schematic diagram of a master-slave phase recovery algorithm

FIG. 6 shows the phase noise of different channels received after 1000km of optical comb transmission

FIG. 7 is a constellation diagram recovered by master-slave phase compensation after 1000km of optical comb system transmission, wherein the star 5 is the main channel

FIG. 8 is a schematic diagram of an optical comb coherent transmission system based on OPC

FIG. 9 is a schematic diagram of an OPC module structure

FIG. 10 is a diagram of different channel constellations obtained by processing an OPC-based optical comb coherent transmission system by using a master-slave phase compensation algorithm, wherein a channel 9 is a main channel

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

Examples

The present embodiment is a master-slave phase compensation method based on optical phase conjugation, which is proposed for an optical comb coherent transmission system, and the overall system structure is shown in fig. 8. The method comprises the following specific steps:

step 1: WDM signal modulation generation based on optical comb

The optical comb used in this embodiment can be expressed as:

wherein E _n Representing the amplitude of the laser, the frequency of 9 comb teeth being f _n ＝f ₀ -n·Δf，f ₀ ＝193.6THz，Δf＝100GHz，φ _T (t) represents phase noise caused by the line width of the transmitting-end seed laser. After the optical comb is filtered by a wavelength division demultiplexer, QPSK signal modulation is carried out on each comb tooth, and then all signals are coupled by a wavelength division multiplexer, wherein the WDM signals after coupling are:

S _n (t) represents the amplitude of the modulation symbols on the nth comb, which is a fixed value, φ, for QPSK signals _n And (t) is the phase corresponding to the symbol, and only pi/4, 3 pi/4, 5 pi/4 and 7 pi/4 take values for the QPSK signal. The QPSK signal rate for each channel modulation in this example is 25GBaud with a transmit power of-1 mW.

Step 2: signal transmission and OPC module configuration

In fig. 8, each fiber span is 100km, and an Erbium Doped Fiber Amplifier (EDFA) is used to compensate the attenuation of the signal experienced by each fiber, after the signal has been transmitted for 500km, the influence of third-order dispersion is ignored, and then the wavelength of channel 1 is the smallest and the transmission speed is the fastest, while the other channels n are slower than channel 1 by about 6.4 × n nanoseconds (ns) according to the formula Δ T = DL Δ λ. At this time, the WDM signal is output to the OPC module, and the specific structure of the OPC module is shown in fig. 9. In the OPC module, the center frequencies of the two pump signals are 191.1THZ and 196.1THZ, and the power is 80mW; inputting two pumping signals and WDM signals into HNLF of 280m to generate four-wave mixing to generate conjugate signals of WDM signals, which are complex conjugates of original signal envelopes and carry information of original signals, but the center frequencies of the signals are changed, and the channels are corresponding to the converted channels k ^* Center frequency of

The conjugate signal was filtered out by a filter and input to the second half of the 500km fiber link. Channel 1 at this time ^* The slowest transmission speed, channel 9 ^* And the fastest speed is opposite to the first half part of the link, so that the transmission time delay of each channel is basically consistent when the data is transmitted to a receiving end, and the transmission time delay difference between the data and the receiving end is greatly reduced. In addition, according to the OPC principle, the conjugate signal can compensate for the dispersion and partial nonlinear damage to the signal in the first half link during the transmission of the second half.

And step 3: multi-channel coherent reception

Comb teeth frequency of receiving end local oscillation optical comb is

Corresponding to the center frequency of the conjugate signal. The conjugate signals and the local oscillator optical comb are respectively demultiplexed through a wavelength division demultiplexer, then the conjugate signals and the local oscillator comb teeth with corresponding frequencies are accessed to a coherent receiver for coherent reception, and then sampling is carried out through an analog-to-digital converter (ADC) to become digital signals.

Step 4, preprocessing the digital signal:

the sampled signal compensates signal damage caused by timing error through a clock recovery algorithm, and then the equalizer is used for compensating residual chromatic dispersion and polarization mode chromatic dispersion to obtain a preprocessed signal

τ _n Is the transmission delay. And 5, master-slave phase compensation:

by channel 5 ^* Is a main channel and the other channels are auxiliary channels, and adopts a main-auxiliary phase compensation algorithm to estimate the phase noise of the main channel to be phi _T (t-τ ₅ )+φ _R (t)]Compensating the phase noise for all channels, the compensated signal

Since the transmission delays of all channels are substantially the same, phi _T (t-τ _n )-φ _T (t-τ ₅ ) Approximately 0, the phase noise of the respective channels can be compensated well.

As shown in fig. 10, for the constellation diagram after the master-slave phase estimation and compensation of the system, it can be seen from the diagram that the phase noise of the slave channel is substantially compensated, and the quality of the constellation diagram of the slave channel is substantially the same as that of the master channel, which illustrates that the present example can effectively recover the signals of each channel by the master-slave phase compensation method in the optical comb coherent transmission system.

Claims

1. A master-slave phase compensation method of an optical comb coherent transmission system based on optical phase conjugation is characterized by mainly comprising the following steps:

step 1, transmitting end signal modulation: the optical frequency comb used as light source filters out N channels of comb teeth used for signal modulation as modulation carrier waves through a wavelength division demultiplexer, and the center frequency of the carrier waves is f _k ＝f ₀ K · Δ f, k =1,2, \8230, N, Δ f is the frequency spacing between the teeth of the optical comb; each filtered comb tooth passes through an I/Q modulator to obtain a signal x _k (m), m represents the mth symbol; all the N paths of modulation signals are input into the optical fiber link for transmission after passing through the wavelength division multiplexing module to obtain WDM signals.

Step 2, signal transmission and OPC module configuration: the WDM signal passes through 2. M section of fixed span optical fiber in the whole transmission process, the optical amplifier is used for compensating signal attenuation after passing through each section of optical fiber, and the OPC device is positioned in the middle position of the whole link; in the OPC signal generation process, the center frequency is set to f _P1 And f _P2 Is coupled with the transmitted WDM signal and obtains the conjugate signal of the input WDM signal through a highly nonlinear fiber HNLF, wherein f _P1 +f _P2 ＝2·f ₀ And f is _P1 <f _N So that the center frequency of the obtained N-path conjugate signal is

Step 3, multi-channel coherent reception: in an optical comb system based on an OPC module, N paths of conjugate signals are separated from each other at a receiving end through a wavelength division demultiplexer, and corresponding center frequency comb teeth obtained after each path of signal and local optical comb demultiplexing are accessed into a coherent receiver to realize coherent reception of the N paths of signals and obtain N paths of digital receiving signals.

Step 4, preprocessing the digital signal: and compensating the signal damage caused by the timing error of the sampling signal through a clock recovery algorithm, and then compensating the residual chromatic dispersion and the polarization mode chromatic dispersion by using an equalizer to obtain a preprocessed signal.

And 5, master-slave phase compensation: and taking the central channel of the N paths of conjugate signals as a main channel, and performing phase estimation and compensation on the N paths of conjugate signals by using a master-slave phase compensation algorithm.

2. The method of claim 1, wherein: the OPC module adopts a double-pumping scheme to generate a conjugate signal of the WDM signal, and when the conjugate signal is transmitted in the rear half part link, the dispersion and nonlinear damage of the signal in the transmission of the front half part link are effectively compensated, and the transmission delay difference among all paths of signals is reduced.