CN107634814B - A kind of removing method of the carrier path crosstalk from homodyne detection mode division multiplexing system - Google Patents

Abstract

The invention discloses a kind of removing methods of carrier path crosstalk from homodyne detection mode division multiplexing system, belong to field of communication technology, the present invention using carrier path pilot tone light transmitting terminal double sideband modulation and the method that receiving end down coversion is restored effectively eliminated due in less fundamental mode optical fibre transmission process carrier path by the crosstalk on signal road caused by be concerned with the influence of beat frequency noise.This method is not in the case where changing existing less fundamental mode optical fibre (FMF) parameter, improve the transmission range from homodyne detection mode division multiplexing (MDM-SHD) system, reduce the requirement of model selection sensitivity of the MDM-SHD system to pattern multiplexer/demultiplexer (MUX/DEMUX), it can guarantee preferable error performance, the impact of performance with higher simultaneously.

Description

A method for eliminating carrier crosstalk in self-homodyne detection mode division multiplexing system

technical field

The invention belongs to the technical field of communications, and in particular relates to a method for eliminating carrier crosstalk in a self-homodyne detection mode division multiplexing (MDM-SHD) communication system.

Background technique

In recent years, with the application and development of mobile Internet, cloud computing, and Internet of Things technologies, data services have shown an explosive growth trend. Among them, the short-to-medium-distance optical communication represented by the optical access network and the optical interconnection of the data center urgently needs to improve the network broadband capacity. At present, the intensity modulation-direct detection (IM/DD) method is generally used in short-distance optical communication. This method can only use the light intensity information, which leads to the loss of phase information and the waste of spectrum resources. Moreover, IM/DD is affected by dispersion and chromatic dispersion. The influence of nonlinear noise is serious, making it difficult to achieve large-capacity transmission. Coherent optical communication (ID) has the advantages of high sensitivity and high spectral efficiency, and has been widely used in long-distance and large-capacity backbone optical fiber networks. Combined with dense wavelength division multiplexing (DWDM) technology, coherent optical communication can even achieve a high transmission rate of Tb/s for a single fiber. However, the coherent optical communication system has complex equipment and high cost, which makes the coherent optical communication unable to meet the transmission cost requirements of short-distance communication. In order to effectively increase the capacity of the transmission system and keep the cost relatively low, the mode division multiplexing (MDM-SHD) communication technology based on self-homodyne detection (MDM-SHD) emerges as the times require.

In the MDM-SHD system, the pilot (PT) light of the carrier channel and the quadrature amplitude modulation (mQAM) signal of the signal channel pass through the mode multiplexer (MUX) into the few-mode fiber (FMF) for transmission. At the receiving end, a mode demultiplexer (DEMUX) separates the signal path and the carrier path. The pilot light of the separated carrier path will be used as the local oscillator light (LO) to perform coherent detection with the signal path, thereby obtaining the transmitted signal. This detection method can save the local oscillator laser source at the receiving end and reduce the requirement of the light source line width at the transmitting end. Since the carrier path light and the signal path light come from the same laser source and experience the same noise channel environment, the received There is no frequency offset damage to the signal, and the phase noise is also suppressed and eliminated, which greatly reduces the complexity and number of operations of DSP signal processing. However, the mode crosstalk in the actual few-mode fiber will cause the pilot light of the carrier path to be subjected to crosstalk from the signal path. These crosstalks will generate beat noise after coherent detection, reducing the transmission performance of the entire system. Therefore, it is an urgent problem to eliminate the influence of the carrier crosstalk in the MDM-SHD system and further improve the transmission performance of the MDM-SHD system.

SUMMARY OF THE INVENTION

Aiming at the problem of carrier channel crosstalk in the existing MDM-SHD system, the present invention proposes a self-homodyne detection mode division multiplexing (DSB-MDM-SHD) system scheme based on carrier channel double sideband modulation. The method can reduce the influence of the coherent beat frequency noise caused by the mode crosstalk of the signal path on the carrier path during the transmission process without changing the parameters of the existing few-mode fiber. The method has good effect and can reduce the bit error rate of the system under the same transmission parameters.

The present invention is achieved through the following technical solutions:

A method for eliminating carrier channel crosstalk in a self-homodyne detection mode division multiplexing system, the specific steps are as follows:

The first step is to modulate the double sideband pilot light of the carrier path and the mQAM optical signal of the signal path, which specifically includes the following steps:

Step S101: Calculate the bandwidth f _s of the signal according to the symbol rate and modulation format of the mQAM signal to be transmitted, and determine the frequency of the sine wave that the carrier path needs to use with the frequency ω ₀ of the first spectral zero point of the mQAM signal, thereby maintaining the carrier wave. The path and the signal path are orthogonal in frequency spectrum;

Step S102: modulate a sine wave with a frequency of ω ₀ to the carrier path light through a Mach-Zehnder intensity modulator (MZ), thereby generating double-sideband pilot light of the carrier path;

Step S103: modulate the mQAM signal to be transmitted on the signal path light through the quadrature IQ modulator, thereby obtaining the mQAM optical signal of the signal path;

Among them, at the transmitting end, the signal path E _S (t) and the carrier path E _P (t) are respectively expressed as:

Among them, P _S and P _P are the average optical power of the signal path and the carrier path, respectively, represents the optical carrier whose center frequency is _WC ;

The carrier path _{EP (t) and the signal path E S (} t) can be expressed as:

where s(t) is the energy-normalized mQAM signal;

The second step is to send the carrier path and the signal path together into the few-mode fiber for transmission, which specifically includes the following steps:

Step S104: the carrier path and the signal path are respectively carried on different linear polarization modes (LP modes) through the mode coupler, and enter the few-mode fiber for transmission;

Step S105: at the receiving end, the mode conversion and separation of the carrier path and the signal path are performed by the mode demultiplexer, and the optical fiber with adjustable length (ODLs) is used to eliminate the differential mode delay between different modes in the few-mode fiber ( DMGD) time difference caused at the receiving end;

Since the signal on each LP mode will be damaged by mode crosstalk, differential mode delay, etc. during the transmission process of the few-mode fiber, and the time difference caused by the differential mode group delay can be eliminated by ODLs, it will not affect the carrier path and the signal path. damage to the spectral orthogonality. Therefore, for a simple and clear analysis, we only consider the effect of mode crosstalk in few-mode fibers in formula expression.

Wherein, after step S104 and step S105, the carrier path E _P (t) and the signal path E _S (t) can be expressed as:

Among them, α and β are the coupling coefficients of the carrier path and the signal path after transmission through the few-mode fiber;

The third step is to down-convert, filter, and amplify the carrier path at the receiving end, and then perform coherent detection with the signal path as a coherent local oscillator (LO).

Step S106: sending the carrier path separated by the mode demultiplexer into the Mach Zender intensity modulator, and down-converting the carrier path;

After step S106, the carrier path can be expressed as:

in, is the required pilot light, which can be used as a coherent local oscillator light source; It is twice the frequency of the double-sideband pilot light in the original carrier path; and are the right band and the left band after down-conversion of the carrier crosstalk signal, respectively; according to formula (7), it can be seen that after the signal of the carrier crosstalk is down-converted, it still maintains the spectrum orthogonal to the required pilot light.

Step S107: use an optical bandpass filter to filter the down-converted carrier path;

After the carrier path goes through step S107, the carrier path can be expressed as:

Step S108: The filtered pilot light of the carrier path is close to the ideal local oscillator light. However, due to the double-sideband modulation depth and the loss of the optical fiber during transmission, the power of the filtered pilot light of the carrier path is low, so optical The amplifier amplifies the filtered carrier circuit to stabilize the optical power at the mw level;

Step S109: use the amplified pilot light of the carrier path as a local oscillator light source for coherent optical communication, and perform coherent detection with the signal path;

Step S110: Send the electrical signal obtained by coherent detection into a digital signal processing (DSP) module for signal processing; wherein, the signal processing includes sampling, normalization, clock synchronization, chromatic dispersion (CD) compensation, MIMO equalization, and judgment error code rate (BER) calculation.

Compared with the prior art, the present invention has the following advantages:

The invention effectively eliminates the influence of coherent beat frequency noise caused by the crosstalk of the signal path caused by the carrier path during the transmission process of the few-mode fiber by adopting the double-sideband modulation of the carrier path pilot light at the transmitting end and the down-conversion recovery method at the receiving end. . The method improves the transmission distance of the self-homodyne detection mode division multiplexing (MDM-SHD) system without changing the existing few-mode fiber (FMF) parameters, and reduces the MDM-SHD system to the mode multiplexer/demultiplexer. It can meet the requirements of the mode selection sensitivity of the user (MUX/DEMUX), and at the same time, it can ensure better bit error performance and have a higher performance effect.

Description of drawings

1 is a schematic structural diagram of a 4×4 self-homodyne detection mode division multiplexing (DSB-MDM-SHD) system based on carrier double sideband modulation of the present invention;

2 is a schematic structural diagram of a 4×4 self-homodyne detection mode division multiplexing (DSB-MDM-SHD) system transmitter based on carrier double sideband modulation of the present invention;

3 is a schematic structural diagram of a 4×4 self-homodyne detection mode division multiplexing (DSB-MDM-SHD) system receiver based on carrier double sideband modulation of the present invention;

The carrier path spectrum diagrams shown in Figures 4-1, 4-2, 4-3, 4-4, and 4-5 correspond to positions 1, 2, 3, 4, and 5 marked in the structure of Figure 1, respectively. Figure 4-1 is the spectrum of the carrier path after the light source at the transmitting end passes through the beam splitter; Figure 4-2 is the spectrum of the carrier path after double-sideband modulation on the transmitter side; Figure 4-3 is the carrier path at the receiving end after delay compensation Figure 4-4 is the frequency spectrum of the receiver carrier path after down-conversion; Figure 4-5 is the spectrum diagram of the receiver carrier path after optical filtering and optical amplification.

FIG. 5 is a system bit error rate (BER) performance diagram of each mode signal path using the 4×4 DSB-MDM-SHD system structure scheme under different optical signal-to-noise ratios (OSNR) after 60km transmission according to the present invention;

FIG. 6 is a system using a mode multiplexer/demultiplexer (MUX/DEMUX) with different coupling strengths in a 4×4DSB-MDM-SHD system and a 4×4MDM-SHD system when the OSNR of the present invention is 20dB Bit Error Rate (BER) performance and comparison chart;

7 is a system bit error rate (BER) performance and comparison diagram of a 4×4DSB-MDM-SHD system and a 4×4MDM-SHD system at different transmission distances when the OSNR of the present invention is 20dB;

Detailed ways

In order to be able to understand the above-mentioned methods and steps of the present invention more clearly, the present invention will be further described in detail below with reference to the accompanying drawings and specific implementation methods.

Example 1

The method proposed by the present invention is simulated and verified by using a 4×4 self-homodyne detection mode division multiplexing (MDM-SHD) system. 1 is a schematic structural diagram of a 4×4 self-homodyne detection modulo division multiplexing (DSB-MDM-SHD) system based on carrier double sideband modulation adopted by the present invention, and its composition includes: one center frequency is 193.1THz (center A DFB laser source module with a wavelength of 1552.5nm) with a linewidth of 1MHz and an average output power of 3mw; two 56Gb/s PDM-QPSK signal transmitter modules; one 1:3 and one 1:2 power beam splitter module ; two 14GHz sinusoidal signal sources and two Mach Zender intensity modulator (MZ) modules with 0.8A drive current and 0A bias current; a pair of mode multiplexer/demultiplexer modules with mode selection The sensitivity can be controlled according to the mode coupling matrix, and the coupling strength introduced by default is -25dB; two single-mode fibers (ODLs) with adjustable lengths; a second-order Gaussian optical bandpass filter (OBPF) module with a bandwidth of 10GHz; two PDM-QPSK signal receiver modules; one DSP data signal processing module. Among them, the simulation parameters of few-mode fiber (FMF) are: the coupling strength between LP01 and LP11 is -34dB/km, the demerger mode coupling between LP11a and LP11b is -28dB/km; the loss of few-mode fiber is 0.2dB/km ; The chromatic dispersion of the LP01 mode is 20ps/nm/km, and the chromatic dispersion of the LP11 mode is 21ps/km/nm; the differential mode delay (MDGD) between the LP01 and LP11 modes is 4ns/km. The present invention is an improvement on the carrier circuit of a self-homodyne detection mode division multiplexing (MDM-SHD) system to achieve better transmission performance.

A method for eliminating carrier channel crosstalk in a self-homodyne detection mode division multiplexing system, the specific steps are as follows:

Step S101: The example of the present invention adopts a 4×4DSB-MDM-SHD system structure, and the transmission signal adopts two 56Gb/sPDM-QPSK signals, the transmission spectrum bandwidth is 28GHz, and the required carrier path sinusoidal signal ω ₀ =B/2 is 14GHz .

Step S102 : a DFB laser source with a center wavelength of 1552.5 nm emits a laser beam with an average power of 3 mw, and the beam is equally divided into three beams through a 1:3 power beam splitter. Figure 4-1 shows the spectrum of the carrier path after the light source at the transmitting end passes through the beam splitter. One of the beams is used as the carrier path, and the double-sideband pilot light of the carrier path is generated by a Mach-Zehnder intensity modulator driven by a sinusoidal signal source. Figure 4-2 shows the spectrum of the carrier circuit after DSB modulation.

Step S103: The other two beams of the power beam splitter are used as signal paths to generate two paths of PDM-QPSK signals through two 56Gbit/s quadrature IQ modulators respectively. FIG. 2 is a schematic structural diagram of a PDM-IQ transmitter. It consists of a PRBS random signal source, two polarization beam splitters (PBS), and two IQ modulators. Among them, the IQ modulator consists of two Mach-Zehnder intensity modulators (MZ) and a π/2 phase shifter. The transmitter performs QPSK mapping on the random signal with a length of 2 ¹⁵ -1 generated by PRBS, and then modulates the QPSK signal to the X and Y polarization states of the laser beam of each signal path to generate two paths of PDM-QPSK signals.

Step S104: Connect the carrier path and the two signal paths into the three modes (LP01, LP11a and LP11b) and their corresponding polarization modes in the few-mode fiber through the mode coupler. The LP01 mode carries the carrier path, and the LP11a and LP11b carry two signal paths respectively.

Step S105: After transmission through the 60km few-mode fiber, use a mode demultiplexer (DEMUX) at the receiving end to separate each mode. Pass the separated LP11a and LP11b signal paths through two ODLs fibers to eliminate the differential mode delay between the LP11 and LP01 modes during the transmission process (according to the parameters of the few-mode fiber, the DMGD between the LP01 and LP11 modes is 240ns after 60km transmission ). Figure 4-3 is the spectrum diagram of the carrier path after demultiplexing, which includes double-sideband pilot light and crosstalk PDM-QPSK signal.

Step S106: After the time delay compensation, the carrier circuit is sent to a Mach-Zehnder intensity modulator (MZ) module, and the frequency is down-converted via a 14GHz sinusoidal signal. Figure 4-4 shows the frequency spectrum of the carrier channel after down-conversion. The double-sideband pilot light of the carrier path will be down-converted to regenerate the required pilot light at the center wavelength, and maintain the orthogonality of the frequency components of the pilot light and the crosstalk signal.

Step S107: After down-conversion, the carrier path passes through a second-order Gaussian optical band-pass filter (OBPF) module with a bandwidth of 10 GHz to filter out the mixed frequency components of the carrier path and out-of-band ASE noise, and retain the ideal pilot frequency of the carrier path Light (PT).

Step S108 : sending the filtered carrier path pilot light (PT) into an erbium-doped fiber amplifier (EDFA) to amplify and compensate for power loss, so that the power is stabilized at the mw level. Figure 4-5 is the spectrum diagram of the pilot light (PT) of the carrier path after filtering and amplification.

Step S109: The pilot light (PT) of the carrier path amplified by the EDFA is sent to a 1:2 optical beam splitter for beam splitting. A PDM-QPSK coherent receiver performs coherent detection. FIG. 3 is a schematic structural diagram of a PDM coherent receiver. It consists of two polarizing beam splitters (PBS), two 90-degree mixers, four balanced detectors and four low pass filters (LPF). The receiver sends the received light in LP11a and LP11b modes together with the split pilot light (PT) into a 90° mixer, and the mixed optical signal is converted into an electrical signal by a balanced detector (BPD). , and then send the electrical signal into the low-pass filter, and finally get the output signal and send it to the DSP module.

Step S110: After coherent detection, the four-channel signals of LP11ax, LP11ay, LP11bx and LP11by enter the DSP signal processing module respectively, and the signals are sampled, normalized, clocked, and the chromatic dispersion ( CD) to compensate. The 4×4 MIMO-CMA algorithm is used to adaptively equalize the signal to eliminate the crosstalk in the LP11 mode. The data is then subjected to hard decision, QPSK demapping and bit error rate (BER) calculation.

Figure 5 is a graph showing the bit error rate (BER) variation of the four-channel signals of LP11ax, LP11ay, LP11bx and LP11by under different OSNRs. After 60km transmission of the four channels, the FEC limit is reached at 17dB (the highest system bit error rate for error-free transmission, and the 7% FEC threshold is 3.8e-3).

Figure 6 is a graph showing the influence of mode multiplexer demultiplexing (MUX/DEMUX) with different coupling degrees on the transmission performance of LP11ax, LP11ay, LP11bx and LP11by when the OSNR is 20dB; Figure 6 also compares the same conditions Next, the change of bit error rate of MDM-SHD system and DSB-MDM-SHD system. It can be seen from FIG. 6 that the DSB-MDM-SHD system solution proposed by the present invention can effectively reduce the bit error rate of the system and reduce the influence of the mode crosstalk of the multiplexer/demultiplexer.

Figure 7 shows the influence of different transmission distances on the bit error rate of LP11ax, LP11ay, LP11bx and LP11by when the OSNR is 20dB; Figure 7 also compares the MDM-SHD system and DSB-MDM-SHD system under different transmission distances Variation in bit error rate. It can be seen from Figure 7 that the system bit error rate of the MDM-SHD system is higher than the FEC limit when it transmits 60km, while the DSB-MDM-SHD system reaches the FEC limit when it transmits 90km. The results show that the DSB-MDM-SHD system has better transmission performance.

The radix 4×4 self-homodyne detection mode division multiplexing system described in the above example is well known in the art, and is obtained by a well-known method.

Terms such as self-homodyne detection, double-sideband modulation, down-conversion, etc. used in the present invention are only for the convenience of describing and explaining the present invention, and should not be used as additional limitations thereof.

Claims (6)

1. a method for eliminating carrier path crosstalk in a homodyne detection mode division multiplexing system, is characterized in that, concrete steps are as follows: The first step is to modulate the double-sideband pilot light of the carrier path and the mQAM optical signal of the signal path; specifically, the steps are as follows: Step S101: Calculate the bandwidth f _s of the signal according to the symbol rate and modulation format of the mQAM signal to be transmitted, and determine the frequency of the sine wave that the carrier path needs to use with the frequency ω ₀ of the first spectral zero point of the mQAM signal, thereby maintaining the carrier wave. The path and the signal path are orthogonal in frequency spectrum; Step S102: modulate a sine wave with a frequency of ω ₀ to the carrier path light through a Mach-Zehnder intensity modulator (MZ), thereby generating double-sideband pilot light of the carrier path; Step S103: modulate the mQAM signal to be transmitted on the signal path light through the quadrature IQ modulator, thereby obtaining the mQAM optical signal of the signal path; The second step is to send the carrier path and the signal path together into the few-mode fiber for transmission; the specific steps include the following: Step S104: the carrier path and the signal path are respectively carried on different linear polarization modes (LP modes) through the mode coupler, and enter the few-mode fiber for transmission; Step S105: at the receiving end, the mode conversion and separation of the carrier path and the signal path are performed by the mode demultiplexer, and the optical fiber with adjustable length (ODLs) is used to eliminate the differential mode delay between different modes in the few-mode fiber ( DMGD) time difference caused at the receiving end; The third step is to down-convert, filter, and amplify the carrier path at the receiving end, and then perform coherent detection with the signal path as a coherent local oscillator (LO); Step S106: sending the carrier path separated by the mode demultiplexer into the Mach Zender intensity modulator, and down-converting the carrier path; Step S107: use an optical bandpass filter to filter the down-converted carrier path; Step S108: using an optical amplifier to amplify the filtered carrier circuit to stabilize the optical power at the mw level; Step S109: use the amplified pilot light of the carrier path as a local oscillator light source for coherent optical communication, and perform coherent detection with the signal path; Step S110: Send the electrical signal obtained by coherent detection into a digital signal processing (DSP) module for signal processing. 2. the elimination method of carrier path crosstalk in a kind of self-homodyne detection mode division multiplexing system as claimed in claim 1, is characterized in that, in the described first step, at transmitting end, signal path E _S (t ) and the carrier path E _P (t) are respectively expressed as: Among them, P _S and P _P are the average optical power of the signal path and the carrier path, respectively, represents the optical carrier whose center frequency is _WC ; The carrier path _{EP (t) and the signal path E S (} t) can be expressed as: where s(t) is the energy-normalized mQAM signal. 3. The method for eliminating carrier channel crosstalk in a self-homodyne detection mode division multiplexing system according to claim 1, wherein in the step 2, after step S104 and step S105, the carrier channel E _P (t) and signal path _ES (t) can be expressed as: Among them, α and β are the coupling coefficients of the carrier path and the signal path after transmission through the few-mode fiber, respectively, _{P S} and PP are the average optical power of the signal path and the carrier path, respectively, and s(t) is the energy-normalized mQAM Signal. 4. the method for eliminating carrier path crosstalk in a self-homodyne detection modulo division multiplexing system as claimed in claim 1, characterized in that, after step S106, the carrier path can be expressed as: in, is the required pilot light, which can be used as a coherent local oscillator light source; It is twice the frequency of the double-sideband pilot light in the original carrier path; and are the right and left bands of the carrier channel crosstalk signal after down-conversion, respectively. 5. a method for eliminating carrier path crosstalk in a self-homodyne detection modulo division multiplexing system as claimed in claim 1, characterized in that, after step S107, the carrier path can be expressed as: 6. The method for eliminating carrier crosstalk in a self-homodyne detection modulo division multiplexing system according to claim 1, wherein the signal processing described in the step S110 comprises sampling, normalization, clocking Synchronization, dispersion (CD) compensation, MIMO equalization, and decision bit error rate (BER) calculations.