A MZM nonlinear compensation method and system based on subcarrier modulation

Abstract

The present invention discloses a MZM nonlinear compensation method based on subcarrier modulation, which belongs to the field of communication technology. The supplementary method uses a PAM4 signal IM / DD back-to-back transmission system built by an Optisystem simulation platform to simulate the nonlinear damage caused by the MZM by adjusting the bias voltage of the MZM; since the modulation curve of the MZM is a cosine-like function, which is periodic and nonlinear, a modulation method based on a sine function as a subcarrier is used to suppress the nonlinear distortion of the signal generated by the MZM during the modulation process, and finally Matlab is used for digital signal processing to balance the inter-symbol crosstalk. The present invention uses a modulation method based on a subcarrier to compensate for the nonlinear damage caused by the modulation characteristics of the MZM itself, reduce the complexity of the DSP at the receiving end, and improve the transmission efficiency of the signal.

Description

Technical Field

[0001] The present invention belongs to the field of communication technology, and relates to compensation for nonlinear damage of a modulator in a coherent optical fiber communication system based on a PAM4 signal, and specifically to a MZM nonlinear compensation method and system based on subcarrier modulation. Background Art

[0002] In the face of growing business needs, higher requirements are placed on the development of optical transmission technology. The traditional intensity modulation / direct detection (IM / DD) optical communication system implemented by non-return-to-zero (NRZ) coding is difficult to meet the requirements of the ever-increasing transmission rate. Therefore, short-distance optical transmission systems usually adopt IM-DD modulation detection technology and combine it with advanced multi-level modulation technology to achieve high-speed transmission, considering cost and energy efficiency. At present, there are three efficient modulation schemes suitable for the mainstream of IM-DD: pulse amplitude modulation (PAM), carrier-less amplitude and phase modulation (CAP), and discrete multi-tone modulation (DMT). Among them, PAM4 is widely used in IM / DD systems because the system power consumption and the complexity of modulation format implementation need to be considered in practical applications.

[0003] The IM / DD system structure based on PAM4 mainly includes digital signal processing (DSP) at the transmitter, optical modulator, optical fiber transmission, photodetector, and digital signal processing at the receiver. Digital signal processing at the transmitter includes channel coding, oversampling, PAM4 symbol mapping and other technologies. Digital signal processing at the receiver includes downsampling, channel equalization, PAM4 symbol demapping, and bit error rate calculation. Currently, there are two common IM / DD system modulation methods: internal modulation and external modulation. Internal modulation mainly includes electro-absorption modulator (EAM) and directly modulated lasers (DML), while external modulation mainly includes Mach-Zender modulator (MZM).

[0004] While the PAM4 modulation format improves the transmission rate, it is also more sensitive to factors such as nonlinear damage caused by the system's optoelectronic devices and inter-symbol crosstalk. As one of the mainstream modulators in the IM / DD system, the nonlinear signal distortion caused by the nonlinear characteristics of its own modulation curve is an inevitable problem for the IM / DD system based on PAM4. The current MZM nonlinear compensation methods all use equalization algorithms to perform digital signal processing on the transmitted signal at the receiving end, and the algorithm complexity is relatively high.

[0005] In summary, there is an urgent need to provide a method for compensating for MZM nonlinear damage. Summary of the invention

[0006] In view of the MZM nonlinear compensation method in the prior art that uses an equalization algorithm to perform digital signal processing on a transmitted signal at a receiving end, the algorithm has the problem of high complexity. The present invention provides an MZM nonlinear compensation method and system based on subcarrier modulation. The nonlinear compensation method of the present invention uses Optisystem and Matlab to jointly simulate an IM / DD back-to-back transmission system based on PAM4. In this system, the generation, digital-to-analog conversion and optoelectronic conversion of PAM4 signals are implemented by Optisystem, and the digital signal processing of the PAM4 signal receiving end is implemented by Matlab, which can reduce the complexity of the DSP at the receiving end.

[0007] The principle of an MZM nonlinear compensation method based on subcarrier modulation is as follows:

[0008] First, the Optisystem simulation platform is used to build a PAM4 signal IM / DD back-to-back transmission system, and the nonlinear damage caused by the MZM is simulated by adjusting the bias voltage of the MZM. Since the modulation curve of the MZM is a cosine-like function, which is periodic and nonlinear, a modulation method based on a sine function as a subcarrier is used to suppress the nonlinear distortion of the signal caused by the MZM during the modulation process. Finally, Matlab is used for digital signal processing to balance the inter-code crosstalk. The present invention uses a subcarrier-based modulation method to compensate for the nonlinear damage caused by the modulation characteristics of the MZM itself, reduce the complexity of the DSP at the receiving end, and improve the transmission efficiency of the signal.

[0009] The present invention is achieved through the following technical solutions:

[0010] A MZM nonlinear compensation method based on subcarrier modulation specifically comprises the following steps:

[0011] Step S1: Generate a PAM4 signal from a pseudo-random bit sequence;

[0012] Step S2: simulate MZM nonlinear damage, add subcarrier modulation, and compensate for nonlinear damage;

[0013] Step S3: Perform digital signal processing on the compensated signal to balance the inter-symbol crosstalk.

[0014] Further, step S1 is specifically as follows:

[0015] First, a pseudo-random bit sequence generator is used in Optisystem to generate a random binary digital signal, and then a PAM sequence generator is used to map the random binary digital signal into a PAM4 signal format, where each symbol of the signal can provide 2 bits of information.

[0016] Furthermore, the process of step S2 is as follows:

[0017] (1) The mapped PAM4 signal enters the multi-level pulse generator for pulse shaping to generate a digital signal;

[0018] (2) The digital signal enters the digital-to-analog converter (DAC) and is converted into an analog signal;

[0019] (3) The PAM4 signal and the sine wave after digital-to-analog conversion enter the optical modulator (MZM) at the same time, and the laser generates a continuous optical signal to drive the optical modulator; the sine wave generated by the sine wave generator suppresses the nonlinear interference of the optical signal generated by the MZM during the modulation process; wherein, the laser generates an amplitude of E0, an angular frequency of ω0, a time of t, and an initial phase of Continuous optical signal j is an imaginary unit; the subcarrier has an amplitude of A, an angular frequency of ω1, and an initial phase of The sine wave signal After MZM modulation, the output optical signal is Where IL is the insertion loss; γ is the power splitting ratio of the two Y-branch waveguides of the MZM (assuming symmetry), that is, ExtRatio is the extinction ratio; v1(t) and v2(t) are the input voltages of the upper and lower waveguides of the modulator, respectively; v bias1 and v bias2 are the first bias voltage and the second bias voltage respectively; V πRF is the switching modulation voltage of MZM; V πDC is the switching bias voltage of MZM;

[0020] (4) The optical signal output by the photoelectric conversion after adding the subcarrier is converted into an electrical signal by a photodetector (PD); the current generated by the photodetector changes with the incident light power, and the output current of the photodetector at the receiving end is positively correlated with the square of the received light field intensity, that is, I out ∝|E out (t)| 2 ;Iout is the output current;

[0021] (5) After the converted electrical signal containing the subcarrier signal is filtered out with a low-pass Gaussian filter to remove the high-frequency component, the PAM4 signal after nonlinear compensation is obtained. The filter transfer function is: Where α is the insertion loss; f c is the filter cutoff frequency; f is the frequency; N is the filter order;

[0022] (6) After filtering out the high-frequency component and nonlinear compensation, the PAM4 signal enters the analog-to-digital converter (ADC) and is converted into a digital signal.

[0023] Further, step S3 is specifically as follows:

[0024] The PAM4 signal after nonlinear compensation is subjected to a series of digital signal processing through Matlab, which mainly includes downsampling, digital equalization, and PAM4 symbol demapping.

[0025] On the other hand, the present invention also provides an MZM nonlinear compensation system based on subcarrier modulation, comprising:

[0026] A digital modulation module is used to map a random bit sequence into a PAM4 signal and transmit it to a pulse shaping module;

[0027] The pulse shaping module is used to shape the PAM4 signal pulse to generate a digital signal and transmit it to the digital-to-analog conversion module.

[0028] A digital-to-analog conversion module is used to convert digital signals into analog signals and transmit them to the optical control module;

[0029] An optical modulation module is used to convert analog signals into optical signals and transmit them to the photoelectric detection module;

[0030] A photoelectric detection module is used to convert the optical signal into an electrical signal and transmit it to the filter module;

[0031] The filter module is used to filter out the high-frequency components of the electrical signal and transmit it to the analog-to-digital conversion module;

[0032] The analog-to-digital conversion module is used to convert the electrical signal with high-frequency components filtered out into a digital signal and transmit it to the digital signal processing module;

[0033] Digital signal processing module, used for downsampling, digital equalization, and PAM4 symbol demapping of digital signals.

[0034] Furthermore, the MZM nonlinear compensation system further includes:

[0035] The subcarrier module is used to generate a subcarrier for suppressing MZM nonlinear damage and is carried on the upper end of the main carrier;

[0036] The laser module is used to generate continuous light signals and drive the optical modulation module.

[0037] Compared with the prior art, the advantages of the present invention are as follows:

[0038] The present invention proposes a MZM nonlinear compensation method and system based on subcarrier modulation. Specifically, the method compensates for the nonlinear damage caused by the MZM device to the signal by adding a sinusoidal subcarrier. The implementation difficulty of the sinusoidal subcarrier is relatively low, and no complicated modeling process is required. The receiving end can adopt the traditional linear equalization method, which reduces the complexity of the DSP algorithm of the receiving end. The solution has low cost and good effect, can ensure the stability of IM / DD system transmission, and can achieve nonlinear compensation for MZM. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following is a brief introduction to the drawings required for the specific embodiments or the description of the prior art. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn according to the actual scale.

[0040] Figure 1 It is a flow chart of the MZM nonlinear compensation method based on subcarrier modulation of the present invention;

[0041] Figure 2 This is a structural diagram of the PAM4-based IM / DD system of the present invention;

[0042] Among them, a) is a system structure diagram without subcarrier, and b) is a system structure diagram with subcarrier added;

[0043] Figure 3 This is an example diagram of Embodiment 1 of the present invention;

[0044] Among them, a) is the system output eye diagram after compensation, and b) is the system output eye diagram without compensation;

[0045] Figure 4 This is a system block diagram of an MZM nonlinear compensation system based on subcarrier modulation according to Embodiment 2. DETAILED DESCRIPTION

[0046] In order to clearly and completely describe the technical solution and its specific working process of the present invention, the specific implementation methods of the present invention are as follows in conjunction with the accompanying drawings of the specification:

[0047] Example 1

[0048] like Figure 1 FIG. 1 is a flow chart of a MZM nonlinear compensation method based on subcarrier modulation according to this embodiment. The compensation method specifically includes the following steps:

[0049] Step S1: Generate a PAM4 signal from a pseudo-random bit sequence, as follows:

[0050] First, a pseudo-random bit sequence generator is used in Optisystem to generate a random binary digital signal. Then, a PAM sequence generator is used to map the random signal into a PAM4 signal format, where each symbol of the signal can provide 2 bits of information.

[0051] Step S2: simulate the nonlinear damage of MZM, add subcarrier modulation, and compensate for the nonlinear damage, as follows:

[0052] (1) The mapped PAM4 signal enters the multi-level pulse generator for pulse shaping to generate a digital signal;

[0053] (2) The digital signal enters the digital-to-analog converter (DAC) and is converted into an analog signal;

[0054] (3) Figure 2 As shown in b), the PAM4 signal and the sine wave after digital-to-analog conversion enter the MZM at the same time and are driven by the laser for photoelectric conversion. The continuous laser generates an amplitude of E0, an angular frequency of ω0, and an initial phase of Continuous optical signal The subcarrier has an amplitude of A, an angular frequency of ω1, a time of t, and an initial phase of The sine wave signal The parameter setting is A=5V, the DC bias voltage is 1V; after MZM modulation, the output optical signal is Where IL is the insertion loss; γ is the power splitting ratio of the two Y-branch waveguides of the MZM (assuming symmetry), that is, ExtRatio is the extinction ratio; v1(t) and v2(t) are the input voltages of the upper and lower waveguides of the modulator; v bias1 and v bias2 is bias voltage 1 and bias voltage 2; V πRF is the switching modulation voltage of MZM; V πDC is the switch bias voltage of MZM, the parameter setting is ExtRatio=20dB, V πDC =4V,V πRF =4V,IL=5dB,v bias1 =3V, v bias2 =-1.4V. No subcarrier modulation is added. Figure 2As shown in a), the DC power supply voltage is set to 0V.

[0055] (4) After adding the subcarrier, the photoelectrically converted optical signal is converted into an electrical signal by a photodetector (PD). The current generated by the photodetector changes with the incident light power, and the output current of the photodetector at the receiving end is positively correlated with the square of the received light field intensity, that is, I out ∝|E out (t)| 2 .

[0056] (5) The converted electrical signal containing the subcarrier signal is filtered out with a low-pass Gaussian filter to remove the high-frequency component and obtain the PAM4 signal after nonlinear compensation. The filter transfer function is: Where α is the insertion loss; f c is the filter cutoff frequency; f is the frequency; N is the filter order, and the parameters are set to α=0dB, N=1.

[0057] (6) Enter the analog-to-digital converter (ADC) to convert it into a digital signal;

[0058] Step S3: Perform digital signal processing on the compensated signal to balance the inter-symbol crosstalk.

[0059] The PAM4 signal after nonlinear compensation is subjected to a series of digital signal processing through Matlab, which mainly includes downsampling, digital equalization, PAM4 symbol demapping, etc. Figure 3 Figures a) and b) are the compensated and uncompensated system output eye diagrams, respectively. From the system output eye diagrams before and after compensation, we can see that the sinusoidal subcarrier can compensate the nonlinearity of the signal well, and the three eyes of the eye diagram are evenly opened.

[0060] Example 2

[0061] like Figure 4 FIG. 1 is a system block diagram of an MZM nonlinear compensation system based on subcarrier modulation according to an embodiment of the present invention. The MZM nonlinear compensation system based on subcarrier modulation includes:

[0062] A digital modulation module is used to map a random bit sequence into a PAM4 signal and transmit it to a pulse shaping module;

[0063] The pulse shaping module is used to shape the PAM4 signal pulse transmitted from the digital modulation module to generate a digital signal and transmit it to the digital-to-analog conversion module.

[0064] A digital-to-analog conversion module is used to convert digital signals into analog signals and transmit them to the optical modulation module;

[0065] An optical modulation module is used to convert analog signals into optical signals and transmit them to the photoelectric detection module;

[0066] A photoelectric detection module is used to convert the optical signal into an electrical signal and transmit it to the filter module;

[0067] The filter module is used to filter out the high-frequency components of the electrical signal and transmit it to the analog-to-digital conversion module;

[0068] The analog-to-digital conversion module is used to convert the electrical signal with high-frequency components filtered out into a digital signal and transmit it to the digital signal processing module;

[0069] Digital signal processing module, used for downsampling, digital equalization, and PAM4 symbol demapping of digital signals.

[0070] Furthermore, the MZM nonlinear compensation system further includes:

[0071] Subcarrier module, used to generate subcarrier to suppress MZM nonlinear damage, which is carried on the upper end of the main carrier;

[0072] The laser module is used to generate continuous light signals and drive the optical modulation module.

[0073] In this embodiment, if Figure 2 As shown in b,

[0074] The digital modulation module uses a PRBS generator and a PAM generator.

[0075] The pulse shaping module adopts an M-ray pulse generator.

[0076] The digital-to-analog conversion module uses a digital-to-analog converter.

[0077] The optical modulation module adopts Mach-Zehnder modulator.

[0078] The photoelectric detection module adopts a photodiode.

[0079] The filter module adopts a low-pass Gaussian filter.

[0080] The analog-to-digital conversion module uses an analog-to-digital converter.

[0081] The digital signal processing module uses Matlab components and combines with Matlab for processing.

[0082] The subcarrier module adopts a sine wave generator.

[0083] The preferred embodiments of the present invention are described in detail above in conjunction with the accompanying drawings. However, the present invention is not limited to the specific details in the above embodiments. Within the technical concept of the present invention, a variety of simple modifications can be made to the technical solution of the present invention, and these simple modifications all belong to the protection scope of the present invention.

[0084] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not further describe various possible combinations.

[0085] In addition, various embodiments of the present invention may be arbitrarily combined, and as long as they do not violate the concept of the present invention, they should also be regarded as the contents disclosed by the present invention.

Claims

1. A MZM nonlinear compensation method based on subcarrier modulation, characterized in that: The specific steps include: Step S1: Generate a PAM4 signal from a pseudo-random bit sequence; Step S2: simulate MZM nonlinear damage, add subcarrier modulation, and compensate for nonlinear damage; Step S3: performing digital signal processing on the compensated signal to balance the inter-symbol crosstalk; Step S1 is specifically as follows: First, a pseudo-random bit sequence generator is used in Optisystem to generate a random binary digital signal, and then a PAM sequence generator is used to map the random binary digital signal into a PAM4 signal format, where each symbol of the signal can provide 2 bits of information. The process of step S2 is as follows: (1) The mapped PAM4 signal enters the multi-level pulse generator for pulse shaping to generate a digital signal; (2) The digital signal enters the digital-to-analog converter and is converted into an analog signal; (3) The PAM4 signal and the sine wave after digital-to-analog conversion enter the optical modulator (MZM) at the same time, and the laser generates a continuous optical signal to drive the optical modulator; the sine wave generated by the sine wave generator suppresses the nonlinear interference of the optical signal generated by the MZM during the modulation process; wherein, the laser generates an amplitude of E0, an angular frequency of ω0, a time of t, and an initial phase of Continuous optical signal j is an imaginary unit; the subcarrier has an amplitude of A, an angular frequency of ω1, and an initial phase of The sine wave signal After MZM modulation, the output optical signal is Where IL is the insertion loss; γ is the power splitting ratio of the two Y-branch waveguides of the MZM (assuming symmetry), that is, ExtRatio is the extinction ratio; v1(t) and v2(t) are the input voltages of the upper and lower waveguides of the modulator, respectively; v bias1 and v bias2 are the first bias voltage and the second bias voltage respectively; V πRF is the switching modulation voltage of MZM; V πDC is the switching bias voltage of MZM; (4) The optical signal output by the photoelectric conversion after adding the subcarrier is converted into an electrical signal by a photodetector (PD); the current generated by the photodetector changes with the incident light power, and the output current of the photodetector at the receiving end is positively correlated with the square of the received light field intensity, that is, I out ∝|E out (t)| 2 ;I out is the output current; (5) After the converted electrical signal containing the subcarrier signal is filtered out with a low-pass Gaussian filter to remove the high-frequency component, the PAM4 signal after nonlinear compensation is obtained. The filter transfer function is: Where α is the insertion loss; f c is the filter cutoff frequency; f is the frequency; N is the filter order; (6) After filtering out the high-frequency component and nonlinear compensation, the PAM4 signal enters the analog-to-digital converter and is converted into a digital signal.

2. The MZM nonlinear compensation method based on subcarrier modulation according to claim 1, characterized in that: Step S3 is as follows: The PAM4 signal after nonlinear compensation is subjected to a series of digital signal processing through Matlab, which mainly includes downsampling, digital equalization, and PAM4 symbol demapping.

3. A MZM nonlinear compensation system based on subcarrier modulation, characterized in that: include: A digital modulation module is used to map a random bit sequence into a PAM4 signal and transmit it to a pulse shaping module; The pulse shaping module is used to shape the PAM4 signal pulse to generate a digital signal and transmit it to the digital-to-analog conversion module. A digital-to-analog conversion module is used to convert digital signals into analog signals and transmit them to the optical control module; An optical modulation module is used to convert analog signals into optical signals and transmit them to the photoelectric detection module; A photoelectric detection module is used to convert the optical signal into an electrical signal and transmit it to the filter module; The filter module is used to filter out the high-frequency components of the electrical signal and transmit it to the analog-to-digital conversion module; The analog-to-digital conversion module is used to convert the electrical signal with high-frequency components filtered out into a digital signal and transmit it to the digital signal processing module; Digital signal processing module, used for downsampling, digital equalization, and PAM4 symbol demapping of digital signals.

4. The MZM nonlinear compensation system based on subcarrier modulation as claimed in claim 3, characterized in that: The MZM nonlinear compensation system further includes: The subcarrier module is used to generate a subcarrier for suppressing MZM nonlinear damage and is carried on the upper end of the main carrier; The laser module is used to generate continuous light signals and drive the optical modulation module.

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