Single-polarization self-coherent optical communication system and polarization control method thereof

Abstract

The invention discloses a single-polarization self-coherent optical communication system and a polarization control method thereof, and relates to the field of optical fiber communication. The single-polarization self-coherent optical communication system mainly comprises a laser, a first polarization controller, an IQ modulator, a digital-to-analog converter, a second polarization controller, a polarization beam combiner, a lithium niobate polarization controller, a polarization beam splitter, a polarization-maintaining coupler, a coherent receiver, a variable optical attenuator, an avalanche photodiode and an analog-to-digital converter. By implementing the single-polarization self-coherent optical communication system and the polarization control method thereof provided by the invention, the complexity of a DSP (Digital Signal Processor) algorithm can be reduced, the power consumption and the cost of the system are reduced, and the stability of the system is improved.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber communication, and more specifically, to a single-polarization self-coherent optical communication system and a polarization control method thereof. Background Art

[0002] The fiber-optic communication system, which uses optical fiber as the signal transmission medium, has many advantages, such as large transmission capacity, long transmission distance, strong anti-interference, high integration, and low unit transmission symbol cost. Therefore, as the carrier of mainstream network traffic, the fiber-optic communication system carries the communication services that connect the world. In recent years, the rapid development and gradual popularization of large data volume services such as cloud computing, virtual reality, metaverse, and real-time high-definition video have driven the increase in people's demand for large-capacity communication networks and high-performance computing capabilities, and also brought huge pressure to the expansion of fiber-optic communication systems. The explosive growth of IP traffic demand requires the existing fiber-optic communication system to expand and upgrade to cope with such a huge challenge.

[0003] For fiber-optic communication networks with different transmission distances, capacities, signal quality, and power consumption cost requirements, the corresponding communication architectures and technologies have different characteristics. For short-distance fiber-optic communication systems between and within data centers, due to their dense distribution, high transmission capacity requirements, and short transmission distances, people are more concerned about the power consumption and cost of both the sending and receiving ends, such as lasers, high-speed analog-to-digital converters, and related digital signal processing (DSP) modules. The cost of fiber resources in short-distance optical interconnection scenarios is relatively low, so in short-distance application scenarios, how to reduce the power consumption and cost required for each bit of transmission while maximizing the transmission capacity has become a more urgent problem to be solved.

[0004] By using the received signal itself as a reference light source, autocoherence technology eliminates the need for an external local oscillator light source, significantly simplifying the system architecture, reducing hardware complexity and power consumption, and is particularly suitable for cost-sensitive scenarios. Its advantages include low requirements for laser line width, wide dynamic range, fewer polarization-related issues, and easy miniaturization and integration, enabling it to perform well in medium and short-distance communications, access networks, and optical sensing. At the same time, autocoherence technology can eliminate phase noise through digital signal processing or optical domain compensation, has good adaptability and scalability, and is suitable for high-speed and high-density optical communication applications.

[0005] However, the existing coherent optical network architecture requires expensive narrow-linewidth lasers to emit light with similar wavelengths, and also requires complex DSPs to process the phase noise of the received signals. The DSP phase compensation part of the coherent receiver uses complex digital signal processing technology to correct the phase of the optical signal. The system design is complex and relies on high-power DSP modules. The system cost is still high, and the signal quality and system stability need to be further improved. Summary of the invention

[0006] The object of the present invention is to provide a single-polarization self-coherent optical communication system and a polarization control method thereof, which can reduce the complexity of DSP algorithm, reduce system power consumption and cost, and improve system stability.

[0007] The present invention provides a single-polarization self-coherent optical communication system, comprising a transmitting end and a receiving end, wherein the transmitting end is used to obtain a communication optical signal according to an electrical signal; the receiving end is used to obtain an electrical signal according to the communication optical signal; the transmitting end comprises a laser, a first polarization controller, an IQ modulator, a digital-to-analog converter, a second polarization controller, and a polarization combiner; the receiving end comprises a lithium niobate polarization controller, a polarization beam splitter, a polarization-maintaining coupler, a coherent receiver, a variable optical attenuator, an avalanche photodiode, and an analog-to-digital converter.

[0008] Furthermore, the output end of the above-mentioned laser is connected to the input end of the first polarization controller, the output end of the laser is also connected to the optical input end of the IQ modulator, the output end of the digital-to-analog converter is connected to the electrical input end of the IQ modulator, the optical output end of the IQ modulator is connected to the input end of the second polarization controller, and the output end of the first polarization controller and the output end of the second polarization controller are connected to the input end of the polarization combiner.

[0009] Furthermore, the above-mentioned laser is used to generate local oscillator light and modulated light; the first polarization controller is used to obtain a first polarization state local oscillator light according to the local oscillator light; the digital-to-analog converter is used to obtain an analog electrical signal according to the digital electrical signal; the IQ modulator is used to obtain an IQ modulated light according to the modulated light and the analog electrical signal; the second polarization controller is used to obtain a second polarization state modulated light according to the IQ modulated light; and the polarization combiner is used to obtain a polarization combined light according to the first polarization state local oscillator light and the second polarization state modulated light.

[0010] Furthermore, the optical output end of the above-mentioned lithium niobate polarization controller is connected to the input end of the polarization beam splitter, the first polarization state output end of the polarization beam splitter is connected to the first input end of the coherent receiver, the second polarization state output end of the polarization beam splitter is connected to the input end of the polarization maintaining coupler, the main output end of the polarization maintaining coupler is connected to the second input end of the coherent receiver, and the output end of the coherent receiver is connected to the input end of the analog-to-digital converter; the auxiliary output end of the polarization maintaining coupler is connected in series with the variable optical attenuator and the avalanche photodiode.

[0011] Furthermore, the above-mentioned lithium niobate polarization controller is used to adjust the polarization state of the polarization combined light; the polarization beam splitter is used to separate the first polarization state local oscillator light and the second polarization state modulated light in the polarization combined light; the polarization maintaining coupler is used to couple part of the second polarization state modulated light to obtain a coupled light signal; the variable optical attenuator is used to adjust the intensity of the coupled light signal; the avalanche photodiode is used to convert the coupled light signal into a coupled electrical signal; the coherent receiver is used to obtain a received electrical signal based on the first polarization state local oscillator light and the second polarization state modulated light; the analog-to-digital converter is used to obtain a digital IQ electrical signal based on the received electrical signal.

[0012] The present invention provides a polarization control method applied to the above-mentioned single-polarization self-coherent optical communication system, comprising the following steps: S1: using an IQ modulator to add a pilot signal to IQ modulated light, and using an avalanche photodiode to obtain a coupled electrical signal; S2: according to the coupled electrical signal, using a fast Fourier transform method to obtain each frequency component; S3: according to each frequency component, obtaining a pilot signal component; S4: according to the pilot signal component, using a gradient descent method to obtain a first control voltage output value, a second control voltage output value, a third control voltage output value and a fourth control voltage output value; S5: using the first control voltage, the second control voltage, the third control voltage and the fourth control voltage to control the lithium niobate polarization controller of the single-polarization self-coherent optical communication system.

[0013] Further, step S4 specifically includes: S41: setting the initial values ​​of the first control voltage, the second control voltage, the third control voltage and the fourth control voltage to 0; S42: increasing the first control voltage from 0 with a preset step size, superimposing and iterating the pilot signal component 80 times, and obtaining the objective function value; when the current value of the first control voltage is within the preset working range and the objective function value becomes smaller, the first control voltage is decreased by twice the preset step size, and when the current value of the first control voltage is within the preset working range and the objective function value becomes larger or unchanged, the first control voltage output value is obtained; when the current value of the first control voltage is not within the preset working range, the second control voltage output value is obtained according to the current value of the first control voltage and the current value of the second control voltage. And the first control voltage is incremented and updated again from 0 with a preset step size; S43: the third control voltage is incremented from 0 with a preset step size, the pilot signal component is superimposed and iterated 80 times, and the objective function value is obtained; when the current value of the third control voltage is within the preset working range and the objective function value becomes smaller, the third control voltage is decreased by twice the preset step size, and when the current value of the third control voltage is within the preset working range and the objective function value becomes larger or remains unchanged, the third control voltage output value is obtained; when the current value of the third control voltage is not within the preset working range, the fourth control voltage output value is obtained according to the current value of the third control voltage and the current value of the fourth control voltage, and the third control voltage is incremented and updated again from 0 with a preset step size.

[0014] Furthermore, the pilot signal components are superimposed and iterated 80 times to obtain the objective function value, as shown in the formula: , in, is the objective function value, for The first control voltage at the moment, for The second control voltage at time, for The third control voltage at time, for A third control voltage at time t; Indicates Overlay, Represents the pilot signal component.

[0015] Furthermore, the second control voltage output value is obtained according to the first control voltage current value and the second control voltage current value, and the first control voltage is incremented and updated again from 0 with a preset step size, as shown in the formula: , , , in, for The second control voltage at time, for The first control voltage at the moment, is the preset step size.

[0016] Furthermore, the preset step size is 10mV, and the preset working range is greater than -4V and less than 4V.

[0017] The implementation of the single-polarization self-coherent optical communication system and polarization control method provided by the present invention has the following beneficial effects: The present invention uses the same laser to emit signal light and local oscillator light, and only needs to use ordinary lasers instead of expensive narrow-linewidth lasers; the signal light and the local oscillator light are modulated to the same wavelength by polarization multiplexing; when the signal light is modulated in I and Q paths, a 5MHz sine wave pilot is added to one of the paths to distinguish the polarization state of the signal and improve the anti-disturbance performance; when receiving, the polarization state of the polarization multiplexed light is adjusted based on the lithium niobate polarization controller; in order to change the polarization state of the optical signal to the X polarization state and the polarization state of the local oscillator light to the Y polarization state, the present invention adopts a gradient descent algorithm, which can quickly adjust the polarization state of the polarization multiplexed optical signal; the lithium niobate is controlled by identifying the pilot size to adjust the polarization state of the lithium niobate. The voltage of the controller is used to control the polarization state of the signal light, so that the local oscillator light and the signal light can be distinguished; the noise caused by the polarization-related effect is effectively suppressed, and the signal quality and system stability are further improved; the self-coherent receiving mode is used at the receiving end to realize the demodulation and signal recovery of the optical signal through the beat frequency of the local oscillator light and the signal light, and the influence of the phase noise of the coherent receiving end is eliminated; the polarization control method of the present invention can replace the DSP phase compensation part of the coherent receiving end, and the same effect is achieved by controlling the polarization state of the light. The step of eliminating phase noise is omitted during DSP processing, the complexity of the DSP algorithm is reduced, the system design is simplified, the dependence on the high-power DSP module is reduced, the power consumption is reduced, and the system cost is reduced at the same time. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The present invention will be further described below with reference to the accompanying drawings and embodiments, in which: Figure 1 It is a schematic diagram of the composition of the single-polarization self-coherent optical communication system provided by the present invention; Figure 2 is a flow chart of the polarization control method provided by the present invention; Figure 3 is a flow chart of the polarization demodulation algorithm provided by the present invention; Figure 4 It is a time domain diagram of two electrical signals after signal demodulation provided by the present invention. DETAILED DESCRIPTION

[0019] In order to have a clearer understanding of the technical features, purposes and effects of the present invention, specific embodiments of the present invention are now described in detail with reference to the accompanying drawings.

[0020] Figure 1 The schematic diagram of the composition of the single polarization self-coherent optical communication system of this embodiment is shown. In this embodiment, the single polarization self-coherent optical communication system includes a transmitting end and a receiving end, the transmitting end is used to obtain a communication optical signal according to an electrical signal; the receiving end is used to obtain an electrical signal according to the communication optical signal; In an exemplary embodiment, the transmitting end includes a laser, a first polarization controller, an IQ modulator, a digital-to-analog converter, a second polarization controller, and a polarization combiner; In an exemplary embodiment, the laser is used to generate local oscillator light and modulated light; the first polarization controller is used to obtain a first polarization state local oscillator light according to the local oscillator light; the digital-to-analog converter is used to obtain an analog electrical signal according to the digital electrical signal; the IQ modulator is used to obtain an IQ modulated light according to the modulated light and the analog electrical signal; the second polarization controller is used to obtain a second polarization state modulated light according to the IQ modulated light; the polarization combiner is used to obtain a polarization combined light according to the first polarization state local oscillator light and the second polarization state modulated light; In an exemplary embodiment, the receiving end includes a lithium niobate polarization controller, a polarization beam splitter, a polarization-maintaining coupler, a coherent receiver, a variable optical attenuator, an avalanche photodiode, and an analog-to-digital converter; In an exemplary embodiment, the lithium niobate polarization controller is used to adjust the polarization state of the polarization combined beam; the polarization beam splitter is used to separate the first polarization state local oscillator light and the second polarization state modulated light in the polarization combined beam; the polarization maintaining coupler is used to couple part of the second polarization state modulated light to obtain a coupled optical signal; the variable optical attenuator is used to adjust the intensity of the coupled optical signal; the avalanche photodiode is used to convert the coupled optical signal into a coupled electrical signal; the coherent receiver is used to obtain a received electrical signal according to the first polarization state local oscillator light and the second polarization state modulated light; the analog-to-digital converter is used to obtain a digital IQ electrical signal according to the received electrical signal; In an exemplary embodiment, the output end of the laser is connected to the input end of the first polarization controller, the output end of the laser is also connected to the optical input end of the IQ modulator, the output end of the digital-to-analog converter is connected to the electrical input end of the IQ modulator, the optical output end of the IQ modulator is connected to the input end of the second polarization controller, and the output end of the first polarization controller and the output end of the second polarization controller are connected to the input end of the polarization combiner; In an exemplary embodiment, the optical output end of the lithium niobate polarization controller is connected to the input end of the polarization beam splitter, the first polarization state output end of the polarization beam splitter is connected to the first input end of the coherent receiver, the second polarization state output end of the polarization beam splitter is connected to the input end of the polarization maintaining coupler, the main output end of the polarization maintaining coupler is connected to the second input end of the coherent receiver, and the output end of the coherent receiver is connected to the input end of the analog-to-digital converter; the secondary output end of the polarization maintaining coupler is connected in series with the variable optical attenuator and the avalanche photodiode; In an exemplary embodiment, the coupling ratio of the polarization-maintaining coupler is 10:90.

[0021] This embodiment provides a polarization control method for the above-mentioned single-polarization self-coherent optical communication system, such as Figure 2The figure shows a flow chart of a polarization control method, which includes the following steps: S1: Use an IQ modulator to add a pilot signal to the IQ modulated light, and use an avalanche photodiode to obtain a coupled electrical signal; S2: According to the coupled electrical signal, each frequency component is obtained by using the fast Fourier transform method; S3: Obtain pilot signal components according to each frequency component; S4: obtaining a first control voltage output value, a second control voltage output value, a third control voltage output value and a fourth control voltage output value by using a gradient descent method according to the pilot signal component; In an exemplary embodiment, step S4 specifically includes: S41: setting the initial values ​​of the first control voltage, the second control voltage, the third control voltage and the fourth control voltage to 0; S42: increasing the first control voltage from 0 with a preset step size, superimposing and iterating the pilot signal component 80 times, and obtaining the objective function value; when the current value of the first control voltage is within the preset working range and the objective function value becomes smaller, decreasing the first control voltage by twice the preset step size, and when the current value of the first control voltage is within the preset working range and the objective function value becomes larger or unchanged, obtaining the first control voltage output value; when the current value of the first control voltage is not within the preset working range, obtaining the second control voltage output value according to the current value of the first control voltage and the current value of the second control voltage, and re-increasing and updating the first control voltage from 0 with a preset step size; In an exemplary embodiment, the pilot signal component is superimposed and iterated 80 times to obtain the objective function value, such as the formula: , in, is the objective function value, for The first control voltage at the moment, for The second control voltage at the moment, for The third control voltage at time, for A third control voltage at time t; Indicates Overlay, represents the pilot signal component; In an exemplary embodiment, the second control voltage output value is obtained according to the first control voltage current value and the second control voltage current value, and the first control voltage is incremented and updated again starting from 0 with a preset step size, such as the formula: , , , in, for The second control voltage at time, for The first control voltage at the moment, is the preset step length; S43: increasing the third control voltage from 0 with a preset step size, superimposing and iterating the pilot signal component 80 times, and obtaining the objective function value; when the current value of the third control voltage is within the preset working range and the objective function value becomes smaller, decreasing the third control voltage by twice the preset step size, and when the current value of the third control voltage is within the preset working range and the objective function value becomes larger or unchanged, obtaining the third control voltage output value; when the current value of the third control voltage is not within the preset working range, obtaining the fourth control voltage output value according to the current value of the third control voltage and the current value of the fourth control voltage, and re-increasing and updating the third control voltage from 0 with a preset step size; In an exemplary embodiment, the preset step size is 10 mV, and the preset working range is greater than -4 V and less than 4 V; S5: Control the lithium niobate polarization controller of the single-polarization self-coherent optical communication system using the first control voltage, the second control voltage, the third control voltage and the fourth control voltage.

[0022] This embodiment provides a single-polarization self-coherent optical communication system, including an LD (Laser Diode), a PC (Polarization Controller), an IQ Modulator, a DAC (Digital-to-Analog Converter), a PBC (Polarization Beam Combiner), a lithium niobate polarization controller, a PBS (Polarization Beam Splitter), a 10:90 polarization-maintaining coupler, a coherent receiver, a VOA (Variable Optical Attenuator), an APD (Avalanche Photodiode), an FPGA (Field-Programmable Gate Array), an ADC (Analog -to Digital - Converter), and an optical fiber.

[0023] This embodiment provides an experiment of a polarization control method for a single-polarization self-coherent optical communication system, comprising the following steps: (1) First, the laser (LD) is used as a light source to generate two beams of light, one as the local oscillator light and the other as the signal modulation light. The DAC is used to modulate the IQ modulator to modulate the electrical signal onto the modulated light so that both the I and Q paths of the modulated light have signals, and a pilot signal is added to one of them. The pilot signal uses a 5MHz sine wave. The modulated light and the local oscillator light are then aligned with the X and Y polarizations of the PBC using the PC control input. The output of the PBC is connected to the PC to disturb the polarization state of the light. After passing through the lithium niobate polarization controller, the polarization state is separated using the PBS. One of the paths is passed through a 10:90 polarization-maintaining coupler, and the 10% path is converted into an electrical signal through an APD and connected to the FPGA's ADC. The sampling rate of the ADC is 125MHz. After processing the signal received by the ADC, the FPGA feedback adjusts the lithium niobate polarization controller. The other end of the PBS and the 90% end of the polarization-maintaining coupler are connected to a coherent receiver. The electrical signal output of the coherent receiver is then observed using a high-speed ADC. The experimental system structure diagram is shown in the figure below. Figure 1 As shown; (2) In the optical fiber link, one end of the PBS output is directly connected to the coherent receiver as the local oscillator light input of the coherent receiver; the other end of the PBS output is connected to the 10:90 polarization-maintaining coupler, and the 90% path is directly connected to the coherent receiver as the modulated light input of the coherent receiver; the output electrical signal of the coherent receiver is connected to the high-speed ADC, and the high-speed ADC is used to observe whether the signal is completely demodulated; since the phase damage in this system can be ignored, the original I and Q path electrical signals can be observed at the output end of the high-speed ADC; (3) 10% of the 10:90 polarization-maintaining coupler is converted into an electrical signal through the APD and connected to the ADC of the FPGA. After the FPGA processes the signal received by the ADC, it feedbacks and adjusts the lithium niobate polarization controller to adjust the polarization state of the optical signal.

[0024] The present embodiment provides a polarization control method for the above-mentioned single-polarization self-coherent optical communication system, in which the collected voltage signal is converted from the APD received light intensity into an electrical signal, and the collected signal is analyzed by an algorithm. The basic working principle of the algorithm is as follows: the ADC module collects the received voltage and processes it in the FFT program module. When the module receives the flag bit after the previous voltage is output through the DAC, it starts to run, performs a 1024-point FFT operation on the signal collected by the ADC, and then takes the absolute value of the 5MHz frequency component and stores it in the register. The absolute value of the 5MHz frequency component of the next FFT operation is taken and added to it, and a total of 80 times are superimposed. After the absolute values ​​of the 5MHz frequency components of the 80 FFT operations are taken and added, a flag signal is sent to indicate that the collection is completed and output to the SGD program module, and the register storing the sum of the 5MHz frequency components is cleared; after receiving the flag signal, the SGD module first changes ,make Increase 10mV, and store the absolute value of the 5MHz frequency component output by the FFT program module 80 times, and then Through the DAC output, at the same time pull up the flag to the FFT program module, wait for the FFT program module output flag to be pulled high again, and compare the absolute value superposition of the 5MHz frequency component collected twice 80 times. If the second collection is larger or unchanged, no processing is performed. If the second collection is smaller, then Reduce by 20mV; process next time , the processing flow is the same as Va1; When it is greater than or equal to 4V or less than or equal to -4V, it is reset to 0V. When reset by 4V, From -4V to 0V or from 0V to 4V, if If it is 4V, it will remain unchanged; similarly, and The same transformation is also carried out; the algorithm flow is as follows Figure 3 shown.

[0025] In an exemplary embodiment, in the above polarization control method, the ADC module collects signals, and the FFT module performs a 1024-point FFT operation on the collected signals to obtain a series of complex values, each of which represents a frequency component. This embodiment only takes real values. For a 5MHz frequency component, assuming that the FFT output is ,in is the corresponding frequency index, is the corresponding frequency value, This is the first FFT; the absolute value of the 5MHz frequency component in the first iteration can be calculated by Get, where is the FFT index corresponding to 5MHz, and: , After each FFT operation, the absolute value of the 5MHz frequency component is taken and stored, and then superimposed 80 times; the signal is controlled by the voltage The calculation formula for each iteration is: , Next, judge Is it within the range (i.e. ),in is the step size, set to 10mV. If it is not satisfied, update : , , Next update : , Then collect another set of FFT operations 80 times and average them to get: , Compare the updated and before update : , If the condition is met, then Keep the current value; if not, reduce : , Then and Perform something similar to and The algorithm flow is as follows: Figure 3 shown.

[0026] It should be noted that , hereinafter referred to as , whose value is controlled by 4 voltages. In this embodiment, the 5MHz component is averaged by 80 FFTs to obtain , then change Increase 10mV to , and then collect the signal and perform 80 FFTs to average the 5MHz component to get the new , comparing before and after the change If the new Greater or no change Remain unchanged, if Smaller or Decrease by 20mV.

[0027] In an exemplary embodiment, the polarization control method comprises the following steps: Step 1: Turn on the laser LD to output local oscillator light and modulated light into the optical fiber; Step 2: Turn on the AWG to modulate the 30Gbaud 16QAM signal to the IQ modulator, and the IQ modulator then modulates the electrical signal into an optical signal; Step 3: Adjust the PC after the IQ modulator and the LD local oscillator light output, so that the polarization states of the two beams are aligned with the polarization state of the PBC input; Step 4: FPGA collects the voltage signal output by APD in real time, processes it through the algorithm, outputs it through the DAC output voltage, and then outputs it to the lithium niobate polarization controller after passing through the voltage amplifier; Step 5: bend the polarization state of the PC perturbation light after PBC, and observe whether the optical signal of the oscilloscope is affected, and whether it recovers quickly after being affected; Figure 4 This is a performance diagram of this solution, which shows the demodulation result of a single-polarization 16QAM signal in a 30G baud rate scenario. As can be seen from the figure, when the polarization controllers are in the optimal state, the receiving end can obtain two independent signals, and the impact of polarization rotation is very small.

[0028] The embodiments of the present invention are described above in conjunction with the accompanying drawings, but the present invention is not limited to the above-mentioned specific implementation modes, which are merely illustrative rather than restrictive. Under the guidance of the present invention, ordinary technicians in this field can also make many forms without departing from the scope of protection of the present invention and the claims, all of which are within the protection of the present invention.

Claims

1. A single polarization self-coherent optical communication system, characterized in that: It includes a transmitting end and a receiving end, the transmitting end is used to obtain a communication optical signal according to an electrical signal; the receiving end is used to obtain an electrical signal according to the communication optical signal; the transmitting end includes a laser, a first polarization controller, an IQ modulator, a digital-to-analog converter, a second polarization controller, and a polarization combiner; the receiving end includes a lithium niobate polarization controller, a polarization beam splitter, a polarization-maintaining coupler, a coherent receiver, a variable optical attenuator, an avalanche photodiode, and an analog-to-digital converter.

2. The single-polarization self-coherent optical communication system according to claim 1, characterized in that: The output end of the laser is connected to the input end of the first polarization controller, the output end of the laser is also connected to the optical input end of the IQ modulator, the output end of the digital-to-analog converter is connected to the electrical input end of the IQ modulator, the optical output end of the IQ modulator is connected to the input end of the second polarization controller, and the output end of the first polarization controller and the output end of the second polarization controller are connected to the input end of the polarization combiner.

3. The single-polarization self-coherent optical communication system according to claim 1, characterized in that: The laser is used to generate local oscillator light and modulated light; the first polarization controller is used to obtain a first polarization state local oscillator light according to the local oscillator light; the digital-to-analog converter is used to obtain an analog electrical signal according to a digital electrical signal; the IQ modulator is used to obtain an IQ modulated light according to the modulated light and the analog electrical signal; the second polarization controller is used to obtain a second polarization state modulated light according to the IQ modulated light; the polarization combiner is used to obtain a polarization combined light according to the first polarization state local oscillator light and the second polarization state modulated light.

4. The single-polarization self-coherent optical communication system according to claim 1, characterized in that: The optical output end of the lithium niobate polarization controller is connected to the input end of the polarization beam splitter, the first polarization state output end of the polarization beam splitter is connected to the first input end of the coherent receiver, the second polarization state output end of the polarization beam splitter is connected to the input end of the polarization maintaining coupler, the main output end of the polarization maintaining coupler is connected to the second input end of the coherent receiver, and the output end of the coherent receiver is connected to the input end of the analog-to-digital converter; the secondary output end of the polarization maintaining coupler is connected in series with a variable optical attenuator and an avalanche photodiode.

5. The single-polarization self-coherent optical communication system according to claim 1, characterized in that: The lithium niobate polarization controller is used to adjust the polarization state of the polarization combined light; the polarization beam splitter is used to separate the first polarization state local oscillator light and the second polarization state modulated light in the polarization combined light; the polarization-maintaining coupler is used to couple part of the second polarization state modulated light to obtain a coupled light signal; the variable optical attenuator is used to adjust the intensity of the coupled light signal; the avalanche photodiode is used to convert the coupled light signal into a coupled electrical signal; the coherent receiver is used to obtain a received electrical signal based on the first polarization state local oscillator light and the second polarization state modulated light; the analog-to-digital converter is used to obtain a digital IQ electrical signal based on the received electrical signal.

6. A polarization control method applied to a single-polarization self-coherent optical communication system as claimed in any one of claims 1 to 5, characterized in that: The following steps are involved: S1: Use an IQ modulator to add a pilot signal to the IQ modulated light, and use an avalanche photodiode to obtain a coupled electrical signal; S2: according to the coupled electrical signal, obtaining each frequency component by using a fast Fourier transform method; S3: Obtaining a pilot signal component according to each frequency component; S4: obtaining a first control voltage output value, a second control voltage output value, a third control voltage output value and a fourth control voltage output value by using a gradient descent method according to the pilot signal component; S5: Control the lithium niobate polarization controller of the single-polarization self-coherent optical communication system using the first control voltage, the second control voltage, the third control voltage and the fourth control voltage.

7. The polarization control method according to claim 6, characterized in that: Step S4 specifically includes: S41: setting the initial values ​​of the first control voltage, the second control voltage, the third control voltage and the fourth control voltage to 0; S42: increasing the first control voltage from 0 with a preset step size, superimposing and iterating the pilot signal component 80 times, and obtaining the objective function value; when the current value of the first control voltage is within the preset working range and the objective function value becomes smaller, decreasing the first control voltage by twice the preset step size, and when the current value of the first control voltage is within the preset working range and the objective function value becomes larger or unchanged, obtaining the first control voltage output value; when the current value of the first control voltage is not within the preset working range, obtaining the second control voltage output value according to the current value of the first control voltage and the current value of the second control voltage, and re-increasing and updating the first control voltage from 0 with a preset step size; S43: Increment the third control voltage from 0 with a preset step size, superimpose and iterate the pilot signal component 80 times to obtain the objective function value; when the current value of the third control voltage is within the preset working range and the objective function value becomes smaller, decrease the third control voltage by twice the preset step size, and when the current value of the third control voltage is within the preset working range and the objective function value becomes larger or remains unchanged, obtain the third control voltage output value; when the current value of the third control voltage is not within the preset working range, obtain the fourth control voltage output value according to the current value of the third control voltage and the current value of the fourth control voltage, and increase and update the third control voltage again from 0 with a preset step size.

8. The polarization control method according to claim 7, characterized in that: The pilot signal component is superimposed and iterated 80 times to obtain the objective function value, as shown in the formula: ， in, is the objective function value, for The first control voltage at the moment, for The second control voltage at the moment, for The third control voltage at time, for A third control voltage at time t; Indicates Overlay, Represents the pilot signal component.

9. The polarization control method according to claim 7, characterized in that: The second control voltage output value is obtained according to the first control voltage current value and the second control voltage current value, and the first control voltage is incremented and updated again from 0 with a preset step size, as shown in the formula: ， ， ， in, for The second control voltage at the moment, for The first control voltage at the moment, is the preset step size.

10. The polarization control method according to claim 7, characterized in that: The preset step size is 10 mV, and the preset working range is greater than -4V and less than 4V.

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