A spatial-mode hybrid diversity receiving system for space laser communication

Abstract

The application discloses a spatial-mode hybrid diversity receiving system for spatial laser communication. After a transmitting light beam passes through atmospheric turbulence, diffusion and distortion and other phenomena occur, spatial diversity and mode diversity are combined at a receiving end, an array of M receiving antennas is used to receive the diffused light beam to perform spatial diversity, a few-mode fiber supporting N modes is connected after each receiving antenna, high-order modes are converted into base modes by a mode demultiplexer and then output to a single-mode fiber to perform mode diversity, then a total of M*N branch signals are converted into electric signals by photoelectric detection modules, and finally, the branch signals are combined after digital signal processing. The application combines spatial diversity and mode diversity, can effectively compensate the influence of atmospheric turbulence on laser transmission, and is beneficial to spatial laser communication under more severe channel conditions.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of space laser communication, and particularly relates to a space-mode hybrid diversity receiving system for space laser communication. BACKGROUND

[0002] Free space optical communication refers to a new type of communication mode in which laser is used as a carrier and transmission media such as optical fibers are not needed for direct transmission in the atmosphere or vacuum. Compared with traditional radio frequency communication, free space optical communication has the advantages of high capacity, high directivity, high security, rich spectrum resources and the like, and has a broad application prospect in military and civilian fields.

[0003] In free space optical communication, optical signals are directly transmitted in the atmosphere, and therefore one of the biggest obstacles is the interference of the atmospheric channel. Atmospheric turbulence can cause phenomena such as light intensity flicker, wavefront distortion and beam drift of the light beam, thereby causing a serious decline in communication quality. Therefore, how to reduce the influence of atmospheric turbulence is an important work in free space optical communication.

[0004] At present, there are various means to combat the effects of atmospheric turbulence, and diversity receiving technology is widely used as a simple and effective method. The basic idea is to increase the system redundancy by transmitting or receiving multiple signals carrying the same information and experiencing independent fading. Common types of diversity technology include time diversity, wavelength diversity, space diversity and mode diversity. Time diversity limits the communication rate of the system, wavelength diversity requires a large wavelength interval to ensure the mutual independence of the signals, thereby having high requirements for the working wavelength of the laser, detector and amplifier. The space diversity receiving system has a simple structure, and can effectively alleviate the effects of atmospheric turbulence by using multi-aperture reception. Mode diversity regards each mode as an independent channel by receiving optical signals through a few-mode fiber, thereby effectively compensating for the effects of atmospheric turbulence.

[0005] Existing diversity receiving schemes are limited to a single diversity technology, and do not consider the compatibility of various diversity technologies. The compensation effect of atmospheric turbulence has further room for improvement. SUMMARY

[0006] The main purpose of the present application is to provide a hybrid diversity receiving system combining space diversity and mode diversity to effectively alleviate the effects of atmospheric turbulence.

[0007] The technical scheme adopted by the present application is as follows:

[0008] The space-mode hybrid diversity receiving system comprises a space diversity module, a mode diversity module, a photoelectric detection module and a digital signal processing module, the space diversity module is connected with the mode diversity module, the mode diversity module is connected with the photoelectric detection module, and the photoelectric detection module is connected with the digital signal processing module.

[0009] The space diversity module is composed of M optical antennas arranged in a matrix array form.

[0010] In the space diversity module, the distance between two adjacent optical antennas is greater than the atmospheric coherence length, so as to ensure the independence of the branch signals.

[0011] The mode diversity module comprises M N-mode few-mode optical fibers, mode demultiplexers and single-mode optical fibers, the input end of each N-mode few-mode optical fiber is connected with the output end of the corresponding optical antenna in the space diversity module, the output end of each N-mode few-mode optical fiber is connected with the input end of the corresponding mode demultiplexer, the mode demultiplexer converts high-order modes into base modes, the output end of each mode demultiplexer is connected with the input end of the corresponding N single-mode optical fibers, and the output ends of the N single-mode optical fibers are respectively connected with the corresponding photoelectric detection modules.

[0012] The greater the number M of optical antennas in the space diversity module or the mode N of few-mode optical fibers in the mode diversity module, the greater the diversity gain.

[0013] The structure of the photoelectric detection module is set according to the laser wavelength and the signal modulation format of the transmitting end.

[0014] The signal processing method of the digital signal processing module is set according to the signal modulation format of the transmitting end.

[0015] In the digital signal processing module, different algorithms are selected according to the signal modulation mode of the transmitting end and the actual situation, specifically, the combining algorithm adopts the maximum ratio combining in the linear combining algorithm or a nonlinear combining algorithm.

[0016] The present application has the following advantages:

[0017] The present application solves the problem of atmospheric turbulence interference in point-to-point space laser communication and is suitable for various modulation formats of space laser communication, has no requirement on the structure of the transmitting end and can determine the diversity number according to the requirement.

[0018] The application combines spatial diversity and mode diversity, can effectively compensate the influence of atmospheric turbulence on laser transmission, and the compensation effect is obviously superior to single diversity receiving mode, and can ensure spatial laser communication under more severe channel conditions. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 is a schematic diagram of a spatial-mode hybrid diversity receiving system for spatial laser communication.

[0020] Figure 2 is a schematic diagram of a hybrid diversity spatial laser communication system device structure with a 2x3 structure in the embodiment.

[0021] Figure 3 is a flow of digital signal processing in the embodiment.

[0022] Figure 4 is a bit error rate curve of hybrid diversity receiving in the embodiment of the application.

[0023] In the figure: laser 1, signal generator 2, electro-optical modulator 3, transmitting antenna 4, eight times beam expander 5, phase plate 6, receiving antenna array 7, mode demultiplexer 8, photoelectric detection module 9, pre erbium-doped optical fiber amplifier 91, optical band-pass filter 92, local oscillator laser 93, photoelectric detector 94, digital signal processing module 10. DETAILED DESCRIPTION

[0024] The application will be further described in detail below in combination with the drawings and embodiments.

[0025] A spatial-mode hybrid diversity receiving system for spatial laser communication is shown in Figure 1 The application includes a spatial diversity module, a mode diversity module, a photoelectric detection module 9 and a digital signal processing module 10, the spatial diversity module is connected with the mode diversity module, the mode diversity module is connected with the photoelectric detection module 9, and the photoelectric detection module 9 is connected with the digital signal processing module 10. The spatial diversity module is used for receiving diffused and distorted light beams, the mode diversity module is used for outputting MxN branch signals after mode diversity of the light beams, the photoelectric detection module is used for converting the MxN branch optical signals into electrical signals and connecting to the digital signal processing module, and the digital signal processing module is used for signal reconstruction, merging and demodulation.

[0026] The spatial diversity module is composed of M optical antennas arranged in a matrix array form. In the spatial diversity module, the distance between two adjacent optical antennas is greater than the atmospheric coherence length, so as to ensure the independence of the branch signals.

[0027] The mode diversity module comprises M N-mode few-mode optical fibers, mode demultiplexers and single-mode optical fibers, the few-mode optical fiber refers to an optical fiber with a mode number between several to tens of modes. The input end of each N-mode few-mode optical fiber is connected with the output end of the corresponding optical antenna in the spatial diversity module, the output end of each N-mode few-mode optical fiber is connected with the input end of the corresponding mode demultiplexer, the mode demultiplexer converts high-order modes into base modes, the output end of each mode demultiplexer is connected with the input end of the corresponding N single-mode optical fiber, and the output end of the N single-mode optical fiber is connected with the corresponding photodetection module 9. The output port of each single-mode optical fiber is connected with one photodetection module, so that there are M*N branch optical signals in the system. The values of M and N can be determined according to actual conditions, considering the performance and cost requirements.

[0028] The greater the number M of optical antennas in the spatial diversity module or the mode N of few-mode optical fibers in the mode diversity module, the greater the diversity gain, but at the same time, the more photodetection modules are needed, and the cost and system volume are also increased.

[0029] The structure of the photodetection module is set according to the laser wavelength and signal modulation format of the transmitting end, for example, a single photodetector can be selected for OOK, a homodyne coherent receiver, a Kramers-Kroning coherent receiver, etc. can be selected for BPSK / QPSK.

[0030] The signal processing method of the digital signal processing module is set according to the signal modulation format of the transmitting end. Different algorithms are selected according to the signal modulation mode of the transmitting end and actual conditions, specifically, a linear combining algorithm such as maximum ratio combining or a nonlinear combining algorithm is used.

[0031] In the present application, the optical antenna can select various existing spatial light collimation mirror devices, the mode demultiplexer can select photon lantern, spatial phase plate and the like, and the digital signal processing module can select a commercial digital signal processing chip or input a computer for digital signal processing after being collected by an oscilloscope.

[0032] As shown in Figure 2 The present embodiment establishes a hybrid diversity spatial laser communication system with a 2*3 structure of double antennas plus three modes, the transmitting end comprises a laser 1, a signal generator 2, an electro-optical modulator 3, a transmitting antenna 4, an eight times beam expander 5 is used to simulate the beam diffusion phenomenon in long-distance transmission, and atmospheric turbulence is simulated by a rotating phase plate 6. The laser 1 and the signal generator 2 are connected with the optical input port and the electrical input port of the electro-optical modulator 3 respectively, the optical output port of the electro-optical modulator 3 is connected with the transmitting antenna 4, and the optical signal is transmitted to the air and passes through the eight times beam expander and the rotating phase plate in sequence.

[0033] The receiving end comprises a spatial diversity module, i.e. a receiving antenna array 7, a mode demultiplexer 8, an optoelectronic detection module 9 comprising a pre-erbium-doped fiber amplifier 91, an optical band-pass filter 92, a local laser 93, an optoelectronic detector 94, and a digital signal processing module 10. The diffused and distorted light beam is received by the receiving antenna array 7, the few-mode fiber port of the mode demultiplexer 8 is connected to the collimator, the single-mode fiber port after mode conversion is connected to the pre-erbium-doped fiber amplifier 91, the output light is connected to the optical band-pass filter 92 for filtering, and then connected to the optoelectronic detector 94 after mixing with the local light. The optoelectronic detector converts the optical signal into an electrical signal, which is finally collected by a real-time oscilloscope and input into a computer for digital signal processing.

[0034] The specific implementation steps of the embodiment are as follows:

[0035] The electro-optic modulator in the embodiment is an IQ modulator composed of two Mach-Zehnder modulators and a 90° phase shifter, and the transmitting and receiving antennas are both collimators.

[0036] First, a light carrier with a center wavelength of 1550.1 nm is emitted by a laser at the transmitting end and input into the IQ modulator, and a signal generator simultaneously generates two 5Gbit / s pseudo-random binary sequences and inputs them into the IQ modulator, thereby generating a 10Gbit / s QPSK signal. The QPSK optical signal is transmitted into the air through a collimator, enlarged by an eight-fold beam expander, and then passes through a rotating phase plate to simulate the beam diffusion and distortion phenomenon in real atmospheric transmission.

[0037] The rotating phase plate in the embodiment is equivalent to a medium turbulence, and the Reynolds number thereof is 0.4665, and the corresponding coherence length is 2 mm.

[0038] The mode demultiplexer in the embodiment is a mode selection type photonic lantern supporting three modes, one end of which is a few-mode fiber supporting three modes, and the other end is three single-mode fibers, wherein the high-order modes are converted into different single-mode fibers.

[0039] The receiving end comprises a spatial diversity module, i.e. a receiving antenna array 7, a mode demultiplexer 8, an optoelectronic detection module 9 comprising a pre-erbium-doped fiber amplifier 91, an optical band-pass filter 92, a local laser 93, an optoelectronic detector 94, and a digital signal processing module 10. The diffused and distorted light beam is received by the receiving antenna array 7, the few-mode fiber port of the mode demultiplexer 8 is connected to the collimator, the single-mode fiber port after mode conversion is connected to the pre-erbium-doped fiber amplifier 91, the output light is connected to the optical band-pass filter 92 for filtering, and then connected to the optoelectronic detector 94 after mixing with the local light. The optoelectronic detector converts the optical signal into an electrical signal, which is finally collected by a real-time oscilloscope and input into a computer for digital signal processing.

[0040] The local oscillator laser generates laser with center wavelength of 1550.14nm, and the frequency offset of signal light is 5GHz, which is just corresponding to the bandwidth of the generated QPSK signal. The local light is divided into six paths, and the power of each path is set to 4dBm. After mixing with six signal lights respectively, the lights enter the photoelectric detector. The photoelectric detector converts the optical signal into an electrical signal. Finally, the six electrical signals are collected by a real-time oscilloscope and input into a computer for digital signal processing.

[0041] The digital signal processing flow is shown in Fig. 6. Figure 3 Compared with the traditional homodyne coherent receiver, the SPD receiver based on KK relationship adds a step of Kramers-Kroning algorithm, that is, the phase information is reconstructed through the intensity information of the signal. When the minimum signal condition is met, that is, the local light power is greater than the signal light power, the intensity and phase of the received signal satisfy the Kramers-Kroning relationship:

[0042]

[0043] Wherein, φ(t) represents the signal phase at time t, p.v. represents the Cauchy principal value, I(t ′ ) represents the signal intensity at time t ′ .

[0044] After reconstructing the phase information of the signal, the sampling clock recovery, channel equalization, frequency offset compensation and carrier phase recovery are sequentially performed. Then, the six branch signals are combined through maximum ratio combination, and finally demodulation decision is performed.

[0045] The bit error rate of the spatial laser communication system adopting spatial-mode hybrid diversity under the atmospheric turbulence is shown in Fig. 7. Figure 4 When the bit error rate is 1x10 -5 , the transmission power budget can be reduced by 3.6dBm by using only three-mode diversity, by 3.9dBm by using only dual-aperture spatial diversity, and by 5.5dBm by using spatial-mode hybrid diversity reception.

[0046] It can be seen that the compensation effect of the present application on atmospheric turbulence is remarkable, and the spatial-mode hybrid diversity is obviously superior to the single spatial diversity or mode diversity.

[0047] The above embodiments are only used for understanding the idea of the present application, and any person skilled in the related art can make slight changes or replacements without departing from the technical scope of the present application, which should be covered in the protection scope of the present application.

Claims

1. A spatial-mode hybrid diversity receiving system for space laser communication, characterized in that, The space diversity module, the mode diversity module, the photoelectric detection module (9) and the digital signal processing module (10) are connected in series.

2. The spatial-mode hybrid diversity receiving system for space laser communication according to claim 1, wherein, The space diversity module is composed of M optical antennas arranged in a matrix array.

3. The spatial-mode hybrid diversity receiving system for space laser communication according to claim 1, wherein, The distance between two adjacent optical antennas in the space diversity module is greater than the atmospheric coherence length.

4. The spatial-mode hybrid diversity receiving system for space laser communication according to claim 1, wherein, The mode diversity module includes M N-mode few-mode fibers, mode demultiplexers and single-mode fibers.

5. The spatial-mode hybrid diversity receiving system for space laser communication according to claim 1, wherein, The greater the number M of optical antennas in the space diversity module or the mode N of few-mode fibers in the mode diversity module, the greater the diversity gain.

6. The spatial-mode hybrid diversity receiving system for space laser communication according to claim 1, wherein, The structure of the photoelectric detection module is set according to the laser wavelength and the signal modulation format of the transmitting end.

7. The spatial-mode hybrid diversity receiving system for space laser communication according to claim 1, wherein, The signal processing method of the digital signal processing module is set according to the signal modulation format of the transmitting end.

8. The spatial-mode hybrid diversity receiving system for space laser communication according to claim 1, wherein, In the digital signal processing module, the combining algorithm adopts the maximum ratio combining in the linear combining algorithm or a nonlinear combining algorithm.

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