Mode diversity receiving system and method based on all-fiber coherent combination
By employing all-fiber coherent combining technology and utilizing a closed-loop system composed of photonic lantern receiver modules and cascaded dual-coupled modules, the problems of signal power imbalance and insertion loss in mode diversity reception are solved, achieving low-cost and high-efficiency mode diversity reception.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- NINGBO UNIVERSITY OF TECHNOLOGY
- Filing Date
- 2025-12-31
- Publication Date
- 2026-04-10
AI Technical Summary
Existing diversity reception techniques are inefficient and costly in turbulent atmospheric environments, and are difficult to effectively suppress signal power imbalance and insertion loss, thus affecting communication performance.
Employing all-fiber coherent combining technology, a closed-loop system is formed through a photonic lantern receiving module, a cascaded dual-coupled module, a signal monitoring module, and a control and feedback module. The 3*3 dual-coupled structure is used to eliminate non-independent fading in the mode diversity channel, and the phase controller is adjusted by a model-free optimization algorithm to achieve low-loss mode separation and combining.
It effectively reduces the transmit power requirements of communication systems, reduces insertion loss, improves mode diversity reception efficiency, and achieves low-cost anti-turbulence performance.
Smart Images
Figure CN121841497A_ABST
Abstract
Description
TECHNICAL FIELD
[0001] The present application relates to the field of optical communication technology, in particular to a mode diversity receiving system and method based on all-fiber coherent combining. BACKGROUND
[0002] Free space optical communication technology has great application potential in satellite communication, deep space exploration, metropolitan network link and emergency communication due to its advantages of large transmission bandwidth, good security and no need for spectrum license. However, laser beams will be affected by atmospheric turbulence in atmospheric channel transmission, resulting in beam deformation, energy distribution dispersion and signal flicker, which seriously increases the difficulty of reception, and requires turbulence suppression receiving technology. In recent years, mode diversity reception (MDR) technology has attracted widespread attention. This technology is based on the principle that the distorted wavefront can be decomposed into a series of spatially orthogonal modes. MDR uses few-mode fibers to receive these modes, and then obtains mode gain through combining technology, thereby effectively suppressing the influence of atmospheric turbulence, and has the advantages of low cost and simple structure, and has broad development prospects.
[0003] At present, the implementation scheme of mode diversity reception mostly adopts digital coherent combining technology, that is, a series of spatially orthogonal modes in the few-mode fiber are first decomposed, then coherently detected into electrical signals, and finally the decomposition gain is obtained by using combining algorithm. Although this scheme has a simple structure, the cost is proportional to the number of modes supported by the system. Another is the beam coherent combining technology, which has been widely used in spatial diversity reception, but there are few reports in the field of mode diversity. However, unlike the characteristics of channel independent fading in spatial diversity reception, the fading of each mode in mode diversity reception has strong statistical correlation because they share the spatial channel. This leads to non-uniform distribution of signal power among modes, and a common phenomenon is that when one mode has high received power, one or more other modes may be in deep fading state. This will greatly reduce the efficiency of beam coherent combining, and further affect the performance of the mode diversity receiver. At the same time, considering the influence of combiner on the performance of the receiver, different combiner structures have different low insertion losses, and the number of stages of the combining scheme is also related to the number of channels supported by the combiner. Low-loss mode conversion and separation are the key to further improve the performance of the receiver. Therefore, eliminating the influence of non-independent fading in each mode channel, realizing low-loss mode separation and designing appropriate combining scheme with fewer stages are important contents of the beam coherent combining technology of mode diversity.
[0004] The coherent beam combination adopts the following scheme to eliminate the influence of non-independent fading, reduce mode separation loss and reduce the number of combinations, specifically including: (1) adopting a double coupling structure to eliminate power imbalance, but this scheme will multiply the number of combinations, causing more insertion loss and cost; (2) an integrated multi-mode interference photoelectric chip, limited by mode mismatch and structure design, the insertion loss of the system is very large, far exceeding the gain effect brought by mode diversity, and still not reaching the actual application level.
[0005] Therefore, it is necessary to propose a mode diversity receiving scheme based on the coherent beam combination technology, which can effectively resist the influence of atmospheric turbulence on the space laser communication system under the limitation of low cost. SUMMARY
[0006] In view of the deficiencies of the prior art, the problems to be solved by the embodiments of the present application are to provide a mode diversity receiving system and method based on all-fiber coherent combination for a mode diversity receiver, to realize efficient and low-loss mode separation and beam combination loss through the designed all-fiber structure, to eliminate power imbalance through double coupling active phase control, and to realize efficient and low-cost mode diversity receiving.
[0007] To achieve the above object, the present application provides a mode diversity receiving system and method based on all-fiber coherent combination for realizing a turbulence-resistant mode diversity receiver, comprising a photonic lantern receiving module, a cascaded double coupling module, a signal monitoring module, a control and feedback module and a coherent detection module. The photonic lantern receiving module is used for receiving multiple mode optical signals in space and converting all modes in a few-mode optical fiber into a fundamental mode with low loss, and outputting from a single-mode optical fiber port. The cascaded double coupling module is connected with the photonic lantern receiving module and is used for beam combination processing of the multiple single-mode optical fiber output signals, and the cascaded coupling module comprises a first coupling and a second coupling connected in sequence. The coupler can select a 3*3 or 2*2 optical fiber coupler according to the number of input signals, and the input ends are respectively configured with 2 or 1 phase controllers. The first coupling is used to eliminate the power imbalance of each mode caused by atmospheric turbulence. The second coupling is connected in series with the first coupling and is used to realize efficient coherent beam combination.
[0008] The signal monitoring module is arranged at the output end of the cascaded double coupling module, and the output optical signal is divided into two beams, one of which is sent to the detector as the final combined optical signal, and the other of which is used for real-time monitoring of the power of the final combined optical signal. The control and feedback module is connected with the signal monitoring module, generates a control quantity by using a model-free optimization algorithm according to the output signal power at the current time and historical data, drives the phase controller in the cascaded double coupling module, and realizes adjustment of the optical signal.
[0009] The cascaded dual-coupling module, signal monitoring module, and control and feedback module form a closed-loop control system, which gradually converges, causing the final combined optical signal to tend to its maximum value.
[0010] The coherent detection module consists of a coherent receiver and a digital signal processing unit. The specific structure and digital signal processing algorithm can be determined according to the parameters of the transmitted signal.
[0011] The present invention has the following beneficial effects: This invention designs a mode diversity receiving system and method based on all-fiber coherent combining. Unlike existing digital mode diversity, the proposed scheme combines the signals of each mode in the optical domain based on coherent combining technology. It uses a 3*3 dual coupling structure to eliminate the influence of the non-independence of each channel in mode diversity, while reducing the number of combining stages. The all-fiber structure has the advantages of simple implementation and low loss. Attached Figure Description
[0012] Figure 1 This is a system architecture diagram of the present invention; Figure 2 This is a schematic diagram of the receiver system of the present invention; Figure 3 This is a flowchart of the beam combining power control algorithm of the present invention; Figure 4 The communication simulation results are for embodiments of the present invention; Figure 5 This is a flowchart of the method of the present invention. Detailed Implementation
[0013] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.
[0014] like Figure 1 The diagram shows the mode diversity receiving system and method based on all-fiber coherent synthesis proposed in this invention, used as a receiver for space laser communication or lidar, including: a photonic lantern receiving module, a cascaded dual-coupling module, a signal monitoring module, a control and feedback module, and a coherent detection module.
[0015] The photonic lantern receiver module couples spatial optical signals into the few-mode fiber end of the photonic lantern and excites multiple modes within it. A few-mode fiber, denoted as N, is an optical fiber that supports a limited number of orthogonal modes as independent channels for transmission. This module consists of a coupling lens and the few-mode fiber. The choice of coupling lens can be adjusted based on the number of modes N supported by the few-mode fiber, such as by selecting an appropriate numerical aperture. The higher-order spatial modes LP in the few-mode fiber... i Transformed into LP by photon lantern 01 The mode, output from N single-mode optical fibers, is further used to achieve coherent beam combining.
[0016] A cascaded dual-coupling module, connected to the single-mode output port of the photonic lantern receiving module, is used to perform bundle combining of the multi-channel single-mode fiber output signals. Each bundle combining operation of the cascaded coupling module includes sequentially connecting a first coupling and a second coupling. Each coupling includes a phase controller and a coupler connected in sequence. The coupler adopts a 3*3 structure. In the first coupling, the phase controller adjusts the phase of the input optical signal so that the power of the three output optical signals after passing through the 3*3 coupler tends to be equal. In the second coupling, the phase controller adjusts the phase of the optical signal so that the optical signal is output from the target port after passing through the 3*3 coupler. After each stage of bundle combining, the number of output optical signal channels becomes 1 / 3 of the original. It can be understood that when N is not an integer multiple of 3, this can be solved by modulating the connection structure and adding a 2*2 coupling.
[0017] The signal monitoring module, located at the output of the cascaded dual-coupled module, monitors the beam combining quality of the cascaded coupler and feeds the results back to the control and feedback module, which includes a 1:99 optical beam splitter and a detector. A very small portion (1%) of the optical power is guided to the corresponding unit of the photodetector, converted into an electrical signal, and transmitted to the control and feedback module; the remaining 99% of the main energy is used for coherent monitoring and is connected to the coherent detection module.
[0018] The control and feedback module, based on the current signal power and historical data provided by the signal monitoring module, uses a machine learning optimization algorithm to generate multiple control signals. These signals drive the phase controller in the cascaded dual-coupled module to regulate the optical signal. After several iterations, the control signals converge, achieving maximum power output at the output port. The optimization algorithm employs a stochastic parallel gradient descent algorithm to address the random variations in phase and power of each mode caused by turbulence.
[0019] The coherent detection module consists of a coherent receiver and a digital signal processing unit. The coherent receiver performs highly sensitive coherent detection on the coherently combined optical signal, converts it into an electrical signal, samples it, and then performs signal compensation and reconstructs the original signal in the digital signal processing unit. The specific structure of the coherent receiver and the digital signal processing algorithm can be determined according to the transmitted signal and performance requirements. Examples include, but are not limited to, commercial coherent receivers and Kramers-Kronig receivers. The digital signal processing unit needs to perform data recovery of the coherent detection signal, mainly including: clock recovery, channel equalization, frequency offset compensation, and phase offset compensation.
[0020] Understandably, the larger the number of patterns N supported by pattern diversity, the greater the pattern gain the system gains, but the higher the system complexity and cost.
[0021] like Figure 2As shown, based on the joint simulation of Optisystem and MATLAB, a further explanation of the mode diversity optical communication receiving scheme based on a tri-mode photonic lantern and dual 3*3 couplers provided in this embodiment of the invention is as follows: The spatial optical signal affected by turbulence is first received by a coupling lens, focused onto the few-mode fiber port of the three-mode photonic lantern, and excited by LP. 01 LP 11a and LP 11b Three optical signals are generated. The photonic lantern can be either a mode-selective or non-mode-selective type. The three optical signals enter a cascaded coupling module. First, the phase of the signal is adjusted by the phase controller in the first coupling. Then, it is coupled by a 3x3 coupler. The output enters the second coupling, where the phase is adjusted by the phase controller, ultimately resulting in the optical signal being output from port 3. The output of the cascaded coupling module enters the 1:99 beam splitter in the signal monitoring module. 1% of the optical signal is converted into an electrical signal by a PD detector for monitoring, while the main beam with 99% energy is used for coherent detection. The monitoring electrical signal is transmitted to the control and feedback module, where data processing and iteration generate four control phase voltages. These drive phase modulators PC1, PC2, PC3, and PC4 respectively. Data processing utilizes a model-free stochastic parallel gradient descent algorithm, iteratively comparing stored historical data. The main beam enters the coherent receiver and is mixed with the local oscillator laser to complete photoelectric conversion. Finally, the signal is sampled by an oscilloscope and processed offline on a computer to recover the signal. The specific flow of the stochastic parallel gradient descent algorithm used in the data processing is as follows: Figure 3 As shown, the merging of the three optical modes requires the generation of four control phase signals. The first two phase control signals are used to balance the power, and the last two phase control signals are used to achieve coherent combination.
[0022] Finally, the optical signal after mode merging is converted into I and Q electrical signals by a coherent receiver. After passing through filtering, normalization, clock recovery, adaptive equalization, frequency compensation, phase compensation and decision processing, the original signal is demodulated.
[0023] When the wavelength of the transmitting light source is set to 1550nm, the linewidth to 100kHz, the modulation format to QPSK, and the data rate to 50Gbps, the optical signal output from the light source is electro-optically modulated by an IQ modulator and then transmitted by a transmitting antenna with a diameter of 5cm. After transmission over 20km, the optical signal is coupled into the mode diversity receiver through a receiving antenna with a diameter of 15cm and a focal length of 23cm. The communication bit error rate test results under moderate atmospheric turbulence are as follows: Figure 4 As shown. At a bit error rate of 1×10⁻⁶. -2The proposed scheme reduces the transmit power budget by approximately 1.2 dBm compared to the scheme without power compensation, and reduces the transmit power budget by 6.3 dBm compared to the single-mode fiber receiving scheme.
[0024] Therefore, the mode diversity reception scheme adopted in the embodiments of the present invention can effectively reduce the power requirements of the communication system at the transmitting end and effectively compensate for atmospheric turbulence effects, providing a low-cost implementation scheme for atmospheric turbulence compensation.
Claims
1. A mode diversity receiving system based on all-fiber coherent combining, characterized in that, include: The photonic lantern receiving module is used to receive spatial optical signals, couple them to at least-mode optical fibers and excite multiple modes of optical signals, and convert them into multiple single-mode optical signals with low loss and output them from the single-mode optical fiber port. The cascaded dual-coupled module is connected to the photon lantern receiving module and is used to perform beam combining processing on the multi-channel single-mode optical signals. It includes a first-stage coupler and a second-stage coupler connected in sequence. The signal monitoring module is located at the output end of the cascaded dual coupling module, which splits the combined optical signal into two paths and monitors the power of the combined optical signal in real time. The control and feedback module is electrically connected to the signal monitoring module and is used to generate control quantities based on the monitored combined optical signal power and historical data, drive the phase controller in the cascaded dual coupling module to adjust the phase of the optical signal, form a closed-loop control to make the combined optical signal power tend to the maximum value. The coherent detection module performs photoelectric conversion and digital signal processing to recover the original signal.
2. The system according to claim 1, characterized in that, The photonic lantern receiving module includes a coupling lens and a photonic lantern. The numerical aperture of the coupling lens is adjusted according to the number N modes supported by the photonic lantern. The photonic lantern converts higher-order spatial modes into N-channel LP. 01 Basic model.
3. The system according to claim 1, characterized in that, The signal monitoring module includes a 1:99 optical beam splitter and a photodetector; the 1:99 optical beam splitter guides 1% of the optical power of the combined optical signal to the photodetector to be converted into an electrical signal and transmitted to the control and feedback module; the remaining 99% of the optical power is transmitted to the coherent detection module as the final combined optical signal.
4. The system according to claim 1, characterized in that, The coherent detection module includes a coherent receiver and a digital signal processing unit; the coherent receiver is a commercial coherent receiver or a Kramers-Kronig receiver; the digital signal processing unit is used to perform clock recovery, channel equalization, frequency offset compensation, and phase offset compensation.
5. The system according to claim 1, characterized in that, After each stage of the cascaded dual-coupling module is combined, the number of output optical signal channels is 1 / 3 of the number of input channels; when the number N of single-mode optical signals output by the photonic lantern receiving module is not an integer multiple of 3, the connection structure is adjusted and a 2×2 type fiber optic coupler is added for adaptation.
6. A mode diversity reception method based on all-fiber coherent combining, characterized in that, Includes the following steps: S1: The photonic lantern receiving module receives spatial optical signals affected by atmospheric turbulence, couples them with at least-mode optical fiber and excites multiple modes of optical signals, and converts them into multiple single-mode optical signals with low loss for output. S2: The phase of the multi-mode optical signals is adjusted by the first coupling unit of the cascaded dual coupling module to eliminate the power imbalance of each mode; then the phase is adjusted by the second coupling unit to achieve efficient coherent beam combining. S3: The combined optical signal is split into multiple beams by the signal monitoring module, the power of one of the optical signals is monitored in real time and transmitted to the control and feedback module; S4: The control and feedback module generates control quantities based on the monitored power data and historical data, drives the phase controller in the cascaded dual-coupled module, and forms a closed-loop control to make the combined optical signal power converge to the maximum value. S5: The coherent detection module performs photoelectric conversion and digital signal processing on the other combined optical signal to restore the original signal.
7. The method according to claim 6, characterized in that, In step S2, the phase adjustment of the first coupling unit and the second coupling unit is achieved by a phase controller. The number of phase controllers is configured according to the type of fiber optic coupler: 2 phase controllers are configured for a 3×3 type fiber optic coupler, and 1 phase controller is configured for a 2×2 type fiber optic coupler.
8. The method according to claim 6, characterized in that, In step S4, the control and feedback module uses a model-free optimization algorithm to generate control quantities to cope with the random changes in mode phase and power caused by atmospheric turbulence.