空间分集信号间光程同步和共相合成方法研究

Optical Path Synchronization and Co-Phasing Combination Methods Among Spatial Diversity Signals

受限于空间传输相差、光学器件误差、光纤长度不一致、外界环境干扰等因素,空间分集信号间不可避免地存在光程差和相位差,极大地影响了光学合成的效果。提出利用光纤延迟线补偿光程差,确保信息间的时域同步;通过光纤相位调制器校正相位差,实现分集信号的共相合成。搭建基于两通道的基础模块传输速率为10 Gbit/s的分集信号光学合成实验系统,结果表明,当实施光程补偿和共相控制之后,通信误码率稳定为零。此外,还探究了分集信号间光程时域同步的容差范围,对于非归零脉冲信号,信号间允许的最大光程差约为比特周期长度的70%。进一步地,完成了四路分集信号的光学合成通信实验,亦实现零误码,证明该方法的可扩展性。

Objective The combination of spatial diversity receiving signals in free space optical communication can be classified into optical and digital combinations.While the digital combining technique has been widely recognized and applied,a significant portion of research on optical combining aims at enhancing combining efficiency.However,in practical applications,the prerequisite for correctly demodulating signals is the temporal synchronization of diversity signals.Constrained by factors such as spatial transmission aberrations,inconsistent fiber optic lengths,optical device errors,and external environmental interference,inevitable optical path differences,and phase differences exist among spatial diversity signals,significantly impacting the effectiveness of the optical combination.Therefore,we explore the influence of optical path differences and phase differences among diversity signals on the demodulation of combined signals and propose an optical combining method for spatial diversity signals.Methods The overall architecture of optical combination of spatial diversity receiving signals is illustrated in Fig.1.The received spatial diversity signals are coupled into optical fibers and connected to optical fiber delay lines to compensate for static optical path difference,ensuring temporal synchronization between information.Then,phase modulators and couplers are introduced to compensate for dynamic wavefront aberrations among beams using the blind optimization SPGD algorithm,achieving independent and parallel control of multiple phases.The real-time detected optical intensity signal from the photodetector is used as feedback to converge toward the direction of maximum output intensity.Furthermore,we analyze the requirements for optical path differences based on pulse broadening and derive the phase control conditions for co-phasing combination based on a 3 dB coupler.Finally,simulation analysis and experimental verification of two-channel diversity signal combination are carried out.Results and Discussions Taking communication bit error rate and combined optical intensity as evaluation indicators,the performance of this optical synthesis scheme is presented in Fig.6.In the open-loop state,the combined optical intensity fluctuates sharply,with an average bit error rate of 6.05×10^(-1) within one minute.After implementing only phase control,the combined optical intensity is stable,with an average bit error rate of 5.35×10^(-1).By adjusting the optical path difference,the drift of the combined optical intensity becomes slower,and the bit error rate drops to 4.03×10^(-4).Under the simultaneous control of optical path difference and phase difference,the bit error rate reaches 0,and the combined optical intensity remains stable.The effective value of the normalized output optical intensity increases from 0.547 to 0.914,and the mean square error decreases from 0.304 to 0.0142.These results demonstrate the significant efficacy of this solution in improving the stability of the communication system.In addition,we explore the tolerance range of optical path time domain synchronization among diversity signals.For non-return-to-zero(NRZ)pulse signals,the maximum allowable optical path difference among signals is approximately 70%of the bit period length,which is verified in both simulation and experimentation(Fig.4 and Table 1).Furthermore,a combining experiment of four-channel diversity signals is conducted and achieved a 0-bit error rate as well,demonstrating the scalability of the proposed method.Conclusions We analyze the impact of optical path difference and phase difference on optical signal combination and communication demodulation.The optical path synchronization requirements and coherent combining conditions for NRZ signals are deduced.We propose using fiber delay lines and fiber phase modulators to achieve optical path synchronization correction and coherent control.Subsequently,diversity receiving signals are simulated in the optical fiber,conducting an optical combination for two channels of signals.Under the optical path difference correction and phase difference control,the combined optical intensity is stable,achieving a coupling efficiency of up to 90%,and a communication bit error rate of 0 within five minutes.Finally,the proposed approach is extended to achieve optical combination for four channel signals,also achieving a 0-bit error rate and demonstrating the feasibility of applying this approach to the combination of signals from multiple diversity channels.