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.

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.

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.

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.