Free space optical communication (FSOC) features large broadband capacity, low cost, high security, small antenna size, small terminal size, and strong resistance to electromagnetic interference, with a wide range of application prospects. However, atmospheric turbulence can seriously affect the performance of satellite downlink laser communication systems. The array receiving system utilizes multiple smaller aperture receiving arrays to form a larger receiving aperture, improving beam coupling efficiency with the advantages of lower cost and more flexibility. The key to array reception lies in how to efficiently combine multiple received signals to improve the signal-to-noise ratio (SNR). Currently, there are mainly two methods of electrical beam combining and optical beam combining. The electrical beam combining method first detects the received signals of each aperture and then performs electrical merging. It requires detectors and signal acquisition processors corresponding to the number of channels, which is not conducive to high-speed communication. The optical beam combining method coherently synthesizes the received signals of each aperture in the optical device and then demodulates them by a single detector. Compared with electrical beam combining, it has a higher SNR but requires higher requirements for optical field regulation and control systems. Currently, there is relatively little research on this topic. Our study builds an optical coherent beam combining array receiving system model for typical satellite downlink communication and analyzes its performance by simulation. Based on the optical coherent beam combining method, a 10 aperture phased array laser communication receiver with equivalent aperture of 100 mm is built. We employ the rotating phase screen to simulate atmospheric turbulence, and complete coherent beam combining experiments and communication experiments, thus verifying the system performance.

We propose an optical coherent synthesis array receiving system model with a planar waveguide coupler structure, as shown in Fig. 1. Based on theoretical foundations such as atmospheric turbulence, fiber coupling, and optical coherent beam combining, we conduct a theoretical analysis of the system performance and simulate it under typical satellite downlink communication parameters. Meanwhile, we analyze the effects of the turbulence intensity, array aperture number, and receiving surface light intensity distribution on the bit error rate (BER) performance of the receiving system. Then, we conduct an array receiving coherent laser communication experiment, as shown in Figs. 7 and 9. Laser beams with wavelengths of 1550.12 nm and 1551.72 nm are loaded with communication signals at a rate of 56 Gbit/s by QPSK modulation, as shown in Fig. 12. After signal light amplification, it enters a simulated strong turbulent channel composed of a parallel light tube and a rotating phase screen. The channels of the array receiving system are preprocessed with equal optical path length, and after receiving the optical signal, the feedback control module performs optical coherent beam combining. Then, the optical power and coherent demodulation of the signal light are detected to obtain the system communication sensitivity.

Figures 2 and 3 show that as the equivalent optical SNR increases, the BER decreases to varying degrees at different aperture numbers. Under strong turbulence, the decrease becomes more significant with the rising aperture number. In Fig. 2, the required equivalent optical SNR decreases by about 2 dB for aperture numbers from 1 to 4 to achieve a 10^-9 BER. In Fig. 3, the required equivalent optical SNR decreases by more than 6 dB in sequence to achieve the same BER, which is mainly due to the coupling efficiency improvement when the number of apertures increases. Additionally, spatial diversity technology itself can also improve the performance of the receiving system. However, as the number of apertures increases, the increase in coherent combination losses such as phase-locked losses will affect system performance, which makes the increment of SNR gain gradually decrease with the rising aperture number. Figure 4 shows the variation curves of the required SNR with the number of apertures for achieving a 10^-7 BER at a zenith angle. It can be found that in this simulation condition, when the aperture number is greater than 10, the gain for increasing aperture number is no longer significant. Figure 5 reveals that with the increasing turbulence intensity, the fluctuation of the received optical power at each aperture increases, which has a greater influence on the amplitude of the combined beam signal. In the simulated parameter conditions, a maximum beam combination loss of 0.2 can be achieved. Under the zenith angle of 80°, this loss will increase the equivalent SNR requirement for eight-aperture reception by about 1 dB to achieve a BER of 10^-9. In the experiment, the array beam phase is controlled by a feedback control system, and a combining efficiency of 0.587 is achieved after phase locking. However, due to factors such as phase screen turbulence disturbance, environmental vibration, and polarization, the beam combining efficiency is not very high and there are still some fluctuations in the light intensity after beam combining. The receiving optical power of -31 dBm is required for single wavelength 56 Gbit/s communication to achieve a receiving BER of 10^-6. The receiving optical power of -27 dBm is required for dual-wavelength 112 Gbit/s communication, and its receiving sensitivity is about 10 dB away from the shot noise limit. Additionally, we conduct system reliability experiments. The experimental results are shown in Fig. 14. At a dual wavelength communication rate of 112 Gbit/s, when the received power is -23 dBm to -26 dBm, the laser link is built for 17 min, and the system availability is 99.77%. This demonstrates the high reliability of the communication system.

We study the performance of suppressing strong turbulence in an array receiving coherent laser communication. For a typical satellite-to-ground downlink communication link, an optical coherent synthetic array receiving system model is built. Numerical simulations are conducted to analyze the variation of the system BER with equivalent SNR under different aperture numbers and turbulence intensities. The results show that increasing the number of apertures can improve system performance, especially in strong turbulence conditions. The aperture number increase significantly enhances fiber coupling efficiency, thereby improving the system's tolerance to beam combining loss. Additionally, the effect of random intensity variations on the amplitude of the beam-combined signal and equivalent SNR is analyzed. In the simulation conditions, when the zenith angle is 80°, the loss requires an increase of approximately 1 dB in the equivalent SNR for an eight-aperture receiver to achieve a BER of 10^-9. Meanwhile, a 10-aperture array laser communication receiver is constructed. In the laboratory environment, we simulate the turbulence environment with r₀=4 cm by employing the rotating phase screen and conduct beam combining and communication experiments, thus achieving phase lock efficiency of 0.587, communication reception sensitivities of 10^-6@-31 dBm&50 Gbit/s and 10^-6@-27 dBm&100 Gbit/s,and availability of 99% for long-term chain building. This proves the feasibility and application potential of the array receiving and optical combining system in strong turbulent communication environments. In the future, we will further control the optical path difference of the array to improve its performance in multi-wavelength communication and conduct out-field experiments to demonstrate its communication effectiveness in the real atmosphere.