The downlink laser communication link from satellites to the ground, a critical component of the Integrated Space-Ground-Air Information Network, demonstrates an increasing demand for high-capacity optical communication with minimal interruption probability. However, laser transmission remains highly susceptible to atmospheric turbulence, which substantially impairs the communication quality of satellite-to-ground laser links. While adaptive optics (AO) technology, the predominant turbulence compensation approach, effectively enhances coupling efficiency into single-mode fibers, its compensation performance proves insufficient under strong turbulence conditions.

This paper presents an atmospheric turbulence compensation method that integrates AO and mode diversity reception (MDR). The system processes the distorted spatial light beam through initial wavefront correction via an AO system, followed by mode-diverse reception using a multi-plane light converter (MPLC) based on multimode fiber. This turbulence compensation method was implemented in a high-orbit satellite-to-ground laser communication experiment at the Lijiang station. The experiment utilized a three-channel digital coherent reception system to achieve stable transmission of a 1 Gbit/s binary phase shift keying (BPSK) signal.

In this high-orbit satellite-to-ground laser communication experiment, bidirectional satellite-to-ground acquisition was achieved within 30 seconds. As shown in Fig. 3, the tracking system error under AO closed-loop conditions maintained below 1 μrad for both X-axis and Y-axis tracking. After AO wavefront correction, the root mean square (RMS) wavefront error reduced from 0.82λ to 0.10λ, as illustrated in Fig. 4, confirming the adaptive optics system’s wavefront compensation capability. At the receiver end, the distorted light beam underwent AO correction before reception through a MPLC based on multimode fiber. As shown in Fig. 5, under AO closed-loop conditions, the output power at the single-mode end of the MPLC exhibited significant improvements. Notably, the SC-based MDR method enhanced the power at a complementary cumulative probability (CCDF) of 0.9 by 3.94 dB compared to the single AO compensation scheme, as illustrated in Fig. 6. Additionally, real-time digital signal processing results based on a field-programmable gate array (FPGA) demonstrate that under AO closed-loop conditions, the selection combining-based MDR method improved the probability of the bit error rate (BER) being less than 1×10^-3 from 72.0% to 91.1%, as illustrated in Fig. 7. These findings demonstrate the substantial advantages of the combined AO and MDR method over individual AO or MDR compensation, markedly enhancing satellite-to-ground laser communication system performance in complex atmospheric environments.

This paper introduces an AO combined with MDR turbulence compensation strategy designed to effectively mitigate turbulence interference in high-orbit satellite-to-ground laser communication links. An on-orbit demonstration and verification experiment, conducted at the Lijiang ground station with a high-orbit satellite, achieved stable transmission of a 1 Gbit/s BPSK signal. The experimental results confirm the advantages of combining AO and MDR compared to single AO implementation. Furthermore, the AO-MDR mechanism demonstrates synergistic effects between the two components. This advancement holds significant practical implications for future high-orbit satellite-to-ground laser communication experiments and may facilitate further development of satellite-to-ground laser communication technology.