High-Speed Free-Space Optical Communication Using Mode Demultiplexers and Coherent Beam Combining

In satellite-to-ground and near-earth links, Free-Space Optical Communication (FSOC) is influenced by atmospheric turbulence. Currently, atmospheric turbulence suppression methods, such as adaptive techniques, multi-transmit diversity reception, and enlarging the receiving aperture, each possess their own limitations. In this paper, mode demultiplexing technology and coherent beam combining technology are applied in FSOC. A scheme of an FSOC system, which has the capacity to counteract the atmospheric turbulence effect, is proposed. It achieves 20 Gbps signal transmission in a 1-km urban horizontal link, with a bit error rate (BER) better than 10⁻⁶. Meanwhile, this paper elaborates on the basic logic of the coherent beam combining control algorithm, analyzes the influence of different types of control schemes on signals, and proposes a coherent beam combining control scheme without signal fading. We fabricated a coherent beam combining system with low insertion loss by using optical fibers and Piezoelectric (PZT) stacked. The energy loss for completing one-stage beam combining is less than 1 dB. This beam combining system can achieve low-loss, high-speed and high-precision beam combining control, with a control frequency of 10 kHz.