Towards high-capacity and robust free-space optical communications

Free-space optical communication is emerging as a cornerstone technology for next-generation wireless networks, driven by the escalating demand for fiber-optic-level data rates in applications ranging from data center interconnects to satellite and deep-space communications. Despite its substantial theoretical capacity advantage, reliable long-distance optical links remain challenging because of atmospheric turbulence, pointing and tracking inaccuracies, and stringent power budgets. These impairments are becoming increasingly intertwined with advanced modulation formats and various multiplexing techniques aimed at boosting throughput. In this Review, we examine the interplay between increasing capacity and maintaining reliability under turbulence in long-distance, high-capacity optical systems. We trace the evolution of the field from carrying more information on a single optical carrier to fully integrated, multi-dimensional multiplexing architectures. Key strategies for mitigating atmospheric impairments are analyzed, and we conclude by highlighting the major challenges and future opportunities. Free-space optical communication promises fiber-like data rates for next-generation wireless networks. This Review examines the interplay between capacity and reliability, covering advanced modulation, multidimensional multiplexing and strategies for robust long-distance transmission, and highlights challenges and future opportunities.