The concept of free-space optical communications (FSOC) goes back to the 1880s and Alexander Graham Bell’s experiments with the photophone. Military applications were responsible for sporadically occurring developments in this technology, particularly during both World Wars, but it was not until the invention of lasers in the 1960s that FSOC could fulfill its promise of secure, directional, and high-data rate transmission over long distances. Already in 1981, the first FSOC downlinks through the atmosphere, the clouds and ocean water from an aircraft flying at 12 km to a submerged submarine were demonstrated.

As of 2024, we are witnessing the coming of age of commercial FSOC due to demands for higher data rate wireless communications due to the limited frequency resources of radio frequency bands. Several major players—such as Amazon, which achieved successful 100-Gbps intersatellite links, and SpaceX, which also announced successful 200-Gbps intersatellite link demonstrations based on the COTS devices—as well as hundred or so smaller companies are pursuing several FSOC concepts and technologies. Space agencies such as NASA and ESA have demonstrated uplink, downlink, and relayed communications with lasers in space. Laser terminals aboard deep-space missions are also becoming a reality, as the recently launched Psyche mission shows.

Closer to Earth, Li-Fi (light fidelity) technology uses light from light-emitting diodes (LEDs) to enable high-speed communications, especially over short ($\sim 10\mathrm{s}$ of meters) distances and indoors. For underwater communications, links based on LEDs and lasers, using classical as well as quantum security, have been demonstrated in laboratories, and first commercial entities offering this technology have emerged.

While the future certainly looks bright for FSOC, there remain research challenges in different areas, like pointing, acquisition, and tracking with narrow beams and moving platforms, hybrid FSO networks, the influence of background light, channel distortions due to scattering, fog, atmospheric turbulence, etc. These research challenges need to be addressed to come to a fully mature commercialization of this technology. This special section offers a broad overview of FSOC sub-disciplines including aspects of light propagation through the atmosphere, adaptive optics, quantum key distribution (QKD), satellite FSOC, and deep-space FSOC.

In this special section, Stotts and Andrews provide a comprehensive tutorial review of optical communications in atmospheric turbulence. Gupta, Dhawan, and Gupta give an overview and recent advances on UAV-based FSOC. For the onboard FSOC systems, Badás et al. offer a survey on opto-thermo-mechanical phenomena for satellite FSOC. As a possible application to deep-space FSOC, Jarzyna et al. discuss the photon information efficiency limited by the Gordon-Holevo capacity bound. Toselli devised a focal-plane analysis for atmospheric turbulence-affected laser beams, Ruff and Bradley experimentally demonstrate self-healing free space optical link in atmospheric turbulence, and Shishter, Ali, and Young carried out Bessel-Gaussian beam propagation through turbulence. Patel et al. propose a fiber bundle-based beam tracking approach for FSOC, Peng, Mao, and Qi present an improved prediction algorithm for the single-mode fiber coupling, and Kiasaleh examines the receiver architecture for on-off-keying FSOC in the presence of residual spatial tracking error. Häusler et al. propose to characterize the atmospheric background light in satellite-to-ground QKD scenarios.

The guest editors extend their gratitude to the authors for their valuable contributions, as well as to the reviewers for their time and thoughtful feedback. We also express our deep appreciation to the Optical Engineering editorial staff for their tremendous assistance throughout the preparation of this special section.