Mid‐Infrared Free‐Space Optical Communications: Technologies, Demonstrations and Future Directions

ABSTRACT Free‐space optical (FSO) communication is rapidly advancing as a high‐capacity wireless technology, with growing interest in extending its operation from the traditional near‐infrared to the mid‐infrared (mid‐IR) spectral region. Particularly, the mid‐wave infrared (MWIR) and long‐wave infrared (LWIR) atmospheric transmission windows in the mid‐IR region provide advantages regarding reduced turbulence effects, scattering and absorption, enabling more robust links under adverse conditions. This review presents recent progress in MWIR and LWIR FSO communication systems, highlighting advances in transmitter and receiver technologies, such as quantum cascade lasers (QCLs), lithium niobate modulators and detectors, including quantum cascade detectors (QCDs), mercury cadmium telluride (HgCdTe and MCT) photodiodes and quantum‐well infrared photodetectors (QWIPs). Experimental demonstrations achieving multigigabit to tens‐of‐gigabit data rates with advanced modulation formats, digital equalisation and coherent detection are summarised. The paper discusses integration trends in MWIR and LWIR photonics, coherent communication strategies and hybrid FSO–radio frequency (RF) architectures. Together, these developments outline the pathway for MWIR and LWIR FSO systems to evolve from laboratory demonstrations towards scalable, practical platforms for next‐generation terrestrial and space communication networks.