Midwave and longwave infrared free-space optical communication systems: status and prospects

Free-space optical (FSO) communication enables high-capacity wireless data transmission through the atmosphere and is increasingly regarded as a key complement to fiber-based networks for terrestrial, airborne, and satellite platforms. Compared to radio-frequency systems, optical carriers offer substantially higher bandwidth, narrower beam divergence, and enhanced spatial confinement, leading to improved energy efficiency and reduced probability of interception. Most state-of-the-art FSO systems operate near 1.55μm, benefiting from mature telecom technologies and enabling terabit-per-second data rates. However, their performance degrades significantly under fog and adverse weather conditions due to Mie scattering and turbulence-induced scintillation. Operation in the mid-wave infrared (MWIR, 3–5μm) and long-wave infrared (LWIR, 8–14μm) spectral bands offers a promising alternative, as these wavelengths experience reduced atmospheric scattering and coincide with favorable transmission windows. In addition, turbulence-induced effects scale inversely with wavelength, resulting in enhanced link stability and robustness. In the MWIR, interband cascade lasers (ICLs) provide high wall-plug efficiency and compact, low-power operation, making them attractive sources for energy-constrained and airborne FSO platforms, although their modulation bandwidth is limited by bipolar carrier dynamics. In the LWIR, quantum cascade lasers (QCLs) represent the dominant technology, offering high output power, broad wavelength tunability, and exceptionally large direct modulation bandwidths exceeding tens of GHz. This paper reviews the current state of MWIR and LWIR FSO systems and discusses future prospects enabled by these laser technologies. Emerging applications including high-speed coherent transmission, encrypted chaos-based communications, LiDAR, and physical quantum random number generation are highlighted as key drivers for next-generation infrared FSO platforms.