Mid-infrared free-space optical communications: a tutorial

This tutorial provides an overview of free-space optical (FSO) communications operating in the mid-infrared (mid-IR) wavelength bands. In the mid-IR region, there are two atmosphere transmission windows: (mid-wave infrared (MWIR) at 3–5 µm and long-wave infrared (LWIR) at 8–12 µm) with relatively low atmospheric absorption. Compared to near-infrared (near-IR) wavelengths (e.g., telecom C-band at ∼1.55 µm), mid-IR tends to be more resilient to atmospheric degradation effects (e.g., fog-induced scattering and atmospheric turbulence). Thus, using mid-IR wavelengths can enable more robust FSO links through challenging atmospheric conditions. This tutorial aims to review recent developments and advances in mid-IR FSO communications. Various devices and approaches for mid-IR data transmitters and receivers are discussed. Advanced demonstrations for single-channel and multi-channel-multiplexed high-capacity mid-IR FSO links are reviewed. This tutorial also discusses the challenges and limitations of current techniques, as well as the outlook for future research.