Free-space mid-IR communications using wavelength and mode division multiplexing

There has been growing interest in free-space optical (FSO) communications in the mid-infrared (mid-IR) region. In this paper, we review FSO communications in the mid-IR using wavelength division multiplexing (WDM) and orbital angular momentum (OAM)-based mode division multiplexing (MDM). We introduce the effects of mid-IR wavelengths in FSO communication links. The methods available to modulate and detect signals on mid-IR wavelengths are discussed. Subsequently, we introduce WDM and OAM-based MDM techniques in the mid-IR. Finally, we focus on the performance of a mid-IR FSO communication system combining both multiplexing techniques.