  The recent development of Quantum Cascade Lasers (QCLs) represents one of the
biggest opportunities for the deployment of a new class of Free Space Optical
(FSO) communication systems working in the mid-infrared (Mid-IR) wavelength
range. As compared to more common FSO systems exploiting the telecom range, the
larger wavelength employed in Mid-IR systems delivers exceptional benefits in
case of adverse atmospheric conditions, as the reduced scattering rate strongly
suppresses detrimental effects on the FSO link length given by the presence of
rain, dust, fog and haze. In this work, we use a novel FSO testbed operating at
\SI{4.7}{\micro m}, to provide a detailed experimental analysis of noise
regimes that could occur in realistic FSO Mid-IR systems based on QCLs. Our
analysis reveals the existence of two distinct noise regions, corresponding to
different realistic channel attenuation conditions, which are precisely
controlled in our setup. To relate our results with real outdoor
configurations, we combine experimental data with predictions of an atmospheric
channel loss model, finding that error-free communication could be attained for
effective distances up to 8~km in low visibility conditions of 1 km. Our
analysis of noise regimes may have a key relevance for the development of
novel, long-range FSO communication systems based on Mid-IR QCL sources
