Estimation of FSO Path Length at Mid-IR Wavelength

With the unprecedented data rates targeted by 5G+, the attention on Free-Space Optics (FSO) is reviving. FSO provides point-to-point wireless connectivity over large and unlicensed bandwidths. However, the severe channel attenuation due to fog limits the maximum path length of optical links when high-availability targets are required. Here, we quantify the effect of radiation fog on the optical channel at two wavelengths, namely 1.550 {mu m} (near-IR), where most of commercial FSO system operate, and 10.6 {mu m} (mid-IR). The approach adopted here is based on the particle size distribution of fog, which is modelled by both monomodal and multimodal functions. If radiation fogs, typical of mid-latitude continental regions, are considered, the ratio between the achievable path length at mid-IR and at near-IR ranges from 2 to 3, but it can be much higher due to the sensitivity of mid-IR radiation to the microphysics of fog.