Effect of Rain on Free Space Optics

Two years of experimental observations gathered by the 319-m optical link installed at the campus of Politecnico di Milano, Italy, are discussed. Ancillary measurements of rain rate intensity collected by a tipping bucket rain gauge and of visibility collected by a visibility sensor are also used. The visibility sensor and the rain gauge are located nearby one of the two link terminals. A data reduction procedure has been carried out on the time series of received laser power to separate the effects due to precipitation from those due to other contributions, which are by non way negligible. Different drop size distributions (DSD) are taken into account during the simulations of path attenuation being the effect of DSD much more evident at optical frequencies that at microwaves.It is shown that rain attenuation can reduce system availability of links with small system margin in areas where fog is absent.