Measured and predicted light attenuation in dense coastal upslope fog at 650, 850, and <inline-formula><math display="inline" overflow="scroll"><mrow><mn>950</mn><mspace width="0.3em" /><mi>nm</mi></mrow></math></inline-formula> for free-space optics applications

Free-space optics (FSO) has gained considerable importance in this decade of demand for high-bandwidth transmission capabilities. FSO can provide the last mile solution, but the availability and reliability issues concerned with it have received increasing attention and need thorough investigation. In this work, we present our results on fog attenuation measurement and prediction at wavelengths 650, 850, and 950 nm with peak values up to 500 dB/km. For the attenuation measurement, optical wavelengths are transmitted over the same path of fog in free air to a receiver, measuring the power at every wavelength. The measurement of fog attenuation was performed at the France Telecom R&D test facility at La Turbie (in the south of France). We compare our measurement data with the commonly used light attenuation models (Kruse and Kim models) and with a new proposed model allowing the prediction of fog attenuation for the 0.69- to 1.55-µm spectral band. We present some interesting insights and discuss the wavelength dependence. The practical measurements described promise to validate the models and therefore should lead to more accurate availability prediction for FSO links.