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.
KEYWORDS: Fiber optic gyroscopes, Signal attenuation, Free space optics, Visibility through fog, Visibility, Particles, Data modeling, Modulation, Light scattering, Aerosols
Terrestrial free space optical (FSO) links are based on the simple concept of a light beam carrying information, thus facilitating very high data rates. Fog remains the major hurdle in increasing the availability and reliability of terrestrial FSO links, as fog particles scatter the propagating light, causing nonnegligible attenuation. We present measurement results from our campaigns carried out at the continental city of Graz and at La Turbie near Nice, the southern coast of France. We perform a detailed analysis of the measurement results providing time-series analysis of these fog measurements and a comparison between maritime and continental fogs. Based on our measurement analysis, we provide insight into designing efficient FSO systems, with better performance and enhanced resilience.
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 can not be ignored, and requires thorough investigations. In this work, we present our results about light attenuation at 950 and 850 nm wavelengths in continental city fog conditions with peak values up to 130 dB/km and compare them with attenuation under dense maritime conditions with peak values up to 480 dB/km. Dense fog is the most severe limiting factor in terrestrial optical wireless applications and light propagation in fog has properties in the spatial, spectral and the time domain, which are of importance to free-space optic data communication. In 2004 (within a short term scientific mission of COST 270) measurements of very dense maritime fog and low clouds were made in the mountains of La Turbie, close to the coast of southern France. Using the same equipment, the measurements were continued for the conditions of the continental city of Graz, Austria. This campaign was done in the winter months from 2004 to 2005 and 2005 to 2006 and allows us to compare fog properties for different environments, and the impact of snow fall. We provide detail analysis of a fog and a snow event for better understanding of their attenuation behavior.
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 acquired more attention, and a need for thorough investigations. In this work, we present our results about fog attenuation at the 950 and 850 nm wavelengths in heavy maritime fog 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 of every wavelength. The RF marker technology employed takes advantage of modulating every optical wavelength with an individual carrier frequency, allowing to use one optical front end for the receiver and to separate individual wavelengths by electrical signal filters. The measurement of fog attenuation at different wavelengths was performed at the France Telecom R & D test facility at La Turbie. Maritime or advection fog, which caused the light attenuation consists of water droplets of larger diameter in the order of 20 μm and can cause visibilities as low as 30 meters. The visibility was measured using a transmissiometer at 550 nm. We compare our measurement data with the commonly used light attenuation models of Kruse and Kim, and present some interesting insights. The practical measurements described try to validate the models and therefore should lead to a more accurate availability prediction for FSO links.
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