Wavelength selection for the free space optical telecommunication technology

Abstract

The principal disadvantage of using Free Space Optical (FSO) telecommunication systems is the disturbing role played by the atmosphere on light propagation and thus on availability and reliability of the link. Atmospheric effects on the electromagnetic waves transmission differ according to the used wavelength. Among these effects, fog is a very important factor in the electromagnetic radiation degradation and especially for the visible and infrared waves. Relationship between the fog characteristics (liquid water content, density, visibility...) and the transmitted signal attenuation for a given wavelength should be known in order to predict the FSO systems performance and link availability. The lasercom literature shows the complexity of the problem related to different types of fog and its inhomogeneity over the propagation path. The problem of atmospheric attenuation for laser radiation in visible and infrared regions was reviewed from an empirical and theoretical point of view. We describe the optical properties of fog calculated from the Mie scattering theory and the particle size distribution. We investigate the laser system performances in presence of fog in 0.4 to 15 µm spectral band from Fascod computation (advection and radiation fog) and from other particle size distributions available in the literature. We compare the extinction coefficient (function of the wavelength) for different particle size distributions in order to see whether the various types of fog may be represented and covered by the models proposed in Fascod. Finally, we consider some particular laser wavelengths (0.69, 0.78, 0.85, 1.55, 5 and 10 µm) used and proposed for the FSO technologies in order to study and compare the effect of fog on equipments operating at these wavelengths.