Free Space Optical communications (FSO) has attracted considerable attention for a variety of applications in telecommunications field. This technology uses the transmission of an optical or infrared (IR) signal in the atmosphere to provide high data rate reliable more economically than traditional fiber. Some of FSO advantages are: spectrum licensing, frequency coordination, no interference, fast and easy installation, multi-Gbps high data rate, low cost. This communication presents various atmospheric considerations relating to the propagation such as absorption and scattering by molecular or aerosols, scintillation due to the air index variation under the temperature effect, attenuation by hydrometeors (rain, snow) like their various modelling (Kim and Kruse, Al Naboulsi, Carbonneau, etc). Some experimental results (attenuation in function of the visibility) are presented. The comparison of measurements with our model shows a good agreement for low visibility where Kruse and Kim models deviate notably from measurements. 1 Introduction Free Space Optical communication (FSO) has attracted a considerable attention for a variety of applications in telecommunications field. This technology uses the transmission of an optical or infrared (IR) signal in the atmosphere to provide high data rate reliable links in a more profitable way and more rapid than the traditional networks fiber. The various aspects of the infrared and visible optical waves propagation in the atmosphere are presented (molecular and aerosol absorption, molecular and aerosol scattering, rain and snow attenuation, scintillations effects). They constitute the key of all good comprehension of the future free space optical communication systems (FSO). Fog appears as the more penalizing element in the free space optical link operation. The comparison of experimental data allows validating the models suggested in the literature. These models allow also the control of the emission power levels of the future free space optical links guaranteeing a sufficient dynamics taking into account the variability of the optical propagation conditions. 2 Light propagation in free space Free Space Optic (FSO) links involve the transmission, absorption and scattering of the light by the Earth's atmosphere. The atmosphere interacts with light due to the atmosphere composition which, under normal conditions, consists of a variety of different molecular species and small suspended particles called aerosols. This interaction produces of wide variety of optical phenomena: selective attenuation, scattering and scintillations. A selective absorption of radiations that propagate at specific optical wavelength in the atmosphere results from the interaction between photons and atoms or molecules (N2, O2, H2, H2O, CO2, O3, Ar, etc.) which leads to the disappearing of the incident photon and an elevation of the temperature. The absorption coefficient depends on the type of gas molecules and on their concentration. Molecular absorption is a selective phenomenon which results in a spectral transmission of the atmosphere presenting transparent zones, called atmospheric transmission window, and opaque zones, called atmospheric blocking windows. Aerosols are extremely fine solids or liquids particles suspended in the atmosphere with very low fall speed by gravity (ice, dust, smoke, etc). Their size generally lies between 10-2 and 100 m. Fog, dust and maritime spindrift particles are examples of aerosols. Aerosols influence the conditions of atmospheric attenuation due to their chemical nature, their size and their concentration. In maritime environment, the aerosols are primarily made up of droplets of water (foam, fog, drizzle, rain), of salt crystals and various particles of continental origin. Type and density of continental particles depend on distance and characteristics of the neighbouring coasts. Atmospheric scattering results from the interaction of a part of the light with the atoms and/or the molecules in the propagation medium. It causes an angular redistribution of this part of the radiation with or without modification of the wavelength. Aerosols scattering occurs when the particle size are of the same order of magnitude as the wavelength of the transmitted wave. In optics it is mainly due to mist and fog. Attenuation is a function of frequency but also of the visibility related to the particle size distribution. This phenomenon constitutes the most restrictive factor to the deployment of Free Space Optical systems at long distance. The visibility is a concept defined for the needs of the meteorology. It characterizes the transparency of the atmosphere estimated by a human observer. It is measured by the Runway Visual Range (RVP), distance that a parallel luminous ray's beam must travel through the atmosphere until its intensity (or luminous flux) drops to 0.05 times its original value. It is measured using a transmissometer or a diffusiometer. Under the influence of thermal turbulence inside the propagation medium, random distributed cells are formed. They have variable size (10 cm 1 km) and different temperature. These various cells have different refractive indexes thus causing scattering, multipath, variation of the arrival angles: the received signal fluctuates quickly at frequencies ranging between 0.01 and 200 Hz. The wave front varies similarly causing focusing and defocusing of the beam. Such fluctuations of the signal are called scintillations 3 Modeling For optical and IR waves, light propagation through the atmosphere is affected by absorption and scattering by air molecules and absorption and scattering by solid or liquid particles suspended in the air. The transmission of the light in the atmosphere is described by the Beer Lambert law: [ ] ( , ) ( , ) exp ( ) ( ,0) P L L L P λ τ λ γ λ λ = = where: ) is the total transmittance of the atmosphere at wavelength P( , L) is the signal power at distance L from the transmitter, P( , 0) is the emitted power, ) is the attenuation or the total extinction coefficient per unit of length Extinction coefficient is composed of absorption and scattering terms. Generally it is the sum of the following terms: ( ) ( ) ( ) ( ) ( ) m a m a γ λ α λ α λ β λ β λ = + + + Where m,a are molecular and aerosol absorption coefficients respectively and m,a are molecular and aerosol scattering coefficients respectively. In the spectrum region used by FSO, attenuation coefficient is only approximated by the coefficient of scattering by the particles present in the atmosphere. 3.1 Kruse and KIM relations Thus, the attenuation coefficient is approximated by the following relation (Kruse relation): 3.912 ( ) ( ) 550 q nm a V λ γ λ β λ − = ; The coefficient q depends on the particle size distribution. It is given by [1]: 1/ 3 1.6 si 50 1.3 si 6 50 0.585 si 6 V km q km V km