BOOK_CONTENT
From Free-Space Optics: Propagation and Communication

4.4. Meteorological Disturbances

4.4.1. Mie Scattering Attenuation

4.4.1.1. Theoretical Aspect

The phenomenon of scattering occurs when t he size of particles is of the same order of magnitude as the wavelength of the transmitted wave. In optics this is mainly due to mist and fog. Attenuation is a function both of frequency and of visibility related to the particle size distribution. This phenomenon constitutes the most restrictive factor to the deployment of Free-Space Optical systems over long distances.

Attenuation can reach 300 dB per km whereas rain attenuation in millimeter waves is only about 10 dB per km. The scattering coefficient ?n is given by the following equation:

(4.17)

where:

  • ? n( ?), in km -1, is the scattering coefficient of the aerosol,

  • ?, in ?m, is the wavelength,

  • dN( r)/ dr, in cm -4, is the particle size distribution per unit of volume,

  • n' is the real part of the refractive index n of the considered aerosol,

  • r, in cm, is the radius of the particles,

  • Q d( 2 ?r/ ?, n') is the scattering cross section for a given type of aerosol.

Mie theory allows the expression of the scattering coefficient Q d due to the aerosol. This is calculated on the assumption that the particles are spherical and sufficiently distant from each other so that the field of scattering by a particle and of interaction with another one can...

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