pagating photons can be also absorbed by the external electrons of the glass molecules, promoting them to higher energy levels. This mechanism called electron absorption, is efficient in the ultraviolet region and decreases increasing the wavelength. Besides intrinsic absorption, impurities in the glass composition cause extrinsic absorp- tion. In the infrared region of interest for fiber optics communication, extrinsic absorption is mainly due to metal impurities and to hydroxyl ions. These last elements in particular cause two absorption peaks to appear in the near infrared that have a great importance in determining wavelength windows for fiber communications. Besides absorption, another important loss element is light scattering, that is, the phe- nomenon causing a change of photons momentum and consequently their jump from the fundamental mode to a mode that does not propagate along the fiber. At low propagating powers, linear scattering is prevalent. In this case, no wavelength change exists between the incident photon and the scattered photon for each scattering process and the scattered photons are lost generally for radiation. Linear scattering can be studied introducing a parameter called scattering scale and defined as (4.5) where dp is the diameter of the typical scattering particle. If S 1, then we are in the presence of Rayleigh scattering, if S = 1, we are in the presence of Meie scattering. The prevalent form of scattering is the Rayleigh scattering, caused by particles and imperfections much smaller than the propagating field wavelength. Rayleigh scattering occurs with silica molecules composing the glass and it is greatly enhanced due to micro- defects that are present in the fiber material [7]. Rayleigh scattering introduces an attenuation that decreases with the wavelength. Finally, fiber imperfections such as imperfect concentricity of core and cladding intro- duce another loss factor in the field propagation. In order to analytically express the behavior of attenuation versus the field wavel