amental modes is given by (4.4) where E(, 0) is the electrical field of a spherical wave in spherical coordinates on the points of the fiber input facet is the scalar product x and y are the unitary axes vectors integral is extended to the fiber input facet This is the maximum possible coupling efficiency. In practice, this value is never reached, due to misalignments of the focusing system both along the Z axis (the focus is not exactly on the fiber facet) and in the x-y plane (the focus is not at the center of the facet). Moreover, fiber defects (like nonconcentricity of core and cladding and core deviation from a perfect cylinder) also contribute to increasing the coupling losses. Considering coupling of a standard step index fiber with a distributed feedback laser, practical values from 1.5-3 dB are achieved, depending on the complexity and on the toler- ance of the focusing lenses and of the assembly process. 4.2.2.2 Propagation Losses Ideally, the fundamental mode does not attenuate during transmission along the fiber. In a real fiber however, there are several mechanisms causing power attenuation. All the loss mechanisms are wavelength dependent and the final fiber attenuation is obtained by their superposition. The first loss mechanism is the photon absorption. The absorption can be divided into intrinsic absorption and extrinsic absorption. Intrinsic absorption is due to the characteristics of the glass composing the fiber. If no impurities were present in the fiber material, all absorption would be intrinsic. Since the glass is an amorphous and isotopic material, it can be described as a disor- dered ensemble of microcrystal structures. Thus, local phonons [6] can be introduced, Telecommunication Networks which models with a good approximation microcrystals vibrations related to the glass temperature. Local phonons can be excited by the photons of the propagating radiation, thus causing photons absorption that is efficient in the infrared region and increases increasing the wavelength. Moreover, the pro