by the multiplication of the input field in the fre- quency domain with the frequency transfer function of the fiber. In real fibers factors breaking fundamental modes degeneracy in terms of the transverse field shape are present, but are not important for telecommunication systems. Thus, we can assume that the transverse shape of the field remains equal to A(p) in every fiber sec- tion and for every field polarization. We have also seen that the field attenuation is a slowly varying function of the field frequency. For practical modulation, even at 100 Gbit/s, the attenuation variation in the signal bandwidth is completely negligible; thus, in writing the fiber transfer function we can assume a constant attenuation. Telecommunication Networks Fiber nonideality influences greatly the field phase evolution. As a matter of fact, in a real fiber it is not possible to assume that the propagation constants of the two LP funda- mental modes are equal and that these modes do not couple during propagation. On the contrary, the propagation constant depends on the field polarization and if a mix of the two fundamental modes is injected into the fiber, they will couple during propaga- tion generating in every fiber section a different elliptical polarization. Last but not least, microscopic imperfections cause coupling between the fundamental modes SO that even if only one fundamental mode is launched into the fiber, that is the input field is perfectly LP, at the output, the field has a generic elliptic polarization reveal- ing the excitation of both the fundamental modes. Moreover, the output polarization will vary slowly in time, due to time variation of the coupling coefficient driven by phenomena like temperature and material stress variation. These empirical observations can be summarized in the following expression of the field at a generic fiber section (4.8) where the matrix Mxx(z,w) Mxy(z,w) (4.9) called Jones matrix, has to be unitary to fulfill energy conservation and represents both birefringence (i.e., the dep