 point will be considered in more detail describing nonlinear propagation. 4.2.5 Polarization Mode Dispersion Polarization mode dispersion (PMD) is the phenomenon causing strong coupling between the fundamental LP modes SO that the propagating field polarization evolves during prop- agation [8,9]. The polarization not only changes from fiber section to fiber section, but also changes in time and wavelength. This is due to the fact that mode coupling is generated by fiber imperfection both at the material level and at a fiber form level. While temperature and mechanical conditions like micro-stresses evolve, mode coupling changes. The fact that the field polarization changes with wavelength is particularly dangerous for modulated signals when the field polarization has to be recovered at the receiver. As a matter of fact, this becomes impossible if different frequencies of the signal spectrum have different polarizations. Moreover, since the diagonal terms of the Jones matrix are not equal to one, different polarizations travel through the fiber at slightly different group velocities, thus causing the so-called polarization dispersion. This means that a pulse of finite duration sent into the fiber experiments a widening during propagation even in the absence of chromatic dispersion due to the PMD. The base for the analysis of PMD impact is the definition of the principal states of polar- ization (PSPs), which for a fiber with small random birefringence somehow take the place of the birefringence axes in a high birefringence medium. Given a fiber piece, the PSPs of that fiber in the considered deploying conditions are defined as the input states of polarization that generate at the fiber output a polarization state that does not depend on W at the first order [8]. The PSPs can be derived directly from their definition and from the general expression Equation 4.8 of the field during linear propagation in the fiber. As a matter of fact, the definition of the PSPs implies that, at first order, their W derivative has t