 Last but not least, macroscopically identical pieces of fiber can have completely differ- ent PSPs, due to the different distribution of micro-defects causing coupling between the fundamental modes. Starting from the PSPs definition, the PMD characteristics can be derived. Let us call p+ and p the unit vectors corresponding to the PSPs output polarizations of the considered fiber piece. A generic field injected into the fiber can be decomposed along the input PSPs SO that, for the properties of the PSPs, the same decomposition holds for the output field with respect to the output PSPs. This decomposition can be written in general as follows: (4.24) Starting from Equation 4.24, it is possible to evaluate the polarization dispersion contribu- tion with a simple derivation exploiting the PSPs properties [12]. Squaring Equation 4.24 and remembering that the PSPs are orthogonal states, the fol- lowing equation is obtained for the pulse envelope: Telecommunication Networks (t)=g+(t)sin2(0)+g-(t)cos2(0) (4.25) where the envelopes g+(t) and g-(t) are both replicas of the input pulse envelope that arrive at the output with different delays due to their travel along the two different PSPs. Taking into account this consideration, we can imagine the propagation of two identical pulses launched through the fiber along the input linear polarization of the LP states. The output envelope in the two cases can be written as in Equation 4.25 with the suitable coefficients obtaining g1(t)=g(t-t+)sin2(0)+g(t-t_)cos2() (4.26) where T_ and T are the delays of the PSPs. The differential delay due to PMD is by definition the time interval separating the instants in which the peaks of two identical pulses launched in the fiber along the LP fun- damental states arrive at the fiber output. Since Equation 4.25 simply gives the expressions of the output pulses envelopes, this can be the starting point for the PMD differential delay calculation. For a finite duration pulse, it is possible to define the moment generating function and the gener