causes the Brillouin threshold for an amplitude modulated signal to increase up to about 100 mW. Thus, as far as the transmitted power is below 50 mW Brillouin scattering can be neglected. Normalized power spectrum (log scale) 0.001 0.0001 0.00001 Carrier 0.000001 Frequency FIGURE 6.3 Baseband power spectrum of an intensity modulated signal. Transmission Systems Architectures and Performances Indicating with (p+,p-) and (pit, Pi-) the output and the input principal states of polariza- tion (PSPs) of the transmission fiber, the average field at the receiver input can be written as (6.20) where 80 is the nonlinear propagation operator Tt, T are the PSPs' delays In compliance with the hypothesis of small nonlinearity, the pulses are assumed to propa- gate independently SO that 80 can be applied to every pulse individually [9]. It is to be noted that not only 80 is nonlinear in general, but also incorporates the chan- nel memory SO that it results to be signal dependent. However, this difficulty can be solved by considering the patterns Pk as independent signals instead of the single bits. Considering NRZ modulation, if SPM is negligible, the pulse broadening due to chro- matic dispersion and PMD can be analyzed separately and added at the end to obtain the overall pulse spread. Chromatic dispersion effect is deterministic, and can be represented with the effect of a linear operator on the pulses. Due to the channel memory, it is necessary to define eight signals that represents the eight patterns Pk. Calling nT(t) the function equal to 1 if O<t<T and to 0 we can write (6.21) It is noteworthy that = = Vsk(t) SO that the signal received when one of the patterns Pk is sent can be rewritten, neglecting SPM and PMD, as (6.22) where Pout is the detected power DGD represents the dispersion operator is the average propagation delay is a generic polarization vector Now is DGD is a well-known operator, and the dispersion related part of Equation 6.22 can be rewritten, passing from the Fourier domain, as (6.23) Telecommunicati