ion. From this qualitative observation, justified by the opposite sign of the phase chirps imposed by chromatic dispersion and SPM, it is intuitive that in opportune conditions SPM can help in attenuating the effects of dispersion. 6.2.3.3 RZ Signal after Propagation If RZ modulation is concerned exploiting again the hypothesis of small distortions, it is possible to demonstrate [10] that a Gaussian pulse propagating along a fiber remains Real pulse shape ps*ps/km (envelope) Transmitted pulse Triangular approximation -0.15 -0.05 Time (ns) FIGURE 6.4 NRZ pulse after propagating through 40km of SSMF with perfect linear propagation and absence of PMD. The triangular approximation of the distorted pulse is also shown. Transmission Systems Architectures and Performances Instantaneous power (AU) FIGURE 6.5 NRZ pulse sequence transmitted to generate the example of Time (ns) Figure 6.6. Instantaneous power (AU) Linear Nonlinear FIGURE 6.6 An example of the nonlinear evolution of an NRZ pattern com- paring linear evolution (at a transmitted power of -10dBm) and strongly nonlinear evolution (at a transmitted power of 13 dBm). An SSMF fiber is assumed with a length of 30 km. Time (ns) Gaussian in shape, undergoing a broadening caused by interaction between chromatic dispersion, SPM, and polarization dispersion. While chromatic dispersion and SPM are deterministic phenomena, and their interaction cannot be neglected due to the fact that both produce a chirp that modifies the signal phase, PMD is a statistical phenomenon and the PMD induced broadenings can be considered additive. In [10] the following approximated equation is found for the final pulse width T1 relative to a Gaussian pulse propagating in the presence of chromatic dispersion and (6.24) where L is the link length Le is defined in Equation 4.36 The dispersion and nonlinear lengths are defined as follows: Telecommunication Networks (6.25) where both the effective area A and the nonlinear index N2 are defined in Chapter 4. Equation 6.64 even if obtained for the fir