Optical Networks: A Practical Perspective - Part 13. This book describes a revolution within a revolution, the opening up of the capacity of the now-familiar optical fiber to carry more messages, handle a wider variety of transmission types, and provide improved reliabilities and ease of use. In many places where fiber has been installed simply as a better form of copper, even the gigabit capacities that result have not proved adequate to keep up with the demand. The inborn human voracity for more and more bandwidth, plus the growing realization that there are other flexibilities to be had by imaginative use of the fiber, have led people. | 90 Propagation of Signals in Optical Fiber Section when we consider four-wave mixing. The component of the nonlinear dielectric polarization at the frequency ui is eox 3 e 2E2 Ex cosier - ftz . When the wave equations and are modified to include the effect of nonlinear dielectric polarization and solved for the resulting electric field this field has a sinusoidal component at a x whose phase changes in proportion to E2 2E2 z. The first term is due to SPM whereas the effect of the second term is called cross-phase modulation. Note that if Ex E2 so that the two fields have the same intensity the effect of CPM appears to be twice as bad as that of SPM. Since the effect of CPM is qualitatively similar to that of SPM we expect CPM to exacerbate the chirping and consequent pulse-spreading effects of SPM in WDM systems which we discussed in Section . In practice the effect of CPM in WDM systems operating over standard single-mode fiber can be significantly reduced by increasing the wavelength spacing between the individual channels. Because of fiber chromatic dispersion the propagation constants Pi of these channels then become sufficiently different so that the pulses corresponding to individual channels walk away from each other rapidly. This happens as long as there is a small amount of chromatic dispersion 1-2 ps nm-km in the fiber which is generally true except close to the zero-dispersion wavelength of the fiber. On account of this pulse walk-off phenomenon the pulses which were initially temporally coincident cease to be so after propagating for some distance and cannot interact further. Thus the effect of CPM is reduced. For example the effects of CPM are negligible in standard SMF operating in the 1550 nm band with 100 GHz channel spacings. In general all nonlinear effects in optical fiber are weak and depend on long interaction lengths to build up to significant levels so any mechanism that reduces the interaction length decreases the .
