km nm) and NDF is -119 ps/(km nm). The spans are 70 km (PDF) and 9 km (NDF). Operation with High Channel Densities We saw in the earlier section on wavelength division multiplexing that one of the main methods of augmenting system capacities is via channel separations of 50 GHz or less. However, crosstalk due to FWM and XPM is greatest with closely spaced channels. The problem can be reduced by launching the channels with alternating orthogonal states of polarization. If this is done and polarization is preserved throughout the system, the FWM is eliminated and the XPM is reduced by two-thirds. Unfortunately, PMD in the fiber and components causes progressive randomization and SO it lowers the effectiveness of the technique. It is also possible to alternate the polarization states between pulses in the same channel with the aim of ameliorating intrachannel nonlinear effects (Bayvel & Killey, 2002). One of the most promising means of enabling the use of closely spaced channels is via alternative low-spectral-bandwidth modulation formats (Matera et al., 1999). Examples are single-sideband generation, multilevel amplitude shift keying, and duobinary transmission. Furthermore, operation with carrier-suppressed return-to-zero modulation tends to suffer from less nonlinear crosstalk than do many other formats (Zhu & Hadjifotiou, 2004; Zhu et al., 2002). The choice of modulation format is complicated by the interactions between chromatic dispersion, nonlinear effects, and the phases and power spectral densities of the pulses. It is outside the scope of this chapter, but the reader is referred to Ho (2001) for further details. Operation Close to the Zero-Dispersion Wavelength Working at or near 20, the wavelength of zero dispersion, is sometimes unavoidable. Two such circumstances are in short-reach applications (such as in PONs or some of the smaller metropolitan networks) and when legacy dispersion-shifted fiber is installed. Operators then have to take great care to avoid nonlinear crosstalk. We briefly consider the i