are a fraction of the width of the bitslots and, despite their higher dispersive broadening, they can expand to a greater extent in the time domain before they overlap significantly with adjacent slots. By careful optimiza- tion of the duty cycle, the increased flexibility for temporal broadening before unacceptable intersymbol interference can often more than compensate for the pulses' sensitivity to chromatic dispersion. Furthermore, the greater dispersive broadening that the pulses Chapter 6 Optical Fiber Transmission experience in RZ modulation can reduce their peak powers and thus the extent to which they are affected by SPM. The robustness to nonlinear crosstalk tends to favor RZ over NRZ channels when oper- ating at higher bitrates. High-datarate communications (such as at 160 Gbit/s and above) are often achieved by the technique of "optical time division multiplexing," in which (nor- mally) four trains of short pulses are interleaved in the time domain (Urquhart, 2004; Ludwig et al., 2002). The interleaving process is more tolerant of time interval errors when the pulses are shorter than the bitslots and SO RZ modulation is used. We have seen that both NRZ and RZ offer advantages. On the whole, RZ is preferred above 10 Gbit/s, while NRZ tends to be preferable below that value. Both formats are used in 10-Gbit/s systems, where the arguments are more finely balanced. The decision on which to use can often depend on transmission span and the number and density of the WDM channels. Alternative Modulation Formats Other modulation formats have been extensively explored in laboratories (Ho, 2001; Kobayashi et al., 2000); the two main reasons for this are to increase the spectral efficiencies in DWDM systems and to reduce nonlinear crosstalk. Operation on the more closely spaced channels, shown in Figure 6.4, at high bitrates confers significant advantages if it can be done without penalties due to nonlinear effects. One clear benefit of very dense channel spacing is to make the best use of the C-band, where the 