e products have an inverse varia- tion with its effective area. FWM is at its worst when the channels are equally spaced in terms of frequencies and have small separations. The number of mixing products varies approximately as the cube of the channel number, N, which means that many products can be generated. The difficulty for WDM optical communications is when one or more of the products is passed to the detectors, which happens to the ones that lie within the passband of the de-multiplexer at the receiver end of the system. The result is increased BER. Other Nonlinear Effects Other nonlinear effects can occur in optical fiber transmission. We draw attention to two that are based on the principles that have been described in this section. When operating at very high datarates (e.g., 40 Gbit/s and above), the pulses are easily broadened beyond their bitslots by the fiber's chromatic dispersion. When the intensities are sufficiently high, neigh- boring pulses (logical 1s) can then overlap and interact via the intensity-dependent refrac- tive index. The spectral components of the pulses can thereby cause what are termed Chapter 6 Optical Fiber Transmission "intrachannel XPM" and "intrachannel FWM" (Bayvel & Killey, 2002; Shake et al., 1998; Essiambre, Mikkelsen, & Raybon, 1999). In common with the other effects described here, intrachannel XPM and FWM depend on the effective length, the fiber's dispersion and effective area, and the launched channel powers. We therefore have the means to reduce their impact on BER by appropriate selec- tion of fiber type(s) and design of the system's power budget. The issue is considered further in the chapter's final section. Modulation Formats In the section on single-channel transmission, it was shown that optical data is most com- monly transmitted by applying the On-Off Keying (OOK) modulation formats illustrated in Figure 6.1, known as Nonreturn to Zero (NRZ) and Return to Zero (RZ). More advanced modulation formats are chosen to make best use of the available bandwidth and 