ors are therefore reluctant to operate intermediate- and long-haul WDM systems where |D| = 0. Fortunately, G.652 fiber is very good from this point of view because, as shown in Figure 6.18, it has values of D between +15 and +22 ps/(km.nm) over the C- and L-bands. The dispersion management schemes shown in Figures 6.21 and 6.22 cause successive pulse expansion and contraction, but the PDF and NDF both have nonzero values of |D| and SO multichannel nonlinear effects can often be well suppressed. Many studies have been performed on optimal dispersion compensation strategies in the presence of nonlinear transmission. The dispersion map chosen depends on how the required total capacity is to be provided (e.g., 4M channels at 10 Gbit/s or M channels at 40 Gbit/s). Different strategies apply for point-to-point systems and networks that include optical add-drop multiplexers. Logistical factors also influence the choice of dispersion map, such as whether existing fiber is to be reused for higher-capacity operation (for financial reasons) or if new fiber can be justified. Moreover, different maps are optimal for subsea and terrestrial applications. We cannot summarize all of the options for chromatic dispersion compensation but we do note one feature. The Nonlinear Optical Transmission section described how SPM in the presence of a positive dispersion fiber can lead to pulse compression. We can take advantage of this fact to mitigate BER deterioration by what is known as "optimal under-compensa- tion" (Bayvel & Killey, 2002). In terms of equation 6.32, we have, for standard single-mode fiber, |DG.652 (6.41) where x is a factor with a typical value of ~0.9, which is found from simulations using the NLSE. An example of an optimally undercompensated dispersion map for use in the pres- ence of nonnegligible SPM is shown in Figure 6.33. Chapter 6 Optical Fiber Transmission G.652 Path Averaged Dispersion Transmission Length (km) Fig. 6.33 Dispersion map illustrating optimally undercompensated dispersion in which PDF is +17 ps/(