e a high absolute value, with 100 to 200 ps/ (km nm) a common range. The use of these values in equation 6.31, together with data from Figure 6.18, shows that if spacing between the DCMs is 50 to 80 km, 5 to 10 km of DCF is needed on the bobbin. There are many proprietary designs of DCF, normally having refractive index profiles that differ significantly from step index. Most of them achieve the required values of D and S by the use of relatively high refractive index but narrow diameter cores. Such fibers have small effective areas and therefore are more susceptible to nonlinear optical crosstalk, as described in the later section Nonlinear Optical Transmission. Unfortunately, the waveguide designs necessary for DCF normally require high levels of GeO2 doping (Mendez & Morse, 2007), which leads to increased RBS and therefore greater loss. RBS causes increased noise in amplified systems, as discussed in the subsection on the noise figure of optical amplifier chains. The overall loss of DCF is also a concern. Whereas Chromatic Dispersion: Mitigation the (positive-dispersion) transmission fiber has loss coefficients in the range a = 0.20 to 0.25 dB/km in the C-band, we can anticipate values that are two to four times as high from the DCF. For this reason, DCF manufacturers strive to design for the largest possible |DDCF|, simultaneously with the lowest value of loss; therefore, they quote a figure of merit, FOM = measured in ps/(nm.dB), which should be as high as possible (Wandel et al. 2001, 2002). The small-core waveguide structure of the DCF causes another problem: splicing loss (Yablon, 2005). The DCF has to be spliced at both ends to the relatively large-core transmis- sion fiber and, even with the use of special mode field adaptation techniques, we can antici- pate a typical loss of 0.5 dB at each end. DCMs often incorporate optical amplification to compensate for their losses. When EDFAs are used, the DCF can be included between two spans of the erbium-doped fiber and each one can be pumped forward, backward