andwidth varies as the inverse of the pulse duration and therefore in proportion to the bitrate. Second, the duration of the neighboring bitslots into which the pulses spread is the inverse of the bitrate. Equation 6.30 is plotted in Figure 6.19 to give approximate values for 1- and 2-dB power penalties and for when the pulses expand to one whole bit period (E = 1). We assume for simplicity that b = 1. The transmission distance reduces greatly with bitrate. For example, transmission at 2.5 Gbit/s permits operation up to 1160 km when E = 1. However, at 10 and 40 Gbit/s, the span is reduced to 74 and 4.6 km, respectively, showing that uncompensated chromatic dispersion can render all but the shortest systems inoperative. It should be noted that the values given in Figure 6.19 are merely indicative. They assume that the linewidth of the unmodulated laser is narrow, that the pulses that constitute the 1s are unchirped, that there are negligible spectral sidelobes, and that nonlinear optical effects in the transmission fiber can be neglected. Still, it is clear that finding the means to overcome the limits imposed by chromatic dispersion is essential, especially as the datarate is increased. This is the subject of the following section. 10000 STM-16 STM-64 STM-256 Fig. 6.19 Maximum transmission distance for NRZ modulation limited by chromatic dispersion in a G.652 fiber for which D = +17 ps/(km.nm) at 1550 nm. The values of E in equation 6.30 are 1.0 (top), 0.491 (center), and 0.306 (bottom). Datarate (Gbit/s) Chromatic Dispersion: Mitigation Mitigation of Chromatic Dispersion Optical Dispersion Electronic Dispersion Compensation Compensation Dispersion Accommodation Alternative Lower- Laser Chirp Modulation Dispersion Control Formats Fibers (e.g., using (e.g., duo- (e.g., external binary) NZ-DSF) modulators) Dispersion Dispersion Precompensation Postcompensation Management Components (e.g., DCF (e.g., chirped modules) FBGs) Fig. 6.20 Categorization of techniques to mitigate chromatic dispersion. Chromatic Dispersion: