bout 80 channels. Such use is most likely to be found in longer-span national and international systems. Figure 6.4 indicates the possibility of even denser grid lines: at 25 or 12.5 GHz. Channels spaced at 25 GHz (approximately 0.2 nm) would be for WDM operation with many closely spaced channels to achieve very high spectral efficiencies. However, such operation can be technically demanding. The laser and filter wavelengths have to be tightly specified, which increases their costs. Very close channel spacing also increases the likelihood of crosstalk due to four-wave mixing and cross-phase modulation (the subject of a later section). Moreover, operation with 25-GHz channels precludes very high datarates (such as 40 Gbit/s) because the modulation bandwidths are SO great that they spread over several grid lines. The grid lines at 12.5 GHz are for a special purpose: to permit operators with a fiber type known as Dispersion-Shifted Fiber (DSF) in their networks to mitigate nonlinear crosstalk by launching unequally spaced channels (see the later sections on cross-phase modulation, four-wave mixing, and zero- dispersion wavelength). These 12.5-GHz lines are of much less interest to users of nonzero- dispersion fibers, such as the G.652 category mentioned earlier. Amplification in Optical Systems Amplification in Optical Systems Wavelength division multiplexed systems were initially developed for long-haul applica- tions. In these systems, it is essential to boost the signal at regular intervals to overcome the attenuation of the fiber and components in the transmission path. Electronic regeneration is one option, but the cost of conversion from optical to electrical and back to optical for each channel is high. Moreover, the process becomes even more demanding as channel datarates are increased. Optical amplifiers offer the exclusive benefit that they can boost all channels together, often irrespective of their datarates, without the need for optical-to-elec- trical conversion. Optical Amplifier Characteristics Optic