persion Fiber The main optical compensation technique is called "dispersion management" (Wilner & Hoanca, 2002). As stated previously, if we use Positive-Dispersion Fiber (PDF), the short- wavelength components of a pulse travel fastest, but in Negative-Dispersion Fiber (NDF) the long-wavelength components travel fastest and arrive first. We can therefore alternate the two types of fiber in our system, with the effect on the pulses illustrated in Figure 6.21, which shows that they expand and then contract again. In order to provide ideal dispersion compensation in this way, the fiber lengths and dispersion coefficients must obey |DPDF LIPDF=IDNDFILINDF (6.31) High-capacity systems have many periods that each conform to equation 6.31, and the overall variation is called a "dispersion map," an example of which is shown in Figure 6.22. The objective is to ensure that the accumulated dispersion is within acceptable bounds by the end of the system. Consequently, the pulse duration at the receiver is close to that at the transmitter and SO the power penalty is tolerable. Chromatic Dispersion: Mitigation Pulse from Shorter-Wavelength Transmitter Transmission Fiber Components of the (logical 1) (positive dispersion) Pulse Travel Faster Reformed Pulse (longer a components Positive DCF (negative travel faster) Dispersion dispersion) Fig. 6.21 Schematic of the role of dispersion-compensating fiber in a simple point-to-point system. (a) Uncompensated; (b) compensated. ps/nm Length (km) ps/(km nm) Length (km) Fig. 6.22 Dispersion map consisting of alternating spans of PDF and NDF according to equation 6.31 (not to scale). Operating with high-datarate channels and/or WDM is a little more complicated. As Figure 6.18 shows, a fiber's dispersion coefficient, D, varies with wavelength. High-speed systems have very broad bandwidth pulses, which means that D varies slightly across the pulse spectrum. Similarly, in WDM operation each channel experiences a different value of D. In both cases, the consequence is that equation 6.31 appl