ter wavelengths travel faster). 3. When utilizing WDMs, multiple wavelengths travel down a fiber by design. Here the approach to address dispersion is to utilize a certain amount of dispersion compensation fiber (DCF) at the receiving end. By properly selecting a terminating fiber that has the opposite effect of the transmission fiber in terms of dispersion, you can reduce the impact of the dispersion phenomenon. The add-on fiber takes the rays that traveled faster and slows them down more than the rays that traveled slower. Unfortunately, a rather long spool of fiber is needed, which is placed in a "pizza box." Such spools could have several kilometers of fiber. Pizza boxes have to be selected based on the length of the transmission link. This requirement constitutes a problem because add-on fiber further impacts attenuation- - the added fiber must be taken into account in the power budget. A relatively new approach is to use tunable dispersion compensators (see Figure 2-14 for examples). 5 Chapter 2 Figure 2-14 Chirped FBG Dispersion Incident compensation External without DCF. Mechanical Ridges are etched Reflected Stretcher onto the fiber No Slope Change at 10 Slope Change at 10 Stretch Stretch Relative Relative Delay (ps) Delay (ps) 10 Wavelength (nm) Wavelength (nm) Linearly Chirped Nonlinearly Chirped (top) Nonlinearly Chirped FBG Circulator 10 cm Incident Epoxy Epoxy Single Mechanical Stretching Element Reflected (bottom) Top: Dispersion Compensation Using a Chirped Fiber Bragg Grating Bottom: Tunable Dispersion Compensator The bottom line is that chromatic dispersion can be managed, even at higher speeds, but only at the cost of system and power. Returning to chromatic dispersion on non-WDM-used fiber, the optimal center wavelength for the transmitter is the point where chromatic disper- sion is zero. This wavelength is called 10 or the zero-dispersion point (ZDP). This value varies from optical fiber to optical fiber and is an important parameter for fiber manufacturers to control. Another parameter of i