d, whereas the fiber is a leaky waveguide for the other polarization. After the fibers having particular polarization properties, a great importance in the design of DWDM systems is the dispersion compensating fibers (DCFs) [30,31]. This kind of fiber is designed to have a strong dispersion in the third transmission win- dow SO that a relatively short fiber piece can compensate an entire SSMF span from 70 to 100 km. This result is achieved by suitably shaping the refraction index in the core and in the cladding of the fiber. A typical index shape of a DCF is the segmented core profile represented in Figure 4.4. Two figures of merit can be given for DCFs: 1. Efficiency (e) that is dispersion divided by the dispersion slope for a standard SSMF divided by the same ratio but for the DCF. 2. Figure of merit (FM) that is minimum dispersion in the C band divided by maxi- mum attenuation in C band. Efficiencies greater than 60% and FM as high as 150 can be obtained from commercial products that are completely sufficient for the field applications in DWDM systems. For example, a DCF length having 11.5 dB of loss can compensate 1640ns dispersion in the C band (1545-1606 nm). Photonic crystal fibers (PCFs) [32] are another type of special fibers that could find wide application in future telecommunication system mainly in active and passive fiber devices. They are an attempt to incorporate the bandgap ideas of photonic crystals into the fiber structure by stacking periodically a regular array of channels and drawing into fiber form (see Figure 4.11). Two classes of PCFs exist [33,34], according to their mechanism for field confinement. Those with a solid core, or a core with a higher average index than the microstructured cladding, operates on the same index-guiding principle as conventional optical fiber. Telecommunication Networks However, they can have a much higher effective refractive index contrast between core and cladding, and therefore, they can have much stronger confinement for applications in nonlinear optical d