ommendation. The fiber covered by this recommendation is the most widely installed in the worldwide telecommunications network. nx > ny Fig. 5.17 Effect of PMD on a pulse. Source: Corning Inc. Table 5.6 Impact of PMD on System Length for Typical Bitrates System length (km) Bitrate (Gbit/s) Timeslot (ps) Link PMD (ps) PMDcoeff 0.02 ps/km PMDcoeff 0.5 ps/ km 4.0 x 106 6.4 X 103 2.5 x 105 4.0 X 102 15.6 x 10 3 100.0 2.5 X 103 Special Single-Mode Fibers Until now the discussion has been limited to what may be called the conventional single- mode fiber. That is a single-mode fiber with 20 =1300 nm, low waveguide dispersion (Dm Dw), and a step index profile. Since the introduction of the conventional single-mode fiber, a number of applications have arisen where a fiber optimized for the application can deliver better performance. We are now going to take a very brief look at these special fibers. Special Single-Mode Fibers As was mentioned in the Single-Mode Fibers section, waveguide dispersion can be used to shift the 20 of a fiber to a direction that the dispersion of the fiber will be substantially altered. Initially, two classes of fiber emerged to exploit this property of waveguide dispersion. The first was the dispersion-shifted fiber in which the value of the waveguide dispersion was large and the 20 shifted to 1550 nm. At this wavelength the zero dispersion and the minimum loss value of silica glass coincided, offering the best of both worlds: dispersion and loss. This fiber design favors Electrical Time Division Multiplexed (ETDM) systems, and it was introduced at a time when the only practical way to increase the capacity appeared to be ETDM. Later, however, a better approach in terms of cost-performance was identified with Wavelength Division Multiplexing (WDM), and another fiber was introduced in which the dispersion presented a plateau enabling the WDM channels to face similar dispersion. Unfortunately, again the fiber could not be employed in large quantities because of the nonlinear behavior of the silic