n without material dispersion, this term is present. A typical waveguide dispersion curve is shown in Figure 5.16. The importance of this term is not SO much in the value of the dispersion but in the sign. In silica fiber the waveguide dispersion is negative, and because the material disper- Table 5.3 Attenuation of Corning SMF-28e+TM TM Single-Mode Fiber Wavelength (nm) Maximum values (dB/km) 0.33-0.35 0.31-0.33 0.21-0.24 0.19-0.20 0.20-0.23 Source: Corning Inc. Data Sheet, December 2007 Issue. Single-Mode Fibers Table 5.4 Single-Mode Fiber Dispersion of Corning SMFe+ in the Third Optical Window Wavelength (nm) Dispersion value [ps/(nm X km)] <18.0 <22.0 Source: Corning Inc. Data Sheet, December 2007 Issue. Table 5.5 Zero-Dispersion Wavelength and Slope of Corning SMF-28e+ Single-Mode Fiber Zero-dispersion wavelength (nm) 1310 20 1324 Zero-dispersion slope [ps/(nm2 2 X km)] So 0.092 20 typical value (nm) So typical value [ps/(nm2 X km)] 0.088 Source: Corning Inc. Data Sheet, December 2007 Issue. A(um) Fig. 5.16 Waveguide dispersion with Ac = 1.2 and A = 0.003. sion is positive above 20, the value of the total dispersion is reduced. The immediate impact is to shift 20 to longer wavelengths. By itself this small shift in 20 is of marginal importance, but if the value of the waveguide dispersion is designed to take much higher values, then the shift is substantial and gives rise to two new classes of single-mode fiber: dispersion shifted and dispersion flattened, which will be discussed in the next section. The third component of dispersion in single-mode fiber is polarization mode dispersion, or PMD. For ease of study it has been assumed tacitly that the fiber is a perfect cylindrical structure. In reality the structure is only nominally cylindrical around the core axis. The core can be slightly elliptical. Also, the cabling process can impose strain on the fiber, or the fiber might be bent as it is installed. In these situations the fiber supports the propagation of two nearly degenerative modes with orthogonal p