h Wavelength (um) FIGURE 4.3 Dispersion contributions and total dispersion versus wavelength in a standard SSMF single-mode fiber for signal transmission. This effect is shown in Figure 4.3 in the case of a step index fiber. In the figure, both the overall dispersion and the individual contributions are shown, evidencing the zero disper- sion point (around 1.33 um) where the two dispersion contributions are opposite. The zero dispersion wavelength divides the wavelength axis in two intervals, that for historical reasons take the name of normal dispersion wavelengths (B" >0) and anomalous dispersion wavelengths (B" <0). The dispersion parameter takes into account the greater contribution to fiber dispersion, but there are several cases in which the other terms of Equation 4.10 cannot be neglected. The most important of these cases is propagation around the zero dispersion frequency. In this case, dispersion is almost all due to the third-order term, which at high transmis- sion speed cannot be neglected. As in the case of second-order dispersion, also in the third-order case, the pulse broad- ening can be approximately expressed as a linear function of the square signal bandwidth, thus defining a third-order dispersion coefficient. In particular, the following equation is obtained: (4.19) The dispersion-related parameters of a standard step index fiber in the second and the third transmission window are reported in Table 4.1. The fact that waveguide dispersion can be opposite in sign with respect to material dis- persion can be used to shape the fiber dispersion curve. To control waveguide dispersion, the refraction index behavior along the fiber diameter is suitably designed. Depending on the application, different types of dispersion-managed optical fibers can be used. Dispersion-shifted (DS) fibers are designed to move the zero dispersion point from the second to the third transmission window, dispersion flattened fibers are engi- neered to have a very low (but nonzero) dispersion in a wide wavelength interval,