index of the cladding next to the core is less than that of pure sil- ica; this is done to better confine the optical power distribution to the core (refer once more to Figure 2-5). The ZDP for these fibers occurs at about 1.3 um; the ZDP can be shifted by changing the index profile to a triangular Chapter 2 Spectral width of light sources Figure 2-19 Spectral scope of Power EE-LED sources. SE-LED 1300 nm LASER Loss of single-mode fiber Loss (dB/km) Wavelength (nm) shape, making the zero occur near 1.55 um. As Figure 2-18 suggests, this is where the lowest loss can be obtained. By trading off waveguide dispersion against material dispersion over a range of wavelengths, a dispersion-flattened fiber can be developed. Flat- tening the dispersion enhances the bandwidth properties of the fiber over a greater range of wavelengths. (The index profile for this design is shown in Figure 2-5[f].) Figure 2-20 plots the dispersion for three kinds of fiber: con- ventional, flattened, and shifted. Figure 2-21 expands upon the trade-off by showing the dispersion values of conventional fiber compared with the fiber's loss. As you can see, conventional fiber loss is minimal at about 1.55 um and has zero dispersion near 1.3 um. Figure 2-22 shows the bandwidth-length product to be greater at the ZDP; however, the bandwidth depends on the spectral width of the source (such as an LD or LED). For example, with a source spectrum or of 40 nm (such as for an LED transmitter), the bandwidth-length product for the fiber at the ZDP is about 10 GHz/km, and for a source spectrum of 2 nm Optics Primer Dispersion (ps/km-nm) Figure 2-20 Single-mode fiber Material dispersion as a function of Flattened wavelength. Shifted Wavelength (um) Modified dispersion. Loss (db/km) Figure 2-21 Characteristics of Conventional fiber dispersion conventional fiber. Dispersion (ps/km-nm) Wavelength (um) 10 = ZDP or = Line width of source spectrum Chapter 2 Bandwidth (GHz-km) Figure 2-22 Single-mode fiber transmission characteristics. 81 2nm 10=ZDP Wavelength (um)