the silica glass, causing unwanted radiation of the signal. Loss peaks at a wavelength of about 1.39 um, caused by OH-radical impurity (see Figure 2-10). OH-radical impurity must be controlled to prevent loss spreading into the usable wavelength bands. Losses for high-quality fiber are typically less than 0.40 dB/km and 0.22 dB/km at 1.31 um and 1.55 um, respectively. New fibers have been developed that eliminate the 1,385-nm water peak. Attenuation is also caused by bending. There are two forms of bending: macrobending and microbending. Macrobending is loss that is attributable to macroscopic deviations of the axis from a straight line. Microbending loss occurs from the sharp curvatures involving local axis displacements of a few micrometers and spatial wavelengths of a few millimeters. These bends result from optical fiber coating, cabling, packaging, installation, and aging.10 2.3.2 Dispersion and Zero-Dispersion Point (ZDP) Intramode dispersion can be subdivided into material dispersion and wave- guide dispersion. Material dispersion refers to the wavelength dependence of the fiber's refractive index and the associated differences in the speed of light. Telecom system light sources have a spectral width of at least 1 nm (see Figure 2-19). As we've noted previously, different components within this spectrum travel at slightly different speeds. The slope of material dis- persion is positive. Waveguide dispersion causes pulse broadening in a fiber channel due to spectral components traveling at slightly different speeds, a wavelength dependence related to the geometry of the fiber, and the shape of the index profile. The geometry of the fiber causes the propagation constant of each mode to change if the wavelength changes. The slope for waveguide disper- sion can be negative. This can be used to shift the point of zero dispersion or to flatten the dispersion. Single-mode silica fibers have one of two refractive-index profiles: matched-cladding fiber and depressed-cladding fiber. In depressed-cladding fiber, the 