nter- Optics Primer est is the slope of chromatic dispersion near 10 called the zero-dispersion slope (So). So is a measure of how rapidly chromatic dispersion increases as you move away from ZDP. ZDP occurs naturally in pure silica glass at 1.27 microns (see Figure 2-15). Because, in principle, single-mode fibers work with a single coher- ent wavelength, one way to exploit ZDP is to find a laser that emits light at 1.27 microns. However, the search for the greatest overall efficiency entails other factors. Usually, glass waveguides suffer from losses due to Rayleigh scattering, which occurs because of density and compositional variations within the glass. (Rayleigh scattering lessens as wavelengths grow longer.) To take advantage of this, dopant materials can be added to glass until its ZDP is shifted into the range between 1.3 and 1.6 microns. Many formulations of glass reach their lowest possible absorption in this range (below this range, molecular thermal scattering becomes a prob- lem). Manufacturing must be precise to avoid impurities in glass that increase absorption. Ideally, in non-WDM situations, the laser would operate at this wave- length so the bandwidth would be very high; however, due to manufactur- ing variations and aging, laser-operating wavelengths cover a broad range of operating wavelengths. For a given transmission path length, a single- mode fiber-optic system exhibits less pulse broadening than multimode sys- items, provided that the central wavelength is sufficiently close to the fiber's NDSF (SMF-28) Figure 2-15 Dispersion behavior Corning LEAF Lucent of two fiber types. NZDSF True Wave 1310 nm 1550 nm Wavelength (nm) Corning LS Chapter 2 For long-distance applications, it is desirable to operate at 1,550 nm because of lower attenuation compared to 1,300 nm. However, a simple step-index single-mode fiber shows a large chromatic dispersion at 1,550 nm. It is possible to design more sophisticated index profiles SO that the point where chromatic dispersion is zero falls near 1,550 nm inste