use low-cost light-emitting diode (LED) transmitters at the 850-nm wavelength where attenuation of a multimode fiber is of the order of 2 dB/km. This was the first wavelength range, that is, the "first window," used for optical transmis- sion. In multimode fibers several modes, reflected light rays, propagate through the fiber. Propagation delay is different for each ray, and the light energy of different rays is received with different delays, which causes disper- sion, that is, spreading of light pulses as they travel through an optical fiber as shown in Figure 4.29. The shorter the light pulses are, the higher the impact of this so-called "modal dispersion" and this makes multimode fibers suitable only for rela- tively low data rates. High attenuation makes them feasible only for short- haul systems. Introduction to Telecommunications Network Engineering Input Output Fiber Figure 4.29 Principle of dispersion. Single-mode fibers with approximate diameters of 125/5 um are used in the telecommunications network in high-data-rate and long-distance applications. They allow only one mode to propagate through the fiber and modal dispersion is greatly reduced. Wavelengths of 1.3 or 1.55 um in the second or third window in Figure 4.30 are used in single-mode fibers and then attenuation is of the order of 0.5 dB/km or even less. Semiconductor lasers are used as transmitting components and systems typically tolerate cable sections of tens of kilometers without intermediate repeaters. Long- haul, high-capacity coaxial cable systems required a repeater after every 1.5-km cable section! This partly explains the cost reduction of long-distance telecommunications during past few decades. Attenuation dB/km Wavelength/micrometers Figure 4.30 Attenuation of an optical fiber. Transmission Note that the single-mode fibers require high-precision optical compo- nents and connectors because of the small core diameter and this makes their cost high compared with components used for multimode fibers. There are several types of single m