 application as well as the properties of optical fiber. For local area networks (LANs), lasers typical- ly operate at wavelengths of 850 and 1300 nm. For cable TV and tele- phone communications, lasers typically have source wavelengths of 1310 and 1550 nm. One of the key properties of optical fiber that governs the use of lasers at certain frequencies is the relationship between attenua- tion and wavelength when light flows through a fiber-optic system. Figure 2.6 provides a general illustration of that relationship Note that one of the major operational requirements of an optical fiber transmission sys- tem is to provide the lowest possible level of attenuation when light is being transmitted. If you examine Figure 2.6, you will note three "win- dows" where attenuation at certain wavelengths are generally lower than at other wavelengths. By using lasers or LEDs tuned to wavelengths with- in these windows, it becomes possible to transmit for longer distances before requiring the use of amplifiers. This, in turn, reduces the cost of Wavelength (nanometers) Figure 2.4 An exploded view of the frequency spec- trum centered on the visible-light region. 1 micron 1 meter Gamma X-rays Infrared Radio Waves Violet Visible Region Understanding Light Wavelength = 700 nanometers Figure 2.5 The electromagnetic Frequency = 428.570 GHz wave corresponding to the color red. the fiber-optic transmission system since it reduces the number of amplifiers required. Because we need to understand some additional terms, we will revisit the transmission of light in optical fiber to include a more detailed examination of the relationship between attenuation and the wavelength of light in Chapter 3 when we discuss optical fiber. ENERGY AND LIGHT If we again think back to days spent in a physics class, we might remember using the term electronvolt (eV) to describe visi- ble light when we characterized light as a stream of photons. As a refresher, the electronvolt represents the energy gained by an electron that passes across a positive volta