125 um. Thus, common size specifications will indicate 8/125 or 10/125 for the size of single-mode fiber. The index of refraction between the core and the cladding for single- mode fiber is near uniform, resulting in light traveling parallel to the core axis. The primary use of single-mode fiber is for long-distance com- munications, with millions of kilometers of this type of fiber installed each year. Optical Attenuation In communications we use the term attenuation to denote a loss of signal power. In the wonderful world of optical communications, we use the rate of optical power loss with respect to distance along the fiber to denote attenuation. The loss is usually measured in decibels per kilome- ter (dB/km) at a specific wavelength. Attenuation of an optical signal has many causes. As photons flow down an optical fiber, they can be absorbed by collisions with impurities in the fiber, or the photons may scatter, reducing their energy level in electronvolts. Because each wavelength is transmitted differently within an optical fiber, attenuation must be measured at specific wavelengths. The amount of optical attenuation due to absorption and scattering of light at a specified wavelength (A) is expressed as follows: A (dB) = where P1 represents the input power and P, represents the output power. CLADDING Figure 3.9 The flow of light via a Light single-mode fiber fol- Source lows the center of the core. CLADDING Chapter Three ATTENUATION AND WAVELENGTH In high school physics classes, instructors often talk about relationships between physical parameters. In the world of electrical communications, we probably heard the expression "high frequencies attenuate more rapidly than low frequen- cies." If we put on our thinking caps and remember that wavelength is inversely proportional to frequency, we can express a general relationship between attenuation and wavelength. That general relationship can be expressed as "attenuation decreases as the wavelength increases." In more technical terms, the dispersion of electro