to denote the position of light emission in the electromagnetic spectrum. The fiber optics of today uses three wavelength bands: (1) around 800 nm (nanometers), (2) 1300 nm, and (3) 1600 nm or near-visible infrared. This is illustrated in Figure 9.22. 17 SCPC stands for single channel per carrier. CONCEPTS IN TRANSMISSION TRANSPORT FIBER-OPTIC COMMUNICATION LINKS Radio Optical Wavelength A 100 cm 10 cm 10 mm 100 um 10 um Microwave Infrared Frequency f Figure 9.22 Frequency spectrum above 300 MHz. The usable wavelengths are just above and below 1 um (1 X 10-6 m). This section includes an overview of how fiber-optic links work, including the more common types of fiber employed, a discussion of light sources (transmitters), light detec- tors (receivers), optical amplifiers, and how lengths of fiber-optic cable are joined. There is also a brief discussion of wavelength-division multiplexing (WDM). 9.4.2 Introduction to Optical Fiber as a Transmission Medium Optical fiber consists of a core and a cladding, as illustrated in Figure 9.23. At present the most efficient core material is silica (SiO2). The cladding is a dielectric material that surrounds the core of an optical fiber. The practical propagation of light through an optical fiber may best be explained using ray theory and Snell's law. Simply stated, we can say that when light passes from a medium of higher refractive index (n1) into a medium of lower refractive index (n2), the refractive ray is bent away from the normal. For instance, a ray traveling in water and passing into an air region is bent away from the normal to the interface between the two regions. As the angle of incidence becomes more oblique, the refracted ray is bent more until finally the refracted energy emerges at an angle of 90° with respect to the normal and just grazes the surface. Figure 9.24 shows various incident angles of light entering a fiber. Figure 9.24b illustrates what is called the critical angle, where the refracted ray Plastic coating Cladding (lower refractive index) Light (h