ou cannot simply shine light into a fiber or place a detector at the end of a fiber, the roles of a coupler and its connector are extremely important. Thus, we will also cover the use of these components in this chapter. How- ever, before beginning our coverage of light sources and detectors, let's make sure that we understand how they are employed. Thus, let's begin this chap- ter by focusing on the components of an optical transmission system. Components of an Optical Transmission System An optical transmission system in many ways is very similar to other types of transmission systems, such as an electrical or a wireless trans- mission system. Each type of transmission requires three key compo- nents: a transmitter, a transmission medium, and a receiver. Figure 4.1 illustrates the relationship among the three components. Transmitter and Receiver In an optical transmission system both the transmitter and the receiver are electricooptical devices. Thus, they either convert an electrical signal into an optical signal (transmitter) or convert an optical signal into an electrical signal (receiver). Medium In an optical transmission system, the medium represents an optical fiber. That fiber can be a single-mode or multimode glass fiber or a plas- tic multimode fiber. Because we already examined the transmission Light Sources and Detectors light optical source detector optical fiber transmitter receiver Figure 4.1 The key components of an optical transmission system include a trans- mitter in the form of a light source, the optical fiber that represents the transmission medium, and a receiver in the form of an optical detector. medium in Chapter 3, readers are referred back to that chapter for infor- mation about optical fiber as a transmission medium. Coupler The connection of a fiber to a light source and light detector is not a simple process. Instead, a special coupler must be used to focus light into a fiber and at the opposite end focus light flowing out of the fiber to a light detector. Although not shown in Fi