or senses light and converts it to electricity. Similar to the existence of mul- tiple light sources, there are several types of photodetectors. However, before describing how they operate and discussing their advantages and Chapter Four disadvantages, let's consider their basic operation within a fiber-optic transmission system. Basic Operation within an Optical System Figure 4.11 illustrates the general relationship between a light source, an optical fiber, and a photodetector for a single-wave full-duplex transmis- sion system. Note that two separate fibers are required in this operating environment, each with a separate light source and photodetector. The light source in the form of an LED or laser diode converts an electrical signal into an optical signal, while the photodetector performs a reverse process. Because each optical fiber represents a simplex or one-way trans- mission path, two optical fibers with attached light sources and pho- todetectors are required to support two-way, full-duplex transmission. Figure 4.11 represents a basic optical transmission system. In this type of system light is modulated for transmission over the optical fiber and a photodetector is able to sense light only at the distant end. In a more complex optical transmission system, a light source can operate at two or more frequencies via the use of a tunable laser diode or multiple lasers operating at different frequencies that are coupled onto a common fiber. In this situation the photodetector must be capable of detecting light at the tunable frequencies generated by the laser diode or the fre- quencies generated by multiple lasers. This type of transmission system supports multiple derived channels via different optical frequencies and is referred to as wavelength division multiplexing (WDM). When the num- ber of derived channels is substantial, the multiplexing technique is referred to as dense wavelength division multiplexing (DWDM), and the photodetector must respond to a wider range of frequencies. Figure 4.11 electrica