gnal. Since 1990 or SO, various materials have been used for infrared photo- diodes, some of which were mentioned earlier in this chapter. While a complete list of photodiode material is beyond the scope of this book, it should be noted that manufacturers developed products that operate at speeds as high as 100 GHz by the mid-1990s. It should also be noted that the selection of an applicable light detector depends on several factors. Those factors can include the operational wavelength, speed of detection (which is a function of response time), S/N ratio, and responsiveness. Another important consideration when selecting a high-speed detector is the difference between time-domain and frequency-domain optimiza- tion. A photodetector optimized for frequency-domain applications exhibits a frequency response that is relatively flat over the frequency band, while the time domain is relatively wide. In comparison, a pho- todetector optimized for time-domain applications has a narrower detec- tor pulse width value but its frequency response falls off with increased frequency. Figure 4.12 provides a general comparison of photodetectors optimized for frequency-domain and time-domain applications. Couplers and Connectors Our discussion of light sources, detectors, and fibers would not be com- plete without a brief discussion of couplers and connectors. Light Sources and Detectors Frequency-domain Time-domain Figure 4.12 A general comparison of the operation of photodetectors opti- mized for frequency- domain and time- domain applications. Frequency Frequency Function The job of the coupler is to couple light from an optical source into a fiber or from a fiber into an optical detector. This deceptively easy task is performed in three key steps: 1. The divergent light from LEDs and to a lesser extent from lasers must be converted into a beam of parallel light. This process, which is referred to as collimation, is performed by placing the end of a fiber at the focal plane of a lens. 2. The collimated beams must be focused int