ansport Technologies Chapter 7 their light-collecting ability is, therefore, somewhat limited. A typical single-mode fiber generates an intense spot of light that is roughly 10 microns in diameter; the point upon which that light impinges upon the semiconductor amplifier is less than a micron in diameter, meaning that a lot of the light is lost. Other problems also crop up including polariza- tion issues, reflection, and variable gain. As a result, these devices are not in widespread use; EDFAs and Raman amplification techniques are far more common. Optical Receivers So far, we have discussed the sources of light, including LEDs and laser diodes; we have briefly described the various flavors of optical fiber and the problems they encounter as transmission media; now, we turn our attention to the devices that receive the transmitted signal. The receive devices used in optical networks have a single responsi- bility: to capture the transmitted optical signal and convert it into an electrical signal that can then be processed by the end equipment. There may also be various stages of amplification to ensure that the signal is strong enough to be acted upon, and demodulation circuitry, which recre- ates the originally transmitted electronic signal. Photodetector Types While there are many different types of photosensitive devices, there are two used most commonly as photodetectors in modern networks: positive-intrinsic-negative (PIN) photodiodes and avalanche photodi- odes (APDs). Positive-Intrinsic-Negative (PIN) Photodiodes PIN photodiodes are similar to the device described earlier in the general discussion of photosensitive semiconductors. Reverse biasing the junction region of the device prevents current flow until light at a specific wave- length strikes the substance, creating electron-hole pairs and allowing current to flow across the three-layer interface in proportion to the inten- sity of the incident light. While they are not the most sensitive devices Downloaded from Digital Engineering Library @ McGraw-H