ABSTRACT
This chapter describes the basic principle of operation and performance characteristics of photodetectors used in optical fiber communication systems. It discusses semiconductor photodiodes, both simple PIN diodes and more sophisticated avalanche photodiodes with internal current gain, because they are most compatible with optical fiber systems and have shown best performance thus far. Diodes with high quantum efficiency require a depletion width of 2 to 3 µm, for which a maximum response time of 20 to 30 psec can be expected. The diode capacitance together with the circuit load resistance gives rise to still another circuit RC time constant relevant to the speed of response of a photodiode. In the wavelength region between 0.4 to 1.0 µm, silicon photodiodes are preferred because of the low dark current and a large ratio of carrier ionization rates. Thus, silicon avalanche photodiodes with high sensitivity, wide bandwidth, and low noise have been produced.
