APD - Computer Definition
A type of diode used as a light detector used in high speed, long haul fiber optic transmission systems (FOTS) employing laser diode light sources and single-mode (SMF) glass optical fiber (GOF) media. APDs are especially sensitive detectors as they use a strong electric field to accelerate the electrons flowing in the semiconductor. As a result, an APD generates an avalanche of electrons with a multiplication factor that can be in the range of 70, i.e., an APD can generate 70 electrons from 1 photon. Therefore, a very weak incoming light pulse will create a much stronger electrical effect that can be interpreted more effectively and understood more clearly. So, an APD can be characterized as a very high gain photodiode receiver, i.e., a one-way photonic receiver with a high ratio of (electrical) output power to (optical) input power in the range of 70:1. Although more sensitive and more effective than PIN detectors used in lesser systems, APDs require more electrical power to operate, are more sensitive to extremes of ambient temperature and are more expensive. See also diode, FOTS, gain, GOF, laser diode, light detector, light source, PIN, and SMF.