The latest improvements in high-performance APDs have enabled impressive applications in aerospace technology. Avalanche photodiodes have gained acceptance in the aerospace industry because of their very high sensitivity in detecting very low light levels, even down to a single photon. Low noise levels and high signal-to-noise ratio properties also provide benefits in aerospace applications. Common applications include laser rangefinders that incorporate APD detectors for more sensitive measurements. Avalanche photodiodes used in these applications can operate with lower light levels and shorter laser pulses, resulting in more eye-safe range finders. High-performance APDs can be tailored to meet these applications and have proven to be valuable tools in aerospace technology. Selecting the proper photodetector and amplifier circuitry to optimize gain for particular applications has played a major role in evaluating overall system performance. Avalanche photodiodes have a built-in gain characteristic that provides a significant advantage in detecting lower light signals. The significance of single-photon detection becomes more apparent when applied to an event, such as measuring atmospheric conditions. To examine cloud formations, for instance, a laser beam is transmitted to the cloud and a return signal provides important distance and spectral information. The reflection contains very low light levels and is severely scattered. The few photons of light that actually reflect back can still be detected by the device to accumulate the data necessary for accurate measurement. Fast receiver modules, confocal microscopy, and particle detection are other common uses for APDs. A silicon APD can detect alpha particles, electrons with energies as high as 150 KeV, and other forms of radiation. Avalanche photodiodes also can be used for light detection and ranging (LIDAR) to measure distance, speed, rotation, and composition of a remote target that can be a clearly defined object, such as a vehicle, or a diffuse object, such as a smoke plume or clouds. Current avalanche photodiodes for communications applications are of much higher cost components than conventional p-i-n photodiodes. The results of the devices developed in this project will enhance the state-of the- art in reliable low-cost avalanche photodiodes for fiber optic communications
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