Cooled avalanche photodiodes used for photon detection

Commercial avalanche photodiodes (APDs) have been operated as single-photon detectors at an optimum operating temperature and bias voltage. These detectors were found to be 1.5 to 3 times more sensitive than presently-available photomultiplier tubes (PMTs). Both single-photon detection probability and detector noise increase with bias voltage, detection probabilities greater than 25% were obtained with detector noise levels comparable to the noise of a PMT; higher probabilities were measured at higher noise levels. The sources of noise and their dependence on temperature and bias voltage are discussed.