Photon counting with silicon avalanche photodiodes

Avalanche photodiodes (APDs) are studied for use as photon-counting detectors. The APD may be biased slightly above (Geiger mode) or slightly below its voltage breakdown point. In the latter case, if the photon absorption rate is low enough, each individual photoelectron current pulse may be resolved with the use of a discriminator. APDs used in this photon-counting mode are shown to give the best performance at low light levels. Experimentally, overall photon detection probabilities of 5.0 and 0.33% were obtained at λ=820 nm and λ=1.064 μm, respectively, with a photon counter dead time as low as 15 ns and a dark current counting rate of 7000/s. The APD photon counter exhibited an exponential photon interarrival time probability density and a near-Poissonian photon-counting probability