Afterpulsing arises from feedback in a photon detector. This means that each real signal pulse can be followed by an afterpulse at a later time. This effect is particularly troubling in photon correlation experiments. Few treatments of this effect have appeared in the literature, and few software programs to solve the problem have been written. We demonstrate the afterpulsing effect in fluorescence correlation spectroscopy by using different avalanche photodiodes. We prove theoretically that under simple and reasonable conditions afterpulsing in autocorrelation can be eliminated to the leading order; we have found it easy to program software for the correction. We compare our results with those from cross correlation. We also discuss some experimental parameters that may affect the afterpulsing.
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