A refined method for characterizing afterpulse probability in single-photon avalanche diodes

Single-photon avalanche diodes (SPADs) are critical components in low-light-level sensing and photonic quantum information applications. For these, it is often necessary that a full characterization of the SPAD is performed, for which a key metric is the afterpulse probability. This study provides a detailed comparison of the common synchronized and non-synchronized methods used to measure afterpulse probability. Measurements on a single SPAD reveal inconsistencies between the afterpulse probabilities obtained by the two methods. By re-deriving the equations from first principles, the discrepancy is traced to the analysis approach for the non-synchronized experiment. An improved analysis approach is presented, leading to better agreement between the non-synchronized and synchronized methods. The study also provides guidance on the experimental conditions required for the valid application of both methods, along with a detailed analysis of the limitations of the non-synchronized method under high photon flux. These findings offer a more accurate approach for characterizing afterpulse probability and for reconciling the results of two methods, which enables better quantification of SPAD performance.