High accuracy calibration of photon-counting detectors

We discuss a practical implementation of a photon-counting detector calibration using correlated photon pairs produced by parametric down-conversion. In this calibration scheme, the detection of a first photon triggers the measurement sequence aimed at detection of a second photon by a detector under test (DUT). We also describe measurements of radiant power with a photon-counting detector, which is important for implementation of a conventional calibration technique based on detector substitution. In the experiment, we obtain a time-delay histogram of DUT detection events consisting of a correlated signal and a background. We present a method for separating the correlated signal from the background signal that appropriately handles complex properties of typical avalanche photodiode (APD) detectors. Also discussed are measurements of relevant APD properties, including count-rate-dependent afterpulsing, delayed (by up to 10 ns) electronic detections and deadtime effects. We show that understanding of these is essential to perform an accurate calibration.