Characterization of an advanced harmonic subtraction single-photon detection system based on an InGaAs/InP avalanche diode

We discuss the performance of a 1.25 GHz gated single-photon avalanche diode (SPAD) with bias gates of 150 ps FWHM and AC amplitude up to 25 V, a high-efficiency, high-speed SPAD system. This system uses an interferometric readout technique known as harmonic subtraction, and recent development efforts have enabled the use of up to the 4th harmonic of the gate to discriminate avalanche signals from the gate transient. With an improved design of the RF control system we have been able to demonstrate an ultra-low minimum detectable charge. We discuss the performance of this system, particularly its afterpulsing performance when counting at rates > 10(8) s(-1). Systems of this type require unique characterization techniques, and we will discuss the methods we have developed for this purpose.