The characterization of single-photon detectors (SPDs) plays an important role in the development and application stages of SPDs. The demand grows as the application of solid-state SPDs rapidly increases in fields such as LiDAR and low-light imaging. Since SPDs typically respond to incident photons over a wide spectral range, measurement of the photon detection efficiency (PDE) of an SPD over its entire spectral response range (i.e., PDE spectrum) becomes a complex and time-consuming task. Traditionally, the correlated-photon method and the standard detector substitution (SDS) method are commonly used for the measurement of PDEs of SPDs. However, these methods require additional measurements and corrections for the dead time, afterpulsing, and dark count effects to obtain an accurate PDE. This limitation can be overcome using our recently proposed event-refreshed time-to-digital converter (ER-TDC)-based method. In this study, we design and develop an automatic PDE spectrum testing system based on ER-TDC to improve measurement efficiency and repeatability, and its performance is fully tested.

A dedicated system, consisting of a light-emitting diode (LED) array and an ER-TDC, is designed and developed for measuring PDE spectra (400?1100 nm) of SPDs [Fig. 2(a)], with the supporting automatic measurement procedure (Fig. 4) implemented in Python. The spectral and temporal emission characteristics of the LEDs at different powers (1.0?800.0 nW) are measured using a fiber optic spectrometer and ER-TDC, respectively. Power calibration measurements are conducted on the automatic system exploiting two calibrated photodiodes (PDs). The performance of the automatic system is tested at various LED emitting powers using a commercially available silicon single-photon avalanche diode (Si-SPAD), and the resulting PDE spectrum is compared with that measured by the traditional SDS method [Fig. 2(b)].

The central wavelength (Fig. 5) and power calibration coefficient (Fig. 8), which are two parameters required for PDE calculations, are determined from the LED emission spectra and power calibration measurements, respectively. The spectral characteristics of each single-color LED employed in the LED array (Fig. 6) and the corresponding measured PDE (Fig. 11) are independent of the emitting powers of the LEDs, which is a significant advantage of the developed technique. The PDE spectrum of the Si-SPAD measured by the automatic system is in excellent agreement with that obtained by the traditional SDS method [Fig. 12(a)], which demonstrates the accuracy and effectiveness of the automatic system. In addition, the developed automatic system achieves a measurement speed of 122 s/point and a relative standard uncertainty of less than 3.1% in the 400?1000 nm range [Fig. 12(b)], and these performance parameters are comparable to those of the traditional SDS method.

An automatic system based on an LED array and an ER-TDC is designed and developed for measuring PDE spectra of SPDs. The spectral and temporal emission characteristics of each single-color LED, and the power calibration coefficients, are presented. The performance of the automatic system is tested and demonstrated using a Si-SPAD. This system enables fully automatic measurements of PDE spectra in the 400?1100 nm range, achieving a measurement speed of 122 s/point while maintaining a relative standard uncertainty below 3.1% over the 400?1000 nm sub-range. The developed automatic system is easy to operate, cost-effective, and eliminates the need for additional measurements and corrections for the dead time, afterpulsing, and dark count effects, which makes it suitable for batch characterization of PDE spectra of visible and near-infrared SPDs.