Calibration of Si-SPAD detectors using the double attenuator technique and a low noise silicon photodiode

Silicon single-photon avalanche diodes (Si-SPADs) are today the most frequently devices used for measuring ultra-weak optical flux in many quantum technology fields. Here the determination of the detection efficiency is a key parameter that has to be accurately known for achieving reliable measurements. At PTB the detection efficiency of Si-SPADs is determined by using the double attenuator technique, which uses a calibrated standard detector traceable to the primary standard. However, an alternative way to calibrate Si-SPADs is by direct comparison against a low optical flux transfer detector (LOFT-Detector). Such a calibration was recently carried out at PTB and compared to the one using the double attenuator technique. The calibration was carried out at a wavelength of 770 nm and at different optical power levels from 90 fW to 13 pW. The mean relative deviation of the detection efficiency determined by the LOFT-Detector and the double attenuator technique was < 1 %, thus within the combined standard uncertainty of the two measurements. Details of the setup and the calibration procedure will be discussed at the conference.