Detection efficiency measurement of single photon avalanche photodiodes by using a focused monochromatic beam tunable from 250 nm to 1000 nm

We demonstrate the measurement of detection efficiency, the probability for the successful detection of a single photon incident on a detector, in a wide wavelength range from 250 nm to 1000 nm. The method is based on the responsivity comparison of two detectors by using a tunable monochromatic light source at a radiant power controlled to the pico-Watt level. As the reference detector, a photodiode with a switched integrating amplifier is used, whose spectral responsivity is calibrated at a photo-current level of pico-Ampere. A fibre-based spatial filter is used to form a beam focus that is focused to a diameter of less than 100 µm at the detector under test. The typical uncertainty of the detection-efficiency measurement for a Si-based single photon avalanche photodiode is evaluated to be less than 1% as a relative standard uncertainty (k = 1), which is currently limited by the characteristics of the tunable source.