(color online) (a) The average voltage () of avalanche signal as the function of the incident flux (μ). (b) The standard deviation (σ) of the average voltage () as the function of the incident flux (μ). With the increase of incident flux, the average voltage increases, the standard deviation of the voltage firstly increases and then decreases to almost 0, that is to say, the voltage is becoming saturated in this process. Here, we measure the amplitude of avalanche signal (V) of the SD-SPD for different incident fluxes by the setup shown in Fig. 1(a) with η setting to 11.7% and the trigger rate of the light pulse setting to ω/100 by a prescaler. A large number of V for a certain incident flux are recorded in the oscilloscope, and and σ are obtained by statistical analysis of V, but the effect of the afterpulse of the SG-SPD and the background noise of the oscilloscope have been subtracted. It must be pointed out that the power fluctuation of our incident light pulse is measured to be 15%, so the σ.