| Summary: | In this paper, we present a method to detect ultra-low level light ranging from several mean photoelectrons (MPEs) down to 10<sup>-5</sup> MPEs. It is based on the Poisson statistics algorithm and a double time windows technique with a silicon photomultiplier (SiPM) and a field programmable gate array (FPGA). It measures the mean incident photoelectron number of pulsed light with zero peak statistics of the Poisson distribution to reduce the influence of the correlation noises and the dark count rate (DCR) fluctuation. The linear measurement ranges from ~10<sup>-4</sup> MPEs to ~10 MPEs (i.e., ~97 dB) were demonstrated with double 35-ns time windows at room temperature. Its upper detection limit is determined by the availability for the zero peak counts of the Poisson distribution, and the lower detection limit is mainly determined by the DCR of the SiPM. By narrowing the time window to ~550 ps or decreasing the operating temperature of SiPM to -30 °C, the detection limit can be further decreased to ~10<sup>-5</sup> MPEs. This method, with capability to record the arrival time of incident photons, demonstrated an instrument response function (IRF) of ~214.9 ps (FWHM), showing its compatibility to the time correlated photon counting (TCPC) technique and the time of flight (TOF) measurement.
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