Double feature, digital and analog, 5×5 SPADs single-photon camera

We present a fully reconfigurable single-photon camera, able to operate in both analog and digital modalities so to exploit the best performance out of the two features while solving usual issues that affect SPAD imagers. At first, one of the problems in SPAD arrays is the presence of hot pixels, with much higher dark count rate (DCR) compared to others, that impair system performance and are inevitable because of the presence of trapped charge or impurities in the semiconductor. Then, analog SiPMs provide photon-number resolved analog output, but they give no information about where photons hit the active area. Instead, digital SiPMs (dSiPMs) are position sensitive, can integrate also on-chip electronics (e.g., to count and to time-stamp single photons) and can enable/disable each single pixel, depending on their DCR or the user application. The latter feature can be mandatory in applications where the background is comparable to the dark-count rate of the SPAD microcell. Instead, applications such as Light Detection and Ranging (LiDAR) may not care about hot pixels because usually ambient light background is much higher (e.g., up to 100 klux). The novel camera employs a versatile dSiPM based on a 5 x 5 SPAD array imager, configured for photon-counting applications: each SPAD has both an individual digital output pulse and a common analog output with programmable pulse-width. Thus, it provides all advantages of SPAD arrays, since each SPAD can count the photons detected therein, by means of FPGA-based programmable counters with integration period ranging from 2 ms to 500 ms. Moreover, the chip also provides the advantages of analog SiPMs, since it detects coincidences in an analog way, by using current pulses generated by the triggered SPADs and summing them together to provide a common output analog current. The pulse widths are adjustable in amplitude and in time duration, from 1 ns to 10 ns, so to select the desired coincidence window. A comparator signals when more than a user-selectable number N of photons get concurrently detected. Such a feature has been profitably exploited in LiDAR and Quantum Imaging applications. Furthermore, the availability of the 25 digital outputs allows to extract also the position of the detected photons. The developed camera is also suitable for TOF measurements. Feeding both the output of the comparator and the sync signal coming from a laser illuminator to a Time-to-Digital Converter (TDC), single point distance measurement can be performed. The great advantage of using coincidences is the reduction of the pile-up effect and the unwelcome ambient light triggering, thus drastically improving background rejection in most applications.