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We present the development and the validation of two SPAD camera systems, based on two SPAD array chips (Figure 1), respectively with 8x8 and 128x1 high-performance CMOS SPAD pixels, able to acquire both photon-counting 2D “intensity” images and photon-timing 3D “time-resolved” (hence, also distance-resolved) maps. Each pixel integrates a 30 μm SPAD detector, an 8-bit in-pixel counter (to counts the number of photons detected during user-selectable timeslots in the nanoseconds and microsecond range), and a 12-bit Time-to-Digital Converter (to timestamp the arrival time of the first photon detected by each SPAD, with sub-nanosecond resolution). In addition, the two array chips have the capability of actively gating the SPADs, driving the SPAD bias voltage above or below breakdown, with subnanosecond transitions allowing efficient time-domain filtering of incoming light. Active gating can be enabling in applications such as non-line of sight 3D ranging and time-domain functional Near-Infrared Spectroscopy (fNIRS), since it allows hiding unwelcome reflections, stray rays, or luminescence/fluorescence excitation signals. In fact, this feature allows the user to selectively avoid “early” photons, for instance those reflected by the sample surface, while measuring only the useful “late” photons, for instance those which interacted with the deeper biological tissue’s layers, preventing the triggering due to the strong reflections, which would saturate the SPADs. For optimizing chip operation in many different applications, both systems are extremely versatile and allow the user to customize the cameras for various measurement setups. The cameras quantum sensitivity allows the reconstruction of faint optical signals through the Time-Correlated Single Photon Counting (1) (TCSPC) technique. In addition, they enable many quantum experiments where information on each photon arrival time is required for example to identify time-coincident events with entangled photons. The 128x1 linear array is perfectly suited for spectroscopy applications, particularly for advanced Raman techniques, thanks to on-chip time-gating and time-tagging capabilities.
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