SPAD imagers for remote sensing at the single-photon level

Combined 2D imaging and 3D ranging sensors provide useful information for both long (some kms) and short (few tens of m) distance, in security applications. To this aim, we designed two different monolithic imagers in a 0.35 μm costeffective CMOS technology, based on Single Photon Avalanche Diodes (SPADs), for long-range time-of-flight (TOF) and short-range phase-resolved depth ranging. The single pixel consists of a SPAD (30 μm diameter), a quenching circuit, and a Time-to-Digital Converter (TDC) for TOF measurements or three up/down synched counters for phaseresolved depth assessments. Such smart pixels operate in two different modalities: single photon-counting for 2D “intensity” images; while either photon-timing or phase-resolved photon-counting for 3D “depth” images. In 2D imaging, each pixel has a counter that accumulates the number of photons detected by the SPAD in the pixel, thus providing single-photon level sensitivity and high (100 kframe/s) frame-rate. In the TOF 3D imager, each pixel measures the photon arrival time with a 312 ps resolution, thanks to a two-stage TDC (with 6 bit coarse counter plus a 4 bit fine interpolator), with a 320 ns full-scale range. The resulting spatial resolution is 9 cm within a 50 m range, centered at any user-selectable distance (e.g. 100 m – 5 km), with linearity of DNLrms=4.9% LSB and INLrms=11.7% LSB, and 175 ps precision. In the phase-resolved 3D imager, the in-pixel electronics measures the phase difference between the modulated light emitted by a laser and the back-reflected light, with both continuous-wave and pulsed-light modulation techniques.