Underwater 3D imaging using a single-photon avalanche diode detector array with multi-event time-to-digital conversion

We present a single-photon light detection and ranging (LiDAR) transceiver system, designed for rapid three-dimensional (3D) imaging in highly scattering underwater environments. The system is based on a silicon single-photon avalanche diode (SPAD) detector array fabricated using complementary metal-oxide semiconductor (CMOS) technology. The detector array features 64 × 32 macro-pixels, where each macro-pixel comprised 4 × 4 SPAD detectors. Each macro-pixel is equipped with its own multi-event time-to-digital converter (METDC), facilitating rapid and simultaneous time-tagged acquisition of multiple photon events across the entire macro-pixel via the time-correlated single-photon counting (TCSPC) method. The detector operates in a time-gated mode that enables single-photon detection within an individual pre-defined timing range or within a sequence of time-gates. This multi-event timing approach in gated mode is particularly suited to single-photon LiDAR in turbid underwater environments which necessarily results in very high levels of optical backscatter. The use of the alternative single event TDC approach means that if back-scattered photons are detected first, this will close the detection chain to incoming target return photons until a later reset, leading to sensor inefficiencies as the target return photons will have an increased likelihood of not being detected. The underwater imaging performance was assessed in a water tank containing controlled levels of a scattering agent, with targets placed at a stand-off distance of approximately 1.65 m. The single-photon imaging system achieved detection up to the equivalent of 6.2 attenuation lengths between transceiver and target with an exposure time of only 1 ms. These results show single-photon depth imaging in turbid underwater environments using the ME-TDC approach, demonstrating 3D image acquisition approximately 50 times more rapidly than previously published work based on a 192 × 128 pixel CMOS SPAD detector array incorporating per-pixel single event TDCs to achieve comparable imaging performance levels.