Per-pixel time-to-digital converter (TDC) architectures have been exploited by single-photon avalanche diode (SPAD) sensors to achieve high photon throughput, but at the expense of fill factor, pixel pitch and readout efficiency. In contrast, TDC sharing architecture usually features high fill factor at small pixel pitch and energy efficient event-driven readout. While the photon throughput is not necessarily lower than that of per-pixel TDC architectures, since the throughput is not only decided by the TDC number but also the readout bandwidth. In this paper, a SPAD sensor with 32 × 32 pixels fabricated with a 180 nm CMOS image sensor technology is presented, where dynamically reallocating TDCs were implemented to achieve the same photon throughput as that of per-pixel TDCs. Each 4 TDCs are shared by 32 pixels via a collision detection bus, which enables a fill factor of 28% with a pixel pitch of 28.5 μm. The TDCs were characterized, obtaining the peak-to-peak differential and integral non-linearity of -0.07/+0.08 LSB and -0.38/+0.75 LSB, respectively. The sensor was demonstrated in a scanning light-detection-and-ranging (LiDAR) system equipped with an ultra-low power laser, achieving depth imaging up to 10 m at 6 frames/s with a resolution of 64 × 64 with 50 lux background light.@en

We present a complete pixel based on a singlephoton avalanche diode (SPAD) fabricated in a backsideilluminated (BSI) 3D IC technology. The chip stack comprises an image sensing tier produced in a 65 nm image sensor technology and a data processing tier in 40 nm CMOS. Using a simple, CMOS-compatible technique, the pixel is capable of passive quenching and active recharge at voltages well above those imposed by a single transistor whilst ensuring that the reliability limits across the gate-source (VGS), gate-drain (VGD) and drain-source (VDS) are not exceeded for any device. For a given technology, the circuit extends the maximum excess bias that SPADs can be operated at when using transistors as quenching elements, thus improving the SPAD sensitivity, timing performance and photon detection probability (PDP) uniformity. Implemented with 2.5 V thick oxide transistors and operated at 4.4 V excess bias, the design achieves a timing jitter of 95 ps FWHM, maximum photon detection efficiency (PDE) of 21.9% at 660 nm and 0.08% afterpulsing probability with a dead time of 8 ns. This is both the lowest afterpulsing probability at 8 ns dead time and the highest peak PDE for a BSI SPAD in a 3D IC technology to date.@en

We present a complete pixel based on a singlephoton avalanche diode (SPAD) fabricated in a backsideilluminated (BSI) 3D IC technology. The chip stack comprises an image sensing tier produced in a 65 nm image sensor technology and a data processing tier in 40 nm CMOS. Using a simple, CMOS-compatible technique, the pixel is capable of passive quenching and active recharge at voltages well above those imposed by a single transistor whilst ensuring that the reliability limits across the gate-source (VGS), gate-drain (VGD) and drain-source (VDS) are not exceeded for any device. For a given technology, the circuit extends the maximum excess bias that SPADs can be operated at when using transistors as quenching elements, thus improving the SPAD sensitivity, timing performance and photon detection probability (PDP) uniformity. Implemented with 2.5 V thick oxide transistors and operated at 4.4 V excess bias, the design achieves a timing jitter of 95 ps FWHM, maximum photon detection efficiency (PDE) of 21.9% at 660 nm and 0.08% afterpulsing probability with a dead time of 8 ns. This is both the lowest afterpulsing probability at 8 ns dead time and the highest peak PDE for a BSI SPAD in a 3D IC technology to date.@en

A 252 × 144 single-photon avalanche diode (SPAD) pixel FLASH LiDAR is implemented in 180nm CMOS with 28.5μm pixel pitch and 28% fill factor. The sensor includes a collision detection bus with dynamic reallocation of 48.8 ps dual-clock time-to-digital converters (TDCs). It can operate in time-correlated single-photon counting (TCSPC), single-photon counting (SPC), peak-detection (PD) and partial-histogramming (PH) modes. The PD and PH modes are enabled by the first implementation of integrated histogramming for a full array via an SRAM based partial histogramming readout (PHR) scheme. This provides 16 5-bit bins for each pixel to enable a 14.9-to-l compression ratio.@en

A 252 × 144 single-photon avalanche diode (SPAD) pixel FLASH LiDAR is implemented in 180nm CMOS with 28.5μm pixel pitch and 28% fill factor. The sensor includes a collision detection bus with dynamic reallocation of 48.8 ps dual-clock time-to-digital converters (TDCs). It can operate in time-correlated single-photon counting (TCSPC), single-photon counting (SPC), peak-detection (PD) and partial-histogramming (PH) modes. The PD and PH modes are enabled by the first implementation of integrated histogramming for a full array via an SRAM based partial histogramming readout (PHR) scheme. This provides 16 5-bit bins for each pixel to enable a 14.9-to-l compression ratio.@en

A 252 × 144 single-photon avalanche diode (SPAD) pixel FLASH LiDAR is implemented in 180nm CMOS with 28.5μm pixel pitch and 28% fill factor. The sensor includes a collision detection bus with dynamic reallocation of 48.8 ps dual-clock time-to-digital converters (TDCs). It can operate in time-correlated single-photon counting (TCSPC), single-photon counting (SPC), peak-detection (PD) and partial-histogramming (PH) modes. The PD and PH modes are enabled by the first implementation of integrated histogramming for a full array via an SRAM based partial histogramming readout (PHR) scheme. This provides 16 5-bit bins for each pixel to enable a 14.9-to-l compression ratio.@en

A 252 × 144 single-photon avalanche diode (SPAD) pixel FLASH LiDAR is implemented in 180nm CMOS with 28.5μm pixel pitch and 28% fill factor. The sensor includes a collision detection bus with dynamic reallocation of 48.8 ps dual-clock time-to-digital converters (TDCs). It can operate in time-correlated single-photon counting (TCSPC), single-photon counting (SPC), peak-detection (PD) and partial-histogramming (PH) modes. The PD and PH modes are enabled by the first implementation of integrated histogramming for a full array via an SRAM based partial histogramming readout (PHR) scheme. This provides 16 5-bit bins for each pixel to enable a 14.9-to-l compression ratio.@en

A 252 × 144 single-photon avalanche diode (SPAD) pixel FLASH LiDAR is implemented in 180nm CMOS with 28.5μm pixel pitch and 28% fill factor. The sensor includes a collision detection bus with dynamic reallocation of 48.8 ps dual-clock time-to-digital converters (TDCs). It can operate in time-correlated single-photon counting (TCSPC), single-photon counting (SPC), peak-detection (PD) and partial-histogramming (PH) modes. The PD and PH modes are enabled by the first implementation of integrated histogramming for a full array via an SRAM based partial histogramming readout (PHR) scheme. This provides 16 5-bit bins for each pixel to enable a 14.9-to-l compression ratio.@en

A 252 × 144 single-photon avalanche diode (SPAD) pixel sensor, called Ocelot, is reported for light detection and ranging (LiDAR). The sensor, fabricated in the 180-nm CMOS technology, features 1728 12-bit time-to-digital converters (TDCs) with 48.8-ps resolution (LSB). Each 126 pixels in a half-column are connected to six TDCs through a collision detection bus, which enables effective sharing of resources, and consequently a fill factor of 28% with a pixel pitch of 28.5 μ text{m}. The column-parallel TDCs, based on dual-clock architecture, exhibit a DNL of +0.48/-0.48 LSB and an INL of +0.89/-1.67 LSB; they are dynamically reallocated in a scalable daisy chain approach that enables a maximum of five photon detections per illumination cycle per half-column. The sensor can operate in time-correlated single-photon counting (TCSPC) and single-photon counting (SPC) modes, while peak detection (PD) and partial histogramming (PH) are included in the operation of the sensor. The PD and PH modes are enabled by the first implementation of integrated histogramming for a full array via 3.32-Mb SRAM-based PH readout (PHR) scheme providing a 14.9-to-1 compression. Telemetry measurements up to 50 m achieve an accuracy of 8.8 cm and worst-case precision of 1.4 mm ( σ ). A flash LiDAR using direct time of flight (dTOF) and based on Ocelot is demonstrated, achieving depth imaging at short distances with a frame rate of 30 frames/s, employing an ultra-low power laser with an average power of 2 mW and peak power of 0.5 W.@en

SPAD (single-photon avalanche diode) arrays are single-photon sensors, which enable photon counting and unparalleled time-resolved imaging. In this paper, we will detail the architecture and characteristics of three representative SPAD arrays, implemented in standard CMOS technologies and targeted to a range of applications such as biophotonics, basic sciences, and engineering. Two sensors feature a high-performance pixel and enable time-gated and TCSPC operation, respectively. The third sensor is a line array whose pixels are coupled on a one-To-one basis with an FPGA for flexible data acquisition and processing.@en

Fluorescence molecular tomography (FMT) emerges as a powerful non-invasive imaging tool with the ability to resolve fluorescence signals from sources located deep in living tissues. Yet, the accuracy of FMT reconstruction depends on the deviation of the assumed optical properties from the actual values. In this work, we improved the accuracy of the initial optical properties required for FMT using a new-generation time-domain (TD) near-infrared optical tomography (NIROT) system, which effectively decouples scattering and absorption coefficients. We proposed a multimodal paradigm combining TD-NIROT and continuous-wave (CW) FMT. Both numerical simulation and experiments were performed on a heterogeneous phantom containing a fluorescent inclusion. The results demonstrate significant improvement in the FMT reconstruction by taking the NIROT-derived optical properties as prior information. The multimodal method is attractive for preclinical studies and tumor diagnostics since both functional and molecular information can be obtained.@en