Time-gated SPAD camera with reconfigurable macropixels for LIDAR applications

Abstract

Many industrial and scientific applications, ranging from 3D imaging (such as LIDAR, surveillance, object tracking) to Time Correlated Single Photon Counting (TCSPC), require the ability to perform time-resolved detection of weak light signals, down to single-photon level. Single-Photon Avalanche Diodes are solid-state detectors capable of single-photon sensitivity in the visible and near-infrared wavelength regions and compatible with standard silicon processes. This demand driven the development of low cost, large size CMOS SPAD imagers. In this work we present the design and characterization of a time-gated 32x32 SPAD image sensor fabricated in a 0.16 μm BCD (Bipolar-CMOS-DMOS) technology. The sensor is based on an innovative 16x16 macropixel structure, each composed by four SPADs with independent sensing front-end and event counters, plus a shared Timeto-Digital Converter (TDC). This approach enables higher fill-factor (9.6% with a pixel pitch of 100 μm) by sharing the costly (in terms of area) TDC resource, as well as reduced power dissipation. The imager provides simultaneous photon-timing and photon-counting data and features a 12 bit, 75 ps bin width TDC, which can perform one conversion per each gate window, with up to 62 windows per data readout frame. Two main operation modes are available: a single-photon mode, where an arbitration circuit within the macropixel is used to share the TDC among the 4 SPADs, but with no loss of X-Y resolution (i.e. keeping information about the triggered SPAD); and a two-photon-coincidence mode, where the TDC performs a conversion only if two SPADs of the macropixel are triggered within a preset coincidence window. Lastly, the sensor features multiple readout modes, with varying amount of output data, in order to fit different end-user applications. The imager is capable of 100 kfps (frames per second) in full readout mode, and up to 400 kfps in a reduced data set mode.

Single Photon Avalanche Diode Arrays for Quantum Imaging and Microscopy

“…A coincidence detection circuit made by full‐adders, half‐adders, and other logic gates signals if two or more photons have been detected in the same pixel (which includes 12 SPADs) within a 4 ns or an 8 ns coincidence window. A similar solution to detect coincidences of two or more photons has been proposed in the gated SPAD imager, [ 62,64 ] yet resulting in a simpler (AND‐gate network) logic, since the pixel includes only four SPADs.…”

“…In fact, solutions based on OR/XOR trees (e.g., SPADnet‐I sensor [ 58 ] and direct ToF sensor [ 59 ] for coincidences at pixel level, and MONDO detector [ 60 ] for coincidences at array level) are not able to detect actual coincidences within sub‐picosecond coincidence windows. Whereas solutions based on more complex logic architectures (e.g., linear dSiPM, gated SPAD imager [ 64 ] and coincidence tree imager [ 68 ] ) are not scalable and are used only to detect coincidences within the same pixel or sub‐parts of the array, as complexity drastically increases with the number of pixel to be combined.…”

“…Pixel architecture of the gated SPAD imager. [ 64 ] Reproduced with permission. [ 64 ] Copyright 2018, SPIE.…”

Time-Gated Single-Photon Detection in Time-Domain Diffuse Optics: A Review

“…Indeed, in the last years several clinical and diagnostics applications [8,131] adopted DOT due to its capability to improve the imaging of deep inhomogeneities of the medium (e.g., tumor, local changes in O 2 Hb/HHb due to brain activation, etc.). The technological improvements in terms of both detectors (matrix of small detectors or large area ones) and detection electronics allows one to have faster measurements, thus going in the direction of real-time tracking of buried objects [132][133][134][135]. Additionally, the use of multiple-view acquisitions coupled to the tomographic reconstruction enables: (i) to overcome the limitation in spatial resolution typical of the diffuse optics; (ii) to improve the quantification of the absorption and scattering coefficient of the inclusion [136][137][138].…”