Vision systems capable of acquiring both two-dimensional and three-dimensional information through Light Detection And Ranging are assuming ever-increasing importance, being this market driven by the push from automotive companies to develop systems to be integrated in selfdriving vehicles. Among others, candidate sensors for these systems are avalanche photodiodes, single-photon avalanche diodes, and silicon photomultipliers. Avalanche Photodiodes provide a good robustness to high background light at the cost of requiring an analog readout, instead Single-Photon Avalanche Diodes offer the possibility to implement digital readout and single-photon sensitivity, but are prone to saturation at extremely high background levels. We compare these three single-and multi-photon detector topologies, operated either in linear or digital regime, aiming at identifying the best suited detector to achieve the highest performance in Light Detection And Ranging applications at the lowest optical power active illumination and in presence of intense background (e.g. 100 klux). We present Matlab modelling and simulations and their experimental validation. Eventually, we propose a nomogram (referred to 100 m target distance) for identifying the most suited sensor topology across different operating areas and constraints, in order to achieve at least 70% success ratio. Index Terms-Single photon detectors, avalanche photodiode (APD), single-photon avalanche diode (SPAD), silicon photomultiplier (SiPM), 3D ranging, light detection and ranging (LiDAR). I. INTRODUCTION D URING the last decade, many automotive companies, such as Waymo, Ford, Toyota, Tesla and others, have been investigating reliable autonomous driving vision systems. While some companies are investigating systems based on 77 GHz radar (mainly used for adaptive cruise control [1]) and pure two-dimensional (2D) vision [2], the great majority of the players are working on three-dimensional (3D) ranging systems, able to reconstruct obstacles' shape and position [3]. The most widespread approaches rely on Light Detection And Ranging (LiDAR), mostly based on shining a laser pulse Manuscript
Single-photon avalanche diode (SPAD) exploitation in high-flux applications is often hindered by the trade-off between the SPAD dead-time and afterpulsing probability. In this paper, we present the architecture and the experimental characterization of two chips including a novel SPAD sensing, and readout scheme designed to minimize dead-time (1.78 ns and 0.93 ns respectively) and afterpulsing probability (0.14% maximum). We have coupled this architecture with high-performance SPADs obtaining an extremely stable dead-time (6.44 ps rms jitter) that can be easily regulated through an external voltage. Thanks to its compact size, this novel pixel architecture can be easily integrated within high-resolution SPAD arrays for GHz applications.
We present a novel architecture for multi-channel Time-to-Digital Converters to be implemented into low-cost FPGAs, achieving 10 ps LSB, 164 µs full-scale range, and good linearity both in terms of DNL and INL. The conceived architecture is based on the carry chain delay line model and wave union A method: the positions of both rising and falling edges that propagate in multiple parallel carry chains are recorded each time there is a HIT input. This technique effectively sub-divides the ultra-wide bins improving the measurement precision, and, combined with the sliding-scale technique and continuous code density calibration, improves the TDC linearity. Employing the proposed architecture, we have implemented in a Xilinx Artix-7 FPGA a TDC with 20 timestamp units and validated the device in a Time-Correlated Single Photon Counting setup, when connected to an array chip with 5 × 5 Single Photon Avalanche Diodes.
We present the design and experimental characterization of a CMOS sensor based on Single-Photon Avalanche Diodes for direct Time-Of-Flight single-point distance ranging, under high background illumination for short-range applications. The sensing area has a rectangular shape (40 × 10 SPADs) to deal with the backscattered light spot displacement across the detector, dependent on target distance, due to the non-confocal optical setup. Since only few SPADs are illuminated by the laser spot, we implemented a smart laser-spot tracking within the active area, so to define the specific Region-Of-Interest (ROI) with only SPADs hit by signal photons and a smart sharing of the timing electronics, so to significantly improve Signal-to-Noise Ratio (SNR) of TOF measurements and to reduce overall chip area and power consumption. The timing electronics consists of 80 Time-to-Digital Converter (TDC) shared among the 400 SPADs with a self-reconfigurable routing, which dynamically connects the SPADs within the ROI to the available TDCs. The latter have 78 ps resolution and 20 ns Full-Scale Range (FSR), i.e., up to 2 m maximum distance range. An on-chip histogram builder block accumulates TDC conversions so to provide the final TOF histogram. We achieve a precision better than 2.3 mm at 1 m distance and 80% target reflectivity, with 3 klux halogen lamp background illumination and 2 kHz measurement rate. The sensor rejects 10 klux of background light, still with a precision better than 20 mm at 2 m.INDEX TERMS Light detection and ranging (LiDAR), laser rangefinder, time-of-flight (TOF), time-todigital converter (TDC), single photon avalanche diode (SPAD), background light rejection.VINCENZO SESTA was born in Ribera, Italy, in 1991. He received the master's degree in electronics engineering and the Ph.D. degree in information technology from the Politecnico di Milano, Milan, Italy, in 2016 and 2020, respectively, where he is currently a Postdoctoral Researcher with the Department of Electronics, Information and Bioengineering. His research activity focuses on the design and development of time-to-digital converters and timing electronics CMOS circuits for arrays of silicon SPADs.KLAUS PASQUINELLI was born in Seriate, Italy, in 1994. He received the bachelor's and M.Sc. degrees in electronics engineering from the Politecnico di Milano in 2016 and October 2018, respectively, where he is currently pursuing the Ph.D. degree in information technology. His research interests include the design, development, and testing of systems with single-photon avalanche diodes matrices and digital silicon photomultiplier.
In recent years, direct Time-of-Flight techniques have been exploited with single-photon detectors to provide long distance ranges and high-frame rates measurements. Detectors based on Silicon Photon Multipliers commonly collect repetitive laser shots to reconstruct a histogram of the TOF of returning pulsed-laser photons, so to discriminate between signal, background light, and detector noise. Instead of performing multishot measurements, we propose a single-shot technique capable to provide just one useful TOF per laser pulse, corresponding to the returning peak signal with the highest number of concurrent photons. In this way, ambient background photons and dark counts are rejected, being mostly randomly distributed over time compared to the laser pulse photons. Therefore, there is no need for repetitive laser shots, neither to acquire and store multiple TOFs, nor to post-process any TOF histogram, since each laser shot can provide the useful distance information. We present a sensor chip based on a pixel with 272 SPADs, which concurrently provide an analog signal to a peak detector triggering a multi-hit TDC each time the number of concurrent photons exceeds the previous peak. We report preliminary tests up to 25 m, rejecting 60 klux solar background, with > 95% single-shot success ratio.
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