On the calibration of a SPAD-based 3D imager with in-pixel TDC using a time-gated technique

The optical characterization of a CMOS 64×64 single-photon avalanche-diode (SPAD) array with in-pixel 11b time-to-digital converter (TDC) is presented. The overall full-width half-maximum (FWHM) of the detector ensemble SPAD plus TDC is 690ps. The sensor has been fabricated in a 0.18μm standard CMOS technology which features an average dark-count rate (DCR) of 42kHz at 1V excess voltage (V e ) and room temperature. The detector successfully uses its time-gating capability to mitigate this large amount of noise enabling the sensor for accurate time-of-flight (ToF) measurements. The effectiveness of the time-gating technique is experimentally demonstrated. According to measurements, a time window of 400ns is enough to ensure that the TDC is triggered by light rather than by spurious events.Office of Naval Research (USA) N000141410355Ministerio de Economía y Competitividad TEC2012-38921- C02, IPT- 2011-1625-430000, IPC- 20111009Junta de Andalucía TIC 2012- 233

Photon Counting and Direct ToF Camera Prototype Based on CMOS SPADs

This paper presents a camera prototype for 2D/3D image capture in low illumination conditions based on single-photon avalanche-diode (SPAD) image sensor for direct time-offlight (d-ToF). The imager is a 64×64 array with in-pixel TDC for high frame rate acquisition. Circuit design techniques are combined to ensure successful 3D image capturing under low sensitivity conditions and high level of uncorrelated noise such as dark count and background illumination. Among them an innovative time gated front-end for the SPAD detector, a reverse start-stop scheme and real-time image reconstruction at Ikfps are incorporated by the imager. To the best of our knowledge, this is the first ToF camera based on a SPAD sensor fabricated and proved for 3D image reconstruction in a standard CMOS process without any opto-flavor or high voltage option. It has a depth resolution of 1cm at an illumination power from less than 6nW/mm 2 down to 0.1nW/mm 2 .Office of Naval Research (USA) N000141410355Ministerio de Economía y Competitividad TEC2015-66878-C3- 1-RJunta de Andalucía P12-TIC 233

A CMOS Imager for Time-of-Flight and Photon Counting Based on Single Photon Avalanche Diodes and In-Pixel Time-to-Digital Converters

The design of a CMOS image sensor based on single-photon avalanche-diode (SPAD) array with in-pixel time-to-digital converter (TDC) is presented. The architecture of the imager is thoroughly described with emphasis on the characterization of the TDCs array. It is targeted for 3D image reconstruction. Several techniques as fast quenching/recharge circuit with tunable dead-time and time gated-operation are applied to reduce the noise and the power consumption. The chip was fabricated in a 0.18 m standard CMOS process and implements a double functionality: time-of-flight (ToF) estimation and photon counting. The imager features a programmable time resolution of the array of TDCs down to 145 ps. The measured accuracy of the minimum time bin is lower than 1LSB DNL and 1.7 LSB INL. The TDC jitter over the full dynamic range is less than 1 LSB.Peer reviewe

A CMOS Imager for Time-of-Flight and Photon Counting Based on Single Photon Avalanche Diodes and In-Pixel Time-to-Digital Converters

The design of a CMOS image sensor based on single-photon avalanche-diode (SPAD) array with in-pixel time-to-digital converter (TDC) is presented. The architecture of the imager is thoroughly described with emphasis on the characterization of the TDCs array. It is targeted for 3D image reconstruction. Several techniques as fast quenching/recharge circuit with tunable dead-time and time gated-operation are applied to reduce the noise and the power consumption. The chip was fabricated in a 0.18 m standard CMOS process and implements a double functionality: time-of-flight (ToF) estimation and photon counting. The imager features a programmable time resolution of the array of TDCs down to 145 ps. The measured accuracy of the minimum time bin is lower than 1LSB DNL and 1.7 LSB INL. The TDC jitter over the full dynamic range is less than 1 LSB.Office of Naval Research (USA) N000141410355Ministerio de Economía y Competitividad TEC2012-38921- C02, IPT-2011-1625-430000, IPC- 20111009 CDTIJunta de Andalucía TIC 2012-233

32 × 32 CMOS SPAD Imager for Gated Imaging, Photon Timing, and Photon Coincidence

We present the design and simulations of a single-photon sensitive imager based on single photon avalanche diodes (SPADs) with an innovative pixel architecture that includes four separate SPADs with independent active time-gating and quenching circuit, a shared time-to-digital converter (TDC) with 50-ps resolution, four independent photon counters, and multiple operation modes. The TDC is driven by smart arbitration logic, which preserves spatial information among the four detectors; furthermore, an alternative operation mode exploits photon-coincidence on multiple detectors to reduce the effect of high background levels, e.g., in light detection and ranging applications with strong ambient light. Key features are the ability to operate in simultaneous photon counting and timing modes for capturing 2-D and 3-D images of the scene in a single shot (frame), the option of a counting-only mode, reducing power consumption, and increasing achievable frame-rate when timing information is not needed, and the ability to individually shut down noisy detectors or to enable just some regions of interests

A 192×128 Time Correlated SPAD Image Sensor in 40-nm CMOS Technology

A 192 X 128 pixel single photon avalanche diode (SPAD) time-resolved single photon counting (TCSPC) image sensor is implemented in STMicroelectronics 40-nm CMOS technology. The 13% fill factor, 18.4\,\,\mu \text {m} \times 9.2\,\,\mu \text{m} pixel contains a 33-ps resolution, 135-ns full scale, 12-bit time-to-digital converter (TDC) with 0.9-LSB differential and 5.64-LSB integral nonlinearity (DNL/INL). The sensor achieves a mean 219-ps full-width half-maximum (FWHM) impulse response function (IRF) and is operable at up to 18.6 kframes/s through 64 parallelized serial outputs. Cylindrical microlenses with a concentration factor of 3.25 increase the fill factor to 42%. The median dark count rate (DCR) is 25 Hz at 1.5-V excess bias. A digital calibration scheme integrated into a column of the imager allows off-chip digital process, voltage, and temperature (PVT) compensation of every frame on the fly. Fluorescence lifetime imaging microscopy (FLIM) results are presented

Time-to-digital converters and histogram builders in SPAD arrays for pulsed-LiDAR

Light Detection and Ranging (LiDAR) is a 3D imaging technique widely used in many applications such as augmented reality, automotive, machine vision, spacecraft navigation and landing. Pulsed-LiDAR is one of the most diffused LiDAR techniques which relies on the measurement of the round-trip travel time of an optical pulse back-scattered from a distant target. Besides the light source and the detector, Time-to-Digital Converters (TDCs) are fundamental components in pulsed-LiDAR systems, since they allow to measure the back-scattered photon arrival times and their performance directly impact on LiDAR system requirements (i.e., range, precision, and measurements rate). In this work, we present a review of recent TDC architectures suitable to be integrated in SPAD-based CMOS arrays and a review of data processing solutions to derive the TOF information. Furthermore, main TDC parameters and processing techniques are described and analyzed considering pulsed-LiDAR requirements

Advanced photon counting techniques for long-range depth imaging

The Time-Correlated Single-Photon Counting (TCSPC) technique has emerged as a candidate approach for Light Detection and Ranging (LiDAR) and active depth imaging applications. The work of this Thesis concentrates on the development and investigation of functional TCSPC-based long-range scanning time-of-flight (TOF) depth imaging systems. Although these systems have several different configurations and functions, all can facilitate depth profiling of remote targets at low light levels and with good surface-to-surface depth resolution. Firstly, a Superconducting Nanowire Single-Photon Detector (SNSPD) and an InGaAs/InP Single-Photon Avalanche Diode (SPAD) module were employed for developing kilometre-range TOF depth imaging systems at wavelengths of ~1550 nm. Secondly, a TOF depth imaging system at a wavelength of 817 nm that incorporated a Complementary Metal-Oxide-Semiconductor (CMOS) 32×32 Si-SPAD detector array was developed. This system was used with structured illumination to examine the potential for covert, eye-safe and high-speed depth imaging. In order to improve the light coupling efficiency onto the detectors, the arrayed CMOS Si-SPAD detector chips were integrated with microlens arrays using flip-chip bonding technology. This approach led to the improvement in the fill factor by up to a factor of 15. Thirdly, a multispectral TCSPC-based full-waveform LiDAR system was developed using a tunable broadband pulsed supercontinuum laser source which can provide simultaneous multispectral illumination, at wavelengths of 531, 570, 670 and ~780 nm. The investigated multispectral reflectance data on a tree was used to provide the determination of physiological parameters as a function of the tree depth profile relating to biomass and foliage photosynthetic efficiency. Fourthly, depth images were estimated using spatial correlation techniques in order to reduce the aggregate number of photon required for depth reconstruction with low error. A depth imaging system was characterised and re-configured to reduce the effects of scintillation due to atmospheric turbulence. In addition, depth images were analysed in terms of spatial and depth resolution