59 research outputs found
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
ToF Estimation Based on Compressed Real-Time Histogram Builder for SPAD Image Sensors
This paper presents a FPGA implementation of a novel depth map estimation algorithm for direct time-of-flight CMOS image sensors (dToF-CISs) based on single-photon avalanche-diodes (SPADs). Conventional ToF computation algorithms rely on complete ToF histograms. The next generation of high speed dToF-CIS is expected to have wide dynamic range and high depth resolution. Applications such as 3D imaging based on dToF-CISs require pixel-level ToF histograms which have to be stored by huge fully-random access memory (RAM) modules. The proposed shifted inter-frame histogram (SiFH) algorithm has the same accuracy but requires a memory footprint 128 times smaller than the conventional algorithm. Thus a much larger number of pixels can be resolved using limited block RAM resources of FPGAs. Moreover the overall frame rate is also remarkably improved compared to the scanning method. The proof of concept of the SiFH algorithm on 15 bits has been implemented on Spartan-3E. An automated testbench was developed to confirm that no ambiguity errors occur along the entire dynamic range.Office of Naval Research (USA) N00014-19-1-2156Ministerio de Economía y Competitividad TEC2015-66878-C3-1-RJunta de Andalucía TIC 2338- 201
Compact Real-Time Inter-Frame Histogram Builder for 15-Bits High-Speed ToF-Imagers Based on Single-Photon Detection
Time-of-flight (ToF) image sensors based on single-photon detection, i.e., SPADs, require some filtering of pixel readings. Accurate depth measurements are only possible if the jitter of the detector is mitigated. Moreover, the time stamp needs to be effectively separated from uncorrelated noise, such as dark counts and background illumination. A powerful tool for this is building a histogram of a number of pixel readings. Future generation of ToF imagers are seeking to increase spatial and temporal resolution along with the dynamic range and frame rate. Under these circumstances, storing the complete histogram for every pixel becomes practically impossible. Considering that most of the information contained by the histogram represents noise, we propose a highly efficient method to store just the relevant data required for the ToF computation. This method makes use of the shifted inter-frame histogram. It requires a memory as low as 128 times smaller than storing the complete histogram if the pixel values are coded on up to 15 bits. Moreover, a fixed 2 8 words memory is enough to process histograms containing up to 2 15 bins. In exchange, the overall frame rate only decreases to one half. The hardware implementation of this algorithm is presented. Its remarkable robustness for a low SNR of the ToF estimation is demonstrated by Matlab simulations and FPGA implementation using input data from a SPAD camera prototype.Office of Naval Research (USA) N000141410355Ministerio de Economía y Competitividad TEC2015-66878-C3-1-RJunta de Andalucía TIC 2338-2013European Union H2020 76586
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
3D LIDAR imaging using Ge-on-Si single–photon avalanche diode detectors
We present a scanning light detection and ranging (LIDAR) system incorporating an individual Ge-on-Si single-photon avalanche diode (SPAD) detector for depth and intensity imaging in the short-wavelength infrared region. The time-correlated single-photon counting technique was used to determine the return photon time-of-flight for target depth information. In laboratory demonstrations, depth and intensity reconstructions were made of targets at short range, using advanced image processing algorithms tailored for the analysis of single–photon time-of-flight data. These laboratory measurements were used to predict the performance of the single-photon LIDAR system at longer ranges, providing estimations that sub-milliwatt average power levels would be required for kilometer range depth measurements
Simulation and Characterization of Single Photon Detectors for Fluorescence Lifetime Spectroscopy and Gamma-ray Applications
Gamma-ray and Fluorescence Lifetime Spectroscopies are driving the development of non-imaging silicon photon sensors and, in this context, Silicon Photo-Multipliers (SiPM)s are leading the starring role. They are 2D array of optical diodes called Single Photon Avalanche Diodes (SPAD)s, and are normally fabricated with a dedicated silicon process. SPADs amplify the charge produced by the single absorbed photon in a way that recalls the avalanche amplification exploited in Photo-Multiplier Tubes (PMT)s. Recently 2D arrays of SPADs have been realized also in standard CMOS technology, paving the way to the realization of completely custom sensors that can host ancillary electronic and digital logic on-chip. The designs of scientific apparatus have been influenced for years by the bulky PMT-based detectors. An overwhelming interest in both SiPMs and CMOS SPADs lies in the possibility of displacing these small sensors realizing new detectors geometries. This thesis examines the potential deployment of SiPM-based detector in an apparatus built for the study of the Time-Of-Flight (TOF) of Positronium (Ps) and the displacement of 2D array of CMOS SPADs in a lab-on-chip apparatus for Fluorescence Lifetime Spectroscopy. The two design procedures are performed using Monte-Carlo simulations. Characterizations of the two sensor have been carried out, allowing for a performance evaluation and a validation of the two design procedures
CMOS SPAD-based image sensor for single photon counting and time of flight imaging
The facility to capture the arrival of a single photon, is the fundamental limit to the detection of quantised
electromagnetic radiation. An image sensor capable of capturing a picture with this ultimate optical and
temporal precision is the pinnacle of photo-sensing. The creation of high spatial resolution, single photon
sensitive, and time-resolved image sensors in complementary metal oxide semiconductor (CMOS) technology
offers numerous benefits in a wide field of applications. These CMOS devices will be suitable to replace high
sensitivity charge-coupled device (CCD) technology (electron-multiplied or electron bombarded) with
significantly lower cost and comparable performance in low light or high speed scenarios. For example, with
temporal resolution in the order of nano and picoseconds, detailed three-dimensional (3D) pictures can be
formed by measuring the time of flight (TOF) of a light pulse. High frame rate imaging of single photons can
yield new capabilities in super-resolution microscopy. Also, the imaging of quantum effects such as the
entanglement of photons may be realised.
The goal of this research project is the development of such an image sensor by exploiting single photon
avalanche diodes (SPAD) in advanced imaging-specific 130nm front side illuminated (FSI) CMOS technology.
SPADs have three key combined advantages over other imaging technologies: single photon sensitivity,
picosecond temporal resolution and the facility to be integrated in standard CMOS technology. Analogue
techniques are employed to create an efficient and compact imager that is scalable to mega-pixel arrays. A
SPAD-based image sensor is described with 320 by 240 pixels at a pitch of 8μm and an optical efficiency or
fill-factor of 26.8%. Each pixel comprises a SPAD with a hybrid analogue counting and memory circuit that
makes novel use of a low-power charge transfer amplifier. Global shutter single photon counting images are
captured. These exhibit photon shot noise limited statistics with unprecedented low input-referred noise at an
equivalent of 0.06 electrons.
The CMOS image sensor (CIS) trends of shrinking pixels, increasing array sizes, decreasing read noise, fast
readout and oversampled image formation are projected towards the formation of binary single photon imagers
or quanta image sensors (QIS). In a binary digital image capture mode, the image sensor offers a look-ahead to
the properties and performance of future QISs with 20,000 binary frames per second readout with a bit error
rate of 1.7 x 10-3. The bit density, or cumulative binary intensity, against exposure performance of this image
sensor is in the shape of the famous Hurter and Driffield densitometry curves of photographic film.
Oversampled time-gated binary image capture is demonstrated, capturing 3D TOF images with 3.8cm
precision in a 60cm range
Millimeter-Precision Laser Rangefinder Using a Low-Cost Photon Counter
In this book we successfully demonstrate a millimeter-precision laser rangefinder using a low-cost photon counter. An application-specific integrated circuit (ASIC) comprises timing circuitry and single-photon avalanche diodes (SPADs) as the photodetectors. For the timing circuitry, a novel binning architecture for sampling the received signal is proposed which mitigates non-idealities that are inherent to a system with SPADs and timing circuitry in one chip
Design of CMOS Digital Silicon Photomultipliers with ToF for Positron Emission Tomography
This thesis presents a contribution to the design of single-photon detectors for
medical imaging. Specifically, the focus has been on the development of a pixel
capable of single-photon counting in CMOS technology, and the associated
sensor thereof. These sensors can work under low light conditions and provide
timing information to determine the time-stamp of the incoming photons.
For instance, this is particularly attractive for applications that rely either on
time-of-flight measurements or on exponential decay determination of the light
source, like positron emission tomography or fluorescence-lifetime imaging,
respectively. This thesis proposes the study of the pixel architecture to optimize
its performance in terms of sensitivity, linearity and signal to noise ratio.
The design of the pixel has followed a bottom-up approach, taking care of
the smallest building block and studying how the different architecture choices
affect performance. Among the various building blocks needed, special emphasis has been placed on the following:
• the Single-Photon Avalanche Diode (SPAD), a photodiode able to detect
photons one by one;
• the front-end circuitry of this diode, commonly called quenching and
recharge circuit;
• the Time-to-Digital Converter (TDC), which determines the timing performance of the pixel.
The proposed architectural exploration provides a comprehensive insight
into the design space of the pixel, allowing to determine the optimum design
points in terms of sensor sensitivity, linearity or signal to noise ratio, thus helping designers to navigate through non-straightforward trade-offs.
The proposed TDC is based on a voltage-controlled ring oscillator, since this
architecture provides moderate time resolutions while keeping the footprint,
the power, and conversion time relatively small. Two pseudo-differential delay
stages have been studied, one with cross-coupled PMOS transistors and the
other with cross-coupled inverters. Analytical studies and simulations have
shown that cross-coupled inverters are the most appropriate to implement
the TDC because they achieve better time resolution with smaller energy per
conversion than cross-coupled PMOS transistor stages.
A 1.3×1.3 mm2 pixel has been implemented in an 110 nm CMOS image sensor technology, to have the benefits of sub-micron technologies along with the
cleanliness of CMOS image sensor technologies. The fabricated chips have been
used to characterize the single-photon avalanche diodes. The results agree with
expectations: a maximum photon detection probability of 46 % and a median
dark count rate of 0.4 Hz/µm2 with an excess voltage of 3 V. Furthermore, the
characterization of the TDC shows that the time resolution is below 100 ps,
which agrees with post-layout simulations. The differential non-linearity is
±0.4LSB, and the integral non-linearity is ±6.1LSB.
Photoemission occurs during characterization - an indication that the avalanches are not quenched properly. The cause of this has been identified to be
in the design of the SPAD and the quenching circuit. SPADs are sensitive devices which maximum reverse current must be well defined and limited by the
quenching circuit, otherwise unwanted effects like excessive cross-talk, noise,
and power consumption may happen. Although this issue limits the operation
of the implemented pixel, the information obtained during the characterization
will help to avoid mistakes in future implementations
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