Linear arrays of single-photon detectors for photon counting and timing

A time-gated 4×128 Single Photon Avalanche Diode (SPAD) line array with a 512-channel Time-to-digital converter (TDC) has been designed and fabricated in a 0.35 μm high voltage CMOS technology for pulsed Raman spectroscopy. In Raman spectroscopy the SPAD array can be designed as a line array with a high fill factor because electronics can be designed on either side of the SPAD array. In this paper cross talk measurements of the array have been carried out to investigate the probability of the cross talk between adjacent SPADs in an array with high density. Measurements showed a cross talk probability of 0.11% with a pitch of adjacent SPADs of 32 μm. In addition, the largest probability of the cross talk was measured to occur with a short delay.

Single-Photon Avalanche Diodes (SPADs) are an emerging pixel technology able to detect the arrival of single photons. Arrays of SPADs can be used to image a scene in binary frames, indicating whether there has been a photon detection in a pixel during a very short frame exposure (e.g., few microseconds). Like many imaging technologies, SPAD sensors suffer from false photon hits, called Dark Count (DC), which increase the photon count, resulting in abnormally bright pixels, called hot pixels. Correction methods used for common CCD/CMOS sensors, such as dark frame subtraction, are ineffective for SPADs, which have non-linear response in observed counts due to their 1-bit quantization. Thus, we derive an analytical model of the count bias introduced by DC, which is signal-dependent, and provide a pixel-wise, closed-form DC correction algorithm. Remarkably, our correction is optimal in expectation, meaning that on average it removes perfectly the distortion. We validate our method on real SPAD acquisitions and synthetic data, showing that it significantly improves image reconstruction.

The integrated circuit design of Single Photon Avalanche Diode (SPAD) with quenching circuit in CMOS is highly desirable photon detection at high rates, but low counting rates constrain image acquisition rates and dynamic range. The characterization should primarily define the dead time of quenching SPAD circuit in order to estimate the SPAD performance prior fabrication. This paper reports the development and characterization of the mathematical model SPAD on passively quenched SPAD circuit with ballast resistor. An improved model in defining the dead time (tD) response for SPAD is used to characterize the performance of 8×1 passively quenched SPAD array. The time response for both quenching and recharging time are developed for 180 nm depletion layer which means low voltage technology. Hence that, the 8×1 passively SPAD array circuit is designed by using Silterra 180nm CMOS technology for uncorrelated time measurements with on-chip 4-bit counter to improve the counting rate. The passive quenching circuit design on-chip would enable the capability to perform at higher speed, which is more than 100 kHz. In addition, are presented in this paper.

We present a novel structured light technique that uses Single Photon Avalanche Diode (SPAD) arrays to enable 3D scanning at high-frame rates and low-light levels. This technique, called “Single-Photon Structured Light”, works by sensing binary images that indicates the presence or absence of photon arrivals during each exposure; the SPAD array is used in conjunction with a high-speed binary projector, with both devices operated at speeds as high as 20 kHz. The binary images that we acquire are heavily influenced by photon noise and are easily corrupted by ambient sources of light. To address this, we develop novel temporal sequences using error correction codes that are designed to be robust to short-range effects like projector and camera defocus as well as resolution mismatch between the two devices. Our lab prototype is capable of 3D imaging in challenging scenarios involving objects with extremely low albedo or undergoing fast motion, as well as scenes under strong ambient illumination.

Conventional sensors for hyperspectral imaging are limited by noise when operated in low illumination conditions. Recent advances in single-photon avalanche diode (SPAD) arrays provide image sensors capable of single photon counting imaging. Such devices provides camera systems which can operate under low illumination levels and high frame rates, with fully digital in-pixel processing capabilities. Here, we present a hyperspectral imaging system based on low level illumination of a target scene using nine LEDs, each with different emission wavelengths. The backscattered light is captured by a SPAD camera, which can recover spectral information based on the number of photon counts received for each illumination wavelength.

The growing interest for fast, compact and cost-effective 3D ranging imagers for automotive applications has prompted
\nto explore many different techniques for 3D imaging and to develop new system for this propose. CMOS imagers that
\nexploit phase-resolved techniques provide accurate 3D ranging with no complex optics and are rugged and costeffective.
\nPhase-resolved techniques indirectly measure the round-trip return of the light emitted by a laser and backscattered
\nfrom a distant target, computing the phase delay between the modulated light and the detected signal. Singlephoton
\ndetectors, with their high sensitivity, allow to actively illuminate the scene with a low power excitation (less than
\n10W with diffused daylight illumination). We report on a 4x4 array of CMOS SPAD (Single Photon Avalanche Diodes)
\ndesigned in a high-voltage 0.35 μm CMOS technology, for pulsed modulation, in which each pixel computes the phase
\ndifference between the laser and the reflected pulse. Each pixel comprises a high-performance 30 μm diameter SPAD, an
\nanalog quenching circuit, two 9 bit up-down counters and memories to store data during the readout. The first counter
\ncounts the photons detected by the SPAD in a time window synchronous with the laser pulse and integrates the whole
\nechoed signal. The second counter accumulates the number of photon detected in a window shifted with respect to the
\nlaser pulse, and acquires only a portion of the reflected signal. The array is readout with a global shutter architecture,
\nusing a 100 MHz clock; the maximal frame rate is 3 Mframe/s.

Single-photon detectors are employed to measure faint light signals with high detection efficiency and time resolution. Silicon photomultipliers (SiPMs) are arrays of Single-photon avalanche diodes (SPADs), each one with its quenching resistor; they are connected in parallel giving a signal proportional to the number of detected photons. They offer the typical advantages of solid-state detectors (e.g. ruggedness, low power consumption, small size), high detection efficiency, good time resolution [1] and very good photon-number resolving capabilities. In FBK we produce SiPMs and SPADs with detection efficiency peaked for blue/ultraviolet light (called NUV technology) [2], or green-red light detection (RGB technology), with different geometries. The former are typically employed in a large number of applications exploiting scintillators [3]. Looking for the maximization of the photo-detection efficiency (PDE), e.g. increasing the fill-factor (FF) of the SiPM cell, noise components such as afterpulsing and optical crosstalk between cells (i.e. the correlated noise) become important issues. Recently, we developed new NUV-SiPMs, with high detection efficiency and with different solutions to reduce the noise of the detectors, both at a technological level [4] and a cell-layout level. Here we show the performance of this new NUV-SiPM, produced at FBK, employing a new silicon substrate, with a reduced minority-carriers lifetime, and new SiPM with trenches between cells and high cell density.

Silicon photomultiplier (SiPMs) is a photon counting device that consists of single photon avalanche diodes (SPADs) arrays being operated at a Geiger mode and has been widely used in many kinds of application including high-energy physics, confocal laser scanning microscopy (CSLM), flow cytometry, and light detection and ranging (LiDAR) instead of photomultiplier tube (PMT). Trench structures basically places at the boarder of every SPAD to guard optical and electrical crosstalk. However, illuminating photons to the trench region restricted the performance of SiPM such as photon detection efficiency (PDE) and timing performance because it does not absorb photon sufficiently. Our previous work demonstrated that guiding incident photons into the photo sensitive area of SPADs using metalens array resulted in improvement of the performance of the SiPM. However, performance is expected to fluctuate due to misalignment caused by slight vibration because metalens and SiPMs are separated configuration. Guaranteeing a good alignment between the metalens and SiPM has been necessary for practical applications. Here, we demonstrate the completely monolithic integration of a metalens with its corresponding SPAD to provide stable PDE improvement while maintaining the compact and flatness of the optical device. Furthermore, we investigated the crosstalk and dark-current comparing with a reference SiPM without the metalens for investigating the negative effect of integrating metalens with the SiPM. From the results of sophisticated investigation, we show the more practical approach toward the improving overall performance of the SiPM not only the PDE and timing performance but also crosstalk and dark current. Finally, we will show our next perspective and progress towards near infrared application.

Vacuum based devices, such as Photo Multiplier Tubes (PMT) and Micro Channel Plates (MCP), have been for many years the sensors of choice for most applications calling for photon counting and timing (Renker, 2004). While providing very good sensitivity, noise and timing characteristics, these photodetectors feature a number of disadvantages: they are bulky, fragile, and sensitive to magnetic fields; they require very high operation voltages, and have large power consumption; in their high-performance models, providing good spatial resolution, they are still very expensive. For high-sensitivity imaging applications, suitable solutions are represented by CCD cameras coupled with either MCP Image Intensifiers (I-CCDs) or Electron Multipliers (EM-CCDs) (Dussault & Hoess, 2004). However, besides being very expensive, their performance is not completely satisfactory in extreme time resolved measurements. For reasons of cost, miniaturization, ruggedness, reliability, design flexibility, integration density, and signal processing capabilities, a fully solid-state solution (and, particularly, CMOS technology) would be highly desirable. Among the advanced CMOS image sensors so far reported, the most promising ones in terms of high sensitivity and fast timing are those based on Single Photon Avalanche Diodes (SPADs). SPADs are avalanche photodiodes operated in the so-called Geiger mode, i.e., biased above breakdown, so as to be sensitive to single photons (Cova et al., 1996). Although these sensors have been developed for more than 30 years, in particular owing to the group of Prof. Cova at Politecnico di Milano, and single devices have reached outstanding performance (Ghioni et al., 2007), only recently the perspective of making a SPAD-based camera has become feasible. The first SPAD-based pixel arrays in CMOS technology have been demonstrated only a few years ago (Rochas et al., 2003a), but since then further developments rapidly followed, also facilitated by the availability of commercial, High-Voltage CMOS technologies (HV-CMOS) aimed at integrated circuits for power electronics, as well as of specially tailored “imaging” processes, which have been boosted by the huge market of

We report on the design and characterization of a multipurpose 64 × 32 CMOS single-photon avalanche diode (SPAD) array. The chip is fabricated in a high-voltage 0.35-μm CMOS technology and consists of 2048 pixels, each combining a very low noise (100 cps at 5-V excess bias) 30-μm SPAD, a prompt avalanche sensing circuit, and digital processing electronics. The array not only delivers two-dimensional intensity information through photon counting in either free-running (down to 10-μs integration time) or time-gated mode, but can also perform smart light demodulation with in-pixel background suppression. The latter feature enables phase-resolved imaging for extracting either three-dimensional depth-resolved images or decay lifetime maps, by measuring the phase shift between a modulated excitation light and the reflected photons. Pixel-level memories enable fully parallel processing and global-shutter readout, preventing motion artifacts (e.g., skew, wobble, motion blur) and partial exposure effects. The array is able to acquire very fast optical events at high frame-rate (up to 100 000 fps) and at single-photon level. Low-noise SPADs ensure high dynamic range (up to 110 dB at 100 fps) with peak photon detection efficiency of almost 50% at 410 nm. The SPAD imager provides different operating modes, thus, enabling both time-domain applications, like fluorescence lifetime imaging (FLIM) and fluorescence correlation spectroscopy, as well as frequency-domain FLIM and lock-in 3-D ranging for automotive vision and lidar.

Silicon photomultipliers (SiPMs) are arrays of many single-photon avalanche diodes (SPADs), all connected in parallel. Each SPAD is sensitive to single photons and the SiPM gives an output proportional to the number of detected photons. These sensors are becoming more and more popular in different applications, from high-energy physics to spectroscopy, and they have been significantly improved over last years, decreasing the noise, increasing the cell fill-factor (FF) and thus achieving very high photon-detection efficiency (PDE). In FBK (Trento, Italy), we developed new SiPM technologies with high-density (HD) and, more recently, ultra-high-density (UHD) of cells (i.e. density of SPADs). These technologies employ deep-trenches between cells, for electrical and optical isolation. As an extreme case the smallest-cell, SiPM, i.e. with 5μm cell pitch, has about 40000 SPADs per squared millimeter. Such small SPAD dimensions gives a significantly high dynamic range to the SiPM. These small-cells SiPM have a lower correlated noise (including lower afterpulsing probability) and a faster recharge time (in the order of few nanoseconds), and they also preserve a very good detection efficiency (despite the small SPAD dimension).

Optical fibre based endoscopes are increasingly used for imaging and sensing within the human body without navigational guidance of the miniaturised fibre probe. Meanwhile, other medical device placement is a standard procedure in clinic. We demonstrate successful imaging of optical device location with centimetre resolution in clinically relevant models, in a realistically lit environment, achieved through the detection of early arriving photons with a time resolved single photon detector array. This prototype has been developed within the UK-EPSRC Proteus project, moving advanced research technologies towards clinical implementation. Short (~100ps) laser pulses are transmitted from the tip of the endoscope at 785nm in the “optical window” where attenuation is less severe in clinical scenarios. Most of the photons that pass through tissue undergo much scattering from the disordered tissue structures providing only low accuracy determination of the location of the light source. However, some photons probabilistically undergo less scattering, travelling through the medium in an almost straight line without a much extended path. Such photons exit the body sooner than the highly scattered light. A camera based upon a 32 × 32 array of Single Photon Avalanche Diodes (SPADs) made with CMOS technology is used to image the small number photons exiting the tissue. The time resolution capabilities of such a single photon detector (50ps time bin resolution, 200ps jitter) allow observation of the photon arrival times simultaneously for all 1024 pixels of the imaging array. Photon arrival statistics distinguish the early arriving photons from the highly scattered light, revealing the endoscope location. Scattered photon arrivals peak at delays of multiple nanoseconds due to the thick tissue samples. The progression of light through complex scattering structures can be observed. Normal fluorescent room lighting has distinct emission peaks. Appropriate choice of operating wavelength between these spectral features, combined with aggressive filtering, allows operation in normal fluorescent lighting. This compact packaged system is demonstrated in a normally lit room to determine optical endomicroscope location in a whole ventilated ovine lung as well as tissue models including bone structure. At the limit of capabilities of this prototype, demonstration through an entire human torso is shown to be possible. System improvements and the potential of the next generation prototype in development will be discussed. This offers the potential for real time (sub second) imaging of device location with a portable system for application in standard medical procedures, such as catheter insertion. The avoidance of the need to confirm device placement with X-ray imaging has potential to decrease disruption to procedures throughout clinical practice.

Quantum imaging and microscopy profit from entangled photons to surpass the boundaries of classical optics, thus improving image resolution. Thanks to their single-photon sensitivity, readout noise absence, low-voltage operation and high frame-rate, detectors based on Single-Photon Avalanche-Diodes (SPADs) are particularly suited for this application field. We discuss strengths and weaknesses of different SPAD based architectures (classified in SiPMs, SPAD arrays, or SiPM arrays), highlighting those to be exploited as quantum imagers. As just SPAD arrays are capable of spatial resolution at single-SPAD level and, through the possible implementation of quantum-specific on-chip processing, we identified them as the forefront detector type for quantum imaging and microscopy.

We describe the integration of techniques and technologies to develop a Point-of-Care for molecular diagnosis PoC-MD, based on a fluorescence lifetime measurement. Our PoC-MD is a low-cost, simple, fast, and easy-to-use general-purpose platform, aimed at carrying out fast diagnostics test through label detection of a variety of biomarkers. It is based on a 1-D array of 10 ultra-sensitive Single-Photon Avalanche Diode (SPAD) detectors made in a 0.18 μm High-Voltage Complementary Metal Oxide Semiconductor (HV-CMOS) technology. A custom microfluidic polydimethylsiloxane cartridge to insert the sample is straightforwardly positioned on top of the SPAD array without any alignment procedure with the SPAD array. Moreover, the proximity between the sample and the gate-operated SPAD sensor makes unnecessary any lens or optical filters to detect the fluorescence for long lifetime fluorescent dyes, such as quantum dots. Additionally, the use of a low-cost laser diode as pulsed excitation source and a Field-Programmable Gate Array (FPGA) to implement the control and processing electronics, makes the device flexible and easy to adapt to the target label molecule by only changing the laser diode. Using this device, reliable and sensitive real-time proof-of-concept fluorescence lifetime measurement of quantum dot QdotTM 605 streptavidin conjugate is demonstrated.

Time-resolved non-line-of-sight (NLOS) imaging based on single-photon avalanche diode (SPAD) detectors have demonstrated impressive results in recent years. To acquire adequate number of indirect photons from a hidden scene in the presence of overwhelming amount of early-arrival photons, a single-gated SPAD is widely employed to mitigate pile-up. However, additional prior knowledge of the hidden range and relay surface profile are required to preset the gating position and implement the reconstruction. With this work, we propose a gradient-gated technique to alleviate these shortcomings (pile-up and prior knowledge) in NLOS imaging with the development of a 6 × 6 SPAD array fabricated in a standard 55-nm Bipolar-CMOS-DMOS (BCD) technology. The SPAD sensor includes a delay-locked loop (DLL) block, a gating generator and a pixel array, which can be flexibly configured to free-running, single-gating and gradient-gating modes. The rise time of the gates is less than 450 ps (from 10% to 90%). In gradient-gating mode, the interval time between adjacent gates can be configured from 300 ps to 2 ns. In single-gating mode, the minimum gate window is measured below 5 ns. We demonstrated the sensor in a confocal NLOS imaging system that reconstructs hidden scenes with both retroreflective and diffuse objects. The presented scheme enables the development of calibration-free NLOS imaging modality for practical applications.