Single-photon Avalanche Diode Array Research Articles

A SiPM-Enabled Portable Delayed Fluorescence Photon Counting Device: Climatic Plant Stress Biosensing.

A portable and sensitive time-resolved biosensor for capturing very low intensity light emission is a promising avenue to study plant delayed fluorescence. These weak emissions provide insight on plant health and can be useful in plant science as well as in the development of accurate feedback indicators for plant growth and yield in applications of agricultural crop cultivation. A field-based delayed fluorescence device is also desirable to enable monitoring of plant stress response to climate change. Among basic techniques for the detection of rapidly fluctuating low intensity light is photon counting. Despite its vast utility, photon counting techniques often relying on photomultiplier tube (PMT) technology, having restricted use in agricultural and environment measurements of plant stress outside of the laboratory setting, mainly due to the prohibitive cost of the equipment, high voltage nature, and the complexity of its operation. However, recent development of the new generation solid-state silicon photomultiplier (SiPM) single photon avalanche diode array has enabled the availability of high quantum efficiency, easy-to-operate, compact, photon counting systems which are not constrained to sophisticated laboratories, and are accessible owing to their low-cost. In this contribution, we have conceived, fabricated and validated a novel SiPM-based photon counting device with integrated plug-and-play excitation LED, all housed inside a miniaturized sample chamber to record weak delayed fluorescence lifetime response from plant leaves subjected to varying temperature condition and drought stress. Findings from our device show that delayed fluorescence reports on the inactivation to the plant’s photosystem II function in response to unfavorable acute environmental heat and cold shock stress as well as chronic water deprivation. Results from our proof-of-concept miniaturized prototype demonstrate a new, simple and effective photon counting instrument is achieved, one which can be deployed in-field to rapidly and minimally invasively assess plant physiological growth and health based on rapid, ultra-weak delayed fluorescence measurements directly from a plant leaf.

High-resolution depth imaging with a small-scale SPAD array based on the temporal-spatial filter and intensity image guidance.

Currently single-photon avalanche diode (SPAD) arrays suffer from a small-scale pixel count, which makes it difficult to achieve high-resolution 3D imaging directly through themselves. We established a CCD camera-assisted SPAD array depth imaging system. Based on illumination laser lattice generated by a diffractive optical element (DOE), the registration of the low-resolution depth image gathered by SPAD and the high-resolution intensity image gathered by CCD is realized. The intensity information is used to guide the reconstruction of a resolution-enhanced depth image through a proposed method consisting of total generalized variation (TGV) regularization and temporal-spatial (T-S) filtering algorithm. Experimental results show that an increasement of 4 × 4 times for native depth image resolution is achieved and the depth imaging quality is also improved by applying the proposed method.

Fast-Gated 16 × 16 SPAD Array With 16 on-Chip 6 ps Time-to-Digital Converters for Non-Line-of-Sight Imaging

We present the design and characterization of a fully-integrated array of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$16 \times 16$ </tex-math></inline-formula> Single-Photon Avalanche Diodes (SPADs) with fast-gating capabilities and 16 on-chip 6 ps time-to-digital converters, which has been embedded in a compact imaging module. Such sensor has been developed for Non-Line-Of-Sight imaging applications, which require: i) a narrow instrument response function, for a centimeteraccurate single-shot precision; ii) fast-gated SPADs, for time-filtering of directly reflected photons; iii) high photon detection probability, for acquiring faint signals undergoing multiple scattering events. Thanks to a novel multiple differential SPAD-SPAD sensing approach, SPAD detectors can be swiftly activated in less than 500 ps and the full-width at half maximum of the instrument response function is always less than 75 ps (60 ps on average). Temporal responses are consistently uniform throughout the gate window, showing just few picoseconds of time dispersion when 30 ns gate pulses are applied, while the differential non-linearity is as low as 250 fs. With a photon detection probability peak of 70% at 490 nm, a fill-factor of 9.6% and up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.6 \cdot 10^{8}$ </tex-math></inline-formula> photon time-tagging measurements per second, such sensor fulfills the demand for fully-integrated imaging solutions optimized for non-line-of-sight imaging applications, enabling to cut exposure times while also optimizing size, weight, power and cost, thus paving the way for further scaled architectures.

Research on the detection probability curve characteristics of long-range target based on SPAD array lidar

Using single photon avalanche diode (SPAD) array lidar to detect long-range target, the difference of target surface characteristics will cause the distortion of the detection probability curve. This distortion severely limits the ability of target feature extraction and recognition. In this paper, a detection probability model for detecting long-range target using SPAD array lidar is established, which considers the influence of the difference of target surface structural characteristics (distance and pose) and reflection characteristics (reflectivity and cross-sectional area). Based on the finite element analysis method, the proposed model can calculate the detection probability when the target surface has the above characteristics difference. The full width at half maximum (FWHM) and skewness are used to evaluate the distortion of the detection probability curve. It reveals that with the increase of the structural characteristic difference of the target, the increasing trend of FWHM satisfies the cubic polynomial, while the decreasing trend of skewness satisfies the cubic polynomial. As the ratio of the reflection characteristic of the target increases, the decreasing trend of FWHM satisfies the first-order polynomial, while the increasing trend of skewness satisfies the first-order polynomial. The experimental results show that the feature vector composed of the detection probability curve features (FWHM, skewness, and total detection probability) can be used to identify the pixels containing target surfaces with different distances. And the identifiable distance difference is less than the system pulse width distance. This paper lays a theoretical foundation for target characteristic analysis and target recognition based on SPAD array lidar.

Deep-learning based photon-efficient 3D and reflectivity imaging with a 64 × 64 single-photon avalanche detector array.

A single photon avalanche diode (SPAD) is a high sensitivity detector that can work under weak echo signal conditions (≤1 photon per pixel). The measured digital signals can be used to invert the range and reflectivity images of the target with photon-efficient imaging reconstruction algorithm. However, the existing photon-efficient imaging reconstruction algorithms are susceptible to noise, which leads to poor quality of the reconstructed range and reflectivity images of target. In this paper, a non-local sparse attention encoder (NLSA-Encoder) neural network is proposed to extract the 3D information to reconstruct both the range and reflectivity images of target. The proposed network model can effectively reduce the influence of noise in feature extraction and maintain the capability of long-range correlation feature extraction. In addition, the network is optimized for reconstruction speed to achieve faster reconstruction without performance degradation, compared with other existing deep learning photon-efficient imaging reconstruction methods. The imaging performance is verified through numerical simulation, near-field indoor and far-field outdoor experiments with a 64 × 64 SPAD array. The experimental results show that the proposed network model can achieve better results in terms of the reconstruction quality of range and reflectivity images, as well as reconstruction speed.

Design and characterization of a time-domain optical tomography platform for mesoscopic lifetime imaging.

We report on the system design and instrumental characteristics of a novel time-domain mesoscopic fluorescence molecular tomography (TD-MFMT) system for multiplexed molecular imaging in turbid media. The system is equipped with a supercontinuum pulsed laser for broad spectral excitation, based on a high-density descanned raster scanning intensity-based acquisition for 2D and 3D imaging and augmented with a high-dynamical range linear time-resolved single-photon avalanche diode (SPAD) array for lifetime quantification. We report on the system's spatio-temporal and spectral characteristics and its sensitivity and specificity in controlled experimental settings. Also, a phantom study is undertaken to test the performance of the system to image deeply-seated fluorescence inclusions in tissue-like media. In addition, ex vivo tumor xenograft imaging is performed to validate the system's applicability to the biological sample. The characterization results manifest the capability to sense small fluorescence concentrations (on the order of nanomolar) while quantifying fluorescence lifetimes and lifetime-based parameters at high resolution. The phantom results demonstrate the system's potential to perform 3D multiplexed imaging thanks to spectral and lifetime contrast in the mesoscopic range (at millimeters depth). The ex vivo imaging exhibits the prospect of TD-MFMT to resolve intra-tumoral heterogeneity in a depth-dependent manner.

High Speed and High Sensitivity InGaAs/InAlAs Single Photon Avalanche Diodes for Photon Counting Communication

A planar InGaAs/InAlAs single photon avalanche diode (SPAD) was fabricated and comprehensively characterized. At room temperature, the dark count rate is 3.1 MHz at 10.4% photon detection efficiency. The corresponding noise-equivalent power of 3.07 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−15</sup> W/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> is achieved, which is the lowest value among reported InGaAs/InAlAs SPADs operated at room temperature. Moreover, it is the first time to report an InGaAs/InAlAs SPAD driven by a sine wave gate (SWG) quenching circuit as a receiver in the photon counting communication system. For achieving high-bit-rate transmission with high sensitivity, a high-frequency (1.8 GHz) SWG quenching circuit is developed and the time jitter for the system is reduced to 53 ps. At bit rates of 100 Mbit/s, 200 Mbit/s, and 400 Mbit/s, the sensitivities of −57.2 dBm, −53.42 dBm, and −51.06 dBm are experimentally demonstrated with a single InGaAs/InAlAs SPAD respectively. In addition, a theoretical sensitivity of −42.7 dBm is predicted at 1.8 Gbit/s with a 4 × 4 InGaAs/InAlAs SPAD array.

High resolution TCSPC imaging of diffuse light with a one-dimensional SPAD array scanning system.

We report a time-correlated single-photon counting (TCSPC) imaging system based on a line-scanning architecture. The system benefits from the high fill-factor, active area, and large dimension of an advanced CMOS single-photon avalanche diode (SPAD) array line-sensor. A two-dimensional image is constructed using a moving mirror to scan the line-sensor field-of-view (FOV) across the target, to enable the efficient acquisition of a two-dimensional 0.26 Mpixel TCSPC image. We demonstrate the capabilities of the system for TCSPC imaging and locating objects obscured in scattering media - specifically to locate a series of discrete point sources of light along an optical fibre submerged in a highly scattering solution. We demonstrate that by selectively imaging using early arriving photons which have undergone less scattering than later arriving photons, our TCSPC imaging system is able to locate the position of discrete point sources of light than a non-time-resolved imaging system.

A Multi-Time-Gated SPAD Array with Integrated Coarse TDCs

Time-gating of single-photon avalanche diodes (SPADs) was commonly used as a method to reduce dark noise in biomedical imaging applications where photon events are correlated with a reference clock. Time-gating was also used to obtain timing information of photon events by shifting the gate windows applied to a SPAD array. However, in this approach, fine timing resolution comes at the cost of a lengthened measurement time due to the large number of counts required for each shift. As a solution, we present a multi-time-gated SPAD array that simultaneously applies shifted gate windows to an array of SPADs, which has the potential to reduce the measurement time compared to a single time gate window. Compared to similar works, this design has fully integrated the multi-gate generation using shared circuitry which also functions as a coarse time-to-digital converter. The proposed array, fabricated in the TSMC 65 nm standard CMOS process, achieved a median dark count rate (DCR) of 37 kHz, 4.37 ns gate widths, 550 ps timing resolution, and a peak photon detection probability (PDP) of 42.9% at 420 nm, all at a 0.8 V excess bias.

Imaging Dynamics Beneath Turbid Media via Parallelized Single-Photon Detection.

Noninvasive optical imaging through dynamic scattering media has numerous important biomedical applications but still remains a challenging task. While standard diffuse imaging methods measure optical absorption or fluorescent emission, it is also well‐established that the temporal correlation of scattered coherent light diffuses through tissue much like optical intensity. Few works to date, however, have aimed to experimentally measure and process such temporal correlation data to demonstrate deep‐tissue video reconstruction of decorrelation dynamics. In this work, a single‐photon avalanche diode array camera is utilized to simultaneously monitor the temporal dynamics of speckle fluctuations at the single‐photon level from 12 different phantom tissue surface locations delivered via a customized fiber bundle array. Then a deep neural network is applied to convert the acquired single‐photon measurements into video of scattering dynamics beneath rapidly decorrelating tissue phantoms. The ability to reconstruct images of transient (0.1–0.4 s) dynamic events occurring up to 8 mm beneath a decorrelating tissue phantom with millimeter‐scale resolution is demonstrated, and it is highlighted how the model can flexibly extend to monitor flow speed within buried phantom vessels.

2-D Photon-Detection-Probability Simulation and a Novel Guard-Ring Design for Small CMOS Single-Photon Avalanche Diodes

CMOS single-photon avalanche diode (SPAD) array is a high-timing-resolution photon-counting image sensor. Pixel size shrinkage for high image resolution is highly desirable but hindered by the lowered photon-detection-probability (PDP) as the edge area becomes significant. We propose and demonstrate a parameter-free method for simulating PDP with the edge effect. Considering the doping profile, electric field, photon generation, and trigger probability distributions in two dimensions, the PDP reduction at the device edge is analyzed in a quantitative way. Besides, a novel guard-ring design for enhancing PDP of small SPAD has been proposed and simulated by our method. A threefold enhancement was obtained with the newly designed guard-ring structure compared with the original one. Our work is useful for developing small-pitch SPAD array for high-resolution imaging applications.

24‐3: Distinguished Student Paper: Towards a Solid‐State LIDAR Using Holographic Illumination and a SPAD‐Based Time‐Of‐Flight Image Sensor

Light Detection and Ranging (LIDAR) systems are finding their way into an increasing number of applications including autonomous vehicles. Imaging based on Single Photon Avalanche Diode (SPAD) arrays has demonstrated great promise for LIDAR, typically using flood illumination or raster‐scanning of a small spot. Holographic illumination, on the other hand, has been successfully applied to display systems such as automotive Head Up Displays (HUDs). It holds promise as the method of illumination for a LIDAR system. We present a novel architecture and report initial results to show that it can improve the depth accuracy of 3D imaging.

A 500 × 500 Dual-Gate SPAD Imager With 100% Temporal Aperture and 1 ns Minimum Gate Length for FLIM and Phasor Imaging Applications

In this article, we report on SwissSPAD3 (SS3), a 500 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times$ </tex-math></inline-formula> 500 pixel single-photon avalanche diode (SPAD) array, fabricated in 0.18- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> CMOS technology. In this sensor, we introduce a novel dual-gate architecture with two contiguous temporal windows, or gates, guaranteed by the circuit architecture to be nonoverlapping and covering the totality of the sensor’s exposure period. The gates can be adjusted with a temporal resolution of 17.9 ps, and the minimum measured gate width is 0.99 ns; to our knowledge, the shortest reported to date among large-format SPAD imagers. In the dual-channel mode, the burst frame rate is 49.8 and 97.7 kframes/s in the single-channel mode. A 2690-MB/s PCI express (PCIe) interface has been added to the data acquisition framework, enabling continuous operation at approximately 44 and 88 kframes/s. Due to optimizations of the gate-signal tree, we achieved a significant reduction to gate skew and gate width variation, which is negligible with respect to the SPAD temporal jitter. These improvements, along with sub-10-cps dark count rate (DCR) per pixel and 50% maximum photon detection probability (PDP), result in a sensor particularly well suited for fast acquisition fluorescence lifetime imaging microscopy (FLIM) experiments, for which we demonstrate reduced dispersion versus a single-gated sensor.

Advantages and Limitations of Fluorescence Lifetime Measurements Using Single-Photon Avalanche Diode (SPAD) Array Detector: A Comprehensive Theoretical and Experimental Study.

Fast fluorescence lifetime (FL) determination is a major factor for studying dynamic processes. To achieve a required precision and accuracy a certain number of photon counts must be detected. FL methods based on single-photon counting have strongly limited count rates because of the detector’s pile-up issue and are suffering from long measurement times in the order of tens of seconds. Here, we present an experimental and Monte Carlo simulation-based study of how this limitation can be overcome using array detectors based on single-photon avalanche diodes (SPADs). We investigated the maximum count rate per pixel to determine FL with a certain precision and accuracy before pile-up occurs. Based on that, we derived an analytical expression to calculate the total measurement time which is proportional to the FL and inversely proportional to the number of pixels. However, a higher number of pixels drastically increases data rate. This can be counteracted by lowering the time resolution. We found that even with a time resolution of four times the FL, an accuracy of 10% can be achieved. Taken all together, FLs between 10 ns and 3 ns can be determined with a 300-pixel SPAD array detector with a measurement time and data rate less than 1 µs and 700 Mbit/s, respectively. This shows the enormous potential of SPAD array detector for high-speed applications requiring continuous data read out.

A Reconfigurable 5-Channel Ring-Oscillator-Based TDC for Direct Time-of-Flight 3D Imaging

Time-correlated single-photon counting (TCSPC) 3D imaging requires the digitization of photon arrival times across an array of single-photon avalanche diodes (SPADs). The most critical performance metrics of time-to-digital converters (TDC) in TCSPC applications are their conversion rate (CR), area, and power consumption. This paper presents a multi-channel RObased TDC architecture whose power consumption scales with its configurable resolution for power-sensitive applications. Further power savings are achieved by sharing one RO among multiple TDC channels. We have demonstrated that sharing one RO among five channels reduces the power consumption by more than 75 relative to non-shared architectures. Here, a 5-channel 12-bit TDC is fabricated in 65 nm CMOS with an area of 1920 um2 per channel. It demonstrates CR up to 125 MHz and offers a resolution configurable over the range of 24-133 ps. At a CR of 125 MHz, the TDC power consumption per channel is 0.1 mW and 1 mW per channel at 133 ps and 24 ps resolution, respectively.

5 Gbps optical wireless communication using commercial SPAD array receivers.

Photon counting detectors such as single-photon avalanche diode (SPAD) arrays can be used to improve the sensitivity of optical wireless communication (OWC) systems. However, the achievable data rate of SPAD-based OWC systems is strongly limited by the nonlinearity induced by the SPAD dead time. In this work, the performance of a SPAD-based OWC system with orthogonal frequency division multiplexing (OFDM) is investigated and compared with that of on-off keying (OOK). We employ nonlinear equalization, peak-to-average power ratio optimization by adjusting the OFDM clipping level, and adaptive bit and energy loading to achieve a record experimental data rate of 5 Gbps. The contrasting optimal regimes of operation of the two modulation schemes are also demonstrated.

SPAD Pixel With Sub-NS Dead-Time for High-Count Rate Applications

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&#x0025; 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.

Fluorescence fluctuation spectroscopy with a cooled SPAD array detector to unravel molecular processes in living cells

The molecular dynamics and protein interactions in living cells remain poorly understood despite their importance to several cellular processes. RNA molecules, for example, play a major role in many processes, from encoding and regulating gene expression to catalyzing biological reactions. Alterations of the metabolism of RNAs and RNA-binding proteins are involved in neurodegenerative diseases, such as ALS in neuronal cells. In particular, the connection between biomolecular dynamic processes and cellular function/organization is still unclear.Here, we propose a new class of Fluorescence Fluctuation Spectroscopy (FFS) techniques performed with a novel single-photon-avalanche-diode (SPAD) array detector to better decipher molecular dynamics, interactions and structural changes directly in living cells with microsecond time resolution.

Design and Analysis of a Photon Counting System Using Covered Single-Photon Avalanche Photodiode

A photon counting system using covered single-photon avalanche diode (SPAD) based on a standard integrated circuit (IC) process ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.18~\mu \text{m}$ </tex-math></inline-formula> ) is designed and analyzed in this work. The SPAD is formed using the medium voltage (MV) doping layers of the process. To reduce the dark count rate (DCR) in the SPAD, a shaded SPAD with the same structure is fabricated on the same chip which is covered by a metal layer and only providing DCR for the DCR correction. This DCR provided by the shaded SPAD can also be used for the real time on chip monitoring of some other parameters, such as temperature, breakdown voltage, and afterpulsing probability. Experimental results show that the SPAD developed is able to detect the visible light from 450 to 850 nm with a 35% peak photon detection probability (PDP) achieved at around 550 nm with a bias voltage of 16 V (excess voltage of 3 V). A timing jitter of 176 ps is measured with an excess voltage of 3 V. The DCR in the SPAD tested is about 1.38 cps/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}^{2}$ </tex-math></inline-formula> with an excess bias voltage of 1 V and 14.62 cps/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}^{2}$ </tex-math></inline-formula> with the excess bias voltage of 3 V without the DCR correction. Results also show that a reduction of more than 85% in the DCR (background noise) can be achieved when the DCR correction is applied resulting in a DCR of 1.68 cps/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}^{2}$ </tex-math></inline-formula> with an excess bias voltage of 3 V. By monitoring the DCR of the shaded SPAD, the breakdown voltage and temperature of other on chip SPAD can be measured. The potential usefulness of the afterpulsing probability monitoring using the shaded SPAD and the crosstalk probability (CTP) between SPADs on the chip are analyzed. In addition, the effects of process variations on the SPAD performance are investigated by testing ten chips with the same SPADs fabricated, and a potential method is proposed for alleviate the process variations in the SPAD arrays.

A High-Throughput Photon Processing Technique for Range Extension of SPAD-Based LiDAR Receivers

There has recently been a keen interest in developing Light Detection and Ranging (LiDAR) systems using Single Photon Avalanche Diode (SPAD) sensors. This has led to a variety of implementations in pixel combining techniques and Time to Digital Converter (TDC) architectures for such sensors. This paper presents a comparison of these approaches and demonstrates a technique capable of extending the range of LiDAR systems with improved resilience to background conditions. A LiDAR system emulator using a reconfigurable SPAD array and FPGA interface is used to compare these different techniques. A Monte Carlo simulation model leveraging synthetic 3D data is presented to visualize the sensor performance on realistic automotive LiDAR scenes.