Single-photon Avalanche Diode Detectors Research Articles

Three-dimensional imaging of stationary and moving targets in turbid underwater environments using a single-photon detector array.

Three-dimensional imaging in underwater environments was investigated using a picosecond resolution silicon single-photon avalanche diode (SPAD) detector array fabricated in complementary metal-oxide semiconductor (CMOS) technology. Each detector in the 192 × 128 SPAD array had an individual time-to-digital converter allowing rapid, simultaneous acquisition of data for the entire array using the time-correlated single-photon counting approach. A picosecond pulsed laser diode source operating at a wavelength of 670 nm was used to illuminate the underwater scenes, emitting an average optical power up to 8 mW. Both stationary and moving targets were imaged under a variety of underwater scattering conditions. The acquisition of depth and intensity videos of moving targets was demonstrated in dark laboratory conditions through scattering water, equivalent to having up to 6.7 attenuation lengths between the transceiver and target. Data were analyzed using a pixel-wise approach, as well as an image processing algorithm based on a median filter and polynomial approximation.

Hot pixel classification of single‐photon avalanche diode detector arrays using a log‐normal statistical distribution

CMOS single-photon avalanche diode (SPAD) detector arrays are commonly used in low-light imaging applications, and are known to suffer from certain defects which cause `hot pixels'. These are detectors which exhibit a significantly larger than average dark count rate, adding noise to the data. Typically, data from these detectors are removed by post-processing or the detectors can, in some cases, be switched off prior to measurement. Users must define and identify these hot pixels, however there exists no consistent methodology of doing so. The authors present a self-consistent method of defining a hot pixel by fitting a log-normal distribution to a histogrammed dark-count map of the array. The approach has proven a robust method of classifying hot pixels in a number of different detector arrays, providing a threshold based on statistical analysis rather than human intuition. This definition provides a reliable and standardised figure of merit, facilitating a more accurate comparison between different single-photon detector arrays.

High Dynamic Range Imaging with TDC-Based CMOS SPAD Arrays

This paper investigates the use of image sensors based on complementary metal–oxide–semiconductor (CMOS) single-photon avalanche diodes (SPADs) in high dynamic range (HDR) imaging by combining photon counts and timestamps. The proposed method is validated experimentally with an SPAD detector based on a per-pixel time-to-digital converter (TDC) architecture. The detector, featuring 32 × 32 pixels with 44.64-µm pitch, 19.48% fill factor, and time-resolving capability of ~295-ps, was fabricated in a 150-nm CMOS standard technology. At high photon flux densities, the pixel output is saturated when operating in photon-counting mode, thus limiting the DR of this imager. This limitation can be overcome by exploiting the distribution of photon arrival times in each pixel, which shows an exponential behavior with a decay rate dependent on the photon flux level. By fitting the histogram curve with the exponential decay function, the extracted time constant is used to estimate the photon count. This approach achieves 138.7-dB dynamic range within 30-ms of integration time, and can be further extended by using a timestamping mechanism with a higher resolution.

Non-Line-of-Sight Three-Dimensional Imaging with a Single-Pixel Camera

Real time, high resolution 3D reconstruction of scenes hidden from the direct field of view is a challenging field of research with applications in real-life situations related e.g. to surveillance, self-driving cars and rescue missions. Most current techniques recover the 3D structure of a non-lineof-sight (NLOS) static scene by detecting the return signal from the hidden object on a scattering observation area. Here, we demonstrate the full colour retrieval of the 3D shape of a hidden scene by coupling back-projection imaging algorithms with the high-resolution time-of-flight information provided by a single-pixel camera. By using a high effciency Single-Photon Avalanche Diode (SPAD) detector, this technique provides the advantage of imaging with no mechanical scanning parts, with acquisition times down to sub-seconds.

Passive CMOS Single Photon Avalanche Diode Imager for a Gun Muzzle Flash Detection System

We present the architecture and design of a novel $64\times64$ CMOS single-photon avalanche diode (SPAD)-based imager for gun muzzle flash detection. The imager is fabricated in a standard front side illuminated $0.18~\mu \text{m}$ CMOS image sensor process. Each pixel comprises a $25~\mu \text{m}$ diameter SPAD, a variable-load passive quenching circuit implemented with 1.8 V PMOS, an 8-bit counter, an 8-bit latch register, and digital processing electronics, and feeds an 8-bit output bus. The array delivers two-dimensional intensity data through photon counting, with integration time as low as $5~\mu \text{s}$ over the full dynamic range. Per-pixel digital memory enables fully parallel processing and global-shutter mode readout. The imager can acquire fast optical events at high frame rate (up to 200 kfps) and at single-photon sensitivity. The imager has an 8-pixel (64-bit) parallel output bus. This imager enables for the first time the detection and arrival-direction determination of individual muzzle flashes in real time, and even in the case of bursts of flashes, at a moderate cost and size. The presented results confirm the feasibility of gun muzzle flash online detection in the visible or NIR spectrum by uncooled silicon SPAD detectors using standard CMOS technology.

High performance planar germanium-on-silicon single-photon avalanche diode detectors

Single-photon detection has emerged as a method of choice for ultra-sensitive measurements of picosecond optical transients. In the short-wave infrared, semiconductor-based single-photon detectors typically exhibit relatively poor performance compared with all-silicon devices operating at shorter wavelengths. Here we show a new generation of planar germanium-on-silicon (Ge-on-Si) single-photon avalanche diode (SPAD) detectors for short-wave infrared operation. This planar geometry has enabled a significant step-change in performance, demonstrating single-photon detection efficiency of 38% at 125 K at a wavelength of 1310 nm, and a fifty-fold improvement in noise equivalent power compared with optimised mesa geometry SPADs. In comparison with InGaAs/InP devices, Ge-on-Si SPADs exhibit considerably reduced afterpulsing effects. These results, utilising the inexpensive Ge-on-Si platform, provide a route towards large arrays of efficient, high data rate Ge-on-Si SPADs for use in eye-safe automotive LIDAR and future quantum technology applications.

Three-dimensional single-photon imaging through obscurants.

We investigate the depth imaging of objects through various densities of different obscurants (water fog, glycol-based vapor, and incendiary smoke) using a time-correlated single-photon detection system which had an operating wavelength of 1550 nm and an average optical output power of approximately 1.5 mW. It consisted of a monostatic scanning transceiver unit used in conjunction with a picosecond laser source and an individual Peltier-cooled InGaAs/InP single-photon avalanche diode (SPAD) detector. We acquired depth and intensity data of targets imaged through distances of up to 24 meters for the different obscurants. We compare several statistical algorithms which reconstruct both the depth and intensity images for short data acquisition times, including very low signal returns in the photon-starved regime.

Behavioral Modeling of Photon Arrival Time for Time-of-Flight Measurement Circuit Simulation

This letter proposes a behavioral model to estimate the statistical photon arrival time for time-of-flight (TOF) measurement circuit simulation. A theoretical model is derived to calculate the photon arrival rate by full consideration of a variety of properties of a TOF measurement system, such as laser source, optics, single-photon avalanche diode detector, target distance, and ambient illumination. For the first time, the newly developed model is implemented in behavioral Verilog-A hardware description language and successfully operated on Cadence Spectre simulator for TOF circuit simulation. It is capable of accurately predicting the statistical performance parameters such as photon arrival time and achievable distance, providing a new candidate tool for the ease of design and simulation of various high-performance TOF measurement circuits.

A Point-of-Care Device for Molecular Diagnosis Based on CMOS SPAD Detectors with Integrated Microfluidics.

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.

A Simple Analytic Modeling Method for SPAD Timing Jitter Prediction

Timing jitter as a key performance of single-photon avalanche diode (SPAD) detectors plays a significant role in determining the fast temporal response behavior of the SPAD device. Nevertheless, few analytic models are developed to directly calculate the characteristic of timing jitter for its modeling difficulty. In this paper, we propose a simple analytic modeling method, which can predict the temporal response of SPADs, without using time-consuming Monte Carlo simulation. Model investigation incorporates avalanche current, avalanche buildup time, and jitter tail under different conditions. Furthermore, the key model parameters provided by Geiger mode technology computer-aided design simulation allow an accurate prediction on timing jitter. Analytical results indicate that for an SPAD device structure with a shallow P+/N-well junction in a 0.18- $\mu \text{m}$ CMOS technology, the Gaussian peak response with about 110-ps full-width at half-maximum and the exponential jitter tail are in good agreement with the measured data, validating the accuracy, and feasibility of this modeling method.

Statistical Study of Intrinsic Parasitics in an SPAD-Based Integrated Fiber Optical Receiver

Recently, a fully integrated optical fiber receiver was reported based on single-photon avalanche diode detectors to reduce the sensitivity gap to the quantum limit. This was realized through an array of four detectors to overcome the parasitic effects dominated by afterpulsing which prevent a single detector from providing an acceptable performance to serve as an optical receiver. However, the array structure imposes an extra effect called “crosstalk,” which needs to be accurately characterized. Thanks to the memoryless nature of dark noise statistics, we present an effective method to investigate the intrinsic parasitic effects of the array based on an all-at-once dark noise measurement. The corresponding detection probabilities and delays versus excess bias voltage are obtained for different parasitics at all detectors. The results are used to model crosstalk and estimate the error probabilities required for bit error ratio analysis and structure optimization. The accuracy of the estimations is verified by experimental data.

High-speed dual color fluorescence lifetime endomicroscopy for highly-multiplexed pulmonary diagnostic applications and detection of labeled bacteria.

We present a dual-color laser scanning endomicroscope capable of fluorescence lifetime endomicroscopy at one frame per second (FPS). The scanning system uses a coherent imaging fiber with 30,000 cores. High-speed lifetime imaging is achieved by distributing the signal over an array of 1024 parallel single-photon avalanche diode detectors (SPADs), minimizing detection dead-time maximizing the number of photons detected per excitation pulse without photon pile-up to achieve the high frame rate. This also enables dual color fluorescence imaging by temporally shifting the dual excitation lasers, with respect to each other, to separate the two spectrally distinct fluorescent decays in time. Combining the temporal encoding, to provide spectral separation, with lifetime measurements we show a one FPS, multi-channel endomicroscopy platform for clinical applications and diagnosis. We demonstrate the potential of the system by imaging SmartProbe labeled bacteria in ex vivo samples of human lung using lifetime to differentiate bacterial fluorescence from the strong background lung autofluorescence which was used to provide structural information.

37ps-Precision Time-Resolving Active Quenching Circuit for High-Performance Single Photon Avalanche Diodes

Time-resolved imaging by means of single-photon avalanche diodes (SPADs) has achieved widespread interest in recent years, especially since technological progress has opened the way to the development of multichannel time-correlated single-photon counting (TCSPC) acquisition systems. Unfortunately, currently available TCSPC imagers feature relatively low performance with respect to state-of-the-art single-channel systems. A real breakthrough in this field would be the exploitation of large arrays of high-performance SPAD detectors developed by means of dedicated fabrication processes, usually referred to as custom technology. Custom-technology SPADs require external electronics potentially leading to interconnection issues for densely integrated arrays. In this paper, we present a new fully integrated front-end circuit able to provide both quenching/reset and timing functionalities while requiring a single connection toward the SPAD. This is the first fully integrated circuit reported in literature that can provide both the timing information about the photon time of arrival with a jitter as low as 37 ps and apply high-voltage pulses up to 50 V in order to meet the requirements of several detectors, including the new red-enhanced SPAD. Combining these two capabilities in a single circuit strongly reduces the complexity of the connection between an array of custom-technology SPADs and the relative external front end, thus paving the way for the exploitation of high-performance SPADs in TCSPC imaging systems.

Characterization of a Time-Resolved Diffuse Optical Spectroscopy Prototype Using Low-Cost, Compact Single Photon Avalanche Detectors for Tissue Optics Applications.

Time-resolved diffuse optical spectroscopy (TR-DOS) is an increasingly used method to determine the optical properties of diffusive media, particularly for medical applications including functional brain, breast and muscle measurements. For medical imaging applications, important features of new generation TR-DOS systems are low-cost, small size and efficient inverse modeling. To address the issues of low-cost, compact size and high integration capabilities, we have developed free-running (FR) single-photon avalanche diodes (SPADs) using 130 nm silicon complementary metal-oxide-semiconductor (CMOS) technology and used it in a TR-DOS prototype. This prototype was validated using assessments from two known protocols for evaluating TR-DOS systems for tissue optics applications. Following the basic instrumental performance protocol, our prototype had sub-nanosecond total instrument response function and low differential non-linearity of a few percent. Also, using light with optical power lower than the maximum permissible exposure for human skin, this prototype can acquire raw data in reflectance geometry for phantoms with optical properties similar to human tissues. Following the MEDPHOT protocol, the absolute values of the optical properties for several homogeneous phantoms were retrieved with good accuracy and linearity using a best-fitting model based on the Levenberg-Marquardt method. Overall, the results of this study show that our silicon CMOS-based SPAD detectors can be used to build a multichannel TR-DOS prototype. Also, real-time functional monitoring of human tissue such as muscles, breasts and newborn heads will be possible by integrating this detector with a time-to-digital converter (TDC).

Note: Optical fiber two-way time transfer based on single photon counting approach.

We are reporting on time transfer parameters of a newly presented method of the Two-Way Time Transfer (TWTT) using the single photon counting approach. The method utilizes two Single Photon Avalanche Diode (SPAD) detectors located on both ends of the optical channel. Each of the SPAD detectors measures the common signal taken from both communication directions on its location. The mixed signal is then mathematically decomposed back to the original, separated signals, whose time shifts are then measured, and the absolute time shift of the two distant time scales is calculated. The photon counting approach was utilized due to its great beneficence for the TWTT as most of the systematic errors can be eliminated, and, additionally, an optical power of only about -100 dBm is necessary for the measurement. It makes the method usable in optical fiber channels with the lengths of more than 500 km without any amplification. The presented TWTT method can be performed for any communication wavelength when proper SPAD detectors are used. In our free-air experiment, we utilized two commonly used multimode small form factor pluggable laser modules and the SPAD detector developed by our group. The long-term timing stability of the SPAD detectors and the timing system used is better than 100 fs. The ultimate precision of the time transfer of less than 1 ps for averaging times of hours was achieved, and the reliability of the new TWTT method was proven.

On Two-Dimensional Rangefinding Using a ∼1 nJ/∼100 ps Laser Diode Transmitter and a CMOS SPAD Matrix

A potentially compact 2-D range profiler for noncooperative targets with a distance range of >10 m is demonstrated. A laser diode utilizing enhanced gain switching is used as a transmitter to provide short, high-energy optical pulses (~1 nJ/140 ps). Cylindrical lenses spread the laser beam in one dimension and collimate the beam in the other dimension to illuminate a line shaped field of view of about 45°. A similarly shaped field of view for the detector is realized with a single-photon avalanche diode (SPAD) detector matrix. Compared with a flash 3-D rangefinder, the 2-D rangefinding scheme relaxes the SPAD and time-to-digital converter design requirements, increases the signal irradiance at the detector, can have better tolerance for background illumination, and it can be sufficient or advantageous in some applications. The distance measurement precision to each direction is inherently within a few centimeters (FWHM) since the laser pulse width and SPAD detector jitter both correspond with an optical pulse back-and-forth flight time of about a centimeter. Demonstration measurements show >15 Hz line rate and a lateral resolution of about 5 mrad to noncooperative targets at distances of more than 10 m.

Fast fully-integrated front-end circuit to overcome pile-up limits in time-correlated single photon counting with single photon avalanche diodes.

Time-Correlated Single Photon Counting (TCSPC) is an essential tool in many scientific applications, where the recording of optical pulses with picosecond precision is required. Unfortunately, a key issue has to be faced: distortion phenomena can affect TCSPC experiments at high count rates. In order to avoid this problem, TCSPC experiments have been commonly carried out by limiting the maximum operating frequency of a measurement channel below 5% of the excitation frequency, leading to a long acquisition time. Recently, it has been demonstrated that matching the detector dead time to the excitation period allows to keep distortion around zero regardless of the rate of impinging photons. This solution paves the way to unprecedented measurement speed in TCSPC experiments. In this scenario, the front-end circuits that drive the detector play a crucial role in determining the performance of the system, both in terms of measurement speed and timing performance. Here we present two fully integrated front-end circuits for Single Photon Avalanche Diodes (SPADs): a fast Active Quenching Circuit (AQC) and a fully-differential current pick-up circuit. The AQC can apply very fast voltage variations, as short as 1.6ns, to reset external custom-technology SPAD detectors. A fast reset, indeed, is a key parameter to maximize the measurement speed. The current pick-up circuit is based on a fully differential structure which allows unprecedented rejection of disturbances that typically affect SPAD-based systems at the end of the dead time. The circuit permits to sense the current edge resulting from a photon detection with picosecond accuracy and precision even a few picoseconds after the end of the dead time imposed by the AQC. This is a crucial requirement when the system is operated at high rates. Both circuits have been deeply characterized, especially in terms of achievable measurement speed and timing performance.

Statistical Modeling of Single-Photon Avalanche Diode Receivers for Optical Wireless Communications

In this paper, a comprehensive analytical approach is presented for modeling the counting statistics of active quenching and passive quenching single-photon avalanche diode (SPAD) detectors. It is shown that, unlike ideal photon counting receiver for which the detection process is described by a Poisson arrival process, photon counts in practical SPAD receivers do not follow a Poisson distribution and are highly affected by the dead time caused by the quenching circuit. Using the concepts of renewal theory, the exact expressions for the probability distribution and moments (mean and variance) of photocounts in the presence of dead time are derived for both active quenching and passive quenching SPADs. The derived probability distributions are validated through Monte Carlo simulations and it is demonstrated that the moments match with the existing empirical models for the moments of SPAD photocounts. Furthermore, an optical communication system with on–off keying and binary pulse position modulation is considered and the bit error performance of the system for different dead time values and background count levels is evaluated.

Comparative study of afterpulsing behavior and models in single photon counting avalanche photo diode detectors

Single-photon avalanche diode (SPAD) detectors, have a great importance in fields like quantum key distribution, laser ranging, florescence microscopy, etc. Afterpulsing is a non-ideal behavior of SPADs that adversely affects any application that measures the number or timing of detection events. Several studies based on a few individual detectors, derived distinct mathematical models from semiconductor physics perspectives. With a consistent testing procedure and statistically large data sets, we show that different individual detectors - even if identical in type, make, brand, etc. - behave according to fundamentally different mathematical models. Thus, every detector must be characterized individually and it is wrong to draw universal conclusions about the physical meaning behind these models. We also report the presence of high-order afterpulses that are not accounted for in any of the standard models.

Long-range depth profiling of camouflaged targets using single-photon detection

We investigate the reconstruction of depth and intensity profiles from data acquired using a custom-designed time-of-flight scanning transceiver based on the time-correlated single-photon counting technique. The system had an operational wavelength of 1550 nm and used a Peltier-cooled InGaAs/InP single-photon avalanche diode detector. Measurements were made of human figures, in plain view and obscured by camouflage netting, from a stand-off distance of 230 m in daylight using only submilliwatt average optical powers. These measurements were analyzed using a pixelwise cross correlation approach and compared to analysis using a bespoke algorithm designed for the restoration of multilayered three-dimensional light detection and ranging images. This algorithm is based on the optimization of a convex cost function composed of a data fidelity term and regularization terms, and the results obtained show that it achieves significant improvements in image quality for multidepth scenarios and for reduced acquisition times.