Search

At the present time, Single Photon Avalanche Diodes (SPADs) are the enabling devices in many applications, ranging from medical imaging to laser ranging and from remote sensing to quantum key distribution. Even though they belong to different scientific domains, these applications share the need for a detector capable of attaining high count rates possibly without trading it off with other key detector’s features, such as afterpulsing probability, photon detection efficiency, and dark counts. In this work, we present the characterization of a fast integrated active quenching circuit capable of driving high-performance external custom-technology SPADs for single photon detection in the visible wavelength range. Combining the prompt intervention of the electronic circuitry and the performance of a custom-technology SPAD, we attained count rates up to 250 MCps while keeping the afterpulsing probability within 2%.

One of the major drawbacks of time-correlated single-photon counting (TCSPC) is generally represented by pile-up distortion, which strongly bounds the maximum acquisition speed to a few percent of the laser excitation rate. Based on a previous theoretical analysis, recently we presented the first, to the best of our knowledge, low-distortion and high-speed TCSPC system capable of overcoming the pile-up limitation by perfectly matching the single-photon avalanche diode (SPAD) dead time to the laser period. In this work, we validate the proposed system in a standard fluorescence measurement by comparing experimental data with the reference theoretical framework. As a result, a count rate of 32 Mc/s was achieved with a single-channel system still observing a negligible lifetime distortion.

Timing measurements with single photon detectors have acquired a prominent role in many applications, especially where they allow the recovery of faint light signals in harsh environments. We present a new fully-integrated multifunctional Time-to-Amplitude Converter (TAC), featuring 8 channels with a full scale range up to 100ns and a linearity better than 1% of the LSB peak to peak. The maximum speed of the converter is obtained in the Fast-TAC configuration (80MHz), while the precision of the converter can be maximized by exploiting multiple channels to perform the same conversion, achieving an overall jitter as low as 1.4ps.

Multispectral/hyperspectral fluorescence lifetime imaging microscopy ( λ FLIM) is a promising tool for studying functional and structural biological processes. The rich information content provided by a multidimensional dataset is often in contrast with the acquisition speed. In this work, we develop and experimentally demonstrate a wide-field λ FLIM setup, based on a novel time-resolved 18 × 1 single-photon avalanche diode array detector working in a single-pixel camera scheme, which parallelizes the spectral detection, reducing measurement time. The proposed system, which implements a single-pixel camera with a compressive sensing scheme, represents an optimal microscopy framework towards the design of λ FLIM setups.

Multispectral Fluorescence Lifetime Imaging Microscopy (FLIM) is a fundamental tool to study multifold processes in biology and material science. The growing demand for acquisition time reduction requires the parallel acquisition of a multi-dimensional dataset and the exploitation of compressive sensing techniques. In this work we present a multispectral FLIM set-up based on wide-field structured illumination coupled with a spectrometer and a novel time-resolved parallel 18x1 SPAD array detector, working in a single pixel camera scheme. We show the system characterization and its imaging properties varying the compression ratio.

Space-based light detection and ranging (LiDAR) sensors have provided valuable insight into the global, vertical distribution of aerosol and cloud layers in Earth’s atmosphere, and, more recently, of the distribution of phytoplankton in the ocean. However, the photodetectors in these sensors lack the performance necessary to capture the vertical structure of cloud tops and ocean phytoplankton to a fidelity sufficient for advancing our understanding of the global water cycle and ocean carbon cycle, respectively. Recent advancements in high-performance single photon avalanche diode (SPAD) arrays promise to enable these measurements, while also offering a sensitivity that will allow significant reductions in laser power and telescope size, with associated sensor-level size, weight, and power (SWaP) savings. To harness the unique benefits of SPADs for these measurements, we propose to develop a large-format array of photon counting SPADs with

We present the first compact Time-Correlated Single-Photon Counting single-channel system, capable of overcoming the typical pile-up limitation of Fluorescence Lifetime Imaging. An ultra-fast acquisition is obtained (40 Mcps), along with excellent timing results and negligible distortion.

Time-Correlated Single Photon Counting (TCSPC) is generally recognized as a powerful tool for Fluorescence Lifetime Imaging (FLIM), thanks to its inherently high sensitivity and timing precision. Nevertheless, one of the major drawbacks of the technique is represented by the so-called pile-up distortion, that typically limits the acquisition rate to few percent of the laser stimulation rate. In recent years, an innovative methodology has been proposed to overcome this restriction: by matching the detector dead time to the laser period an average acquisition rate of 40 Mcps is achieved, along with negligible distortion. In this work, we present the first single-channel system that implements the new measurement technique. To this aim, two modules have been specifically developed to accommodate a custom-technology Single-Photon Avalanche Diode (SPAD) and its dedicated acquisition chain. On one hand, a compact Detection Module hosts both a fully-integrated Active Quenching Circuit (AQC) to provide a finely-tunable dead time and a differential Pick-Up Circuit (PUC) to extract a picosecond-precision timing signal. On the other hand, a Time Conversion module is intended to acquire the fast timing signal thanks to a mixed-architecture Fast Time to Amplitude Converter (F-TAC). The experimental characterization proved that the modules feature excellent performance both in terms of timing precision (55 ps FWHM) and Differential Nonlinearity (4 % LSB peak to peak) and we’re now ready to compare the new technique with the classic pile-up limited approach in a real application on field.

Time-resolved imaging by means of Time Correlated Single Photon Counting (TCSPC) is the enabling technology for several powerful analytical techniques, such as Fluorescence Lifetime Imaging Microscopy (FLIM), F¨orster Resonance Energy Transfer (FRET) and Fluorescence Lifetime Correlation Spectroscopy (FLCS). Even more, TCSPC could further extend its application also to other research fields, for example in-vivo analysis, if only its relatively long acquisition time could be reduced. To achieve such a challenging result, multi-channel integrated systems have been developed to increase the measurement speed, but posing at the same time tight constraints on power dissipation, area occupation and interconnection complexity inside the chip. Moreover, the high rate of events potentially produced by a huge number of detectors can easily saturate the transfer bandwidth. To address this problem, we designed a dynamic router optimized to use the transfer bandwidth of the system at its best. The router is based on a fully-integrated, 150nm-technology, distributed routing logic that permits to interconnect different parts of the system, each one optimized by exploiting a different technology node selected on purpose. In this work, we present a 32-channel prototype of the router, but we’ll show that it has been designed to be easily up scalable and to be the building block of a densely integrated system.

The measurement of the time of arrival of single photons with picoseconds precision has been subject to a growing interest in recent years. In this field, conversion electronics with ultimate precision becomes of the utmost importance not only for demanding applications but even for most advanced sensors characterization. We present the first fully-integrated Time-to-Amplitude Converter providing a state-of-art precision better than 6ps Full Width at Half Maximum (2.4ps rms) on a 12.5ns Full Scale Range. The TAC structure has been proven effective to combine picosecond precision with high linearity, i.e. lower than 1% rms. The single channel can achieve a conversion rate of several MHz and thanks to a low power dissipation and area occupation, it is also suitable to be the building block of a multichannel structure.

Time-correlated measurements by means of single photon counting have been historically subject to a speed-distortion tradeoff, mainly due to the so-called pile-up effect. Recently, a new theoretical solution has been proposed showing that such tradeoff can be overcome using a detector dead time matched to the excitation period. This result opens the way to unprecedented speed operation in Time Correlated Single Photon Counting combined with low distortion. To achieve this goal, dedicated electronics has to be designed on purpose. In this paper, we report on the design of the first module that implements the proposed solution. Based on the exploitation of a high-performance Single Photon Avalanche Diode, the module features a fast full-integrated Active Quenching Circuit able to drive the sensor and a dummy cell with a finely tunable dead time as short as 12.5ns, a fully-differential avalanche current readout circuit providing the timing information with a jitter lower than 54ps and the logic circuitry that is needed to prevent distortion at high rates. The timing information is converted by a Fast-TAC providing picosecond timing and high linearity along with a count rate as high as 80Mcps..

Time-resolved multispectral imaging has recently found many applications ranging from biomedical to environmental field. Multidimensional approach measuring spectral and ultrafast temporal dynamics of fluorescence signal combined with spatial information (imaging) allows one to characterize biological processes at both microscopic and macroscopic level, representing a fundamental step towards development of diagnostic strategies. Long acquisition time is the main drawback of multidimensional approach because it is not compatible with biological system dynamics. In order to reduce the measurement times, it is necessary to parallelize the acquisition (hardware level) and to optimize the acquisition strategy to reduce the measurements number while preserving the information content. In this work we have developed a time-resolved multispectral fluorescence imaging system based on a spad array combined with compression techniques which allows to reduce the number of time-resolved acquisitions by a factor < 70%. The system is based on a double DMD configuration (excitation and detection) coupled to a 32x1 SPAD array, each one with its own TCSPC circuit, placed after an imaging spectrometer. This allows one to use the spatial modulation of the excitation/detection light to acquire images at different wavelengths following the single pixel camera (SPC) scheme. In order to compress the number of acquisitions, a CW fluorescence image is acquired through a CCD and Hadamard transform is applied to select most significative coefficients. The patterns related to these coefficients are subsequently used for SPC acquisition for time and spectral resolution. A Total-Variation based algorithm is used for the reconstruction of the 4D images.

Recently, there has been a growing research interest towards multichannel Time Correlated Single Photon Counting systems. To break the tradeoff between number of channels and performance that currently affects available systems, we have proposed an approach based on resource sharing and on the exploitation of different technologies. Here, we present a new standalone high-performance Time-to-Amplitude Converter featuring a timing jitter of 4.3ps rms and a DNL lower than 1% rms. The area occupation is 0.2mm2 , the power dissipation 70mW and the conversion rate 13.7MHz, making this circuit suitable to be the building block of a densely integrated, high-performance system.

The Time Correlated Single Photon Counting (TCSPC) technique is a powerful tool to analyze extremely fast and faint optical signals; however, its main drawback relies in its intrinsic slowness which is due to the necessity to acquire a large number of events to make an accurate reconstruction of the analog waveform in the time domain. In recent years, a significant research effort has been put in the design of multichannel systems: indeed, parallelization could in principle increase the overall speed of the acquisition system. In this scenario, we focused on the investigation of both singlechannel and multichannel systems potentiality to push the speed of TCSPC acquisition towards its ultimate limits. In this paper we report a solution to increase the speed of a single-channel system by almost an order of magnitude with respect to the state of the art and a smart routing architecture to provide a true increment of the acquisition speed based on the exploitation of a single photon detector array.

Photonic quantum technologies promise a revolution of the world of information processing, from simulation and computing to communication and sensing, thanks to the many advantages of exploiting single photons as quantum information carriers. In this scenario, single-photon detectors play a key role. On the one hand, superconducting nanowire single-photon detectors (SNSPDs) are able to provide remarkable performance on a broad spectral range, but their applicability is often limited by the need of cryogenic operating temperatures. On the other hand, single-photon avalanche diodes (SPADs) overcome the intrinsic limitations of SNSPDs by providing a valid alternative at room temperature or slightly below. In this paper, we review the fundamental principles of the SPAD operation and we provide a thorough discussion of the recent progress made in this field, comparing the performance of these devices with the requirements of the quantum photonics applications. In the end, we conclude with our vision of the future by summarizing prospects and unbeaten paths that can open new perspectives in the field of photonic quantum information processing.

Single-molecule localization microscopy is a powerful technique with vast potential to study light-matter interactions at the nanoscale. Nanostructured environments can modify the fluorescence emission of single molecules and the induced decay-rate modification can be retrieved to map the local density of optical states (LDOS). However, the modification of the emitter’s point spread function (PSF) can lead to its mislocalization, setting a major limitation to the reliability of this approach. In this paper, we address this by simultaneously mapping the position and decay rate of single-molecules and by sorting events by their decay rate and PSF size. With the help of numerical simulations, we are able to infer the dipole orientation and to retrieve the real position of mislocalized emitters. We have applied our ap-proach of single-molecule fluorescence lifetime imaging microscopy (smFLIM) to study the LDOS modification of a silver nanowire over a field of view of ~10 µm2 with a single-molecule localizatio...

A widefield system for multidimensional fluorescence imaging capable of resolving space, time and wavelength is developed and validated on a synthetic fluorescence sample. The system enables structured illumination and compressing detection. A compression strategy based on an a-priori information obtained by a camera is validated and proved to be effective at compression ratio of about 90%.

Following on the success of the NASA-CNES CALIPSO/CALIOP space-based cloud-aerosol lidar, which is now approaching its 14th year of continuous operation, NASA Langley Research Center has been advancing space-based lidar mission concepts and related technologies to address future cloud, aerosol and ocean science objectives identified in the 2017 Decadal Survey for Earth Science and Applications from Space. Recently, scientists at NASA Langley have been able to use data from CALIOP to retrieve global ocean biomass, which is a measurement that was never originally envisioned for this instrument, and represents a new scientific frontier for space-based lidar. CALIOP’s measurements complement ocean color records, extending ocean retrievals into nighttime and to high latitudes, and creating a more complete picture of global ocean biomass. Due to detector bandwidth limitations, however, CALIOP, as with ocean color, only obtains column-averaged measurements that are biased towards the surface. To address this limitation, Politecnico di Milano is developing a lidar detector based on a single photon avalanche diode (SPAD) array. This detector is expected to achieve a depth resolution of

Time Correlated Single Photon Counting (TCSPC) is a widely diffused technique used in scientific experiments requiring the analysis of optical pulses with high timing precision. One of the major limitations affecting this tool are distortion phenomena at high count rates happening due to pile-up. As a result, experiments must be carried out at a slower operating rate than the laser excitation frequency (1%-5%). It has been recently demonstrated that matching the detector dead time with the duration of the laser excitation period allows to overcome the aforementioned speed limitation, while still keeping distortion low. Theoretical results envision a speed improvement by almost an order of magnitude. In this work we present dedicated integrated electronics to implement the proposed idea. The selected detector for this design is a custom technology SPAD in order to achieve high performance. The SPAD is externally driven by an Active Quenching Circuit (AQC) that senses the avalanche current and provides a prompt quenching and reset of the detector. The AQC features a finely tunable dead time and a low reset time, two key aspects to achieve a very-low distortion regime and high efficiency. The detector electric signal is read out by a fully differential pick-up circuit, delivering a timing differential signal with picosecond precision and rejecting disturbances thanks to a dummy cell. A fast time-to-amplitude converter is used to measure the time of arrival of the photons with picoseconds precision and high linearity.

In recent years, time-correlated single photon counting (TCSPC) has become the technique of choice in many life science analyses, where fast and faint luminous signals are recorded with picosecond accuracy. Nevertheless, the maximum operating frequency of a single TCSPC acquisition channel limits the measurement speed, especially when scanning point systems are exploited. In order to increase the speed of TCSPC experiments, many multichannel systems based on single photon avalanche diode arrays have been proposed in the literature, which integrate thousands of pixels on the same chip. Unfortunately, the huge number of data generated by this kind of system can easily bring to the saturation of the transfer bandwidth to the external processing unit. For this reason, several different readout architectures have been proposed in the literature, attempting to exploit at best the limited bandwidth under TCSPC operating conditions. In this paper, some typical readout approaches, namely clock-driven and event-driven readouts, are discussed and compared, along with a recently-introduced router-based algorithm that is specifically designed to obtain maximum bandwidth exploitation under any condition. Quantitative comparisons are performed starting from imager response of the systems, which is the rate of recorded events in the case of uniform illumination of the detector array.

Recent developments on Single Photon Avalanche Diodes (SPADs) have opened the way to the design of single-photon time of flight systems based on very large arrays of detectors. In particular, the exploitation of 3D stacking now allows the use of different technologies to optimize both the detector and the electronics. Very high performance in terms of Photon Detection Efficiency, Dark Count Noise and Afterpulsing probability can be achieved with a dedicated custom technology fabrication process, as the one developed by Politecnico di Milano. Custom SPADs require external high-performance electronics to be properly operated. In 2019, an active quenching circuit able to operate an external custom SPADs with a dead time as short as 6ns has been developed. These results open the way to the exploitation of these detectors in many applications as spaceborne remote sensing. The very short dead time, indeed, means having a quick recovery, that is paramount to investigate the layers below a very bright surface, e.g. to measure the backscatter from plankton immediately below the ocean surface. Targeting the exploitation of a 256x256 SPAD array, we designed a fully integrated front end and processing circuit able to provide the number of impinging photons during time windows as short as 8ns.

Single-photon detection capabilities of Single Photon Avalanche Diodes (SPADs), in laser ranging, enable the measurement of significantly long ranges, the minimization of the optical power of the laser source and the implementation of high frame rate 3D imaging systems, thanks to the possibility of using an array. However, some disadvantages intrinsically affect the Geiger mode operation: above all, each time a photon is detected, the SPAD cannot record another photon during the so-called dead time. The minimization of this dead time is of paramount importance in many cases. For example, in airborne LIDAR altimeters that scan the terrain topography through a semiporous obscuration (e.g. tree canopies, clouds, ground fog, etc.), the first photons that are reflected can mask the photons actually scattered by the terrain: in this scenario, a dead time in the nanosecond range allows the record of photons reflected by surfaces having a distance of few meters. Moreover, a fast recovery of the detector is crucial in presence of a strong background when the LIDAR receiver can fall into paralyzation due to the high rate of photon detections. Here, we present a new Active Quenching Circuit (AQC) able to operate external high-performance custom technology SPAD detectors at extremely high rates. In particular, the circuit can drive a thin custom-technology SPAD with a dead time as low as 6.2ns, corresponding to a maximum photon count rate of more than 160 Mcps, and a RED-Enhanced SPAD up to 100Mcps.

Graphical Processing Units (GPUs) are a powerful alternative to central processing units, especially for data-parallel, video-rate processing of large data volumes. In the complex scenario of high-performance, multichannel Time Correlated Single Photon Counting (TCSPC), a huge amount of data is potentially generated by the acquisition system. Exploiting a dedicated, external, programmable elaboration unit enables a high degree of flexibility to perform different types of analysis. In this paper, we present a GPU-based application that leverages the common unified device architecture application programming interface for video-rate and accurate lifetime extraction from TCSPC data acquired at a rate of up to 10 Gbit/s.Graphical Processing Units (GPUs) are a powerful alternative to central processing units, especially for data-parallel, video-rate processing of large data volumes. In the complex scenario of high-performance, multichannel Time Correlated Single Photon Counting (TCSPC), a huge amount of data is potentially generated by the acquisition system. Exploiting a dedicated, external, programmable elaboration unit enables a high degree of flexibility to perform different types of analysis. In this paper, we present a GPU-based application that leverages the common unified device architecture application programming interface for video-rate and accurate lifetime extraction from TCSPC data acquired at a rate of up to 10 Gbit/s.

The minimization of the dead time necessary for the operation of a single-photon avalanche diode (SPAD) plays a crucial role in many demanding single-photon applications. Among these, it is worth mentioning the implementation of airborne light detection and ranging systems exploited to scan the terrain topography through semiporous obscurations. In this letter, we present the development and the experimental characterization of a fully integrated active quenching circuit able to drive custom-technology SPADs with a dead time as low as 6.2 ns, corresponding to a maximum photon count rate of more than 160 Mcps. Thanks to the use of a high-voltage CMOS fabrication technology, the circuit is able to operate also SPADs that require an excess bias of few tens of Volts, like the recently developed red-enhanced SPAD up to a maximum photon count rate of 100 Mcps.

Single Photon Avalanche Diodes (SPADs) provide remarkable features including single-photon sensitivity and picosecond timing capability. For these reasons, they are the detectors of choice in a large variety of applicationsincluding Light Detection and Ranging, Fluorescence Lifetime Imaging, quantum cryptography and many others. In most cases, the electronics used to drive the SPAD plays a crucial role in determining the performance of the overall system. In particular, high speed is still an open challenge for SPAD-based systems. In this paper, we discuss fully integrated electronics and system architectures to maximize the speed of these systems while providing overall high performance both for counting and timing applications.

Single Photon Avalanche Diodes (SPADs) have been proven to be extremely powerful sensors for single photon detection. Thanks to the advantages of solid state devices (ruggedness, small size, low supply voltage, high reliability) combined to photon detection efficiency inherently higher than PMTs, especially in the red and near-infrared regions of the spectrum, SPADs have become the detectors of choice in a steadily increasing number of applications, such as Forster Resonance Energy Transfer (FRET), Laser Imaging Detection and Ranging (LIDAR) and Quantum Key Distribution (QKD). The development of specific fabrication processes, usually referred to as custom technologies, has given the designers the degrees of freedom necessary to pursue the best device performance. Nevertheless, custom processes do not easily allow the integration of complex front-end and processing electronics on the same chip of the detector. Therefore, external high-performance electronics is required to extract the best performance from these sensors. We report the latest results we achieved with a fully-planar custom technology process, that allows the fabrication of SPAD arrays, and specifically designed external front-end and timing electronics with particular focus on solutions to achieve high speed in counting and timing applications.

In spaceborne LIDAR, the measurement of both intensity and time of flight of a luminous signal is widely used to investigate the atmosphere and the earth surface. In this scenario, a laser flash is sent from a satellite towards the target and a receiver records the intensity versus time: the recorded time correlates with the distance of the scatterer from the source while the intensity of the signal carries information on scatterer type, number density and intermediate extinction. Starting from an 8x8 array of high-performance Single Photon Avalanche Diodes (SPADs) fabricated with a fully planar custom-technology, we developed a module prototype for spaceborne LIDAR. An alignment board is able to provide the alignment of the trigger signal coming from the laser with the start of the acquisition time with an accuracy better than 1ns. Data coming from the SPAD are then summed and a digital word corresponding to the number of counts in time bins as short as 8.3ns.

Time-Correlated Single-Photon Counting (TCSPC) is an excellent technique used in a great variety of scientific experiments to acquire exceptionally fast and faint light signals. Above all, in Fluorescence Lifetime Imaging (FLIM), it is widely recognized as the gold standard to record sub-nanosecond transient phenomena with picosecond precision. Unfortunately, TCSPC has an intrinsic limitation: to avoid the so-called pile-up distortion, the experiments have been historically carried out, limiting the acquisition rate below 5% of the excitation frequency. In 2017, we demonstrated that such a limitation can be overcome if the detector dead time is exactly matched with the excitation period, thus paving the way to unprecedented speedup of FLIM measurements. In this paper, we present the first single-channel system that implements the novel proposed methodology to be used in modern TCSPC experimental setups. To achieve this goal, we designed a compact detection head, including a custom single-photon avalanche diode externally driven by a fully integrated Active Quenching Circuit (AQC), featuring a finely tunable dead time and a short reset time. The output timing signal is extracted by using a picosecond precision Pick-Up Circuit (PUC) and fed to a newly developed timing module consisting of a mixed-architecture Fast Time to Amplitude Converter (F-TAC) followed by high-performance Analog-to-Digital Converters (ADCs). Data are transmitted in real-time to a Personal Computer (PC) at USB 3.0 rate for specific and custom elaboration. Preliminary experimental results show that the new TCSPC system is suitable for implementing the proposed technique, achieving, indeed, high timing precision along with a count rate as high as 40 Mcps.

Single photon counting and timing are powerful tools in many applications. In recent years, there has been a fast trend towards the development of multichannel systems. Parallelism, indeed, can speed up the measurement and it allows advanced measurements based on the simultaneous acquisition of multiple wavelengths or multiple spots of the same sample at the same time. However, there currently is a tradeoff between number of channels and performance mainly because systems with hundreds of pixels have been developed so far relying on the exploitation of the standard CMOS technology to integrate both the detector and the electronics on the same chip. A breakthrough in this field would be the exploitation of different technologies, each one specifically selected to optimize a different part of the acquisition chain. In particular, we present fast fully integrated electronics able to operate external custom technology SPAD detectors to achieve high performance.

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.

Time-resolved imaging by means of Single Photon Avalanche Diodes (SPADs) has been subject to a widespread interest in recent years, especially since technological breakthroughs have opened the way to the development of multichannel Time Correlated Single Photon Counting (TCSPC) acquisition systems. Nevertheless, a main drawback of TCSPC has to be taken into account: it is an intrinsically slow technique because it requires the collection of a statistically-significant number of events to build a histogram that accurately reconstructs the time-domain waveform. As a result, the acquisition time needed for imaging can be relatively long. Two main solutions can be adopted to push the speed of a TCSPC measurement: the increment of the acquisition rate of the single channel and the exploitation of a high number of channels operating in parallel. The actual implementation of these solutions requires complex highperformance electronics designed on purpose. In this paper we report and discuss fully-integrated solutions for the development of a high-throughput and high-performance TCSPC acquisition system.

In this letter, we present the timing performance of an optimized red-enhanced single-photon avalanche diode (RE-SPAD) manufactured with a fully custom fabrication process. The detector has been characterized with an external CMOS front end circuit able to read out the RE-SPAD avalanche current and provide the timing information with state-of-the-art jitter. In particular, the careful minimization of the RE-SPAD capacitive parasitics, along with the transimpedance architecture and the gigahertz bandwidth of the front end circuit, allowed the achievement of a timing jitter as low as 83 ps full width at half maximum that increases only up to 103 ps for a detection threshold as high as 600 mV. This feature makes it possible to implement high-threshold detection systems insensitive to the electrical disturbances coming from other SPADs integrated in the same chip, demonstrating that the extension of this work to multichannel systems is feasible.

The Time Correlated Single Photon Counting (TCSPC) technique represents a key tool in many fields where picosecond precision is required to record fast and faint luminous signals. Unfortunately, TCSPC experiments involve a relatively long acquisition time to accumulate a statistically relevant number of events. Indeed, the maximum operating rate of a single acquisition channel is usually kept below 5% of the excitation frequency in order to guarantee an acceptable pileup distortion. In this work, we propose a novel technique to speed up TCSPC experiments by almost an order of magnitude, avoiding at the same time any distortion of the recorded curve. First of all, we demonstrate that zero-distortion operation is feasible if an extremely fast conversion electronics is used and the dead time of the detector is matched to the excitation period. In this scenario, the excitation power can be increased well above the pile-up limit, thus enabling unprecedented measurement speed. It is worth highlighting that our technique can be easily extended to a multichannel approach to further speed up the measurement. We also discuss that a practical use of our solution is already feasible exploiting recently-proposed time-measurement circuits with negligible dead time and a Single Photon Avalanche Diode coupled to a fast Active Quenching Circuit, featuring a short and finely tunable dead time. We provide a deep theoretical analysis of the technique to show its potential; in addition, the main issues related to nonidealities of a practical implementation have been widely investigated by means of numerical simulations.

Time-Correlated Single Photon Counting (TCSPC) is a powerful tool that permits to record extremely fast optical signals with a precision down to few picoseconds. On the other hand, it is recognized as a relatively slow technique, especially when a large time-resolved image is acquired exploiting a single acquisition channel and a scanning system. During the last years, much effort has been made towards the parallelization of many acquisition and conversion chains. In particular, the exploitation of Single-Photon Avalanche Diodes in standard CMOS technology has paved the way to the integration of thousands of independent channels on the same chip. Unfortunately, the presence of a large number of detectors can give rise to a huge rate of events, which can easily lead to the saturation of the transfer rate toward the elaboration unit. As a result, a smart readout approach is needed to guarantee an efficient exploitation of the limited transfer bandwidth. We recently introduced a novel readout architecture, aimed at maximizing the counting efficiency of the system in typical TCSPC measurements. It features a limited number of high-performance converters, which are shared with a much larger array, while a smart routing logic provides a dynamic multiplexing between the two parts. Here we propose a novel routing algorithm, which exploits standard digital gates distributed among a large 32x32 array to ensure a dynamic connection between detectors and external time-measurement circuits.

In this letter, we demonstrate a dynamic range as high as 152 dB with a detection head based on a single-photon avalanche diode (SPAD). This result has been attained by exploiting two key factors: on one hand, the low dark count rate (9 cps) of a custom-technology SPAD; and on the other hand, the maximum count rate (120 Mcps) achievable with a new generation active quenching circuit and an optimized assembly of the system. To the best of our knowledge, this is the highest dynamic range reported so far in the literature for a SPAD. Even more important, this value has been attained without sacrificing other detector parameters, such as active area diameter (50 $\mu$ m), photon detection efficiency (50% at 550-nm wavelength), and afterpulsing probability (2%).

In recent years, single-photon timing techniques have been employed in a steadily increasing number of applications. Most of these applications require high detector performance in terms of noise, photon detection efficiency, time resolution, and number of pixels operating in parallel. The detectors best fitting these requirements are single-photon avalanche diode (SPAD) arrays built in custom technology, although the systems based on such detectors are limited to a few pixels. In this paper, we present a novel read-out circuit, developed in a 0.18- $\mu $ m high voltage-CMOS technology, for the detection of the SPAD avalanche current: the designed circuit is based on a 2.2-GHz bandwidth integrated transimpedance amplifier, followed by a low-pass filter, to reduce crosstalk, and by an integrated comparator. The pick-up circuit has a total power dissipation of 1.1 mW, occupies an overall area of 15500 $\mu $ $\mathrm{m}^{2}$ and shows a time resolution down to 48 ps and a negligible crosstalk between two different pixels. All these features can open the way to the development of large SPAD arrays, characterized by a performance comparable with that of the single-pixel structures. Moreover, the good agreement between the simulated and the measured resolution (with a 14% maximum error) makes the future improvement of the evaluated performance possible.