Search

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.

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.

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.

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.

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, the development and the experimental characterization of a new photon detection module, based on a 32 x 1 red-enhanced single-photon avalanche diode (RE-SPAD) array, are presented. A custom-developed technology has been exploited to design a detector having large-area pixels (50-mu m diameter) with optimized performance. With an excess bias voltage V-ov = 15 V, a photon detection efficiency as high as 57% at 600 nm (33% at 800 nm) is achieved, along with a dark count rate in the kHz range and optical crosstalk probability as low as 0.29%. The remarkable detection efficiency of the RE-SPAD array makes the module particularly suitable for all applications where high-detection efficiency in the red/ near-infrared range is mandatory. As an example, the performance of the array module is demonstrated to match the demanding requirements of multispot single-molecule fluorescence spectroscopy.

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%).

This paper describes an eight-channel differential pulse generator based on a fast differential bipolar transistor output stage. It has been designed specifically to test new multichannel time-correlated single-photon counting (TCSPC) instruments, while maintaining a wide range of regulation possibilities in terms of frequency, amplitude, delay, and duty cycle. The input signal for each channel can be provided either by an on-board programmable frequency synthesizer or by an external reference shared by all channels. The chosen input is delivered to a delayer loop controlled by a field programmable gate array (FPGA), which provides the desired delay, minimizing the jitter contribution. The delayed signal is fed to an analog differential stage that provides the fast output, whose amplitude and offset are adjusted by a digital-to-analog converter, controlled by the FPGA. The experimental results on the instrument show an output slew rate of 1600 V/ $\mu \text{s}$ over the complete full-scale range, which is between −2 and 6 V. The feedback delayer regulates the delay between 0 ns and 30 $\mu \text{s}$ whereas the high time is adjustable from a minimum of 10 ns to a maximum of 30 $\mu \text{s}$ . Finally, the frequency range is from 2 kHz to 50 MHz. All these adjustable parameters are managed by a microcontroller that works as an interface between the FPGAs and dedicated PC software. The implemented structure allows the system to achieve a time resolution down to 6 ps across all channels, making this instrument perfect for testing high time resolution TCSPC systems.

Time-Correlated Single Photon Counting (TCSPC) is a very efficient technique for measuring weak and fast optical signals, but it is mainly limited by the relatively "long" measurement time. Multichannel systems have been developed in recent years aiming to overcome this limitation by managing several detectors or TCSPC devices in parallel. Nevertheless, if we look at state-of-the-art systems, there is still a strong trade-off between the parallelism level and performance: the higher the number of channels, the poorer the performance. In 2013, we presented a complete and compact 32 × 1 TCSPC system, composed of an array of 32 single-photon avalanche diodes connected to 32 time-to-amplitude converters, which showed that it was possible to overcome the existing trade-off. In this paper, we present an evolution of the previous work that is conceived for high-throughput fluorescence lifetime imaging microscopy. This application can be addressed by the new system thanks to a centralized logic, fast data management and an interface to a microscope. The new conceived hardware structure is presented, as well as the firmware developed to manage the operation of the module. Finally, preliminary results, obtained from the practical application of the technology, are shown to validate the developed system.

In recent years, the Time-Correlated Single Photon Counting (TCSPC) technique has gained a prominent role in many fields, where the analysis of extremely fast and faint luminous signals is required. In the life science, for instance, the estimation of fluorescence time-constants with picosecond accuracy has been leading to a deeper insight into many biological processes. Although the many advantages provided by TCSPC-based techniques, their intrinsically repetitive nature leads to a relatively long acquisition time, especially when time-resolved images are obtained by means of a single detector, along with a scanning point system. In the last decade, TCSPC acquisition systems have been subjected to a fast trend towards the parallelization of many independent channels, in order to speed up the measure. On one hand, some high-performance multi-module systems have been already made commercially available, but high area and power consumption of each module have limited the number of channels to only some units. On the other hand, many compact systems based on Single Photon Avalanche Diodes (SPAD) have been proposed in literature, featuring thousands of independent acquisition chains on a single chip. The integration of both detectors and conversion electronic in the same pixel area, though, has imposed tight constraints on power dissipation and area occupation of the electronics, resulting in a tradeoff with performance, both in terms of differential nonlinearity and timing jitter. Furthermore, in the ideal case of simultaneous readout of a huge number of channels, the overall data rate can be as high as 100 Gbit/s, which is nowadays too high to be easily processed in real time by a PC. Typical adopted solutions involve an arbitrary dwell time, followed by a sequential readout of the converters, thus limiting the maximum operating frequency of each channel and impairing the measurement speed, which still lies well below the limit imposed by the saturation of the transfer rate towards the elaboration unit. We developed a novel readout architecture, starting from a completely different perspective: considering the maximum data rate we can manage with a PC, a limited set of conversion data is selected and transferred to the elaboration unit during each excitation period, in order to take full advantage of the bus bandwidth toward the PC. In particular, we introduce a smart routing logic, able to dynamically connect a large number of SPAD detectors to a limited set of high-performance external acquisition chains, paving the way for a more efficient use of resources and allowing us to effectively break the tradeoff between integration and performance, which affects the solutions proposed so far. The routing electronic features a pixelated architecture, while 3D-stacking techniques are exploited to connect each SPAD to its dedicated electronic, leading to a minimization of the overall number of interconnections crossing the integrated system, which is one of the main issues in high-density arrays.

Single-molecule spectroscopy on freely-diffusing molecules allows detecting conformational changes of biomolecules without perturbation from surface immobilization. Resolving fluorescence lifetimes increases the sensitivity in detecting conformational changes and overcomes artifacts common in intensity-based measurements. Common to all freely-diffusing techniques, however, are the long acquisition times. We report a time-resolved multispot system employing a 16-channel SPAD array and TCSPC electronics, which overcomes the throughput issue. Excitation is obtained by shaping a 532 nm pulsed laser into a line, matching the linear SPAD array geometry. We show that the line-excitation is a robust and cost-effective approach to implement multispot systems based on linear detector arrays.

Time-Correlated Single Photon Counting (TCSPC) is acknowledged as one of the most effective techniques for measuring weak and fast optical signals, since it provides very high temporal resolution and sensitivity. Nevertheless, the long acquisition time needed to perform a measurement it’s still the main drawback. To overcome this limitation, multidimensional TCSPC systems have been developed, but they still suffer for a strong trade-off between performance and number of channels: the higher the number of channels, the poorer the performance. In this work we present the design of a complete TCSPC acquisition system which is meant to overcome this trade-off. Since the best state-of-the-art detectors and sensing circuits developed so far are designed with different technologies, following the same approach those circuits will be designed onto different chips to achieve the best performance from both sides. Through Silicon Vias (TSVs) will be investigated as a possible solution for connecting a custom technology SPAD array to a CMOS pick-up circuit. Since a high number of detectors will cause the count rate to saturate due to the limited transfer rate of a PC, the target throughput has been set to 10 Gb/s, well beyond the state of the art. Consequently, the number of acquisition chains has been tailored on the affordable throughput, and a dynamic-routing logic connects the detectors to this lower number of acquisition channels. Five fast Time-to-Amplitude Converters (TACs), able to reach 80 Mconv/s, have been designed to get high temporal resolution along with low dead time.

In recent years the development of Single-Photon Avalanche Diodes (SPADs) had a big impact on single-photon counting applications requiring high-performance detectors in terms of Dark Count Rate (DCR), Photon Detection Efficiency (PDE), afterpulsing probability, etc. Among these, it is possible to find applications in single-molecule fluorescence spectroscopy that suffer from long-time measurements. In these cases SPAD arrays can be a solution in order to shorten the measurement time, thanks to the high grade of parallelism they can provide. Moreover, applications in other fields (e.g. astronomy) demand for large-area single-photon detectors, able also to handle very high count rates. For these reasons we developed a new single-photon detection module, featuring an 8 × 8 SPAD array. Thanks to a dedicated silicon technology, the performance of the detector have been finely optimized, reaching a 49% detection efficiency at 550 nm, as well as low dark counts (2 kcount/s maximum all over the array). This module can be used in two different modes: the first is a multi-spot configuration, allowing the acquisition of 64 optical signals at the same time and considerably reducing the time needed for a measurement. The second operation mode instead exploits all the pixels in a combined mode, allowing the detection of a 64-times higher maximum photon rate (up to 2 Gcount/s). In addition, this configuration provides also an extended dynamic range and allows to attain photon number resolving capabilities. Dark counts, detection efficiency, linearity, afterpulsing and crosstalk probability have been characterized at different operating conditions.

Time-Correlated Single Photon Counting (TCSPC) is one of the most effective techniques for measuring weak and fast optical signals. It outperforms traditional “analog” techniques due to its high sensitivity along with high temporal resolution. Despite those significant advantages, a main drawback still exists, which is related to the long acquisition time needed to perform a measurement. In past years many TCSPC systems have been developed with higher and higher number of channels, aimed to dealing with that limitation. Nevertheless, modern systems suffer from a strong trade-off between parallelism level and performance: the higher the number of channels the poorer the performance. In this work we present the design of a 32x32 TCSPC system meant for overtaking the existing trade-off. To this aim different technologies has been employed, to get the best performance both from detectors and sensing circuits. The exploitation of different technologies will be enabled by Through Silicon Vias (TSVs) which will be investigated as a possible solution for connecting the detectors to the sensing circuits. When dealing with a high number of channels, the count rate is inevitably set by the affordable throughput to the external PC. We targeted a throughput of 10Gb/s, which is beyond the state of the art, and designed the number of TCSPC channels accordingly. A dynamic-routing logic will connect the detectors to the lower number of acquisition chains.

In the last years, Time Correlated Single Photon Counting (TCSPC) has become the technique of choice in fluorescence lifetime measurements, given its remarkable sensitivity, accuracy and timing resolution. Nevertheless, a major drawback of this technique lies in the relatively long acquisition time. In order to overcome this issue, many multichannel systems have been proposed in literature, but the presence of many independent acquisition chains gives rise in principle to a huge data rate at the output, which cannot be processed in real time by a PC. Typically adopted solutions involve a limitation of the maximum detection frequency of each channel, so the measurement speed of currently available systems has not increased accordingly with the number of acquisition chains and is still limited well below the saturation of the transfer rate towards the elaboration unit. We present a completely different approach: starting from the maximum manageable data rate imposed by the transfer towards the PC, a proper number of high-performance external channels has been chosen to be shared among a much larger number of Single Photon Avalanche Diode (SPAD) detectors. Then, at each excitation period a dynamic routing mechanism performs a selection among the whole set of detectors carrying a valid signal and routes them towards the external channels. The selection logic relies on a pixelated architecture and on 3D-stacking techniques to connect each SPAD to its dedicated electronic, leading to a minimization of the number of interconnections crossing the integrated system.

We describe an 8-spot confocal setup for high-throughput smFRET assays and illustrate its performance with two characteristic experiments. First, measurements on a series of freely diffusing doubly-labeled dsDNA samples allow us to demonstrate that data acquired in multiple spots in parallel can be properly corrected and result in measured sample characteristics identical to those obtained with a standard single-spot setup. We then take advantage of the higher throughput provided by parallel acquisition to address an outstanding question about the kinetics of the initial steps of bacterial RNA transcription. Our real-time kinetic analysis of promoter escape by bacterial RNA polymerase confirms results obtained by a more indirect route, shedding additional light on the initial steps of transcription.Finally, we discuss the advantages of our multispot setup, while pointing potential limitations of the current single laser excitation design, as well as analysis challenges and their solutions.

The analysis of optical signals by means of Single Photon Avalanche Diodes (SPADs) has been subject to a widespread interest in recent years. The development of multichannel high-performance Time Correlated Single Photon Counting (TCSPC) acquisition systems has undergone a fast trend. Concerning the detector performance, best in class results have been obtained resorting to custom technologies leading also to a strong dependence of the detector timing jitter from the threshold used to determine the onset of the photogenerated current flow. In this scenario, the avalanche current pick-up circuit plays a key role in determining the timing performance of the TCSPC acquisition system, especially with a large array of SPAD detectors because of electrical crosstalk issues. We developed a new current pick-up circuit based on a transimpedance amplifier structure able to extract the timing information from a 50-μm-diameter custom technology SPAD with a state-of-art timing jitter as low as 32ps and suitable to be exploited with SPAD arrays. In this paper we discuss the key features of this structure and we present a new version of the pick-up circuit that also provides quenching capabilities in order to minimize the number of interconnections required, an aspect that becomes more and more crucial in densely integrated systems.

In this paper, we describe a novel solution to increase the speed of Time-Correlated Single Photon Counting (TCSPC) measurements by almost an order of magnitude while providing, in principle, zero distortion regardless of the experimental conditions. Typically, the relatively long dead time associated with the conversion electronics requires a proper tune of the excitation power in order to avoid distortions of the reconstructed waveform due to pileup and counting loss. As a result, the maximum operating rate of a TCSPC channel is now limited between 1% and 5% of the excitation frequency, thus leading to relatively long acquisition times. We show that negligible distortion (below 1%) is guaranteed if the dead time associated with the converter is kept below the dead time of the detector, and at the same time the detector dead time is matched to the duration of the excitation period. In this way, unprecedented high-speed operation is possible. In this paper, we provide a theoretical analysis of the technique, including the main non-idealities which are introduced by a generic physical implementation. The results are supported by both numerical simulations and analytical calculations.

The minimization of Single Photon Avalanche Diodes (SPADs) dead time is a key factor to speed up photon counting and timing measurements. We present a fully integrated Active Quenching Circuit (AQC) able to provide a count rate as high as 100 MHz with custom technology SPAD detectors. The AQC can also operate the new red enhanced SPAD and provide the timing information with a timing jitter Full Width at Half Maximum (FWHM) as low as 160 ps.

Timing measurements with silicon photomultipliers (SiPMs) are affected by the major issue of the so-called time-walk effect; this is due to the intrinsic architecture of SiPM devices. A new technique for time-walk compensation in timing measurements with SiPMs is presented and discussed, feasible with a low-complexity and low-cost circuitry and able to improve the timing accuracy. The validity of the presented technique has been experimentally proven with a 3 × 3 mm2 Hamamatsu MPPC, achieving a jitter reduction of a factor 2.85 compared with the case without time-walk compensation. To perform the experimental validation, a discrete-components transimpedance amplifier is developed with large bandwidth and low jitter, specifically designed to read-out and amplify the SiPM signal.

In the last years. custom-technology single-photon avalanche diodes (SPADs) have demonstrated remarkable performance, thanks to the fine engineering process performed on the electric field profile. However, the integration of custom SPADs in detector arrays often conflicts with the low-threshold current sensing that is mandatory to extract the timing information. A new custom-technology SPAD is introduced, that exploits an extra epitaxial layer in order to reduce the capacitive coupling between the anode and the chip substrate. The new detector shows a state-of-the-art timing jitter of 34 ps full width at half maximum and a weak dependence on the detection threshold, even when the substrate is kept at a fixed voltage, as required by the implementation of multichannel photon timing systems.

Recently developed Active Quenching Circuits (AQCs) with fast-gating capabilities allow us to control a single photon avalanche diode with gate windows in the nanosecond and sub-nanosecond range, thus paving the way to advanced applications, especially in the field of time-correlated single photon counting. In this scenario, an accurate measurement of the time needed by the AQC to turn-on the detector is of utmost importance. Indeed, it permits us to evaluate the impact of the system in specific applications and provides a tool to designers to understand AQC limitations and to enhance its performance. Here we propose a simple non-invasive technique to accurately measure the time needed by a gated system to turn on the detector. The effectiveness of the measure has been proved on a gated system, and results have been compared to those obtained starting from the distribution of recorded photons under constant illumination, which is a widely used approach in the literature. The great advantage of the proposed approach is that it avoids typical artifacts that affect other kinds of measurements.

Single photon avalanche diodes (SPADs) have been recently studied as photodetectors for applications in space missions. In this presentation we report the results of radiation hardness test on large area SPAD (actual results refer to SPADs having 500 mu m diameter). Dark counts rate as low as few kHz at -10 degrees C has been obtained for the 500 mu m devices, before irradiation. We performed bulk damage and total dose radiation tests with protons and gamma-rays in order to evaluate their radiation hardness properties and their suitability for application in a Low Earth Orbit (LEO) space mission. With this aim SPAD devices have been irradiated using up to 20 krad total dose with gamma-rays and 5 krad with protons. The test performed show that large area SPADs are very sensitive to proton doses as low as 2 x 10(8) (1 MeV eq) n/cm(2) with a significant increase in dark counts rate (DCR) as well as in the manifestation of the "random telegraph signal" effect. Annealing studies at room temperature (RT) and at 80 degrees C have been carried out, showing a high decrease of DCR after 24-48 h at RT. Lower protons doses in the range 1-10 x 10(7) (1 MeV eq) n/cm(2) result in a lower increase of DCR suggesting that the large-area SPADs tested in this study are well suitable for application in low-inclination LEO, particularly useful for gamma-ray astrophysics. (c) 2013 Elsevier B.V. All rights reserved.

Single-molecule techniques probe the spectroscopic properties of biomolecules one molecule a time. These approaches allow detecting and quantifying subpopulations which would be masked in ensemble measurements. Single-molecule FRET (smFRET), which indirectly measures the distance between fluorophores, has been extensively used to characterize the conformation of biomolecules or molecular interactions in complex biomolecular assemblies among many applications. FRET efficiency is typically computed ratiometrically from photon counts detected in two spectral channels. However, this approach requires careful correction and calibrations to account for background and differences in quantum yield and detection efficiency between the two channels. Alternatively, time-correlated single-photon counting (TCSPC) measurements can provide the same result from the donor fluorescence lifetime only, which is much less sensitive to these experimental aspects.In both approaches, a fundamental limitation remains the long acquisition time needed to accumulate sufficient statistics from freely-diffusing molecules, severely limiting the throughput of these techniques and preventing the study of rapidly evolving systems. To circumvent these limitations, we have developed a multi-spot setup using sensitive SPAD arrays, achieving high-throughput data acquisition for FCS and smFRET. These systems had so far been limited to counting applications.In this work, we present the first proof-of-principle TCSPC multispot system employing a simple line-excitation, a new dedicated SPAD array with low jitter and integrated multichannel TCSPC electronics[1]. This system allows single-molecule detection on all 16 channels using a total excitation power of 100 mW. We report measurements on short doubly-labeled dsDNA molecules and demonstrate uniform performance across the 16 detection channels. These results are a first step toward extending current single-spot TCSPC spectroscopic techniques (ns-ALEX or PIE) to a multispot geometry for high-throughput time-resolved smFRET measurements.[1] Cuccato, A et al. IEEE Phot. J. 5-5, p6801514, 2013.

In this letter, we present a compact photon detection module, based on an $8 \times 8$ array of single-photon avalanche diodes. The use of a dedicated silicon technology for the fabrication of the sensors allows us to combine large active areas (50- $\mu \text{m}$ diameter), high photon detection efficiency (49% at 550-nm wavelength), and low dark count rate. Because of a fully parallel architecture, the module provides voltage pulses synchronous to each photon detection for a maximum global count rate exceeding 1 Gcps. These properties make the system suitable for operation in two different free-running modes. The first, suitable to acquire faint signals, allows multi-spot acquisitions and can be used to considerably reduce the measurement time in applications, such as single-molecule analysis. With the second, it is possible to use all the pixels in a combined mode, to extend and move the dynamic range of the module to very high count rates and to attain number resolving capabilities.

In recent years, lifetime measurements by means of the Time Correlated Single Photon Counting (TCSPC) technique have led to a significant breakthrough in medical and biological fields. Unfortunately, the many advantages of TCSPC-based approaches come along with the major drawback of a relatively long acquisition time. The exploitation of multiple channels in parallel could in principle mitigate this issue, and at the same time it opens the way to a multi-parameter analysis of the optical signals, e.g., as a function of wavelength or spatial coordinates. The TCSPC multichannel solutions proposed so far, though, suffer from a tradeoff between number of channels and performance, and the overall measurement speed has not been increased according to the number of channels, thus reducing the advantages of having a multichannel system. In this paper, we present a novel readout architecture for bi-dimensional, high-density Single Photon Avalanche Diode (SPAD) arrays, specifically designed to maximize the throughput of the whole system and able to guarantee an efficient use of resources. The core of the system is a routing logic that can provide a dynamic connection between a large number of SPAD detectors and a much lower number of high-performance acquisition channels. A key feature of our smart router is its ability to guarantee high efficiency under any operating condition.

Single photon avalanche diodes (SPADs) have been subject to a fast improvement in recent years. In particular, custom technologies specifically developed to fabricate SPAD devices give the designer the freedom to pursue the best detector performance required by applications. A significant breakthrough in this field is represented by the recent introduction of a red enhanced SPAD (RE-SPAD) technology, capable of attaining a good photon detection efficiency in the near infrared range (e.g. 40% at a wavelength of 800 nm) while maintaining a remarkable timing resolution of about 100ps full width at half maximum. Being planar, the RE-SPAD custom technology opened the way to the development of SPAD arrays particularly suited for demanding applications in the field of life sciences. However, to achieve such excellent performance custom SPAD detectors must be operated with an external active quenching circuit (AQC) designed on purpose. Next steps toward the development of compact and practical multichannel systems will require a new generation of monolithically integrated AQC arrays. In this paper we present a new, fully integrated AQC fabricated in a high-voltage 0.18 μm CMOS technology able to provide quenching pulses up to 50 Volts with fast leading and trailing edges. Although specifically designed for optimal operation of RE-SPAD devices, the new AQC is quite versatile: it can be used with any SPAD detector, regardless its fabrication technology, reaching remarkable count rates up to 80 Mcounts/s and generating a photon detection pulse with a timing jitter as low as 119 ps full width at half maximum. The compact design of our circuit has been specifically laid out to make this IC a suitable building block for monolithically integrated AQC arrays.

In order to fulfill the requirements of many applications, we recently developed a new technology aimed at combining the advantages of traditional thin and thick silicon Single Photon Avalanche Diodes (SPAD). In particular we demonstrated single-pixel detectors with a remarkable improvement in the Photon Detection Efficiency in the red/near-infrared spectrum (e.g. 40% at 800nm) while maintaining a timing jitter better than 100ps. In this paper we discuss the limitations of such Red-Enhanced (RE) technology from the point of view of the fabrication of small arrays of SPAD and we propose modifications to the structure aimed at overcoming these issues. We also report the first preliminary experimental results attained on devices fabricated adopting the improved structure.

Since single-molecule FRET (smFRET) was demonstrated 20 years ago, tremendous progresses have been made in sample preparation and data analysis, allowing the study of increasingly complex systems and the extraction of precise structural information. However, technical improvements have been mostly confined to combining the technique with microfluidics or complementary techniques such as optical tweezers or atomic force microscopy.Problematically, the approach remains a low throughput one, due to the fact that “single-molecule” FRET measurements are really “multiple single-molecule” measurements, requiring thousands of single-molecule transits to be recorded in order to accurately characterize a sample. Combined with the necessary low molecular concentration, this translates into several minute-long acquisition times, preventing the study of dynamic processes occurring on faster time-scales.In order to overcome these limitations, we have embarked in a long-term project to develop a high-throughput smFRET system using a parallel multispot confocal platform and new detector arrays, with the aim to reduce the acquisition time by close to two orders of magnitude compared to conventional single-spot systems[1,2].Here, we review the current performance of our system for smFRET and FCS measurements, benchmarked using different doubly-labeled DNA samples. In particular, we show that the different confocal spots probe independent volumes of the sample and individually obtain comparable results, allowing pooling of their data into a single higher-statistics result. As an illustration of the advantage of a faster data acquisition rate, we present results on the kinetics of the initial stage of E. coli RNA transcription, inaccessible by a conventional single-spot smFRET approach.[1] Ingargiola A. et al. 2013. Proc of SPIE. p.85900E-85900E-11.[2] Michalet X. et al. 2014. IEEE J. Sel. Top. Quantum Electron. 20: 1-20.

Multi-dimensional time correlated single-photon counting has reached a prominent position among analytical techniques employed in the medical and biological fields. The development of instruments able to perform temporal and spectral fluorescence analysis (sFLIM) at the same time is limited by the performance of single-photon detectors, since currently available arrays cannot simultaneously satisfy all the requirements. To face this rising quest, a fully-parallel eight-channel module, based on a monolithic single-photon avalanche diode (SPAD) array with great temporal resolution, high photon detection efficiency and low dark count rate, has been designed and fabricated. The system relies on a novel architecture of the single pixel, based on the integration of the timing pick-up circuit next to the photodetector, making the negative effects of electrical and optical crosstalk on photon timing performance negligible. To this end, the custom technological process used to fabricate the SPAD has been modified,...

Excellent performance of custom technology SPAD detectors have been widely demonstrated in recent years. Low-jitter timing measurements with these detectors require front end electronics able to sense the avalanche current at a very low level when the multiplication process is still confined in a very small area around the photon absorption point. Best in class results (35 ps full width at half maximum) have been obtained with discrete circuits not suitable to be used in densely integrated systems of SPAD arrays required by modern demanding applications. A new fully integrated front end able to read out the avalanche current with a timing jitter as low as 32 ps and suitable to be exploited with SPAD arrays is presented.

Over the past few years there has been a growing interest in monolithic arrays of single photon avalanche diodes (SPADs) for parallel detection of faint ultrafast optical signals. SPADs implemented in CMOS-compatible planar technologies offer the typical advantages of microelectronic devices (small size, ruggedness, low voltage, low power, etc.). Furthermore, they have inherently higher photon detection efficiency than photomultiplier tubes and they are able to provide very high acquisition speeds. The development of fully-parallel multichannel systems is a challenge both for the detector (with new problems like optical and electrical crosstalk, isolation elements size reduction and the need for higher yields) and for the electronics. The development of a parallel detector must be accompanied by that of the time-to-amplitude converters and analysis electronics necessary in time-correlated single photon counting applications. In this paper we present the fundamental building blocks necessary to develop a p...

In recent years, time-correlated single-photon counting techniques have been applied to time-of-flight measurements for long-distance range-finding and depth imaging. Depth imaging has been performed by obtaining timing information from an individual single-photon detector and scanning the optical field to obtain a full depth image. Typically, the measurement is made by dwelling on each individual depth pixel for a pre-defined integration time and completing the data acquisition for that pixel before steering the beam to the adjacent spatial position. We present a novel photon-counting data acquisition mode where the time-of-flight histograms for each depth pixel are gradually populated. The system repeatedly scans the same spatial frame with short per-pixel dwell times, and sufficient photon statistics are built up over many frames by cumulating photon events from all acquired frames. The technique is used to compare the depth imaging performance of two single-photon avalanche diode detectors: a novel, r...

In recent years, a growing number of applications demand better timing resolution from single-photon avalanche diodes (SPADs). The challenge is pursuing improved timing resolution without impairing other device characteristics such as quantum efficiency and dark count rate. This task requires a clear understanding of the statistical phenomena involved in the avalanche current growth in order to drive the device engineering process. Past studies state that in Si SPADs the avalanche injection position statistics is the main contribution to the photon-timing jitter. However, in recent re-engineered devices, this assumption has been questioned. To address this issue, we developed an experimental setup capable of characterizing the photon-timing jitter as a function of the injection position by means of a laser focused on the device active area. The results not only confirmed that the injection position statistics is not the main contribution to photon-timing jitter, but also evidenced interesting dependences of the timing performances on the injection position. Furthermore, we found a relationship between the photon-timing jitter and the specific resistance of the devices, which has been investigated by means of photoluminescence measurements.

In free-space single-photon quantum key distribution (QKD), the error rate due to daytime background photons can be reduced with strong temporal filtering. In this case, the improvement in performance is determined by the receiver's ability to resolve signal-photon arrival times. We use fast clock recovery and commercially available single-photon detectors with timing resolution enhanced by additional electronic circuitry to implement temporal gating down to 50 ps in a free-space QKD system. The single-photon channel operates at 850 nm, and the improved timing resolution enables transmission rates of 1.25 GHz. We observe daytime quantum bit error rates of 0.04, which is less than one-third of the ungated error rate. We present the design and performance of the system and demonstrate its benefit to free-space QKD.

Single-molecule Forster resonance energy transfer (smFRET) allows measuring distances between donor and acceptor fluorophores on the 3-10 nm range. Solution-based smFRET allows measurement of binding-unbinding events or conformational changes of dye-labeled biomolecules without ensemble averaging and free from surface perturbations. When employing dual (or multi) laser excitation, smFRET allows resolving the number of fluorescent labels on each molecule, greatly enhancing the ability to study heterogeneous samples. A major drawback to solution-based smFRET is the low throughput, which renders repetitive measurements expensive and hinders the ability to study kinetic phenomena in real-time. Here we demonstrate a high-throughput smFRET system that multiplexes acquisition by using 48 excitation spots and two 48-pixel single-photon avalanche diode array detectors. The system employs two excitation lasers allowing separation of species with one or two active fluorophores. The performance of the system is demonstrated on a set of doubly labeled double-stranded DNA oligonucleotides with different distances between donor and acceptor dyes along the DNA duplex. We show that the acquisition time for accurate subpopulation identification is reduced from several minutes to seconds, opening the way to high-throughput screening applications and real-time kinetics studies of enzymatic reactions such as DNA transcription by bacterial RNA polymerase. Published by AIP Publishing.

Time-correlated single-photon counting (TCSPC) is a powerful optical technique, which permits recording fast luminous signals with picosecond precision. Unfortunately, given its repetitive nature, TCSPC is recognized as a relatively slow technique, especially when a large time-resolved image has to be recorded. In recent years, there has been a fast trend toward the development of TCPSC imagers. Unfortunately, present systems still suffer from a trade-off between number of channels and performance. Even worse, the overall measurement speed is still limited well below the saturation of the transfer bandwidth toward the external processor. We present a routing algorithm that enables a smart connection between a 32×32 detector array and five shared high-performance converters able to provide an overall conversion rate up to 10 Gbit/s. The proposed solution exploits a fully digital logic circuit distributed in a tree structure to limit the number and length of interconnections, which is a major issue in densely integrated circuits. The behavior of the logic has been validated by means of a field-programmable gate array, while a fully integrated prototype has been designed in 180-nm technology and analyzed by means of postlayout simulations.

In life science, optical techniques for the characterization of biological processes are well established and widely used. In most of them, to obtain the best performances, detectors with single-photon detection capability are required. Moreover, the growing demand for this type of information is pushing the technology towards a parallelization of the analysis. These requirements make it very challenging to develop new detection heads, because state-of-the-art detectors, such as photomultiplier tubes (PMTs) and charged coupled devices (CCDs), have some drawbacks. For example, in fluorescence correlation spectroscopy (FCS) fluorescence fluctuations must be monitored on a short time scale because typical time constants range from hundreds of nanoseconds to milliseconds. In this case, developing parallel modules for this application is very challenging. In fact, PMTs are bulky and they cannot be integrated, while the use of imaging detectors, such as CCDs and electron multiplying CCDs, is strongly limited by...

Inductor current sensing is becoming widely used in current programmed controllers for microprocessor applications. This method exploits a low-pass filter in parallel with the inductor to provide lossless current sense. A major drawback of inductor current sensing is that accurate sense the dc and ac components of the current signal requires precise matching between the low-pass filter time constant and the inductor time constant (L/RL). However, matching accuracy depends on the tolerance of the components and on the operating conditions; therefore it can hardly be guaranteed. To overcome this problem, a novel digital autotuning system is proposed that automatically compensates any time constant mismatch. This autotuning system has been developed for voltage regulator module (VRM) current programmed controllers. It makes it possible to meet the adaptive voltage positioning requirements using conventional and low cost components, and to solve problems such as aging effects, temperature variations, and process tolerances as well. A prototype of the autotuning system based on a field-programmable gate array and a commercial dc/dc controller has been designed and tested. The experimental results fully confirmed the effectiveness of the proposed method, showing an improvement of the current sense precision from about 30% up to 4%. This innovative solution is suitable to fulfill the challenging accuracy specifications required by the future VRM applications.

New detector modules exploit recent progress in the technology of silicon single-photon avalanche diodes (SPADs) and in the design of associated electronic circuits. SPAD detectors with diameter up to 100 mu m, good photon detection efficiency (48% peak at 550 nm wavelength) and low dark counting rate are used in these modules. Monolithic integrated active-quenching circuits (iAQC) and fast time-pickoff circuits ensure efficient photon counting and timing, with better than 50 ps FWHM time resolution and less than 50 ps centroid shift for counting rates LIP to 4 Mc/s. Experimental tests of the module performance and an application example are presented.

Time correlated photon counting techniques have been proved to be very effective, especially when very fast and faint optical signals have to be recorded with extremely high precision. Nowadays, a steadily increasing number of applications require not only high performance in terms of photon detection efficiency, time resolution and linearity but also a high number of pixels operating in parallel. In order to combine the features of the most performing detectors and state of art timing electronics, an innovative architecture has been conceived and the main circuits have been designed. The system will employ dense arrays of custom technology Single Photon Avalanche Diode (SPAD) detectors, in order to perform a truly concurrent analysis of the sample, while it will have only a small number of acquisition chains with the ultimate purpose of obtaining very high performance while limiting area occupation and power dissipation. To this aim, three main circuits have been designed: first of all, a pick-up circuit capable of directly reading the signal coming from the sensor; secondly, a timing circuit to measure the arrival time of the each photon with picoseconds resolution and very high linearity and finally, a circuit to perform a dynamic binding between the many sensors and the few conversion chains.