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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.

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.

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.

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.

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-molecule fluorescence spectroscopy (SMFS), based on the detection of individual molecules freely diffusing through the excitation spot of a confocal microscope, has allowed unprecedented insights into biological processes at the molecular level, but suffers from limited throughput. We have recently introduced a multispot version of SMFS, which allows achieving high-throughput SMFS by virtue of parallelization, and relies on custom silicon single-photon avalanche diode (SPAD) detector arrays. Here, we examine the premise of this parallelization approach, which is that data acquired from different spots is uncorrelated. In particular, we measure the optical crosstalk characteristics of the two 48-SPAD arrays used in our recent SMFS studies, and demonstrate that it is negligible (crosstalk probability ≤ 1.1 10−3) and is undetectable in cross-correlation analysis of actual single-molecule fluorescence data.

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 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 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.

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.

A new algorithm for controlling the temperature of a thermoelectric cooler is proposed. Unlike a classic proportional-integral-derivative (PID) control, which computes the bias voltage from the temperature error, the proposed algorithm exploits the linear relation that exists between the cold side's temperature and the amount of heat that is removed per unit time. Since this control is based on an existing linear relation, it is insensitive to changes in the operating point that are instead crucial in classic PID control of a non-linear system.

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.

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.

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.

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.

Fluorescence correlation spectroscopy (FCS) is a well-established technique to study binding interactions or the diffusion of fluorescently labeled biomolecules in vitro and in vivo. Fast FCS experiments require parallel data acquisition and analysis which can be achieved by exploiting a multi-channel Single Photon Avalanche Diode (SPAD) array and a corresponding multi-input correlator. This paper reports a 32-channel FPGA based correlator able to perform 32 auto/cross-correlations simultaneously over a lag-time ranging from 10 ns up to 150 ms. The correlator is included in a 32 × 1 SPAD array module, providing a compact and flexible instrument for high throughput FCS experiments. However, some inherent features of SPAD arrays, namely afterpulsing and optical crosstalk effects, may introduce distortions in the measurement of auto- and cross-correlation functions. We investigated these limitations to assess their impact on the module and evaluate possible workarounds.

Single Photon Avalanche Diodes (SPADs) are widely employed for photon counting and timing in a variety of scientific and industrial applications. However they are affected by the afterpulsing effect, which can significantly increase the effective dark count rate of the detector. Being correlated with previous photon pulses, afterpulses can cause sharp distortions in photon correlation experiments. In this paper we exploit autocorrelation to characterize the afterpulsing of SPADs. A FPGA based autocorrelator has been specifically designed to this purpose, featuring a lag-time range from 10ns to more than 10ms, covering a range well beyond the longest expected trapped carrier lifetime.

Modern Time-Correlated Single-Photon Counting applications require to detect spectral and temporal fluorescence data simultaneously and from different areas of the analyzed sample. These rising quests have led the development of multichannel systems able to perform high count rate and high performance analysis. In this work we describe a new 32-channel TCSPC system designed to be used in modern setups. The presented module consists of four independent 8-channel TCSPC boards, each of them including two 4-channel Time-Amplitude Converter arrays. These TAC arrays are built-in 0.35 μm Si-Ge BiCMOS technology and are characterized by low crosstalk, high resolution, high conversion rate and variable full-scale range. The 8-channel TCSPC board implements an 8-channel ADC to sample the TAC outputs, an FPGA to record and organize the measurement results and a USB 2.0 interface to enable real-time data transmission to and from an external PC. Experimental results demonstrate that the acquisition system ensures high performance TCSPC measurements, in particular: high conversion rate (5 MHz), good time resolution (down to 30 psFWHM with the full scale range set to 11 ns) and low differential non-linearity (rms value lower than 0.15% of the time bin width). We design the module to be very compact and, thanks to the reduced dimensions of the 8-channel TCSPC board (95×40 mm), the whole system can be enclosed in a small aluminum case (160×125×30 mm).

Nowadays, many research fields like biology, chemistry, medicine and space technology rely on high sensitivity imaging instruments that allow to exploit modern measurement techniques; among these, Time-Correlated Single-Photon Counting (TCSPC) provides extremely high time resolution. Single-photon detectors play a key role in these advanced imaging systems, and in recent years Single-Photon Avalanche Diodes (SPADs) have become a valid alternative to Photo Multiplier Tubes (PMTs). Moreover scientific research has recently focused on single photon detector arrays, pushed by a growing demand for multichannel systems. In this scenario, we developed a compact, stand-alone, 32-channel system for time-resolved single-photon counting applications. The system core is represented by a 32×1 SPAD array built in custom technology, featuring high time resolution, high photon detection efficiency (> 45%) and low dark count rate. The SPAD avalanche signal is exported through an integrated inverter which is placed close to the photo detector, this way the avalanche event is detected with high time resolution while achieving negligible crosstalk between adjacent pixels. SPAD proper operation is guaranteed by a 32×1 mixed passive-active quenching circuit (AQC) array built in 0.18 μm HV-CMOS technology; its digital outputs are fed to an FPGA that performs on-board processing of photon counting information. On the contrary, photon timing information is directly extracted from the pixel array and exported in Current Mode Logic (CML) standard. Preliminary experiments have been carried out on the developed system, resulting in a high time resolution (< 60 ps FWHM) and mean dark count rate lower than 8500 counts/s at 25°C.

Single-photon detectors play a key role in many research fields such as biology, chemistry, medicine, and space technology, and in recent years, single-photon avalanche diodes (SPADs) have become a valid alternative to photo multiplier tubes (PMTs). Moreover, scientific research has recently focused on single-photon detector arrays, pushed by a growing demand for multichannel systems. In this scenario, we developed a compact 32-channel system for time-resolved single-photon counting applications. The system is divided into two independent modules: a photon detection head including a 32 × 1 SPAD array built in custom technology, featuring high time resolution, high photon detection efficiency (44% at 550 nm), and low dark count rate (mean value

Over the past years an always growing interest has arisen about the measurement technique of time-correlated single photon counting (TCSPC) and many applications exploiting TCSPC have been developing in several fields, such as medicine and chemistry. The use of multianode PMTs and of single photon avalanche diode arrays led to the development of acquisition systems with several parallel channels, to employ the TCSPC technique in even more applications. Since TCSPC basically consists in the photons arrival time measurement, the most important part of an acquisition chain is the time measurement block, which must have high resolution and low differential nonlinearity and, in order to implement multidimensional systems, it has to be integrated to reduce both cost and area. To accomplish all these specifications, we have designed and fabricated a 4 channel fully integrated time-to-amplitude converter (TAC), built in 0.35 mm Si-Ge technology, characterized by a variable full scale range from 11 ns to 89 ns, very good time resolution (better than 20 ps FWHM), low differential nonlinearity (better than 0.04 LSB peak-peak and less than 0.2% LSB rms), high counting rate (16 MHz), low and constant power dissipation (50 mW) and low area occupation (340 t 390 mm2 per channel). Our measurements also show a very little crosstalk between converters integrated on the same chip; this feature together with low power and low area make the fabricated converter suitable for parallelization, so that it can be the starting point for future acquisition chains with a high number of parallel channels.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Thanks to the steady improvement in the detectors' performance, single-photon techniques are nowadays employed in a large number of applications ranging from single molecule dynamics to astronomy. In particular, silicon Single Photon Avalanche Diodes (SPAD) play a crucial role in this field thanks to their remarkable performance in terms of Photon Detection Efficiency (PDE), temporal response and Dark Count Rate (DCR). While CMOS technology allows the fabrication of large arrays of SPAD with built-in electronics, it is only resorting to custom fabrication processes that is possible to attain detectors with high-end performance required by most demanding applications. However, the fabrication of arrays for timing applications, even with a small number of pixels, is quite challenging with custom processes owing to electrical coupling between pixels. In this paper we will discuss technological solutions for the fabrication of arrays of high-performance SPAD for parallel photon timing.

Over the past years an always growing interest has arisen about the measurement technique of time-correlated single photon counting (TCSPC), since it allows the analysis of extremely fast and weak light waveforms with a picoseconds resolution. Consequently, many applications exploiting TCSPC have been developing in several fields such as medicine and chemistry. Moreover, the use of multianode PMT and of single photon avalanche diode arrays led to the development of multichannel acquisition systems, employed in even more applications. Since TCSPC basically consists of the measurement of the arrival time of a photon, a high resolution and high linearity time measurement block is of the utmost importance, and in order to realize multidimensional systems, it has to be integrated to reduce both cost and area. We have designed and fabricated a 4 channel fully integrated time-to-amplitude converter (TAC), built in 0.35 μm Si-Ge technology, characterized by a very good time resolution (less than 50 ps), low differential nonlinearity (better than 2% peak-peak and less than 0.1% rms), high counting rate (16 MHz), low and constant power dissipation (50 mW), and low area occupation (2.58x1.28 mm 2 ). Moreover our measurements show a very little crosstalk between the converters integrated on the same chip; this feature together with low power and low area make the fabricated converter suitable for parallelization, so it can be the starting point for future large scale multi-channel acquisition chains.

Recent advances in fluorescence measurements for single molecule spectroscopy, genomics, proteomics and medical diagnostics require single-photon detectors with high quantum efficiency in the extended red spectral range (from 600 nm to 900 nm) and low noise (

One of the main drawbacks of single-photon avalanche diode arrays is optical crosstalk between adjacent detectors. In the past, this phenomenon was basically ascribed to light propagating from one detector to another through a direct optical path. Accordingly, deep trenches coated with metal were introduced as optical isolation barriers between pixels. This solution, however, was unable to completely prevent the crosstalk. In this letter, we demonstrate that a strong contribution to optical crosstalk comes from photons reflected at the bottom of the chip. These photons can bypass trenches making them less effective.

One of the main drawbacks of Single Photon Avalanche Diode arrays is the optical crosstalk between adjacent detectors. This phenomenon represents a fundamental limit to the density of arrays, since the crosstalk increases with reducing the distance between adjacent devices. In the past, crosstalk was mainly ascribed to the light propagating from one detector to another through a direct optical path. Accordingly, deep trenches coated with metal were introduced as optical isolation barriers between pixels. This solution, however, was unable to completely prevent the crosstalk. In this paper we present experimental evidence that a significant contribution to crosstalk comes from photons reflected internally at the bottom of the chip. These photons can bypass trenches making them ineffective. We also propose an optical model suitable to predict the dependence of crosstalk on the position within the array.

A modification of a standard Perkin Elmer SPCM-AQR photon detector module that remarkably improves the photon timing performance is presented here. The modification consists of an additional timing circuit board, which is inserted in the module without modifying the original circuit board. The essential feature is a pulse pickup linear network, connected to the high-voltage terminal of the photodetector, which extracts a short pulse signal with fast rise, coincident with the rise of the avalanche current. The information about the photon arrival time is obtained by sensing the onset of the rise. At low counting rates (

One of the main issues of Single Photon Avalanche Diode arrays is optical crosstalk. Since its intensity increases with reducing the distance between devices, this phenomenon limits the density of integration within arrays. In the past optical crosstalk was ascribed essentially to the light propagating from one detector to another through direct optical paths. Accordingly, reflecting trenches between devices were proposed to prevent it, but they proved to be not completely effective. In this paper we will present experimental evidence that a significant contribution to optical crosstalk comes from light reflected internally off the bottom of the chip, thus being impossible to eliminate it completely by means of trenches. We will also propose an optical model to predict the dependence of crosstalk on the distance between devices.