Time-gated Single-photon Avalanche Diode Research Articles

Gate-Width Optimisation Based on Time-Gated Single Photon Avalanche Diode Receiver for Optical Wireless Communications

Using a single photon avalanche diode (SPAD) as a receiver in an optical wireless communications (OWC) system can effectively expand the transmission distance. However, the performance of the SPAD receiver is usually affected with the bit error rate (BER) lower bound determined by background light and the inter-symbol interference (ISI) distortion caused by dead time. In this paper, external time-gated technology is employed, and the SPAD is only activated within the set gate-ON time to alleviate the influence of background light and ISI distortion. The SPAD photon counting model and the communication BER model are established according to the external time-gated characteristics. Considering the functional relationship among the gate-ON time, signal light flux, background light flux and blocking time, we take the minimum BER as the optimization target, the optimal gate-ON time is derived. The numerical results show that whenever the signal light flux or the background light flux is higher, the BER performance of the time-gated mode is apparently better than the free-running mode. For example, when signal photons and background photons are 30 and 10 per symbol time, respectively, the BER of free-running SPAD converges to 0.1, while the BER of the time-gated scheme is about 10−6.

Time-coordinated SPAD-based receiver for high-speed optical wireless communication

In an optical wireless communication system (OWC), the single photon avalanche diode (SPAD) can effectively improve the sensitivity of the receiver. However, the inter-slot interference (ISI) distortion and counting losses are inevitable because of the deadtime of the SPAD device. This effect is especially serious when the symbol time is similar than the deadtime. In this work, we propose a time-coordinated SPAD-based receiver scheme that enables multiple time-gated SPADs to reduce ISI distortion and counting losses in the high-speed transmission. The photon counting models and bit error rate (BER) model are established under the effect of ISI to the time-coordinated scheme. The results show that the proposed time-coordinated scheme not only can effectively reduce the ISI distortion and counting losses, but also can significantly improve the signal to noise ratio (SNR) of the receiver. In addition, compared with the free-running SPADs receiver under the same size, the background light tolerance and BER performance of the proposed time-coordinated receiver are better at the high signal power.

In vitro and in vivo NIR fluorescence lifetime imaging with a time-gated SPAD camera.

Near-infrared (NIR) fluorescence lifetime imaging (FLI) provides a unique contrast mechanism to monitor biological parameters and molecular events in vivo. Single-photon avalanche diode (SPAD) cameras have been recently demonstrated in FLI microscopy (FLIM) applications, but their suitability for in vivo macroscopic FLI (MFLI) in deep tissues remains to be demonstrated. Herein, we report in vivo NIR MFLI measurement with SwissSPAD2, a large time-gated SPAD camera. We first benchmark its performance in well-controlled in vitro experiments, ranging from monitoring environmental effects on fluorescence lifetime, to quantifying Förster resonant energy transfer (FRET) between dyes. Next, we use it for in vivo studies of target-drug engagement in live and intact tumor xenografts using FRET. Information obtained with SwissSPAD2 was successfully compared to that obtained with a gated intensified charge-coupled device (ICCD) camera, using two different approaches. Our results demonstrate that SPAD cameras offer a powerful technology for in vivo preclinical applications in the NIR window.

Time-Gated Photon Counting Receivers for Optical Wireless Communication

Photon counting detectors such as single-photon avalanche diode (SPAD) arrays are commonly considered for reliable optical wireless communication at power limited regimes. However, SPAD-based receivers suffer from significant dead time induced intersymbol interference (ISI) especially when the incident photon rate is relatively high and the dead time is comparable or even larger than the symbol duration, i.e., sub-dead-time regime. In this work, we propose a novel time-gated SPAD receiver to mitigate such ISI effects and improve the communication performance. When operated in the gated mode, the SPAD can be activated and deactivated in well-defined time intervals. We investigate the statistics of the detected photon count for the proposed time-gated SPAD receiver. It is demonstrated that the gate-ON time interval can be optimized to achieve the best bit error rate (BER) performance. Our extensive performance analysis illustrates the superiority of the time-gated SPAD receiver over the traditional free-running receiver in terms of the BER performance and the tolerance to background light.

Noise and Breakdown Characterization of SPAD Detectors with Time-Gated Photon-Counting Operation.

Being ready-to-detect over a certain portion of time makes the time-gated single-photon avalanche diode (SPAD) an attractive candidate for low-noise photon-counting applications. A careful SPAD noise and performance characterization, however, is critical to avoid time-consuming experimental optimization and redesign iterations for such applications. Here, we present an extensive empirical study of the breakdown voltage, as well as the dark-count and afterpulsing noise mechanisms for a fully integrated time-gated SPAD detector in 0.35-m CMOS based on experimental data acquired in a dark condition. An “effective” SPAD breakdown voltage is introduced to enable efficient characterization and modeling of the dark-count and afterpulsing probabilities with respect to the excess bias voltage and the gating duration time. The presented breakdown and noise models will allow for accurate modeling and optimization of SPAD-based detector designs, where the SPAD noise can impose severe trade-offs with speed and sensitivity as is shown via an example.

Fluorescence lifetime imaging with a megapixel SPAD camera and neural network lifetime estimation

Fluorescence lifetime imaging microscopy (FLIM) is a key technology that provides direct insight into cell metabolism, cell dynamics and protein activity. However, determining the lifetimes of different fluorescent proteins requires the detection of a relatively large number of photons, hence slowing down total acquisition times. Moreover, there are many cases, for example in studies of cell collectives, where wide-field imaging is desired. We report scan-less wide-field FLIM based on a 0.5 MP resolution, time-gated Single Photon Avalanche Diode (SPAD) camera, with acquisition rates up to 1 Hz. Fluorescence lifetime estimation is performed via a pre-trained artificial neural network with 1000-fold improvement in processing times compared to standard least squares fitting techniques. We utilised our system to image HT1080—human fibrosarcoma cell line as well as Convallaria. The results show promise for real-time FLIM and a viable route towards multi-megapixel fluorescence lifetime images, with a proof-of-principle mosaic image shown with 3.6 MP.

Wide-field time-gated SPAD imager for phasor-based FLIM applications

We describe the performance of a new wide area time-gated single-photon avalanche diode (SPAD) array for phasor-FLIM, exploring the effect of gate length, gate number and signal intensity on the measured lifetime accuracy and precision. We conclude that the detector functions essentially as an ideal shot noise limited sensor and is capable of video rate FLIM measurement. The phasor approach used in this work appears ideally suited to handle the large amount of data generated by this type of very large sensor (512 × 512 pixels), even in the case of small number of gates and limited photon budget.

Single-Photon, Time-Gated, Phasor-Based Fluorescence Lifetime Imaging through Highly Scattering Medium.

Fluorescence lifetime imaging (FLI) is increasingly recognized as a powerful tool for biochemical and cellular investigations, including in vivo applications. Fluorescence lifetime is an intrinsic characteristic of any fluorescent dye which, to a large extent, does not depend on excitation intensity and signal level. In particular, it allows distinguishing dyes with similar emission spectra, offering additional multiplexing capabilities. However, in vivo FLI in the visible range is complicated by the contamination by (i) tissue autofluorescence, which decreases contrast, and by (ii) light scattering and absorption in tissues, which significantly reduce fluorescence intensity and modify the temporal profile of the signal. Here, we demonstrate how these issues can be accounted for and overcome, using a new time-gated single-photon avalanche diode array camera, SwissSPAD2, combined with phasor analysis to provide a simple and fast visual method for lifetime imaging. In particular, we show how phasor dispersion increases with increasing scattering and/or decreasing fluorescence intensity. Next, we show that as long as the fluorescence signal of interest is larger than the phantom autofluorescence, the presence of a distinct lifetime can be clearly identified with appropriate background correction. We use these results to demonstrate the detection of A459 cells expressing the fluorescent protein mCyRFP1 through highly scattering and autofluorescent phantom layers. These results showcase the possibility to perform FLI in challenging conditions, using standard, bright, visible fluorophore or fluorescence proteins.

A range-gated CMOS SPAD array for real-time 3D range imaging

A range image of a scene is produced with a solid-state time of- flight system that uses active illumination and a time-gated single photon avalanche diode (SPAD) array. The distance to a target from the imager is measured by delaying the time gate in small steps and counting the photons of the pixels in each delay step in successive measurements. To achieve a high frame rate, the number of delay steps needed is minimized by limiting the scanning of the depth only to the range of interest. To be able to measure scenes with objects in different ranges, the array has been divided into groups of pixels with independently controlled time gating. This paper demonstrates an algorithm that can be used to control the time gating of the pixel groups in the sensor array to achieve depth maps of the scene with the time-gated SPAD array in real time and at a 70 Hz frame rate.

In vivo time-gated diffuse correlation spectroscopy at quasi-null source-detector separation

We demonstrate time domain diffuse correlation spectroscopy at quasi-null source-detector separation by using a fast time-gated single-photon avalanche diode without the need of time-tagging electronics. This approach allows for increased photon collection, simplified real-time instrumentation, and reduced probe dimensions. Depth discriminating, quasi-null distance measurement of blood flow in a human subject is presented. We envision the miniaturization and integration of matrices of optical sensors of increased spatial resolution and the enhancement of the contrast of local blood flow changes.

Advances in Gated CW STED Microscopy: Toward a Versatile Implementation

Stimulated emission depletion (STED) microscopy based on time-gated detection is a straightforward implementation, which substantial reduces the STED beam (peak) intensity to obtain effective sub-diffraction imaging. Furthermore, when combined with STED beam operating in continuous-wave (CW) lowers the complexity and the cost with respect to the early STED implementations, the so called gated CW-STED implementation (gCW-STED). However, the time-gated detection reduces the fluorescence signal that forms the images. Hence, in a situation of high background and/or weak signal, the images degrade in terms of signal-to-noise/background ratio (SNR and SBR) and the benefits of time-gated detection could vanish.In this work we reported about synergetic combinations of hardware and software methods able to compensate for the SNR and SBR reduction. In particular we demonstrate the benefits of (i) low intensity-noise laser to further reduce the STED beam intensity [1]; (ii) dedicated time gating electronics to optimize the fluorescence collection efficiency [2]; (iii) synchronous detection methods to remove the anti-Stokes emission background caused by the direct excitation from the STED beam [3]; (iv) tailored image deconvolution algorithms to improve the contrast. The combination of the proposed methods with two-photon-excitation (TPE) gCW-STED microscopy are also demonstrated.[1]I. Coto Hernandez, M. D’Amora, a Diaspro, and G. Vicidomini, “Influence of laser intensity noise on gated CW-STED microscopy,” Laser Phys. Lett., vol. 11, no. 9, p. 095603, 2014.[2]I. C. Hernandez, M. Buttafava, G. Boso, A. Diaspro, A. Tosi, and G. Vicidomini, “Gated STED microscopy with time-gated single-photon avalanche diode,” Biomed. Opt. Express, vol. 6, no. 6, p. 2258, 2015.[3]I. Coto Hernandez, C. Peres, F. C. Zanacchi, M. Amora, S. Christodoulou, P. Bianchini, A. Diaspro, and G. Vicidomini, “A new filtering technique for removing anti-Stokes emission background in gated CW-STED microscopy,” J. Biophotonics, 1-5, 2014.

Non-line-of-sight imaging using a time-gated single photon avalanche diode.

By using time-of-flight information encoded in multiply scattered light, it is possible to reconstruct images of objects hidden from the camera's direct line of sight. Here, we present a non-line-of-sight imaging system that uses a single-pixel, single-photon avalanche diode (SPAD) to collect time-of-flight information. Compared to earlier systems, this modification provides significant improvements in terms of power requirements, form factor, cost, and reconstruction time, while maintaining a comparable time resolution. The potential for further size and cost reduction of this technology make this system a good base for developing a practical system that can be used in real world applications.

Gated STED microscopy with time-gated single-photon avalanche diode.

Stimulated emission depletion (STED) microscopy provides fluorescence imaging with sub-diffraction resolution. Experimentally demonstrated at the end of the 90s, STED microscopy has gained substantial momentum and impact only in the last few years. Indeed, advances in many fields improved its compatibility with everyday biological research. Among them, a fundamental step was represented by the introduction in a STED architecture of the time-gated detection, which greatly reduced the complexity of the implementation and the illumination intensity needed. However, the benefits of the time-gated detection came along with a reduction of the fluorescence signal forming the STED microscopy images. The maximization of the useful (within the time gate) photon flux is then an important aspect to obtain super-resolved images. Here we show that by using a fast-gated single-photon avalanche diode (SPAD), i.e. a detector able to rapidly (hundreds picoseconds) switch-on and -off can improve significantly the signal-to-noise ratio (SNR) of the gated STED image. In addition to an enhancement of the image SNR, the use of the fast-gated SPAD reduces also the system complexity. We demonstrate these abilities both on calibration and biological sample. The experiments were carried on a gated STED microscope based on a STED beam operating in continuous-wave (CW), although the fast-gated SPAD is fully compatible with gated STED implementations based on pulsed STED beams.

Effects of time-gated detection in diffuse optical imaging at short source-detector separation

The adoption of a short source-detector distance, combined with a time-resolved acquisition, can be advantageous in diffuse optical imaging due to the stricter spatial localization of the probing photons, provided that the strong burst of early photons is suppressed using a time-gated detection scheme. We propose a model for predicting the effect of the time-gated measurement system using a time-variant operator built on the system response acquired at different gate delays. The discrete representation of the system operator, termed Spread Matrix, can be analyzed to identify the bottlenecks of the detection system with respect to the physical problem under study. Measurements performed on tissue phantoms, using a time-gated single-photon avalanche diode and an interfiber distance of 2 mm, demonstrate that inhomogeneities down to 3 cm can be detected only if the decay constant of the detector is lower than 100 ps, while the transient opening of the gate has a less critical impact.

Towards a portable Raman spectrometer using a concave grating and a time-gated CMOS SPAD.

A low-cost, compact Raman spectrometer suitable for the on-line water monitoring applications is explored. A custom-designed concave grating for wavelength selection was fabricated and tested. The detection of the Raman signal is accomplished with a time-gated single photon avalanche diode (TG-SPAD). A fixed gate window of 3.5ns is designed and applied to the TG-SPAD. The temporal resolution of the SPAD was ~60ps when tested with a 7ps, 532nm solid-state laser. To test the efficiency of the gating in fluorescence signal suppression, different detection windows (3ns-0.25ns) within the 3.5ns gate window are used to measure the Raman spectra of Rhodamine B. Strong Raman peaks are resolved with this low-cost system.

Fast-gated single-photon counting technique widens dynamic range and speeds up acquisition time in time-resolved measurements

In many time-domain single-photon measurements, wide dynamic range (more than 5 orders of magnitude) is required in short acquisition time (few seconds). We report on the results of a novel technique based on a time-gated Single-Photon Avalanche Diode (SPAD) able to increase the dynamic range of optical investigations. The optical signal is acquired only in well-defined time intervals. Very fast 200-ps gate-ON transition is used to avoid the undesired strong signal, which can saturate the detector, hide the fainter useful signal and reduce the dynamic range. In experimental measurements, we obtained a dynamic range approaching 8 decades in few minutes of acquisition.