Custom-Technology Single-Photon Avalanche Diode Linear Detector Array for Underwater Depth Imaging

Time-correlated single-photon counting (TCSPC) has been established as the preferred detection approach for high performance lidar and depth profiling due to its excellent surface to surface resolution and high optical sensitivity. This presentation will show the results of laboratory-based experiments obtained using different optical transceiver configurations. Particular attention will be given to underwater depth imaging using silicon single photon avalanche diode (Si-SPAD) detector arrays fabricated in complementary metal-oxide semiconductor (CMOS) and Si SPAD detector arrays built in custom fabrication technology.

We report a time-correlated Raman spectroscopy technique based on a silicon photomultiplier (SiPM) and a time-correlated single photon counting (TCSPC) technique to exploit the natural temporal separation between Raman and fluorescence phenomena to alleviate the high fluorescence background with conventional Raman detection. The TCSPC technique employed can greatly reduce the effect of high dark count rate (DCR) and crosstalk of SiPM that seriously hinder its application in low light level detection. The operating principle and performance of the 400 ps time resolution system are discussed along with the improvement of the peak-to-background ratio (PBR) for bulk trinitrotoluene (TNT) Raman spectrum relative to a commercial Raman spectrometer with charge coupled device (CCD). The fluorescence lifetime for solid TNT and Surface Enhanced Raman Scattering (SERS) spectrum for 10(-6) mol/L trace TNT have also been obtained by this system, showing excellent versatility and convenience in spectroscopy measurement.

This paper outlines an experimental demonstration of a Bayesian image reconstruction approach to achieve rapid single-photon color imaging of moving objects. The capacity to extract the color of objects is important in a variety of target identification and computer vision applications. Nonetheless, it remains challenging to achieve high-speed color imaging of moving objects in low-photon flux environments. The low-photon regime presents particular challenges for efficient spectral separation and identification, while unsupervised image reconstruction algorithms are often slow and computationally expensive. In this paper, we address both of these difficulties using a combination of hardware and computational solutions. We demonstrate color imaging using a Single-Photon Avalanche Diode (SPAD) detector array for rapid, low-light-level data acquisition, with an integrated color filter array (CFA) for efficient spectral unmixing. High-speed image reconstruction is achieved using a bespoke Bayesian algorithm to produce high-fidelity color videos. The analysis is conducted first on simulated data allowing different pixel formats and photon flux scenarios to be investigated. Experiments are then performed using a plasmonic metasurface-based CFA, integrated with a 64 × 64 pixel format SPAD array. Passive imaging is conducted using white-light illumination of multi-colored, moving targets. Intensity information is recorded in a series of 2D photon-counting SPAD frames, from which accurate color information is extracted using the fast Bayesian method introduced herein. The per-frame reconstruction rate proves to be hundreds of times faster than the previous computational method. Furthermore, this approach yields additional information in the form of uncertainty measures, which can be used to assist with imaging system optimization and decision-making in real-world applications. The techniques demonstrated point the way towards rapid video-rate single-photon color imaging. The developed Bayesian algorithm, along with more advanced SPAD technology and utilization of time-correlated single-photon counting (TCSPC) will permit live 3D, color videography in extremely low-photon flux environments.

Diffuse optical tomography resorting to time-domain measurements (TD-DOT) provides information-rich data that have not yet been fully exploited for image reconstruction, notably to increase imaging spatial resolution. Current TD-DOT scanners suffer from a very low sensitivity owing to their small number of detection channels. This leads to excessively long acquisition times for in vivo imaging. To obtain a higher number of detection channels, thus increasing detection density, a low-cost time-correlated single photon counting (TCSPC) system dedicated to TD-DOT was designed, resorting solely to off-the-shelf electronic components to reduce costs, in distinction to custom application-specific integrated circuit solutions. The system features four input channels, each of them including a leading-edge discriminator for direct interfacing with off-the-shelf photodetector modules and a programmable delay line to compensate for undesired propagation delays, thus also avoiding manual adjustment of cable lengths. The system supports a large number of TCSPC channels using a daisy-chain configuration through an onboard Ethernet switch. It also features a 13.02 ps bin width with a 12.5 ns dynamic range. Results show excellent linearity, with a $\sigma _{\mathrm{ DNL}}$ of 1.7% least significant bit (LSB), peak differential nonlinearity of 5% LSB, and peak integral nonlinearity of 10% LSB and an average accuracy of 19 ps full-width at half-maximum (FWHM) throughout two separate four-channel systems. The system was tested in a TD-DOT acquisition setup, and 37 ps FWHM instrument response functions were obtained using a single-photon avalanche diode (SPAD) detector. Diffuse intrinsic measurements were also acquired and compared with a reference TCSPC system showing similar results.

Time-resolved fluorescence measurement screening and sorting has been proven to be interesting for biological applications. Fluorescence lifetime (FLT) measurement provides additional information to the measurement of the intensity or polarization of the fluorescence emission. Furthermore, droplet microfluidics enables higher throughput than well plate readers. Few researches have been recently done about the microfluidic droplet sorting based on the fluorescence lifetime measurement. Some of the proposed solutions support high throughput but they do not extract the FLT directly from the fluorescence decay. In this paper, we present an alternative low-cost system for the microfluidic droplets sorting. We implemented a FLT measurement system based on the time-correlated single photon counting (TCSPC) technique on a cyclone V SoC-FPGA. For the excitation light source, we use a simple pulsed laser diode and a single photon avalanche diode (SPAD) as a photodetector. The optical system is an ad hoc microscope. The droplet generation is done with flow focusing technique in a PDMS-based microfluidics chip. This system was successfully tested in real-time at a droplet rate of more than 3000 droplets per second.

We have constructed a simple, all solid-state, time-correlated single photon counting device for collecting decay profiles of chromophores attached to DNA fragments moving through a capillary tube filled with a sieving gel under the influence of an applied electric field (capillary electrophoresis). The major components of the instrument consist of an actively pulsed GaAlAs diode laser (λexcitation=780 nm; τp<200 ps; repetition rate=80 MHz; average power=5.0 mW), single photon avalanche diode (dark count rate <50 cps; quantum efficiency=65% at 800 nm) and a PC board containing a constant fraction discriminator, time-to-amplitude converter, and an analog-to-digital converter (maximum processing count rate=3×106 cps). The instrument possessed a response function of approximately 275 ps (full width at half-maximum), adequate for measuring fluorescence lifetimes in the subnanosecond regime. To demonstrate the utility and the sensitivity of the instrument, dynamic measurements of fluorescence lifetimes for near-IR dye-labeled DNA fragments were measured during capillary electrophoresis for the identification of the dye-labeled nucleotide bases via temporal discrimination. The results indicated that in a two-dye experiment, in which only two of the four constituent bases which comprise DNA were labeled with unique fluorescent probes, the characteristic lifetime of the probe could be used to readily identify the terminal nucleotide base. Decay profiles were constructed from roughly 17 500 photoelectrons accumulated over a 2 s counting interval at a loading level of approximately 6.2×10−21 mol (3900 molecules) of DNA per electrophoretic band. The lifetimes of the two labeling dyes were determined to be 669 ps (±42 ps) and 528 ps (±68 ps).

Maximum-Likelihood Position Sensing of Single Molecules in a Confocal Microscope

Active depth imaging approaches have numerous potential applications in a number of disciplines, including environmental sensing, manufacturing and defense. The high sensitivity and picosecond timing resolution of the singlephoton counting technique can provide distinct advantages in the trade-offs between required illumination power, range, depth resolution, and data acquisition durations. These considerations must also address requirements for eye-safety, especially in applications requiring outdoor, kilometer range sensing. We present a scanning time-of-flight imager based on high repetition-rate (>MHz) pulsed illumination and a silicon single-photon detector. In advanced photon-counting experiments, we have employed the system for unambiguous range resolution at several kilometer target distance, multiple-surface resolution based on adaptive algorithms, and a cumulative data acquisition method that facilitates detector characterization and evaluation. We consider a range of optical design configurations and discuss the performance trade-offs in more detail. Much of this work has been performed at wavelengths around 850nm for convenient use with Si-based single photon avalanche diode detectors, however we will also discuss the performance at wavelengths around 1550 nm employing superconducting nanowire single photon detectors. The extension of this depth profiling technique to longer wavelengths will lead to relaxed eye safety requirements, reduced solar background levels and improvements in atmospheric transmission.

Single-Photon Avalanche Diodes (SPADs) are an emerging pixel technology able to detect the arrival of single photons. Arrays of SPADs can be used to image a scene in binary frames, indicating whether there has been a photon detection in a pixel during a very short frame exposure (e.g., few microseconds). Like many imaging technologies, SPAD sensors suffer from false photon hits, called Dark Count (DC), which increase the photon count, resulting in abnormally bright pixels, called hot pixels. Correction methods used for common CCD/CMOS sensors, such as dark frame subtraction, are ineffective for SPADs, which have non-linear response in observed counts due to their 1-bit quantization. Thus, we derive an analytical model of the count bias introduced by DC, which is signal-dependent, and provide a pixel-wise, closed-form DC correction algorithm. Remarkably, our correction is optimal in expectation, meaning that on average it removes perfectly the distortion. We validate our method on real SPAD acquisitions and synthetic data, showing that it significantly improves image reconstruction.

Planar semiconductor InGaAs/InP single photon avalanche diodes with high responsivity and low dark count rate are preferred single photon detectors in near-infrared communication. However, even with well-designed structures and well-controlled operational conditions, the performance of InGaAs/InP SPADs is limited by the inherent characteristics of avalanche process and the growth quality of InGaAs/InP materials. It is difficult to ensure high detection efficiency while the dark count rate is controlled within a certain range at present. In this paper, we fabricated a device with a thick InGaAs absorption region and an anti-reflection layer. The quantum efficiency of this device reaches 83.2%. We characterized the single-photon performance of the device by a quenching circuit consisting of parallel-balanced InGaAs/InP single photon detectors and single-period sinusoidal pulse gating. The spike pulse caused by the capacitance effect of the device is eliminated by using the characteristics of parallel balanced common mode signal elimination, and the detection of small avalanche pulse amplitude signal is realized. The maximum detection efficiency is 55.4% with a dark count rate of 43.8 kHz and a noise equivalent power of 6.96 × 10−17 W/Hz1/2 at 247 K. Compared with other reported detectors, this SPAD exhibits higher SPDE and lower noise-equivalent power at a higher cooling temperature.

In this study, we describe a time-correlated single photon counting (TCSPC) technique for multi-wavelength lifetime imaging in laser-scanning microscopes. The technique is based on a four-dimensional histogramming process that records the photon density versus the time in the fluorescence decay, the x-y coordinates of the scanning area and the wavelength. It avoids any time gating or wavelength scanning and, therefore, yields a near-ideal counting efficiency. The decay functions are recorded in a large number of time channels, and the components of a multi-exponential decay can be resolved down to less than 30 ps. A single TCSPC imaging channel works with a high detection efficiency up to a photon count rate of about 5 . 10 6 s -1 . A modified version of the TCSPC fluorescence lifetime imaging (FLIM) technique uses several fully parallel detector and TCSPC channels. It operates at a count rate of more than 10 7 photons

Single-photon avalanche diodes (SPADs) must be integrated on modern highly-scaled process nodes to achieve high array fill factors and imager pixel counts. Integration also enables applications where rapid, complex data processing needs to be co-located with these detectors and other photonic components. In this work we implement 5μm, 10μm, and 15μm-diameter circular SPADs using a p+/n-well structure and an STI guard ring in a 90nm bulk CMOS process, and in contrast to previous work on silicon SPADs carry out detailed study of dark count rate (DCR) and afterpulsing at cryogenic temperatures. With passive quenching these SPADs are saturated by dark counts at room temperature and by afterpulsing at cryogenic temperatures, motivating the development of an active quenching circuit (AQC) to characterize and ultimately suppress the processes responsible for these non-idealities. A novel AQC topology based on a linear output stage, able to detect avalanches and drive SPADs connected by a coaxial cable faster than previously-demonstrated circuits, is proposed and demonstrated. This circuit enables device testing using a coaxial microprobe inside a Helium cryostat. To quantify the SPADs’ performance, we operate them in free-running mode and collect histograms of the pulse interarrival times from which we extract the DCR and afterpulsing probability. We use this technique to measure these parameters with varying temperature and overbias voltage, showing there is an optimal temperature for operating each SPAD. With active quenching we achieve 100Hz-level DCR, 10% 405nm quantum efficiency at 140K, and are presently characterizing the photon detection probability.

We study the possibility to use Single Photon Avalanche Diodes (SPADs) optically coupled to scintillating fibers as a novel type of gamma-ray detector for space applications. SPADs are silicon devices operating under polarization conditions above the junction breakdown voltage (typical overvoltage of 5V), for which a single photon interacting in the active region is sufficient to trigger a self sustainable avalanche discharge. SPADs can thus be used for the detection of very low light levels with an absolute timing accuracy of about 30 ps for single photon detection, without spectroscopic capabilities. In this presentation we report the preliminary results on large area SPAD (actual results refers to SPADs having 200 μm diameter, with the aim to grow up to 500 μm SPADs) coupled to scintillating fibers as the basic module for a particle tracker for space application. Dark counts rate as low as few tens of kHz at room temperature, lowering down to few kHz at -10°C have been obtained for the 200 μm devices, in accordance with the basic requirements for the proposed application. Similar instruments based on silicon photomultiplier (SiPM) readout have already been studied, but none based on SPAD has been realized up to now. Moreover, since very few information is available on SPADs for the use in a space environment, 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 the SPAD devices have be irradiated using up to 20 krad total dose with gamma-rays and 5 krad with protons.

This paper introduces the field of metamaterials, details various optical uses of metasurfaces and demonstrates their suitability for imaging with single-photon avalanche diode (SPAD) detector arrays as an integrated optical component. A design for a metasurface-based color filter array (CFA) is presented, the fabrication methodology detailed, and a sample is integrated with a SPAD array. Examples of imaging applications using the integrated assembly are demonstrated, including passive and fluorescence imaging microscopy. The limitations of current metasurface color filtering techniques are highlighted and directions for future advances and applications discussed.

We present an advanced time-correlated single photon counting (TCSPC) technique that delivers traditional fluorescence correlation (FCS) or cross correlation (FCCS) and fluorescence lifetime data simultaneously. Newly developed electronics allow for detection and registration of single photon events over time periods of hours with picoseconds accuracy. Subsequent software-correlation yields correlation curves covering more than 12 orders of magnitude in time. At the same time, the original data, containing all information accessible by single photon counting techniques, can be analyzed conventionally according to common single molecule fluorescence techniques. Potential applications of the new technique using pulsed or cw laser excitation are discussed.