KEYWORDS: Single photon avalanche diodes, Monte Carlo methods, Quenching, Simulations, Stochastic processes, Modeling, Ionization, Power consumption, Picosecond phenomena, Capacitance
We present a study of the main SPAD figures of merit using a multiscale approach, from Monte Carlo simulations to SPICE simulations. We explore novel stochastic approaches capable of predicting accurately experimental measurements such as the Breakdown Probability, and the jitter. Additionally, the SPAD avalanche dynamics that is a stochastic process, is discussed within a transient Monte Carlo simulation perspective. We also derived a VerilogA model, making possible the analysis of the stochastic responses of the SPAD, including the buildup of the avalanche but also its quench. This latter quench probability of these diodes once in avalanche, rarely discussed in literature, is related to the dynamics of the voltage change of the floating cathode node. If the cathode voltage recovery (after the debiasing due to the quench circuit) is quicker than the time needed for the carrier evacuation within the avalanche junction, small additional avalanches can occur.
Single Photon Avalanche diodes (SPADs) were first realized more than five decades ago[1][1], and have now been industrialized for mass production in the 130 nm CMOS technology node by STMicroelectronics (STM). In this paper we present the latest STM SPAD with an excellent NIR photon detection probability (>5% at 850nm), a dark count rate median of 100 cps at room temperature and a low breakdown voltage of 14.2V. The dead time of the SPAD is approximately 25 ns, leading to a maximum count rate of 40 Mcps.
Thanks to the 130 nm gate length of the CMOS technology used and the associated high digital gate density, complex digital signal processing can be implemented allowing fully integrated systems to be realized. The low bias required by the SPAD makes it possible for voltage generation to be achieved on-chip (e.g. charge pumped).
We introduce our first generation time-of-flight system (VL6180) based on the STM SPAD technology, which is capable of ranging up to 60 cm in 60 ms. Ranging capabilities and accuracy are measured using a set of moving targets with reflectance of 5%, 17% and 88% in a fully automated test bed. To the best of our knowledge this was the first high volume SPAD-based device.
To our knowledge this is the first time details of SPAD performance over production volumes and lifetime have been presented.
Micro light-emitting diode (micro-LED) arrays based on an AlInGaN structure have attracted much interest recently as
light sources for data communications. Visible light communication (VLC), over free space or plastic optical fibre (POF), has become a very important technique in the role of data transmission. The micro-LEDs which are reported here contain pixels ranging in diameter from 14 to 84μm and can be driven directly using a high speed probe or via complementary metal-oxide semiconductor (CMOS) technology. The CMOS arrays allow for easy, computer control of
individual pixels within arrays containing up to 16×16 elements. The micro-LEDs best suited for data transmission have
peak emissions of 450nm or 520nm, however various other wavelengths across the visible spectrum can also be used.
Optical modulation bandwidths of over 400MHz have been achieved as well as error-free (defined as an error rate of
<1x10-10) data transmission using on-off keying (OOK) non-return-to-zero (NRZ) modulation at data rates of over
500Mbit/s over free space. Also, as a step towards a more practical multi-emitter data transmitter, the frequency response of a micro-LED integrated with CMOS circuitry was measured and found to be up to 185MHz. Despite the reduction in bandwidth compared to the bare measurements using a high speed probe, a good compromise is achieved from the additional control available to select each pixel. It has been shown that modulating more than one pixel simultaneously can increase the data rate. As work continues in this area, the aim will be to further increase the data transmission rate by modulating more pixels on a single device to transmit multiple parallel data channels simultaneously.
This paper presents the simulation modelling of a typical experimental setup for time-resolved fluorescence
measurement. The developed model takes into account the setup geometry, characteristics of light source, detector and
fluorescent sample as well as the adopted measurement technique. A qualitative verification of the model has been
reported before. In this paper, we present a quantitative analysis and verification of the system versatility. For this we
conducted time-resolved fluorescence measurements using a two-chip based micro-system, including a blue micro-LED
array as a light source and a CMOS SPAD array as a detector. The sample of interest (CdSe/ZnS quantum dots in
toluene) in a micro-cavity slide and an excitation filter were placed in the gap between the excitation and detection
planes. A time-correlated single photon counting module was used to build fluorescence decay curves. A range of
experiments with different excitation light pulse widths and using several setups have been performed. The simulated
data are in good agreement with measured results and the model proves to be flexible enough to simulate different light
sources and detector quenching/recharging circuits. This model can be used to predict qualitative and quantitative results
for specific experimental setups, supporting the explanations of observed effects and allowing the realisation of virtual
experiments.
A new, simple, high-speed, and hardware-only integration-based fluorescence-lifetime-sensing algorithm using a center-of-mass method (CMM) is proposed to implement lifetime calculations, and its signal-to-noise-ratio based on statistics theory is also deduced. Compared to the commonly used iterative least-squares method or the maximum-likelihood-estimation-based, general purpose fluorescence lifetime imaging microscopy (FLIM) analysis software, the proposed hardware lifetime calculation algorithm with CMM offers direct calculation of fluorescence lifetime based on the collected photon counts and timing information provided by in-pixel circuitry and therefore delivers faster analysis for real-time applications, such as clinical diagnosis. A real-time hardware implementation of this CMM FLIM algorithm suitable for a single-photon avalanche diode array in CMOS imaging technology is now proposed for implementation on field-programmable gate array. The performance of the proposed methods has been tested on Fluorescein, Coumarin 6, and 1,8-anilinonaphthalenesulfonate in water/methanol mixture.
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