KEYWORDS: Photons, Sensors, Single photon, Detector arrays, Signal detection, Imaging systems, Electronics, Single photon detectors, Analog electronics, Quantum imaging systems
Single-Photon Avalanche-Diode (SPAD) arrays find extensive use in quantum imaging techniques that exploit entangledphotons states to overcome sensitivity limitations of classical imaging. Thanks to their compactness, low-voltage operation, single-photon sensitivity, absence of readout noise, and high frame-rate, SPAD arrays are particularly suited to detect temporally correlated photons over a scattered background. This work presents a scheme useful to model a generic quantum imaging measurement set-up, with its losses and non-idealities, and it provides the resulting calculations of pair rate (in case of quantum states made of two photons) and spurious single-photon rate at detector level. The computed rates are used to evaluate the performance in terms of signal-to-noise ratio of a possible SPAD array architecture based on an onchip photon coincidences detection, followed by an event-driven readout, which transfers only the addresses of those pixels involved in the coincidence event. Although bringing plenty of advantages in terms of power consumption, data storage, and readout time, especially as the pixels number increases, the intrinsic non-ideal operation timings of this architecture are linked to three possible cases of wrong detection. A detailed computation of these error probabilities is provided, together with a discussion about which design parameters most influence the detected signal quality. Since every on-chip coincidence detection and event-driven architecture is characterized by those same finite operation timings, the presented computation method can be considered a useful tool to optimize the design of detection systems used in quantum imaging and microscopy framework.
Quantum imaging and microscopy profit from entangled photons to surpass the boundaries of classical optics, thus improving image resolution. Thanks to their single-photon sensitivity, readout noise absence, low-voltage operation and high frame-rate, detectors based on Single-Photon Avalanche-Diodes (SPADs) are particularly suited for this application field. We discuss strengths and weaknesses of different SPAD based architectures (classified in SiPMs, SPAD arrays, or SiPM arrays), highlighting those to be exploited as quantum imagers. As just SPAD arrays are capable of spatial resolution at single-SPAD level and, through the possible implementation of quantum-specific on-chip processing, we identified them as the forefront detector type for quantum imaging and microscopy.
KEYWORDS: Silicon photomultipliers, Silicon photonics, Digital photography, Analog electronics, Positron emission tomography, Photodetectors, Near infrared spectroscopy, LIDAR, Single photon, Silicon
An innovative general-purpose Digital Silicon-PhotoMultiplier (dSiPM) with 32 × 32 SPADs, designed in 160 nm BCD technology, is presented. The main goals of this device are to enhance the dynamic range, still keeping the single-photon resolution, and minimize the timing jitter. Both an analog and a digital approach are used to distinguish between 1 to ~300 incoming photons. A voltage-controlled current generator converts the pixel’s digital output pulse in a current pulse, tunable in amplitude (10 μA ÷ 350 μA) and duration (from 1 ns to the SPAD holdoff time). The digital option is useful in low photon flux applications. Instead, in high photon flux applications, the digital output misses information, due to an overlap among the photon pulses, so the analog option is to be preferred. Moreover, a double threshold algorithm is implemented in order to reduce the timing jitter of the output. Basically, the concept behind this procedure is to refer the timing measurement to the crossing of the lower threshold, while the higher threshold is only used as a validation for the measurement. Finally, a Time-to-Digital Converter (TDC), with a resolution of 75 ps, is integrated to provide the timing information. The SPAD frontend design works in a free running photon detection modality, and there is the possibility to enable or disable the pixels individually. Thanks to its programmable number of photon resolution and the improved timing performance, the detector can be exploited in many different scientific applications.
Quantum communication is a fast-growing field that takes advantage of the quantum physics laws to protect and secure sensitive data. This work takes part of the European project UNIQORN (Affordable Quantum Communication for Everyone: Revolutionizing the Ecosystem from Fabrication to Application) whose aim is to develop a Quantum System on Chip (QSoC) for telecom application. The Integrated Circuit (IC) designed contributes in the QRNG block of the system, tailored to communicate with the integrated non-linear optics circuit. Such detector is a 32×1 linear array based on Single-Photon Avalanche-Diode (SPAD) detectors for the generation of a raw random number, by revealing the position on the array of the single photon impinging on it, realized in a BCD 0.16 μm technology. The linear array architecture consists of 32 pixels, pitched at 125 μm, each made by 4 SPADs with different diameter (5 μm, 10 μm, 20 μm, 50 μm). Two operation modes are implemented: Single-Hit Mode, needed to reveal the (5-bit) position of the pixel triggered by a single photon, representing a random number, in a time window synchronous with the laser emission. Multi Hit Mode, used to identify a coincidence of a certain number of photons(more than one, two, three or four) detected within a specified time window, thanks to a multi threshold coincidence detection logic.
KEYWORDS: Photons, Imaging systems, Super resolution microscopy, Logic, Data storage, Sensors, Super resolution, Single photon, Quenching (fluorescence), Quantum information
Microscopy resolution below the diffraction limit can be achieved by exploiting quantum light properties. NitrogenVacancy (NV) color centers in diamond, dye molecules and quantum dots are examples of single-photon emitters, whose antibunching property allows super-resolution imaging through the measurement of high-order autocorrelation functions. In this work, we present a novel Single Photon Avalanche Diode (SPAD) array architecture optimized for n-fold photon coincidence counting, in each point across the whole sensitive area. It is implemented in a 160 nm Bipolar-CMOS-DMOS (BCD) technology, and it includes 24 × 24 SPAD pixels with 50-μm pixel pitch and 10-μm SPAD diameter. Multi-photon coincidences (within time windows ranging from 2 ns to 500 ns) are identified by post-processing of the in-pixel timing data. Given the expected low photon rate on the detector in quantum imaging applications, on-chip logic discards unwanted information to limit readout throughput and data storage. In fact, reading the whole array would take 3 µs, while skipping rows detecting no photon reduces the readout time to 240 ns in case of no photon detected over the entire array. Moreover, we implemented a multi-gate approach, which avoids halting the array during readout, thus enabling multiple data acquisitions. Thanks to these power-saving expedients and efficient readout, the architecture is scalable towards multiple modules, such as 48 × 48 or 96 × 96-pixel arrays. Finally, it features the possibility of being coupled with a micro-lens array to reach a 78% equivalent fill-factor.
Light Detection and Ranging (LiDAR) is a technique that can be applied to identify the position of objects in an industrial environment, which usually suffer by strong background illumination. In this work we present a novel architecture of a Single Photon Avalanche Diode (SPAD) array optimized for a direct Time Of Flight (dTOF) single-point rangefinder system, with a distance range of about 2 m and a resolution of a few centimeters. The ASIC has been implemented in a 0.16 µm Bipolar-CMOS-DMOS (BCD) technology and includes 10 × 40 pixels, 80 Time-to-Digital Converters (TDCs), and a histogram builder. The peculiarity of this work is the ability of performing a Region-Of-Interest (ROI) selection of just those pixels illuminated by the laser spot, as well as a smart sharing of timing electronics. ROI selection is performed through SPADconnected up/down counters, that are decremented whenever the connected SPAD is triggered within the time window where the laser spot is expected, whereas they are incremented when the connected SPAD is triggered within a time window where the laser pulse is not present. If the counter stores a negative value, the pixel is considered to be within the laser spot, and just those pixels might trigger a TDC during the following 500 samples frame. Each TDC is shared among 5 non-adjacent pixels that should not be hit at the same time, considering the expected laser spot dimension. The implemented TDCs have 75 ps resolution and 19.2 ns Full Scale Range (FSR).
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