Large-Area, Fast-Gated Digital SiPM With Integrated TDC for Portable and Wearable Time-Domain NIRS

Abstract

We present the design and characterization of a large-area, fast-gated, all-digital single-photon detector with programmable active area, internal gate generator, and time-todigital converter (TDC) with a built-in histogram builder circuit, suitable for performing high-sensitivity time-domain nearinfrared spectroscopy (TD-NIRS) measurements when coupled with pulsed laser sources. We used a novel low-power differential sensing technique that optimizes area occupation. The photodetector is a time-gated digital silicon photomultiplier (dSiPM) with an 8.6-mm 2 photosensitive area, 37% fill-factor, and ∼300 ps (20%-80%) gate rising edge, based on low-noise single-photon avalanche diodes (SPADs) and fabricated in 0.35-µm CMOS technology. The built-in TDC with a histogram builder has a least-significant-bit (LSB) of 78 ps and 128 time-bins, and the integrated circuit can be interfaced directly with a low-cost microcontroller with a serial interface for programming and readout. Experimental characterization demonstrated a temporal response as good as 300-ps full-width at half-maximum (FWHM) and a dynamic range >100 dB (thanks to the programmable active area size). This microelectronic detector paves the way for a miniaturized, stand-alone, multi-wavelength TD-NIRS system with an unprecedented level of integration and responsivity, suitable for portable and wearable systems. Index Terms-Digital silicon photomultiplier (dSiPM), fastgated single-photon avalanche diode (SPAD) array, photon counting, time-to-digital converter (TDC), time-domain near-infrared spectroscopy (TD-NIRS). I. INTRODUCTION T IME-DOMAIN near-infrared spectroscopy (TD-NIRS) is a powerful technique for obtaining non-invasive, in vivo measurements of tissue constituents and structure [1]. This can be exploited in many scientific fields, from a clinical Manuscript

Time-domain diffuse optics with 8.6 mm2 fast-gated SiPM for extreme light harvesting

“…In a previous work [11], the design and electrical characterization of a new class of photodetectors for TDDO was introduced [namely, fast-gated digital silicon photomultiplier (FG-dSiPM)], featuring both sub-ns gating capability and a wide active area (>8 mm 2 ), thanks to the parallelization of 1728 SPAD pairs (i.e., pixels) on a single digital chip, which can be separately enabled or disabled to tune the light harvesting efficiency. Each SPAD has a square footprint with about 50 µm side and rounded corners.…”

“…1 (left), reporting the response in case of activation of (i) a single pixel with the smallest pinhole size (5 µm), (ii) a single pixel without a pinhole (50 µm), and (iii) 1708 pixels [i.e., all pixels that can be tuned ON, including those with pinholes, but excluding 20 pixels with the highest dark count rate, i.e., higher than 32,000 counts per second (cps)]. For comparison, the contribution of the detector response to the IRF can be found in Conca et al [11]. Other combinations have been omitted from the picture for clarity, but do not exhibit relevant differences.…”

“…Preliminary in vivo investigations have also demonstrated its suitability in following hemodynamic changes. As discussed in [11], this detector was designed to target the future fabrication of portable and wearable time-gated TDDO systems. Together with the recent development of miniaturized pulsed laser sources [9,19], this work can open the way to a new class of TDDO systems, permitting the fabrication of fully operative optodes with a level of compactness close to wearable continuous-wave diffuse optics devices, but with superior performances.…”

SOLUS: a novel multimodal approach to ultrasound and diffuse optics imaging of breast cancer

“…Controllable active area and fast gating are key features of the digital SiPM detector [7] to allow operation with sourcedetector distances varying between <1 cm and >5 cm, with collected signal ranging over several orders of magnitude. The active area can be increased from an individual single photon avalanche diode (SPAD, <40 m 2 ) up to a maximum of 8.6 mm 2 .…”