TACImager – A High Frame Rate 256 x 256 SPAD Time to Amplitude Converter Array With Adjustable Time Zoom

Neil Finlayson, Parmesan Luca, Neale Dutton, Neil Calder, Robert Henderson

Research output: Chapter in Book/Report/Conference proceedingConference contribution

Abstract / Description of output

Single-photon avalanche diodes (SPAD) in the form of high-resolution imaging pixel arrays are used in 3D cameras, motion-tracking, biomedical and time-correlated single photon counting (TCSPC) applications [1]. Rapid spatial and temporal zoom onto objects of interest is an attractive feature. We present here novel high-speed time-zoom functionality achieved with the digital readout mode of the TACImager [2], a 256 x 256 TCSPC image sensor array based on sample and hold Time to Amplitude Converter (TAC) pixels, an 8 μm Pixel Pitch (PP), 19.63% Fill Factor (FF), and time jitter of 368 ps.
In our previous work we utilised a relatively slow but precise analogue readout employing an off-chip 14-bit differential ADC to deliver 10 fps TCSPC images, with maximum 60ps time resolution. The slow readout rate required a short exposure time to avoid significant pixel leakage. This reduced the sensitivity of the imager by around 2 orders of magnitude with respect to full-frame exposure negating the advantage of the high fill-factor and sensitivity of the SPAD array. To overcome these limitations, a column-parallel flash Analogue to Digital Converter (ADC) is also implemented in the TACImager to support fast digital readout, allowing per-pixel, 3-bin TCSPC histogramming at frame rates of 4 kfps. New results related to this high-speed mode of operation are presented.
The TACImager utilises a global ramp voltage as a timing reference, allowing time-zoom to be achieved through dynamic adjustment of comparator voltages, ramp offset voltages and ramp waveforms. Range adjustment of the ramp voltage 3-bin span from 1V to 10mV enables time resolution of up to 200ps to be achieved with good linearity over a 20ns range. Optical features of interest can be discovered through exhaustive exploration of voltage ranges or more rapid interval search. Continuous streaming of TCSPC images is achieved through utilisation of FPGA DRAM storage capable of high-speed buffering of up to 6000 TACImager frames. Videos demonstrating differential time-translation of objects through 1μs ranges with nanosecond resolution are presented, together with leakage, dark current, per-pixel jitter and offset results.
Our work will be placed in the context of other recent advances in SPAD imager arrays. Dutton et al reported on the SPCImager [3], a high-FF, single-photon counting SPAD imaging array based on CMOS SPADs with 8-µm pixel pitch, 26.8% fill factor (FF), analogue pixel electronics and photon shot-noise-limited statistics. Binary operation enables the device to achieve 16-kfps. Burri et al have described a 512 x 128 pixels gated SPAD image sensor known as the SwissSPAD [4]. A fill-factor of 5% is increased to 30% with the use of microlenses. In-pixel digital counting enables frame rates as high as 156 kfps.
We will review the potential of the TACImager, and other high-resolution, high fill-factor single-photon detector arrays in several emerging application areas of research: “light-in-flight” imaging of light propagation; imaging in harsh environments, fluorescence lifetime measurements such as turbid media; fluorescence lifetime measurements; quantum or ghost imaging; imaging of targets that are non-line-of-sight and measurements of quantum-entangled photon pairs.
Original languageEnglish
Title of host publicationEmerging Imaging and Sensing Technologies, Proceedings of SPIE
Editors Keith L. Lewis, Richard C. Hollins
Place of PublicationBellingham, WA
Publication statusPublished - 2016


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