A 128x192 SPAD array (QuantiCam) with an on-chip time-to-digital converter in each pixel is used as a camera in a single-photon time-resolved fluorescence microscope. The SPAD array introduces systematic nonlinearities and timing offset to the measured photon arrival times. This limits the fidelity of the experimental results. A Monte-Carlo algorithm was developed to transform the raw photon time-stamp stream coming from the SPAD array into a corrected virtual “photon” time-stamp stream devoid of the systematic measurement errors. This data is compatible with existing downstream data processing pipelines used in time-correlated single-photon counting. We discuss the calibration measurement, the algorithm, their performance and application to live fluorescence lifetime imaging of photosynthetic organisms.
Time-correlated single-photon counting (TCSPC) has emerged as a key detection technology for lidar and depth profiling in a number of emerging application areas due to its high optical sensitivity and excellent surface to surface resolution. We have applied this technique to measure three-dimensional scenes of stationary and moving targets in several underwater environments. The presentation will show the results of laboratory-based experiments obtained using several different optical transceiver configurations. Particular attention will be given to underwater depth imaging using silicon single photon avalanche diode (Si-SPAD) arrays with in-pixel picosecond timing electronics.
Time-domain microfluidic fluorescence lifetime flow cytometry enables observation of fluorescence decay of particles or cells over time using time-correlated single photon counting (TCSPC). This method requires the fluorescence lifetime measured from a limited number of photons and in a short amount of time. In current implementations of the technique, the low throughput of state of the art detectors and lack of real-time statistical analysis of the current technology, the timedomain approaches are usually coupled with off-line analysis which impedes its use in flow cell sorting, tracking and capturing. In this work, we apply a 32×32 CMOS SPAD array (MegaFrame camera) for real-time imaging flow cytometry analysis. This technology is integrated into a 1024-beam multifocal fluorescence microscope and incorporating a microfluidic chip at the sample plane enables imaging of cell flow and identification. Furthermore, the 1.5% native pixel fill-factor of the MegaFrame camera is overcome using beamlet reprojection with <10 μW laser power at 490 nm for each beam. Novel hardware algorithms incorporating the center-of-mass method (CMM) with real-time background subtraction and division are implemented within the firmware, allowing lossless recording of TCSPC events at a 500 kHz frame rate with 1024 histogram bins at 52 ps time resolution. Live calculation of background compensated CMM-based fluorescence lifetime is realized at a user-defined frame rate (typically 0.001 ~ 27 kHz) for each SPAD pixel. The work in this paper considers the application of the SPAD array to confocal fluorescence lifetime imaging of multiple coincident particles flowing within a microfluidic channel. Compared to previous flow systems based on single-point detectors, the multi-beam flow system enables visualization, detection and categorization of multiple groups of cells or particles according to their fluorescence lifetime.
This paper presents the Time-Correlated Single-Photon Counting (TCSPC) technique applied to underwater environments in order to reconstruct three-dimensional scenes. Two different transceiver systems approaches are described. The first transceiver comprised a single-pixel monostatic scanning unit, which used a fiber-coupled silicon single-photon avalanche diode (SPAD) detector, and a fiber-coupled supercontinuum laser source used in conjunction with an acousto-optic tunable filter (AOTF) for wavelength selection. The experiments were performed using the supercontinuum pulsed laser source operating at a repetition rate of 19.5 MHz, fiber coupled to the AOTF in order to select one operational wavelength, tuned for best performance for the level of scattering of the particular underwater environment. Laboratory-based experiments were performed using average optical powers of less than 1 mW and depth profiles were acquired at up to 8 attenuation lengths between the transceiver and target. The second transceiver system was based on a complementary metal-oxide semiconductor (CMOS) SPAD detector array in a bistatic configuration. It comprised an array of 192 × 128 SPAD detectors, with each detector having an integrated time to digital converter, and a laser diode operating at a wavelength of 670 nm, a repetition rate of 40 MHz, and average optical power up to 9 mW. The experiments demonstrated the recovery of intensity and depth profiles associated with moving targets at up to 4 attenuation lengths. Using data from both systems, various image processing techniques were investigated to reconstruct target depth and intensity profiles in highly scattering underwater environments.
We present Time-of-Flight (TOF) distance, velocity and acceleration characterisation of a multi-event Time-to-Digital- Converter (TDC) optical sensor featuring a 32x32 Single Photon Avalanche Diode (SPAD) array, a 14 GS/s TDC and on-chip histogram generation. Events are continuously recorded on-chip in 264 70 ps-wide histogram bins. High TDC throughput enables the device to be operated in Doppler mode with pulse-trains moving at hypervelocity speeds relative to the operational sensor frequency. Electrical frequency-detuned signals of 50 kHz are resolved by the TDC module. Optical frequency-detuned signals of 1 kHz are resolved, corresponding to a TOF velocity resolution of 15.8 km/s. Linear, sine-wave, and chirp frequency modulation techniques are used to demonstrate these characteristics.