|Thesis abstract: |
n recent years a growing interest has arisen in noninvasive optical analysis, in particular photon-counting measurements have gained wide acceptance in chemical and biomedical research fields, because of their extremely high sensitivity. Pushed by the outstanding performance achieved by state-of-the-art single-photon detectors, such as single-photon avalanche diodes (SPADs) and photomultiplier tubes (PMTs), the time-correlated single-photon counting (TCSPC) technique is currently one of the preferable solutions if fast and weak light signals have to be measured. Two classical TCSPC measurement block architectures are reported in literature: one is based on a time-to-amplitude converter (TAC) followed by an analog-to-digital converter (ADC), one implements a time-to-digital converter (TDC) that generates the digital value of the measured time delay exploiting the transit time of the timing signal within a chain of logic gates. Over the last years these two architectures have been implemented in high performance single-channel TCSPC systems, but modern applications are increasingly demanding for instruments capable of carrying out multiple TCSPC measurements at the same time or as a function of different experiment parameters. Multichannel TCSPC systems currently available on the market achieve high performance in terms of time resolution, differential non-linearity (DNL) and count rate; nevertheless, their structure is mainly a parallelization of a single-channel architecture. This approach is critical since area occupation and power consumption increase linearly with the number of channels. Also multichannel systems that can integrate thousands of single-photon detectors on the same chip have been developed but, since the area occupation and the power dissipation of the single acquisition chain must be very low, the reached performance is far from the state of the art.
Aim of this thesis work is to design an inherently multichannel TCSPC system able to break the trade-off between performance and number of channels. The exploited structure for the measurement block is the TAC/ADC configuration since it achieves the best-in-literature performance in terms of differential non-linearity (DNL). During this work, several TCSPC systems have been realized; the first one allows to acquire a single TCSPC measurement: it exploits the time-to-amplitude converter realized in another thesis work, a single channel A/D converter to sample the TAC measurement, an FPGA to build up the histogram and a USB transceiver to export the data through a USB connection. The achieved performance is very high, comparable with state-of-the-art instruments, in particular in terms of high conversion rate (up to 4 MHz), high time resolution (< 50 ps full width at half maximum, with the full-scale range fixed at 45 ns) and low differential non-linearity (< 4% peak-to-peak of LSB over almost the entire range). Moreover, with this system, two important goals have been reached: very small dimensions (95x40 mm2) and very low power consumption (2.5 W). These characteristics have allowed to start the development of larger TCSPC systems. Indeed, in the second designed board, eight acquisition channels have been implemented keeping the same small area occupation of the single-channel with only 6 W of power consumption. Two 4-channel TAC arrays are employed in this system, each of them including four converters with variable full-scale range. High performance has been obtained also with this system, in particular: very high time resolution (down to 20 ps FWHM selecting the ns FSR), low differential non-linearity (6% peak-to-peak of LSB), high conversion rate (up to 5 MHz per channel) and low crosstalk among the channels (lower than 6% of the time bin width in the worst case). Finally, four 8-channel boards have been parallelized in a compact module to realize a complete system able to manage 32 independent TCSPC channels with performance comparable with the state-of-the-art instruments, a total power consumption lower than 30 W and overall size equal to 160x125x30 mm3.