|Thesis abstract: |
The precise measurement of very short time-intervals is of essential importance in many fields of science, medicine, engineering and industry. Many applications in medicine, biology and chemistry make use of precise time-interval measurements in order to reconstruct very-low intensity fast-changing optical waveforms. In this applications the optical signals are very faint, consisting of just few photons per cycle, thus the discrete nature of the signal itself prohibits analog sampling. Furthermore, in many cases the signals are very fast, therefore a photodetector with very high bandwidth is required if analog sampling is to be employed. To overcome those limitations, the reconstruction of the time-resolved optical waveform is achieved by Time-Correlated Single Photon Counting (TCSPC) technique based on the detections of single photons that compose the optical signal and on the measurement of their time of arrival within the signal period. A single-photon detector, usually a Photomultiplier Tube (PMT) or a Single-Photon Avalanche Diode (SPAD), and a time-measurement device, i.e. a Time-to-Digital Converter (TDC), represent the core of the TCSPC setup.
The general requirements of TCSPC setups are very demanding: the time-measurement precision of the systems has to be very high (tens of picoseconds or less) with very low differential non-linearity (DNL around 1% LSB or less). Several commercial TCSPC modules exist, however, those setups are bulky and consumes a lot of power, thus limiting the number of measurement channels to one or very few. On the other hand, dense arrays of single-photon detectors and time-measurement circuits also exist, however those arrays do not reach nor the resolution nor the linearity required by many TCSPC applications. However, many application would largely benefit from the availability of high-performance compact multi-channel TCSPC systems. The current luck of those systems and their potential to provide important improvements in many fields, such as medicine, biology and chemistry, were the main drivers of this Ph.D. research, setting the main objective of the work: the design and development of high-performance, low-power, compact multi-channel TCSPC instrumentation.
The first step in this direction was the design of a new TDC ASIC capable of reaching very-high performances and with an architecture easily expandable into an array of TDCs. The second step was the development of a compact stand-alone, easily-employable, time-measurement module having the chip as the core. Finally, based on those devices, the final step for demonstrating and developing compact high-performance multi-channel TCSPC device was reached by the development of an array chip and module containing 16 channel with SPAD detectors and TDCs. The high-performances, suitable for most demanding TCSPC applications, and very compact dimensions of the stand-alone module, make this instrument the state-of-the-art multichannel TCSPC system.