Present position: Temporary researcher
|Thesis title:||Microelectronics and Instrumentationwith picosecond resolutionfor Single Photon Detectors|
|Research area:||Sensors and instrumentation|
Several scientific experiments require the measurement of very faint or very fast light emissions. Applications vary from biology (DNA and protein analysis), to medicine (e.g. analysis of tissues for cancer diagnostics), chemistry (single molecule fluorescence spectroscopy) and astrophysics (adaptive optics systems in large telescopes). In the industrial field we can remember non invasive verification of ULSI circuits by hot electron emission, or the characterization of optical fiber links using optical time domain reflectometry (OTDR).
When the measurement of light emissions in the time domain with picosecond resolution is desired, the time correlated single photon counting (TCSPC) technique can be employed. Typical applications of this technique are: measurement of optical waveforms, time resolved laser scanning microscopy, time resolved optical near field microscopy, optical tomography, optical correlation experiments.
As the resolution of TCSPC measurements is limited by the intrinsic time jitter of the photodetector, in the last years devices that are characterized by a high temporal resolution have gained widespread popularity. Typical examples are MCP PMT and single photon avalanche diodes (SPAD), that are both able to obtain timing resolution as low as 20÷30ps.
To perform a measurement using the TCSPC technique, it is necessary to use a quite complex setup that is able to extract the timing information from the output signal of the detector. The classical setup requires the use of an “electronic stopwatch” to measure time intervals with picosecond resolution, an acquisition board to record the raw data and suitable software to allow the elaboration, analysis and interpretation of the acquired data.
The block that constitutes the core of a TCSPC device is certainly the time measurement block (the “electronic stopwatch”, as said earlier). This instrument can be built around a time to amplitude converter (TAC), that converts a time interval into a voltage, and a high linearity A/D converter that is used to translate the voltage in the digital form needed by the acquisition software.
Such appliances are commercially available, either as individual components or as “all in one” devices. Unfortunately they provide only a few acquisition channels and their photon counting rate constitutes a limitation in some kind of experiments. Applications such as optical tomography require the acquisition of timing signals from many detectors operating in parallel; the availability of TCSPC instrumentation with real multi detector capability would allow the realization of acquisition systems characterized by great compactness, higher performance and higher efficiency. By suitable design it would be possible to increase the number of timing acquisition channels exploiting the statistical nature of the TCSPC technique and the redundancy of the components in the setup, while optimizing the space occupation and cost of the whole system.
Aim of this work is to design a complete and compact TCSPC acquisition system with high timing resolution that is especially suited for the use with single photon detectors.
To increase the performance of the TCSPC device and to allow the use of many parallel timing channels, while reducing the space occupation and the cost of the setup, a fully integrated and a hybrid version of time to amplitude converter have been realized.
The acquisition system is completed by a high linearity A/D conversion board that can be connected to a PC for data elaboration by means of an USB link.
The results of this work open the way to further developments of the instrumentation for TCSPC, and in particular a better integration with photodetectors such as SPADs. Monolithic detectors in silicon technology offer a great potential for the integration of complex electronics around the detector itself; for example “timing matrices” could be realized in order to obtain 2D images in the time domain with high timing resolution.
This thesis is organized as follows: the general principles of the Time Correlated Single Photon Counting technique are presented in chapter 1. Chapter 2 will introduce the main components of a TCSPC setup, the time to amplitude converter, and will describe how to evaluate its performance. The integrated TAC (iTAC) and hybrid TAC (hTAC) that were realized during this thesis work will be presented in chapter 3 and 4; both the design procedure and preliminary experimental results will be described. At last, chapter 5 will describe the realization of the A/D acquisition board that can be used with our earlier TAC prototypes or with the new converters that are here presented.