Contatore di singoli fotoni a due finestre ultraveloci per spettroscopia ottica

Low-level optical signals can be acquired with picosecond time resolution by means of the Time-Correlated Single-Photon Counting (TCSPC) technique. TCSPC setups are based on expensive timing boards and, when a large number of detectors must be used, the total cost could be very high and the overall system would be very complex and bulky. Therefore, different approaches must be investigated. Many applications do not really need to reconstruct the whole waveform, but some of them just need to extract few information. For example, when the time constant of a decaying exponential curve has to be calculated, just few acquisitions along the curve are sufficient. A specific case study is the time-resolved reflectance spectroscopy at null source-detector distance, where it could be possible to compute the contrast function by integrating only photons arriving during a short time window at a well-defined delay with respect to the pulse peak. To such purpose, an ultra-fast gated counter is required to generate short counting windows. However, commercially available gated counter are not suitable for this application since they have long counting window (more than 1 ns). We designed our own fast gated counter with minimum gate window 80ps FWHM wide, rising and falling edges of 50ps, count rate of 100MHz.