Breaking the rules of time-domain diffuse optics: working with 1 cm2 SiPM and well-beyond the single-photon statistics

Time domain diffuse optics (TD-DO) relies on the injection of ps laser pulses and on the collection of the arrival times of scattered photons. To reach the ultimate limits of the technique (allowing to investigate even structures at depth >5 cm), a large area detector is needed. To this extent, we realized and present a new silicon photomultiplier featuring a 1 cm2 area. To the best of our knowledge, it represents the largest detector ever proposed for TD-DO and shows a light harvesting capability which is more than 1 decade larger than the state-of-the-art technology system. To assess its suitability for TDDO measurements, we tested the detector with several procedures from shared protocols (BIP, nEUROPt and MEDPHOT). However, the light harvesting capability of a detector with large area can be proficiently exploited only if coupled to timing electronics working in sustained count-rate CR (i.e., well above the single photon statistics). For this reason, we study the possibility to work in a regime where (even more than) one photon per laser pulse is detected (i.e., more than 100% laser repetition rate) exploiting in-silico technology. The results show that the possibility to use sustained count-rate represents a dramatic improvement in the number of photons detected with respect to current approaches (where count-rate of 1-5% of the laser repetition rate are used) without significant losses in the measurement accuracy. This represents a new horizon for TD-DO measurements, opening the way to new applications (e.g., optical investigation of the lung or monitoring of fast dynamics never studied before).