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Dipartimento di Fisica - Politecnico di Milano

Dispositivi Gated a Singolo Fotone (GAP)

The GAP lab is devoted to the development of new instrumentation and novel techniques and strategies for Diffuse Optics. Hence, the activity is focused on two main work steams strongly connected. On one side there is the development and characterization of new instruments for time-domain diffuse optics while, on the other side, new techniques for Time-domain Diffuse Optics (TD-DO) are investigated. For both research lines (whose activity is detailed in the following), strong and continuous collaboration with national and international institutions/universities are carried on in order to reach highest level of quality and impact of the research.

Development and characterization of new instruments for TD-DO (responsible: Dr. L. Di Sieno)
TD-DO is at the dawn of a next-generation of systems, also thanks to the research lead by GAP lab, with potential breakthrough in performance, size, cost, and probable great impact on new application of TD-DO systems and their widespread diffusion. Such a perspective has been enabled by few key recent advancements opening the way to a new class of optoelectronics components and devices.
The GAP lab contributes to this “revolution” designing also new systems embedding for the first time directly on the probe both the pulsed laser diode and the time-gated detector. Such a solution allows an enormous (>10x) improvement in terms of signal-to-noise ratio and the envisage of future additional orders of magnitude improvements.
Additionally, the GAP lab introduced and validated the use of silicon photomultipliers (SiPMs) in TD-DO. Such detectors merge together the advantages of solid-state detectors with the large area of the PhotoMultipliers. The use of SiPMs allows a considerable improvement of the light harvesting, thus improving the signal-to-noise ratio by 2 orders of magnitude.
Key achievements using SiPM were: i) the fabrication of a broadband TD-DO spectroscopy system with increased light harvesting efficiency, which has been exploited in a pioneer study of in vivo non-invasive characterization of human bones; ii) in vivo studies of functional brain imaging with probe-hosted detectors thanks to a miniaturized front-end circuit; iii) demonstration of the suitability of SiPMs in TD-DO tomography; iv) design of the first battery-operated and portable TD-DO system that can be easily hosted into a small backpack.
Further researches are currently on-going with a dual aim: i) additional increase of the collection area to increase the light harvesting and allowing to probe new organs (e.g. the lung or the heart); ii) operate single-photon detectors in fast-gated mode so as to increase the dynamic range of the measurements by different orders of magnitude (see below).


New approaches for TD-DO (responsible: Prof. A. Dalla Mora)
The GAP lab played a pioneering role for the use of new physical concepts and approaches for TD-DO.
Since 2008, the lab developed the “short source-detector distance approach” coupled to the “fast-gating technique” to increase the spatial resolution, the sensitivity to perturbation and the dynamic range (DR) of optical measurements (which is fundamental to detect faint signal).
We have demonstrated that a way to obtain a strong increase of the DR is the use of solid-state detectors with a sub-nanosecond transition from the OFF to the ON state. Any time-resolved optical signal can be measured by acquiring different temporal slices. The idea is to exploit the entire DR of the detection chain for every slice by properly adjusting the signal intensity on the detector so as to reach the maximum detectable level. It is possible to reconstruct the whole waveform by combining slices acquired at different delays after amplitude normalization, as illustrated in Figure 1.

FIG. 1. Schematic representation of the method used for reconstructing a curve with
high dynamic range by dividing it in many slices and by progressively increasing
the optical power for improving the signal-to-noise ratio along the tail.


The advantage of the use of the fast-gating technique is evident from Figure 2. Indeed, a DR of seven orders of magnitude acquired in few seconds with the fast-gating technique could be obtained with standard non-gated technique only over 3 years of continuous measurement time.
Thanks to this striking improvement, a wide field of novel applications has been explored by our research group (often in collaboration with other national and international research centers) which include: functional brain imaging with time-resolved reflectance spectroscopy at very small source-detector separation; non-contact diffuse imaging; optical tomography with short source-detector separation; single-fiber diffuse spectroscopy; investigation of photon-induced background phenomena in solid-state detectors (i.e. the so-called “memory effect”).

FIG. 2. (left) The typical instrument response function of a standard non-gated SPAD detector: the low dynamic range
limits the maximum delay at which photons can be detected. (right) A wide dynamic range of more than 7 orders
of magnitude is obtained by fast-gating the SPAD detector, thus increasing the maximum delay for detecting photons.


Recently, we also explored new approaches for TD-DO, combining it to endoscopy or surgical procedures, potentially reaching any part of the body. This approach can be exploited using, for example, bioresorbable optical fibers, developed by Politecnico di Torino. These fibers could be placed inside the body during surgical interventions to monitor the postoperative healing process or detect anomalous tissue response and they will spontaneously decay without dangers for the subject. Externally, a compact wearable TD-DO device can be connected to such fibers, similarly to an Holter monitor. The suitability of such fibers in TD-DO has been demonstrated on tissue-mimicking phantom and ex-vivo. Currently, further investigations towards the use of a single bioresorbable fiber are going on.



  • A. Behera, L. Di Sieno, A. Pifferi, F. Martelli, and A. Dalla Mora, "Instrumental, optical and geometrical parameters affecting time-gated diffuse optical measurements: a systematic study," Biomed. Opt. Express vol 9 no 11, 5524–5542 (2018). DOI: 10.1364/BOE.9.005524
  • M. Renna, M. Buttafava, A. Behera, M. Zanoletti, L. Di Sieno, A. Dalla Mora, D. Contini, and A. Tosi, "Eight-wavelength, dual detection-channel instrument for near-infrared time-resolved diffuse optical spectroscopy," IEEE J. Sel. Top. Quantum Electron. vol 25 no 1, (2019). DOI: 10.1109/JSTQE.2018.2863570
  • M. Pagliazzi, S. K. V. Sekar, L. Di Sieno, L. Colombo, T. Durduran, D. Contini, A. Torricelli, A. Pifferi, and A. Dalla Mora, "In vivo time-gated diffuse correlation spectroscopy at quasi-null source-detector separation," Opt. Lett. vol. 43 no 11, 2450–2453 (2018). DOI: 10.1364/OL.43.002450
  • A. Farina, S. Tagliabue, L. Di Sieno, E. Martinenghi, T. Durduran, S. Arridge, F. Martelli, A. Torricelli, A. Pifferi, and A. Dalla Mora, "Time-Domain Functional Diffuse Optical Tomography System Based on Fiber-Free Silicon Photomultipliers," Appl. Sci. 7, (2017). DOI: 10.3390/app7121235
  • J. Zouaoui, L. Di Sieno, L. Hervé, A. Pifferi, A. Farina, A. Dalla Mora, J. Derouard, and J.-M. Dinten, "Chromophore decomposition in multispectral time-resolved diffuse optical tomography," Biomed. Opt. Express 8, 4772–4787 (2017). DOI: 10.1117/1.JBO.22.8.085004
  • A. Farina, M. Betcke, L. Di Sieno, A. Bassi, N. Ducros, A. Pifferi, G. Valentini, S. Arridge, and C. D’Andrea, "Multiple-view diffuse optical tomography system based on time-domain compressive measurements," Opt. Lett. 42, 2822–2825 (2017). DOI: 10.1364/OL.42.002822
  • L. Di Sieno, N.G. Boetti, A. Dalla Mora, D. Pugliese, A. Farina, S. Konugolu Venkata Sekar, E. Ceci-Ginistrelli, D. Janner, A. Pifferi, D. Milanese, “Towards the use of bioresorbable fibers in time-domain diffuse optics,” Journal of Biophotonics, in press.
  • S. Konugolu Venkata Sekar, I. Bargigia, A. Dalla Mora, P. Taroni, A. Ruggeri, A. Tosi, A. Pifferi, A. Farina, “Diffuse optical characterization of collagen absorption from 500 nm to 1700 nm,” Journal of Biomedical Optics 22(1), 015006, 2017. DOI: 10.1117/1.JBO.22.1.015006.
  • M. Buttafava, E. Martinenghi, D. Tamborini, D. Contini, A. Dalla Mora, M. Renna, A. Torricelli, A. Pifferi, F. Zappa, A. Tosi, “A compact two-wavelength time-domain NIRS system based on SiPM and pulsed diode lasers,” IEEE Photonics Journal 9(1), 7800114, 2017. DOI: 10.1109/JPHOT.2016.2632061.
  • R. Lussana, F. Villa, A. Dalla Mora, D. Contini, A. Farina, L. Di Sieno, F. Zappa, “Non-contact inclusion detection in food through a single-photon time-of-flight imager,” IEEE Sensors Journal 17(1), 78-83, 2017. DOI: 10.1109/JSEN.2016.2621409.
  • S. Konugolu Venkata Sekar, M. Pagliazzi, E. Negredo, F. Martelli, A. Farina, A. Dalla Mora, C. Lindner, P. Farzam, N. Pérez-Álvarez, J. Puig, P. Taroni, A. Pifferi, T. Durduran, “In vivo, non-invasive characterization of human bone by hybrid broadband (600-1200 nm) diffuse optical and correlation spectroscopies,” PLOS ONE 11(12), e0168426, 2016. DOI: 10.1371/journal.pone.0168426.
  • L. Di Sieno, J. Zouaoui, L. Hervé, A. Pifferi, A. Farina, E. Martinenghi, J. Derouard, J.-M. Dinten, A. Dalla Mora, “Time-domain diffuse optical tomography using silicon photomultipliers: feasibility study,” Journal of Biomedical Optics 21(11), 116002, 2016. DOI: 10.1117/1.JBO.21.11.116002.
  • R. Re, E. Martinenghi, A. Dalla Mora, D. Contini, A. Pifferi, A. Torricelli, “Probe-hosted silicon photomultipliers for time-domain functional near-infrared spectroscopy: phantom and in vivo tests,” Neurophotonics 3(4), 045004, 2016. DOI: 10.1117/1.NPh.3.4.045004.
  • J. Zouaoui, L. Di Sieno, L. Hervé, A. Pifferi, A. Farina, A. Dalla Mora, J. Derouard, J.-M. Dinten, “Quantification in time-domain diffuse optical tomography using Mellin-Laplace transforms,” Biomedical Optics Express 7(10), 4346 – 4363, 2016. DOI: 10.1364/BOE.7.004346.
  • E. Martinenghi, L. Di Sieno, D. Contini, M. Sanzaro, A. Pifferi, A. Dalla Mora, “Time-resolved single-photon detection module based on silicon photomultiplier: a novel building block for time-correlated measurement systems,” Review of Scientific Instruments 87(7), 073101, 2016. DOI: 10.1063/1.4954968.
  • A. Pifferi, D. Contini, A. Dalla Mora, A. Farina, L. Spinelli, A. Torricelli, “New frontiers in time-domain diffuse optics, a review,” Journal of Biomedical Optics 21(9), 091310, 2016. DOI: 10.1117/1.JBO.21.9.091310.
  • S. Konugolu Venkata Sekar, A. Dalla Mora, I. Bargigia, E. Martinenghi, C. Lindner, P. Farzam, M. Pagliazzi, T. Durduran, P. Taroni, A. Pifferi, A. Farina, “Broadband (600-1350 nm) time resolved diffuse optical spectrometer for clinical use,” IEEE Journal of Selected Topics in Quantum Electronics 22(3), 7100609, 2016. DOI: 10.1109/JSTQE.2015.2506613.
  • L. Di Sieno, H. Wabnitz, A. Pifferi, M. Mazurenka, Y. Hoshi, A. Dalla Mora, D. Contini, G. Boso, W. Becker, F. Martelli, A. Tosi, R. Macdonald, “Characterization of a time-resolved non-contact scanning diffuse optical imaging system exploiting fast-gated single-photon avalanche diode detection,” Review of Scientific Instruments 87(3), 035118, 2016. DOI: 10.1063/1.4944562.
  • L. Di Sieno, G. Bettega, M. Berger, C. Hamou, M. Aribert, A. Dalla Mora, A. Puszka, H. Grateau, D. Contini, L. Hervé, J.-L. Coll, J.-M. Dinten, A. Pifferi, A. Planat-Chrétien, “Toward noninvasive assessment of flap viability with time-resolved diffuse optical tomography: a preclinical test on rats,” Journal of Biomedical Optics 21(2), 025004, 2016. DOI: 10.1117/1.JBO.21.2.025004.

Complete list of publications