Fast-Gated Single-Photon Avalanche Diode for Wide Dynamic Range Near Infrared Spectroscopy

We present a novel technique for wide dynamic range optical investigations. It is based on a fast-gated silicon single-photon avalanche diode (SPAD) in time-correlated single-photon counting (TCSPC) setup. The SPAD is gated-ON and OFF in 500 ps so as to detect photons only within a given time interval. This technique is particularly useful in applications where a large amount of unnecessary photons precede or follow the optical signal to be detected, such as in time-resolved near infrared (NIR) spectroscopy, optical mammography, and optical molecular imaging. In particular, in time-resolved reflectance spectroscopy, it is desirable to minimize the source-detector separation to improve system performance. This leads to the saturation of the detection electronics because of the huge amount of early photons back scattered by superficial layers. Our setup is able to reject these photons and detect only late photons from the sample, thus allowing an increase in the dynamic range and the injected power. We acquired diffusive curves of two phantoms with 95 ps time resolution and 107 dynamic range with a measurement time three orders of magnitude shorter than what is currently possible with a standard TCPSC setup.