Depth imaging through obscurants using time-correlated single-photon counting

We present a scanning depth imaging system which was used to investigate free-space imaging of targets through a variety of highly obscuring media. The system utilized the time-correlated single-photon counting (TCSPC) technique to obtain photon time-of-flight information. This was used to reconstruct high-resolution three-dimensional depth and intensity profiles of targets at stand-off distances of up to 24 meters in scenarios with very poor visibility. The results demonstrate the benefits of short-wave infrared (SWIR) wavelengths compared to visible band sensors for imaging through several forms of obscurants. The system was configured for operation at a wavelength of 1550 nm and measurements were performed using a 26 meter long fog tunnel facility which was filled with obscurants of several different types and densities. The system was comprised of a custom-built scanning transceiver unit, fiber-coupled to a Peltier cooled InGaAs/InP single-photon avalanche diode (SPAD) detector. A picosecond pulsed laser was used to provide a fiber-coupled illumination wavelength of 1550 nm at an approximate average optical power level of just under 1.5 mW for all measurements. Bespoke image processing algorithms were developed to reconstruct high resolution depth and intensity profiles of obscured targets in challenging environments with low visibilities. Such algorithms can allow for target reconstruction using low levels of optical power and shorter data acquisition times, thus enabling image acquisition in the sparse photon regime.