Underwater depth imaging using time-correlated single-photon counting at video frame rates

This paper presents the Time-Correlated Single-Photon Counting (TCSPC) technique applied to underwater environments in order to reconstruct three-dimensional scenes. Two different transceiver systems approaches are described. The first transceiver comprised a single-pixel monostatic scanning unit, which used a fiber-coupled silicon single-photon avalanche diode (SPAD) detector, and a fiber-coupled supercontinuum laser source used in conjunction with an acousto-optic tunable filter (AOTF) for wavelength selection. The experiments were performed using the supercontinuum pulsed laser source operating at a repetition rate of 19.5 MHz, fiber coupled to the AOTF in order to select one operational wavelength, tuned for best performance for the level of scattering of the particular underwater environment. Laboratory-based experiments were performed using average optical powers of less than 1 mW and depth profiles were acquired at up to 8 attenuation lengths between the transceiver and target. The second transceiver system was based on a complementary metal-oxide semiconductor (CMOS) SPAD detector array in a bistatic configuration. It comprised an array of 192 x 128 SPAD detectors, with each detector having an integrated time to digital converter, and a laser diode operating at a wavelength of 670 nm, a repetition rate of 40 MHz, and average optical power up to 9 mW. The experiments demonstrated the recovery of intensity and depth profiles associated with moving targets at up to 4 attenuation lengths. Using data from both systems, various image processing techniques were investigated to reconstruct target depth and intensity profiles in highly scattering underwater environments.