Underwater time of flight depth imaging using an asynchronous linear single photon avalanche diode detector array

We investigate three-dimensional profiling of targets in highly scattering underwater environments, obtained using the time-correlated single-photon counting (TCSPC) technique. This approach was implemented in laboratory conditions by using an optical detection system based on a linear array of single-photon detectors and a dedicated TCSPC module. The depth imaging system comprised a single-photon detection module, a TCSPC acquisition system, and a laser diode source in a bi-static transceiver configuration. The laser operated at a wavelength of 670 nm with a pulse duration of 120 ps at a repetition rate of 40 MHz, equivalent to a period of 25 ns. The laser provided a collimated line focus which horizontally illuminated the area of interest. The photon detection module consisted of an array of 16 × 1 silicon single photon avalanche diode (Si-SPAD) detectors built in custom silicon fabrication technology, with each SPAD having a photon detection efficiency of up to 28% at a wavelength of 670 nm. The timing information was measured by a dedicated TCSPC acquisition module, which included four four-channel time to amplitude converter (TAC) arrays. The system acquired time-correlated images with a timing bin duration of 1.6 picoseconds, which was equivalent to 180 µm depth resolution in water. The targets were placed in a 110 liter capacity tank, at a distance of approximately 1.65 meters in several underwater scattering environments, and were moving at a speed of 10 mm/s. These laboratory based experiments demonstrate depth profiles performed in scattering conditions equivalent up to 7.4 attenuation lengths between the transceiver and target, using acquisition times per line of approximately 30 ms, and the pulsed laser had an average optical power of less than 14.6 mW.