Time-correlated underwater depth imaging using an asynchronous linear single photon avalanche diode detector array

Time-correlated single photon counting (TCSPC) has emerged as a detection approach for high performance lidar and depth profiling due to its high optical sensitivity and excellent surface to surface resolution. The TCSPC technique measures, with picosecond temporal resolution, the timing difference between an optical input pulse and a photon detection event recorded by a single-photon detector. This process is repeated, and accurate return time and depth information is acquired. This approach typically uses high repetition rate (i.e. > 1 MHz) pulsed laser sources. This presentation will show how this approach was applied to highly scattering underwater scenarios to obtain three dimensional profiles of the target area, by using a system based on a linear array of single photon detectors. The system comprised a photon detection module, a TCSPC acquisition system, and a laser diode source in a bi static configuration. The laser operated at a wavelength of 670 nm and used a repetition rate of 40 MHz. The photon detection module consisted of an array of 16 × 1 silicon single photon avalanche diode (Si-SPAD) detectors built in custom technology. Laboratory based experiments demonstrate depth profiles performed in scattering conditions equivalent up to 7 attenuation lengths between the transceiver and target, using acquisition times per line which varied from 5 μs to approximately 100 ms, and average optical power of up to 14 mW. The presentation will show intensity and depth profile results obtained with the system in a range of environmental conditions. Other key experimental parameters will be discussed, such as the optimum wavelength and the acquisition time versus depth resolution trade-off. Additionally, we will discuss bespoke image reconstruction algorithms used in these measurements to improve the target profile in photon-starved conditions.