Submerged lidar acquires 3D images underwater

10 May 2023

Heriot-Watt device uses single-photon detection for real-time imaging of moving targets.

Lidar's ability to capture 3D data about an object's surroundings is already put to use in several application areas, including autonomous vehicles and security systems.

Another application of obvious potential interest is underwater imaging, with lidar mounted on autonomous sub-sea vehicles offering a potential way to carry out mapping of the sea floor at centimeter-resolution.

A project from Heriot-Watt University and the University of Edinburgh has now developed what it says is the first fully submerged lidar underwater transceiver system based on single-photon detection, and published the results in Optics Express.

Obtaining lidar images through ocean water can be challenging, because of the limited light penetration possible and the scatter and distortion created by any particles in the water. However, recent advances in single-photon detection can help to overcome these hurdles.

Heriot-Watt employed a CMOS-fabricated silicon single-photon avalanche diode (SPAD) detector to measure photon time-of-flight using picosecond resolution time-correlated single-photon counting, according to the project. The high sensitivity of this architecture allows it to capture detailed information even in extremely low-light conditions found underwater.

"This technology could be useful for a wide range of applications," said Aurora Maccarone from Heriot-Watt University. "For example, it could be used to inspect underwater installations, such as underwater wind farm cables and the submerged structure of the turbines. Underwater lidar can also be used for monitoring or surveying submerged archaeology sites and for security and defense applications."

Less invasive monitoring and maintenance

The new system operates at a central wavelength of 532 nanometers, and is based on a Si-CMOS SPAD detector array of 192 x 128 pixels, according to the project's published paper.

Two recently developed analysis algorithms were also used by the team, one being specifically developed by Heriot-Watt for real-time 3D scene reconstruction from single-photon data in lidar applications, and the other recently proposed for distance estimation in highly scattering underwater environments.

"Heriot-Watt University has a long track record in single-photon detection techniques and image processing of single-photon data, which allowed us to demonstrate advanced imaging in extremely challenging conditions," said Maccarone. "The University of Edinburgh has achieved fundamental advances in the design and fabrication of single-photon avalanche diode detector arrays, which allowed us to build compact and robust imaging systems based on quantum detection technologies."

During trials in a water tank measuring 4 x 3 x 2 meters, experiments at three different turbidity levels demonstrated successful imaging of stationary and moving 3D target objects in controlled highly scattering scenarios at distances of 3 meters. Real-time video of the moving targets at ten frames per second were successfully recorded.

"Our work aims to make quantum detection technologies available for underwater applications, which means that we will be able to image the scene of interest in very low light conditions," said Aurora Maccarone.

"This will impact the use of offshore cable and energy installations, which are used by everyone. The technology could also allow monitoring without the presence of humans, which would mean less pollution and a less invasive presence in the marine environment."