Researchers have developed a lidar system that utilizes quantum detection technology to acquire 3D images while underwater. This is the first demonstration of a prototype system that can capture detailed information in extremely low-light conditions found underwater. The high sensitivity of this system allows it to be useful for a wide range of applications, including inspecting underwater installations such as wind farm cables and turbines, surveying submerged archaeology sites, and for security and defense purposes.

Capturing 3D images through ocean water can be challenging because of the light-limited conditions and particles in the water that scatter light and distort the image. However, single-photon detection, a quantum-based technique, allows for high penetration and works in low-light conditions.

Researchers from Heriot-Watt University and the University of Edinburgh describe experiments in which an entire single-photon lidar system was submerged in a large water tank in Optics Express. The new demonstrations bring the technology closer to practical applications compared to the team’s earlier experiments with underwater single-photon detection, which were performed in carefully controlled laboratory conditions with the optical setup placed outside the water tank and data analysis performed offline.

The researchers implemented new hardware and software developments that allow the 3D images acquired by the system to be reconstructed in real-time. This 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. This will impact the use of offshore cable and energy installations, which are used by everyone. This technology could also allow monitoring without the presence of humans, which would mean less pollution and a less invasive presence in the marine environment.

The lidar system designed by the researchers uses a green pulsed laser source to illuminate the scene of interest, and the reflected pulsed illumination is detected by an array of single-photon detectors, allowing ultrafast low light detection and reducing measurement time in photon-starved environments such as highly attenuating water. “Our approach also allows us to distinguish the photons reflected by the target from those reflected by particles in the water, making it particularly suitable for performing 3D imaging in highly turbid waters where optical scattering can ruin image contrast and resolution,” said Aurora Maccarone, a Royal Academy of Engineering research fellow from Heriot-Watt University.

The researchers developed algorithms specifically for imaging in highly scattering conditions and applied them in conjunction with widely available graphics processing unit (GPU) hardware to process the data necessary to reconstruct the 3D image in a short time, especially for real-time applications. After optimizing the optical setup on a laboratory optical bench, the researchers connected the lidar system to a GPU to achieve real-time processing of the data while also implementing a number of image processing approaches for three-dimensional imaging.

Experiments were conducted in a tank that was 4 meters long, 3 meters wide, and 2 meters deep, with the system submerged in the water and a scattering agent added in a controlled manner to make the water more turbid. Successful imaging was demonstrated at three different turbidity levels at distances of 3 meters.

The researchers are now working to reduce the size of the system so that it could be integrated into an underwater vehicle. They are partnering with industry through the UK Quantum Technology Hub Network and InnovateUK to make the technology accessible for a range of underwater applications.

By Impact Lab