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Researchers have designed a single-photon time-of-flight LiDAR system that can acquire a high-resolution 3D image of an object or scene up to 1 kilometer away. The new system could help enhance security, monitoring, and remote sensing by enabling detailed imaging even in challenging environmental conditions or when objects are obscured by foliage or camouflage netting.

"Our system uses a single-photon detector approximately twice as efficient as detectors deployed in similar LiDAR systems reported by other research groups and has a system timing resolution at least 10 times better," said research team member Aongus McCarthy, from Heriot-Watt University in the UK.

"These improvements allow the imaging system to collect more scattered photons from the target and achieve a much higher spatial resolution."

In Optica, a multi-institutional group of researchers from the UK and U.S. shows that the new system can construct a 3D image depicting a clearly recognizable human face of a person 325 meters away.

The researchers were from Gerald Buller's group at Heriot-Watt University, Robert Hadfield's group at the University of Glasgow, Matthew Shaw's group at the NASA Jet Propulsion Laboratory, and Karl Berggren's group at MIT.

"This type of measurement system could lead to improved security and monitoring systems that could, for example, acquire detailed depth images through smoke or fog and of cluttered scenes," said McCarthy, first author of the new paper.

"It could also enable the remote identification of objects in various environments and monitoring of movement of buildings or rock faces to assess subsidence or other potential hazards."

Light-based range finding

The single-photon time-of-flight depth imaging system uses the time it takes for a laser pulse to travel from the system to a point on an object and back to calculate the distance to the object. These time-of-flight measurements are then repeated for points across the object to obtain 3D information.

The new system uses an ultrasensitive detector called a superconducting nanowire single-photon detector (SNSPD) developed by the MIT and JPL research groups. The SNSPD can detect a single photon of light, which means that lasers with very low powers, including eye-safe lasers, can be used to perform measurements in a very short time and over long distances.

To reduce noise levels, the detector was cooled to just below 1 Kelvin in a compact cryocooler system designed and built by the University of Glasgow group.

The researchers combined the cooled SNSPD with a new custom single-pixel scanning transceiver operating at a 1550-nm wavelength that was designed by McCarthy at Heriot-Watt University. They also added advanced timing equipment to measure extremely precise time intervals—accurate down to trillionths of a second (picoseconds).

To put that into perspective, in just 1,000 picoseconds, light can travel about 300 millimeters (about 1 foot). This precision made it possible to distinguish surfaces separated by about 1 mm in depth from 325 meters away.

"These factors all provide improved flexibility in the trade-off between standoff distance, laser power levels, data acquisition time and depth resolution," said McCarthy.

"Also, since SNSPD detectors can operate at wavelengths longer than 1550 nm, this design opens the door to developing a mid-infrared single-photon LiDAR system, which could further enhance imaging through fog and smoke and other obscurants."

3D measurements of distant objects

The researchers performed field tests of their LiDAR system on the Heriot-Watt University campus by taking measurements from objects that were 45 meters, 325 meters or 1 kilometer away.

To evaluate the spatial and depth resolution, they scanned a custom 3D-printed target with varying pillar sizes and heights. The system resolved features as small as 1 mm in daylight at 45 and 325 meters—a depth resolution approximately 10 times better than they had achieved previously. They also captured a 3D image of a human face at these distances using a 1 ms per-pixel acquisition time, an eye-safe 3.5 mW laser, and minimal data processing.

"The excellent depth resolution of the system means that it would be particularly well suited for imaging objects behind clutter, such as foliage or camouflage netting, a scenario that would be difficult for a digital camera," said McCarthy. "For example, it could distinguish an object located a few centimeters behind a camouflage netting while systems with poorer resolution would not be able to make out the object."

While the field trials for the LiDAR system were limited to a range of 1 kilometer, researchers plan to test the system at distances of up to 10 km and explore imaging through atmospheric obscurants like smoke and fog.

Future work will also focus on using advanced computational methods to accelerate data analysis and enable imaging of more distant scenes.