Researchers develop foliage-penetrating LIDAR

LiDAR’s applications are growing with the push to develop self-driving vehicles and the technology remains a standard method for 3D measurement.

This is a specially designed laser system and a new methodology based on gated digital holography enable LiDAR to see through obscuring elements like foliage and netting. Photo courtesy of US Naval Research Laboratory

One area that LiDAR performs extremely well is land surveying. By scanning areas of land with lasers, often from airplanes, LiDAR’s travel-time measurements for light reflected back from the scanned area provide the distances that make up a resulting high-resolution topography.

However, over the years there have been several notable limitations when it comes to this particular application. But researchers at Washington, DC’s Naval Research Laboratory are using a specially designed laser system and a new methodology based on gated digital holography, to develop a method to give LiDAR an enhanced ability to see through otherwise obscuring environmental elements.

“This was an attempt to address one of the problems with something called foliage-penetrating LiDAR,” Paul Lebow of the Naval Research Laboratory said. “You can actually use it to detect three-dimensional images behind an obscuration such as a tree canopy… You can illuminate using LiDAR through the leaves and get enough light coming back through to be able to recreate a three-dimensional, topographic view of what’s going on beneath.”

For the most part, LiDAR measurements have been limited to wherever light can penetrate. With surfaces hidden behind obstructions, the original light gets thrown away and the camera is unable to detect or only receives a limited signal, providing minimal readings.

“We have been working with a process called optical phase conjugation for quite some time and it dawned on us that we might be able to use that process to essentially project a laser beam through the openings of the leaves and be able to see through a partial obscuration,” Lebow said. “It was something that until maybe the last five years was not viable just because the technology wasn’t really there.”

Lebow and colleague Abbie Watnik developed the new system with a specifically designed laser. And according to Watnik, the key to the system working is the interface between two laser beams on the sensor.

“We send one laser beam out to the target and then it returns, and at the exact same time that return [beam] hits the detector, we interfere it locally with another laser beam,” Watnik said. The researchers needed to design the laser system to ensure the camera captures absolute coherence when the laser beams interfere with one another.

Using a pulsed laser with pulse widths of several nanoseconds, and gated measurements with similar time resolution, the holographic system selectively blocks the earliest-to-arrive light reflecting off obscurations. The camera then only measures light coming back from the partially hidden surface below.

“We’ve done this earlier using a CW (continuous-wave) laser as a demo, but now we’re using a pulsed laser and a very fast gated sensor that can turn on at the appropriate time to basically only let us respond to the light coming from where we want it to come from, from the target,” Lebow said. “The laser is designed so that the time difference between the local reference pulse and the signal pulse that comes back from the target is completely adjustable to accommodate distances from a few feet to several kilometers.”

This enables the laser system to be used in the lab on a tabletop setup as well as out in field, adds Watnik.

This preliminary, laboratory-based system has provided substantial evidence of its power and potential real-world value.

“We were able to verify what our computer model says using our real data – matching it to what we actually see using the spatial light modulator, so I think that was an interesting verification of our results,” Watnik said.

Watnik and Lebow, along with their research team, hope to continue with the project and make the adaptations to their prototype necessary to making the foliage-penetrating LiDAR system field-ready.

“That would be our next plan, if we got funding for it,” Lebow said. “There have been several other follow-on projects, not specifically for LiDAR, such as beam steering and other digital holographic work that we’re doing for imaging through fog and turbid water based on very similar properties and principles.”

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