Method Pairs Gated Digital Holography with Laser to See Obscured Objects

A new methodology based on gated digital holography paired with a specially designed laser system allows lidar to see through obscured elements of terrain like foliage or netting.

Researchers from the Naval Research Laboratory in Washington D.C. developed the system, which can help lidar overcome environments such as dynamic weather patterns to detect 3D images behind obscurities. Potentially, the new lidar system could be used in disaster relief situations where help was needed to find people in trouble.

A specially designed laser system and a new methodology based on gated digital holography enables lidar to see through obscuring elements like foliage and netting. Courtesy of US Naval Research Laboratory.

“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,” said Paul Lebow from the Naval Research Laboratory.

Previously, lidar measurements of surfaces hidden behind foliage have been difficult to acquire. A majority of the original light in these cases gets thrown away, as far as the camera detecting light from the ground is concerned, since the light hitting the leaves never reaches the ground in the first place. The light that is blocked or reflected before getting to the ground often overpowers the signal hitting the camera and hides the fainter signal that does make it to the ground and back.

"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," said Lebow. "It was something that until maybe the last five years was not viable just because the technology wasn't really there. The stuff we had done about 20 years ago involved using a nonlinear optical material and was a difficult process. Now everything can be done using digital holography and computer generated holograms, which is what we do."

The new system uses a specially designed laser that took a year and a half to develop. Researcher Abbie Watnik said the system is essential for this process to work.

"The real key to making our system work is the interference 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," said Watnik.

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 anything that is blocking the view. The camera then only measures light coming back from the partially hidden surface below.

The researchers say their preliminary, laboratory-based system has provided substantial evidence of its power and potential real-world value. Using a perforated index card to pose as foliage, the group was able to image what the holed index card would have otherwise hidden and recreate the topology of the would-be foliage.

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.

The research was presented at The Optical Society’s Imaging and Applied Optics Congress, held in San Francisco.