The US Naval Research Laboratory in Washington, DC has developed a technology that enables lidar to scan through obscurants such as foliage. As the lab’s Paul Lebow explained, it’s all thanks to a holographic system built to exploit a process called optical phase conjugation.
“It was something that until maybe the last five years was not viable just because the technology wasn’t really there,” Lebow said. “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.”
According to Photonics Online, the system works by emitting a pulsed laser with pulse widths of several nanoseconds. The returning pulses are received by a gated sensor “that selectively blocks the earliest-to-arrive light reflecting off foliage, and allows the reflected light that comes later from the obscured ground surfaces.”
Abby Watnik, one of the authors of the paper announcing this breakthrough, explained that system works by exploiting “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. We need complete coherence between those beams such that they interfere with one another, so we had to have a specially designed laser system to ensure that we would get that coherence when they interfere on the camera.”
Lebow said the team can adjust the coherence on the sensor to “respond to the light coming from where we want it to come from, from the target. 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.”
The research team has proved the concept using a perforated index card that simulated foliage. Using their new system, they were able to capture what would have been hidden by the index card, as well as the index card itself.
According to Phys.org, the team will continue to develop the project and hopes to secure funding for a prototype lidar sensor that would be suitable for use in the field.