Detecting landmines and other buried objects might become easier with a new laser-based sensor being developed at the University of Mississippi.
At the Optical Society's (OSA) Laser Congress in Vienna, Austria later this month, the researchers will present a research paper on the sensor, which is termed as a Laser Multi Beam Differential Interferometric Sensor (LAMBDIS). The sensor is reportedly a significant improvement over existing technologies, which cannot operate when in motion and lose accuracy when sound and vibration are present.
Laser Doppler vibrometers (LDVs) combined with vibration excited in the ground have shown promise for detecting landmines and other buried objects, but their sensitivity to environmental vibrations requires them to be operated from a special stable platform. The LAMBDIS device is said to provide comparable detection capabilities but is far less sensitive to motion, so it can work aboard a moving vehicle.
In the new sensor, the researchers used a linear array of 30 laser beams directed onto the interrogated area. Optical elements, including a receiver lens and a shearing interferometer, are used to combine the light reflected from different points on the ground on a photodetector array (PDA), resulting in interference signals on the PDA outputs. The frequency of the signals is proportional to the vibration velocity between illuminated points due to the Doppler effect. Processing of the PDA signals reveals vibrations between illuminated points on the surface.
"The lingering scourge of landmines presents a serious challenge to rapid and accurate interrogation of large areas from moving vehicles," said lead researcher Dr. Vyacheslav Aranchuk, in a statement. "Our new device overcomes this challenge by using a series of laser beams and then combining their signals to create a rapid-detection scheme that also is robust enough to compensate for motion and other 'noise' that could overwhelm other techniques."
Aranchuk added, "Unlike LDVs, the LAMBDIS doesn't use an internal reference beam, but detects a Doppler shift by using interference of light reflected from different points on the object. Due to the lack of a reference beam, the Doppler frequency caused by the sensor motion is practically the same for all reflected beams and is automatically subtracted from the interference signals. As a result, LAMBDIS has very low sensitivity to the motion of the sensor itself, while having high sensitivity to relative vibration between points on the object."
In field tests, researchers reported that LAMBDIS was able to detect buried objects 7.5 meters to 20 meters away and from a vehicle traveling at 3.8 meters per second (about 8.5 miles per hour) with comparable results to a stable platform-mounted LDV. Researchers tested the device both with airborne and seismic sound sources and with different scanning angles, suggesting the device could produce accurate results in various real-world conditions.
Besides landmines, LAMBDIS could benefit other applications where environmental noise or movement hinders the use of LDVs. This could include inspecting automobiles and aircraft components, assessing bridge and structure vibrations, and calibrating equipment.