Sep 17 2019
The detection of landmines could be a slow and difficult process. Although the process can be expedited by detecting the landmines from a moving vehicle, this will have an impact on accuracy.
This is vibration imaging of buried object using LAMBDIS. (Image credit: V. Aranchuk, University of Mississippi)
Scientists at the University of Mississippi, United States, will present a novel laser-based sensor at The Optical Society’s (OSA) Laser Congress that will be held in Vienna, Austria from September 29th to October 3rd, 2019. The sensor can effectively detect hidden objects even when the detector is moving.
This innovative device provides a major enhancement over other currently available technologies, which are impossible to use on the go and also tend to lose precision when external sources of vibration or sound are present.
Laser Doppler vibrometers (LDVs) in combination with vibration triggered in the ground have demonstrated potential to identify various buried objects, including landmines. However, LDVs are sensitive to environmental vibrations, meaning they can be used only from an exclusive stable platform.
By contrast, the new device, known as a Laser Multi-Beam Differential Interferometric Sensor (or LAMBDIS in short), has similar detection capabilities and is also much less susceptible to motion. This means the device can be utilized aboard a moving vehicle.
The lingering scourge of landmines presents a serious challenge to rapid and accurate interrogation of large areas from moving vehicles. 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.
Dr Vyacheslav Aranchuk, Study Lead Researcher, University of Mississippi
Aranchuk continued, “LAMBDIS provides measurement of vibration fields with high sensitivity, while having low sensitivity to the whole-body motion of the object, or sensor itself, allowing for the operation from a moving vehicle.”
Measurements Without a Reference Beam
In order to identify hidden objects, LDVs are used along with a seismic source, for example, a mechanical shaker, or an audio source like a loudspeaker. The ground vibrates due to the sound or seismic waves. The LDV is capable of detecting slight variations in the vibration pattern where an object is concealed. But for this to happen, the environment should be adequately vibration-free and the detector should also be immobile.
Operation of conventional LDVs depends on the interference of light. This light is reflected from an object with a reference beam that is internal to the LDVs. Consequently, the LDV’s motion itself can cause the LDV signals to be considerably higher than—and indistinguishable from—the signals induced by the vibration of the object.
In the latest study, the scientists employed a linear array of 30 laser beams focused onto the interrogated region.
Optical elements, such as a shearing interferometer and a receiver lens, were utilized to integrate the light reflected from varied points on the ground on a photodetector array, or PDA. This leads to interference signals on the PDA outputs.
The signals’ frequency is in proportion to the vibration velocity present between illuminated points. This can be attributed to the Doppler effect. When the PDA signals were processed, vibrations between illuminated points on the surface were observed.
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.
Dr Vyacheslav Aranchuk, Study Lead Researcher, University of Mississippi
Aranchuk continued, “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.”
Successful Field Tests
The team reported that the new LAMBDIS device worked well under a variety of conditions in both lab and field tests. The device was able to identify concealed objects placed 7.5 to 20 m away and from a vehicle moving at a speed of 3.8 m per second (around 8.5 miles per hour) with similar outcomes to an LDV mounted on a stable platform.
The scientists tested the LAMBDIS device both with different scanning and with airborne and seismic sound sources. This indicates that the device can deliver precise results in a range of real-world situations.
Apart from sensing landmines, LDVs are often used to calibrate equipment and explore materials, to evaluate structure and bridge vibrations, to examine aircraft and automobile components, as well as in biomedical and dental applications. Such applications may benefit from the LAMBDIS device in situations where movement or environmental noise prevents the use of LDV devices.
This study is funded by the Department of the Navy, Office of Naval Research under ONR award number N00014-18-2489.