Laser multi-beam differential interferometric sensor for acoustic detection of buried objects

Laser Doppler vibrometers (LDVs) have been successfully used for ground vibration imaging in acoustic detection of buried objects. LDVs operating from a stationary platform or from a moving platform with a beam looking down can provide high sensitivity vibration measurement of the ground. However, operation from a moving vehicle with laser beams looking forward induces Doppler shift in the LDV beam. This shift can be much greater than the modulation bandwidth of the LDV. The demodulation must allow for the shift either by increasing the processing bandwidth, or by tracking the Doppler shift. The former increases the LDV noise while the latter can result in complex LDV design and signal processing. We developed a novel Laser Multi Beam Differential Interferometric Sensor (LAMBDIS) which provides measurement of vibration fields of objects with high sensitivity, while having low sensitivity to the whole body motion of the object, or sensor itself. The principle of operation of the LAMBDIS is based on the interference of light reflected from different points on the object surface illuminated with a linear array of laser beams. The Doppler shift induced by the sensor motion is approximately the same for all beams and is automatically subtracted from the measurements. Scanning the linear array of laser beams in the transverse direction provides a vibration image of the surface. Performance of the sensor for vibration imaging of a buried object was experimentally investigated. The experimental results and description of the sensor are presented in the paper.