Optical returns from weakly illuminated targets would be processed by advanced techniques.

A proposed development of laserbased instrumentation systems would extend the art of laser Doppler vibrometry beyond the prior limits of laserassisted remote hearing and industrial vibrometry for detecting defects in operating mechanisms. A system according to the proposal could covertly measure vibrations of objects at distances as large as thousands of kilometers and could process the measurement data to enable recognition of vibrations characteristic of specific objects of interest, thereby enabling recognition of the objects themselves. A typical system as envisioned would be placed in orbit around the Earth for use as a means of determining whether certain objects on or under the ground are of interest as potential military targets. Terrestrial versions of these instruments designed for airborne or land- or sea-based operation could be similarly useful for military or lawenforcement purposes.

Prior laser-based remote-hearing systems are not capable of either covert operation or detecting signals beyond modest distances when operated at realistic laser power levels. The performances of prior systems for recognition of objects by remote vibrometry are limited by low signal-to-noise ratios and lack of filtering of optical signals returned from targets. The proposed development would overcome these limitations.

A system as proposed would include a narrow-band laser as its target illuminator, a lock-in-detection receiver subsystem, and a laser-power-control subsystem that would utilize feedback of the intensity of background illumination of the target to adjust the laser power. The laser power would be set at a level high enough to enable the desired measurements but below the threshold of detectability by an imaginary typical modern photodetector located at the target and there exposed to the background illumination. The laser beam would be focused tightly on the distant target, such that the receiving optics would be exposed to only one speckle. The return signal would be extremelynarrow- band filtered (to sub-kilohertz bandwidth) in the optical domain by a whispering-gallery-mode filter so as to remove most of the background illumination. The filtered optical signal would be optically amplified. This combination of optical filtering and optical amplification would provide an optical signal that would be strong enough to be detectable but not so strong as to saturate the detector in the lock-in detection subsystem.

The laser emission would be modulated by an optical modulator driven by a low-frequency oscillator. The same oscillator would drive a lock-in amplifier in the lock-in-detection receiver subsystem. It has been estimated that the lock-in amplification would contribute 30 dB to the signal-to-noise ratio.

It has been estimated that a system of this type operating at a laser power of 0.2 W could enable recognition of an object at a distance of 1,000 miles (≈1,600 km). Examples of objects of potential military significance that could be recognized include particular machines shielded under the roof of a factory or deep underground, fake garages or factories, fake weapons, land mines, and improvised explosive devices. Vibrations induced by nearby motorized vehicles are expected to be sufficient to enable recognition of buried land mines.

This work was done by Lute Maleki, Nan Yu, Andrey Matsko, and Anatoliy Savchenkov of Caltech for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Physical Sciences category.

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