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
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.
In accordance with Public Law 96-517,
the contractor has elected to retain title to this
invention. Inquiries concerning rights for its
commercial use should be addressed to:
Innovative Technology Assets Management
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
Refer to NPO-45309, volume and number of this NASA Tech Briefs issue, and the page number.
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Very-Long-Distance Remote Hearing and Vibrometry (reference NPO-45309) is currently available for download from the TSP library.
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