Remote target identification using laser Doppler vibrometry
A laser Doppler vibrometer architecture and detection technique that can remotely identify targets based on their natural vibration frequencies using a scanning Fabry-Pérot interferometer. The proposed systems and methods can have stand-off distances longer than the coherence length of the laser by using spectroscopic detection methods instead of coherent heterodyne detection using a local oscillator. Pulsed lasers can be used which have high power output. In addition, by not using an acousto-optic modulator, the speed of the detectable target is not limited. Also the mixing efficiency of the return signal can be improved.
1. A system comprising:
a laser device;
an optical circulator optically coupled to said laser device;
a telescope optically coupled to said optical circulator;
a tunable interferometer optically coupled to said optical circulator;
a photodetector optically coupled to said tunable interferometer; and
a signal processor electrically coupled to said photodetector,
wherein said tunable interferometer is a scanning Fabry-Perot interferometer comprising a plurality of piezoelectric transducers, and said signal processor is electrically coupled to said piezoelectric transducers; and
wherein said signal processor is configured to perform the following operations:
(a) supplying scanning voltages to said piezoelectric transducers which have the effect of adjusting a wavelength of said tunable interferometer so that the wavelength varies within a range of wavelengths that includes a wavelength wavelengths of light emitted by said laser device and a wavelength of light which was first emitted by said laser device and then Doppler shifted by a target;
(b) monitoring a detection voltage produced by said photodetector as a function of the scanning voltage supplied to said piezoelectric transducers;
(c) identifying scanning voltages where peak detection voltages occur;
(d) comparing differences in scanning voltages where the peaks occur to reference voltages in a calibration table to determine the relative speed of the target as well as the resonant vibration frequency of the target; and
(e) identifying a target based on the resonant vibration frequency determined in operation (d).
2. The system as recited in claim 1 , wherein said tunable interferometer is configured to detect Doppler shifts within a range that includes 1 MHz.
3. The system as recited in claim 1 , wherein said optical circulator is configured to pass a first portion of light emitted by said laser device to said telescope and a second portion of light emitted by said laser device to said tunable interferometer.
4. The system as recited in claim 3 , wherein said optical circulator is further configured to pass light from said telescope to said tunable interferometer.
5. The system as recited in claim 1 , wherein said optical circulator is configured to pass a first portion of light emitted by said laser device to said telescope and to pass light from said telescope to said tunable interferometer.
6. The system as recited in claim 5 , wherein said optical circulator is further configured to pass a second portion of light emitted by said laser device to said tunable interferometer.
7. The system as recited in claim 1 , further comprising a ground-based platform that supports at least said laser device, said optical circulator, said telescope, said tunable interferometer, and said photodetector.
8. The system as recited in claim 1 , further comprising an airborne platform that supports at least said laser device, said optical circulator, said telescope, said tunable interferometer, and said photodetector.
9. The system as recited in claim 1 , further comprising a space-based platform that supports at least said laser device, said optical circulator, said telescope, said tunable interferometer, and said photodetector.
10. A system comprising:
a laser device;
an optical circulator optically coupled to said laser device;
a telescope optically coupled to said optical circulator;
a scanning Fabry-Perot interferometer optically coupled to said optical circulator, said scanning Fabry-Perot interferometer comprising a plurality of piezoelectric transducers;
a photodetector optically coupled to said scanning Fabry-Perot interferometer; and
a signal processor electrically coupled to said photodetector and to said piezoelectric transducers,
wherein said optical circulator is configured to pass a first portion of light emitted by said laser device to said telescope, a second portion of light emitted by said laser device to said scanning Fabry-Perot interferometer, and light from said telescope to said scanning Fabry-Perot interferometer, and
wherein said signal processor is configured to perform the following operations:
(a) supplying scanning voltages to said piezoelectric transducers which have the effect of adjusting a wavelength of said scanning Fabry-Perot interferometer so that the wavelength varies within a range of wavelengths that includes a wavelengths of light emitted by said laser device and a wavelength of light which was first emitted by said laser device and then Doppler shifted by a target;
(b) monitoring a detection voltage produced by said photodetector as a function of the scanning voltage supplied to said piezoelectric transducers;
(c) identifying scanning voltages where peak detection voltages occur;
(d) comparing differences in scanning voltages where the peaks occur to reference voltages in a calibration table to determine the relative speed of the target as well as the resonant vibration frequency of the target; and
(e) identifying a target based on the resonant vibration frequency determined in operation (d).
11. A method comprising:
directing laser light into an optical circulator;
sending respective portions of said laser light to a telescope and to an interferometer;
directing the laser light received from the optical circulator by the telescope toward a target;
recapturing laser light reflected from the target using the telescope;
sending recaptured laser light from the telescope to the interferometer via the optical circulator;
supplying scanning voltages to piezoelectric transducers of the interferometer;
photodetecting recaptured laser light that passes through the interferometer;
monitoring detection voltage as a function of the scanning voltages;
identifying scanning voltages where peak detection voltages occur;
comparing differences in scanning voltages where peaks occur to data in a calibration table to determine a resonant vibration frequency of the target; and
identifying the target based on the determined resonant vibration frequency.
12. The method as recited in claim 11 , further comprising:
comparing differences in scanning voltages where peaks occur to data in a calibration table to determine a relative speed of the target.