US 5,477,324 AGranted

Method and apparatus for detecting surface wave vector dynamics using three beams of coherent light

AssigneeGeorgia Tech Research Corporation

InventorsYves H. Berthelot, Jacek Jarzynski, Ming Yang

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Abstract

A method of detecting and measuring separately the components of vector dynamics on the surface of a vibrating object comprises directing three beams of coherent light onto a common spot on the surface. Each of these beams has a known optical frequency. Two are arranged symmetrically about a normal to the surface at the point of incidence and the third is arranged asymmetrically about the normal with respect to both the first and second beams. Superposition of each beam with the others on the surface produces scattered light embodying characteristic beat or carrier signals equal to the difference in frequencies between the beams. Vector dynamics including velocity and displacement of the vibrating surface induces Doppler shifts in the beams, which, in turn results in a modulation of the carrier frequencies resulting from the superposition of the beams. Each carrier frequency is then isolated and demodulated to produce a signal corresponding to various components of the vector dynamics on the surface that cause the modulation. Further analysis extracts information about the frequency and magnitude of resolved vector dynamics on the surface. The method can be applied at several locations on the object's surface to determine vector dynamics at several points simultaneously.

A method of detecting and specifying resolved vector dynamics on the surface of a vibrating object, said method comprising the steps of:

(a) directing a first beam of coherent light having an optical frequency f1 onto the surface at a point P along a path that forms a first predetermined angle with respect to a normal to the surface at the point P;
(b) directing a second beam of coherent light having an optical frequency f2 onto the surface at the point P along a path that forms a second predetermined angle with respect to the normal to the surface at the point P, said second predetermined angle being substantially equal to said first predetermined angle so that said first and said second beams of coherent light traverse paths that are symmetrically oriented about the normal to the surface at point P;
(c) directing a third beam of coherent light having an optical frequency f3 onto the surface at the point P along a path that forms a third predetermined angle with respect the normal to the surface at point P, said third predetermined angle being substantially different from said first and said second predetermined angles so that said third beam of coherent light traverses a path that is asymmetrically oriented about the normal with respect to said first beam and with respect to said second beam;
(d) said light beams combining on the surface at point P with the resulting light scattered from the surface being a superposition of said first, second, and third light beams and exhibiting carrier signals due to interference at frequencies approximately equal to the difference between the optical frequency of each beam and the optical frequency of each of the other beams, said carrier signals being frequency modulated by vibrational motion of the surface of the object;
(e) detecting the superimposed scattered light from point P on the surface;
(f) analyzing the detected light at the frequencies of at least one of the carrier signals to extract modulation characteristics therefrom;
and
(g) converting the extracted modulation characteristics to vibrational vector dynamics of the surface that induced the modulation of the analyzed carrier signals.

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Abstract

Claims17