Broadband optical detection of transient motion from a scattering surface.

摘要

The invention is concerned with a method and apparatus for optically detecting transient motion from a scattering surface (14). A laser beam (10) having a predetermined frequency is directed onto such a surface (14) to thereby scatter the laser beam and produce a scattered laser beam (16) defining an optical wavefront and having an optical spectrum with a central peak at the laser frequency (FL) and a sideband on either side of the central peak. The laser beam (16) scattered by the surface (14) is caused to interfere with a reference beam (22) derived from the scattered laser beam (16) and having an optical wavefront substantially matching the wavefront of the scattered beam and an optical spectrum with a single peak at the laser frequency (FL) and no sidebands, to obtain an optical signal which is detected and converted into an electrical signal representative of the transient motion. The optical detection technique according to the invention has a broad frequency bandwidth and a large étendue providing greater sensitivity of detection, and is particularly useful for detecting small surface deformations of a material or workpiece subjected to ultrasonic energy.