Acoustic emission analyzer

Abstract

A periodic structure such as a diffraction grating is disposed on the surface of an object from which acoustic emissions are to be detected. A plane monochromatic light beam is incident on the periodic structure and forms a number of Bragg diffraction orders at predetermined angles. A detector positioned to intercept one of the diffraction orders receives light whose frequency has been Doppler shifted by surface motion on the object. A reference beam, also incident on the detector heterodynes with the Doppler-shifted beam so that the Doppler component can be recovered. In one embodiment, the reference beam is also incident on the surface and itself produces Bragg diffraction orders. The geometry produces up Doppler in the Bragg diffraction orders from one beam and down Doppler from the Bragg diffraction orders from the other beam and selected Bragg diffraction orders from both beams are heterodyned in the detector.