Remote excitation and sensing of mechanical resonances in structures using laser diodes and an optical fibre interferometer

The non-contact excitation of resonant flexural oscillations in structures of significant size, using repetitive pulses from a low-power diode laser, is reported. These vibrations are produced by photothermally induced bending moments which are of sufficient amplitude to produce deflections which are readily measured using a simple optical fibre interferometer. The interferometer uses the reflected signal from the natural surface of the structure and requires only that the surface be aligned approximately perpendicular to the interferometer beam. Applications of this technique for non-contact, non-destructive testing for changes in properties such as size, shape and structural integrity are discussed.