Remote vibration measurement of rough surfaces by laser interferometry

A He-Ne laser interferometer employing a focusing telescope is used remotely to measure transient longitudinal displacements of rough surfaces by monitoring the resulting Doppler shift of the backscattered light. The feasibility and accuracy of this technique are considered theoretically relative to laser output, range, telescope aperture, sensor efficiency, bandwidth, noise, surface characteristics, and coherence. The technique was successfully demonstrated on various test surfaces at distances up to 100 m using an unstabilized 4-mW laser, a 15-cm reflector telescope, and a PIN photodiode.