Heterodyne interferometry and holography for vibration measurements

Abstract

Two frequency or heterodyne interferometry transforms the optical interference phase into the phase of an electronically detectable beat‐frequency signal. This allows to use powerful signal processing for increased sensitivity far below the optical wavelength λ and high temporal resolution, especially for vibration analysis. The principle and the properties of this heterodyne technique will be illustrated by presenting laser interferometry for displacement and velocity measurements. With an extended version [R. Dändliker and J.‐F. Willemin, Opt. Lett. 6, 165–167 (1981)] amplitude and phase of in‐plane and out‐of‐plane microvibrations (amplitudes 1 nm, spatial resolution 30 μm) at frequencies from 5 kHz to 5 MHz can be analyzed in real time. Experimental results for piezoelectric ultrasound transducers will be presented. Two‐reference‐beam holographic interferometry allows to use the heterodyne technique for accurate fringe evaluation and interpolation (λ/1000), since the two object fields are stored and accessible independently by the corresponding reference beam [R. Dändliker, Progress in Optics (North‐Holland, Amsterdam, 1980), Vol. 17, pp. 1–84.]. For vibration analysis, however, the object has to be recorded holographically by two or more coherent light pulses, synchronized to the object movement, using pulsed lasers or stroboscopic light modulation. Experimental results of deformation measurements by heterodyne holographic interferometry will be presented. The strength and weakness of this technique applied to vibration analysis will be discussed in particular.