Study of Lamb Wave Dispersion Characterization Using Multiplexed Two-Wave Mixing Interferometer

A multiplexed two-wave mixing interferometer (MTWM) system has been developed, which is able to perform optical detection of ultrasonic motion for multiple points in an array simultaneously. In this MTWM system, optical phase gratings are used to create an array of detection laser beams that are directed to the specimen. The detection array can be arranged in several ways on the test object as needed. The scattered optical beams from the detection laser array are collected and mixed with a single reference beam in a photorefractive crystal to form a multiplexed two-wave mixing. Each of the output beams from the photorefractive crystal is imaged on to one of the elements in the array of photodetectors. The MTWM system is capable of providing simultaneous optical detection at several points on a test object, with high spatial resolution and sub-nanometer displacement sensitivity. The applications of this MTWM system include the crack imaging detection and quantification, material characterization analysis, and other nondestructive detection. In this paper, how to realize fast acquirement of Lamb wave dispersion curves using the MTWM system is presented. The dispersive time-position domain Lamb wave signals at multiple source-to-receiver distances are obtained. Following the algorithms of Alleyne and Cawley, these time-position domain signals are transformed to the frequency-wavenumber domain using two-dimentional fast Fourier transformation technique. This application shows the MTWM system has the ability to rapidly characterize the dispersion characteristics of Lamb waves.