Application of All-Optical Laser Ultrasonics for Characterization of Sub-Mm Layers in Multilayer Structure

The last decades have seen the rapid development of laser ultrasonic (LU) techniques for remote inspection for composite structures. However, it is still challenging for current LU approaches to characterize the sub-mm layers in a multilayer structure. This study aims to apply the all-optical (AO) LU technique for the characterization of the sub-mm layers in a multilayer structure. A specific AOLU system is developed to acquire the transmitted longitudinal waves from the metallic composites. Following the acquisition of the transmitted signals, the spectrum analysis is utilized to overcome the ultrasonic signal aliasing in the time domain. The least-squares method is subsequently adopted to estimate the thicknesses of the layers simultaneously. This study has carried out a successful application of the AOLU technique for spectrum analysis of the sub-mm multilayer structure with the advantages of non-contact remote detection, reliable single-mode acquisition, and overcoming waveform aliasing.