Development of a femtosecond laser-based ultrasonic inspection technique for moving object and its application

Abstract

In this study, a femtosecond laser-based ultrasonic inspection technique for a moving object is developed in a pump-probe setup. A pump laser pulse generates ultrafast ultrasound up to THz in the moving object, and a probe laser pulse reflected by the deformed object is used for detecting the pump induced ultrafast ultrasound. The detection is based on reflectometry with an iris partially blocking the reflected probe beam. The developed technique owns advantages that the femtosecond laser enables an ultra-short contact time with the moving object and minimizes distortion of the measured ultrasound. This research has validated that almost identical ultrasound can be acquired from the moving object compared to a stationary condition in both the pulse-echo and pitch-catch modes. The feasibility of the developed technique for nondestructive evaluation of the moving object has been demonstrated by thickness estimation of a sub-micron layer deposited on a silicon wafer.