Frequency spectrum analysis of laser generated ultrasonic waves in ablative regime

In this paper, laser ultrasonic signals generated in ablative regime are measured in a number of metal samples (2024 Al, 6061 Al, 7075 Al, mild steel, and copper) with a broadband laser interferometer. The frequency spectra are analyzed and compared for different thicknesses (50.8mm, 25.4mm, 12.7mm, and 6.4mm), and for different power densities. Harming windowing is applied before frequency analysis is performed. The experimental data match the theoretical predictions very well. The results show that the frequency spectrum extends from 0 to 15MHz, while the center frequency occurs near 2MHz. The detailed distribution of the spectrum is dependent on the material, thickness, and laser power density.