Laser generated surface acoustic wave using diffractive optical elements

Laser-induced generation of surface acoustic waves (SAW) has emerged as a highly promising method for ultrasonic testing due to its non-contact nature and elimination of the need for physical coupling, making it suitable for a wide range of applications. However, a major limitation in current research is frequency dispersion, which complicates signal interpretation and reduces the accuracy of material characterization. This article explores the use of a beam splitter-based diffractive optical element (DOE) to optimize SAW generation and address this issue. A single DOE was used to generate a multipoint laser source array, and a series of lenses were utilized to reshape these into point and line sources of different dimensions. The comb-like design was based on the SAW wavelength. The influence of the number of sources and the types of sources, i.e., line vs. point, on the SAW generation was investigated. Additionally, we analyze how different line-source lengths influence SAW generation. Theoretical models were first developed to understand the use of DOE and the effect of source width and frequency on SAW propagation. The optical models were further used to design the experiments and study the influence of various parameters, including laser source energy. Compared to the point source, the line sources were observed to result in a significantly higher temporal and spectral response with a narrower bandwidth of the SAW wave. The results suggest that the comb-like arrangement increases the generation efficiency of the SAW wave.