Ultrasonic Non-Destructive Testing (NDT) uses many types of sources mostly based on piezoelectric transducers. This paper focuses on the generation and detection of acoustic waves by Laser Ultrasonics which is a wellknown non-contact inspection method. More precisely, the main goal of this work is to analyze the interaction between a flaw and Rayleigh waves simultaneously excited by a grating of thermoelastic line sources. Experimentally, a pulsed Nd:YAG laser, combined with different optical devices is used for ultrasound excitation in an aluminum sample with a surface defect. Moreover, the normal displacement of Rayleigh waves is detected by a Mach-Zehnder type interferometer. The results obtained using different configurations are compared to determine the advantages of each of them for NDT. A finite element model also allows to confirm the interpretation of the complex phenomena studied.