Influence of laser beam profile on the generation of ultrasonic waves

The different ultrasonic fields generated in metallic materials by a laser beam with flat and Gaussian profile are investigated experimentally and using the finite element method (FEM). A high power laser beam irradiating a solid surface produces elastic waves with a mechanics that depends on many parameters, including the profile of the laser beam. The influence of the beam profile is investigated with the FEM analysis, considering the temperature dependence of material properties. Numerical and experimental results indicate that increasing the size of the source the maximum in the angular directivity of the longitudinal wave moves from around 65? toward smaller angles of detection. Numerical results showed that the regression of the maximum strongly depends on the non-uniform distribution of energy in the laser beam and can be disregarded for areas with a diameter smaller than 3 mm. Comparison of temperature distribution between a Gaussian and a uniform energy density shows how the decrease of temperature, from center source to outer, acts like a hoop-wrapping effect of the outer annulus on the inner annulus. Numerical analysis allowed a better understanding of the mechanism involved in the laser-generation and propagation of elastic waves, providing a theoretical support to the hypothesis made through the experimental data. ? 2011 Springer-Verlag.