The effect of laser characteristics in the generation and propagation of laser generated guided waves in layered-skin model

This paper concerns the use of surface acoustic waves for accurate characterization of human skin, it presents a Finite Element (FE) study of the laser generated ultrasonic waves in a 3-layered models of human skin to study the effects of laser characteristics has on the properties of the generated Surface Acoustic Waves (SAWs). Using the commercially available FE code ANSYS, the effects of laser beam width, pulse rise time and laser wavelength has been taken into consideration on the generated guided waves in the skin models. The simulation is a sequential coupled field analysis, with the heating of the multilayered skin model due to a short laser pulse is simulated by a dynamic thermal analysis with the laser pulse represented as a volumetric heat generation. The results of the thermal analysis are then subsequently applied as a load in the mechanical analysis where the out-of-plane displacement histories and stress fields are analyzed. The two analyses can be assumed to be uncoupled in this work as in the timescale of interest the elastic effects do not feed back into the thermal problems. In order to keep the generation of ultrasonic waves in the thermoelastic regime the energy incident on the tissue must be kept below the threshold at which irreversible changes occur, yet sufficient to produce thermoelastic waves that can be readily detected. Results shows that the laser generated SAW in skin models is dominated by the optical penetration depth which is determined by the properties of the material and the laser wavelength used.