Initial Second Harmonic Generation in Narrowband Surface Waves by Multi-Line Laser Beams for Two Kinds of Spatial Energy Profile Models

  Acoustic nonlinearity of surface waves is an effective method to evaluate the micro damage on the surface of materials. In this method, the A₁ (magnitude of the fundamental wave) and A₂ (magnitude of the second-order harmonic wave) are measured for evaluation of acoustic nonlinearity. However, if there is another source of second-order harmonic wave other than the material itself, the linear relationship between A₁² and A₂ will not be guaranteed. Therefore, the second-order harmonic generation by another source should be fully suppressed. In this paper, we investigated the initial second-order harmonic generation in narrowband surface waves by multi-line laser beams. The spatial profile of laser beam was considered in the cases of Gaussian and square-like. The temporal profile was assumed to be Gaussian. In case of Gaussian spatial profile, the generation of the initial second-order harmonic wave was inevitable. However, when the spatial profile was square-like, the generation of the initial second-order harmonic wave was able to be fully suppressed at specific duty ratio. These results mean that the multi-line laser beams of square-like profile with a proper duty ratio are useful to evaluate the acoustic nonlinearity of the generated surface waves.

Abstract
1. Introduction
2. Generation of Narrowband Laser-Generated Surface Waves
3. Numerical Analysis of Spectrum H(f)
4. Simulation Results and Discussion
5. Conclusions
References