Applying optics in surface wave laser ultrasonic microscopy

Abstract

Laser ultrasonics has many advantages compared to contacting ultrasonics, especially when both optical generation and detection are used. In addition to the obvious advantages of remote inspection there are several other benefits. In this paper we concentrate on the flexibility that can be conferred by employing both optical technology and optical concepts in our laser ultrasonic systems. The papers shows how spatial light modulators (SLMs) can be used to control the optical beam profile used to generate the ultrasonic waves. Initially, SLMs were introduced to improve the signal-to-noise ratio of the system. This is achieved by exciting the tone burst with a grating structure so that large signals can be generated without ablation. The SLMs can also be used to produce optical profiles that generate self-focusing surface waves; this also increases the ultrasonic amplitude. Our recent work has used SLM generation to overcome aberrations or distortions in the ultrasonic field during propagation. To this end an adaptive system using integrated electronics to overcome the phase aberrations during propagation has been developed. We also show how the SLM can be used to perform acoustic wavelength spectroscopy, which has proved invaluable in the characterization of texture in industrially important materials.