Holographically-Induced SAWs at a Solid- or Liquid-Solid Interface

Abstract

Lasers offer a non-contact method for generating ultrasound. The main advantages are that specimen can be scanned more easily and the complications of a coupling layer or liquid are eliminated. Also, more accurate velocity measurements can be obtained using lasers and for holographic generation, higher frequency coherent waves can be obtained. Lasers have been used to generate ultrasonic waves in many configurations, including the generation of bulk longitudinal and shear waves, Rayleigh waves along solid surfaces, and Lamb modes in solid layers. General reviews of laser generation techniques can be found in [1] and [2].Excitation of surface waves along a solid boundary by transient laser heating was first demonstrated by Lee and White [3]. In the simplest configuration, a point source of light is directed at a solid boundary. Line [4] and ring [5] sources have been used to modify the directivity pattern of the resulting surface acoustic waves (SAWs) and increase the acoustic energy at the location of interest. To generate surface waves of predominately one frequency (or wavelength), masks [6] have been used to produce a periodic heating pattern. Alternatively, coherent waves can be generated by the harmonic interference pattern of two crossed laser pulses [7][8].The analysis presented here models the holographic wave generation by them1al surface heating due to a pulsed laser interference pattem. The exact elastic wave solution accounting for the finite depth of heating in the substrate will be presented here, rather than using an approximate boundary condition to model the applied field as is typically found in the literature. The resulting SAW wavelength A will be equal to the interference pattem spacingwhere .\0 is the wavelength of the laser light and (} is the angle between the two incident laser pulses (Fig. 1). The SAW amplitude can be determined by diffraction of a third time-delayed laser pulse of lower intensity. In this situation, only the standing wave pattern within the area illuminated by the generation laser is of interest. From measurements of diffraction efficiency as a function of the time delay of the detection pulse, the SAW amplitude, wavespeed, and attenuation can be determined. The main advantage of this laser interference technique is that coherent, high-frequency (up to -lOOGHz) waves can be generated. In addition, the SAW wave frequency is continuously tunable by adjusting the angle (} between the two generation laser pulses.

Characterization of Moment of Inertia Variations by Holographic Interferometry

“…Holographic interferometry (HI) is a powerful tool for mapping of surface defects. In conjunction with various stressing techniques [1][2][3][4], it offers an NDT tool for the detection of flaws within the volume of materials. The method is advantageous for integrity characterization of components to serve under mechanical stress, where the detailed shape, size and depth of the flaw within the material are of no interest.…”