Measurement of the complex amplitude of transient surface acoustic waves using double-pulsed TV holography and a two-stage spatial Fourier transform method

Abstract

We present a technique to measure the mechanical complex amplitude, i.e. the mechanical amplitude and phase of vibration, of an ultrasonic plane wavefield of nanometric amplitude that propagates on a surface. Our aim is to detect perturbations of the initially smooth wavefronts that indicate the presence of flaws in the material. We use bursts of surface acoustic waves (SAWs) and a double-pulsed TV holography system that records two correlograms with time separations down to 1.5 µs. The phases of the correlograms are calculated separately using the spatial Fourier transform method (SFTM) and operated on to obtain the phase change between exposures. In the resultant optical phase map, the field of instantaneous displacements of the surface (that comprises several periods of the SAW) acts as a modulated spatial carrier, now related to the mechanical phase and amplitude, that are extracted by applying the SFTM again.

Video Ultrasonics by Pulsed TV Holography: A New Capability for Non-Destructive Testing of Shell Structures

“…At least one hologram is recorded for each mechanical state to be compared. However, there is no optical reconstruction of the recorded holograms; instead their intensity distribution is electronically processed to render the optical phase-difference map at the specimen surface, losing the three-dimensional character of HI and introducing extra noise due to the necessity to resolve the speckle in the recorded intensity distributions (Mast and Gordon 2001;Trillo et al 2003a;Trillo et al 2003b). The main advantages of TVH over HI are easier use, real-time display of results, and lower light power needed, being possible to maintain a refreshing cadence (limited by the camera and associated electronics) of tens of Hz with megapixel resolution and, not less important for its spreading in industrial applications, lower cost of the equipment.…”

“…However, to the best of our knowledge, only our group has reported the detection of SAW by pulsed TVH (Cernadas et al 2002;Trillo et al 2003a;Cernadas et al 2006). Moreover, we developed a specific phase evaluation technique that renders quantitative data of transient SAW, namely the instantaneous out-of-plane acoustic displacement field and the out-of-plane acoustic displacement amplitude and phase fields, with a minimum measurable displacement of the order of one nanometer (Trillo et al 2003b). …”

“…As stated in the introduction, it has been developed by our research group over the last decade, with successive improvements in the optical and electronic hardware and, especially, in the information processing scheme. Some parts of the current system have been incorporated from former prototypes already described elsewhere (Trillo et al 2003b;Cernadas et al 2002), for which the details will not be repeated here.…”

“…Spatial evaluation. This variant (Trillo et al 2003b) is based on the spatial periodicity of the signal. For its practical realization, we have used a singlemode incident plane wavetrain in the form of a tone-burst of smooth envelope.…”

“…An estimation of the main contributions to the system noise was made in (Cernadas et al 2002). Also, information processing procedures like the employment of differential phase evaluation algorithms (Trillo et al 2003b) and the numerical cancellation of the phase mismatch between the laser cavities, made by a technique similar to that reported in , contribute to the quality of the measurements.…”