Fast adaptive multi-frequency all-optical scanning acoustic microscope

A noncontact and completely damagefree instrument capable of rapid high resolution vector contrast imaging using surface acoustic waves is described. The laser source of the ultrasound generates around 30 pulses at 82MHz and harmonics thereof, and it is possible to image at several discrete frequencies - 82MHz, 164MHz, 246MHz and above -simultaneously. Adaption of the spatial distribution of the excitation source to accommodate a wide range of materials and circumstances is trivial, since a spatial light modulator is used to produce an arbitrary distribution of incident optical energy. This is used to good effect in anisotropic materials with random grain structures, since the material microstructure perturbs the surface waves as they propagate on the material surface. The wavefront aberrations are detected by an acoustic wavefront sensor, and the excitation profile is adjusted to correct for the aberrations. This increases the accuracy and reliability of the measurements, and can increase the usable frequency range or propagation distance for a given material. The adaptable source enables the instrument to be used for purposes other than linear surface wave detection and c-scan imaging; for instance, the SAW phase velocity of anisotropic crystals may be determined by wavenumber spectroscopy achieved by adapting the source k-vector and finding the peak in excitation. Also, there is considerable interest in nonlinear and harmonic detection and imaging, and the adaptable source may be used to suppress the harmonics naturally present in the excitation source. This is important for harmonic imaging, where it is necessary to ensure that the relatively weak harmonics generated by the nonlinear material properties are not swamped by those present in the source.