Phase variation of focused surface acoustic wave

Converging and diverging surface acoustic waves (SAW) have been generated and detected using a noncontacting method. A yttrium aluminum garnet laser together with an axicon lens produces a focused optical ring which excites both converging and diverging SAW on the test sample. An optical interferometer is used to measure the vertical displacement of the SAW. Variations of the SAW pulse waveforms in different positions along the radial direction of the ring are given. It is found that about 10 SAW wavelengths away from the focus, a 90° phase difference exists between the Fourier components of the SAW going toward the focus and of that having passed through and moving away.