FOCUSING FEATUKES OF SAW EXCITED BY A CURVED IDT ON METALLIZED SUEFACE OF YZ-LiNbO₃ SUBSTEATE

In this letter, the foaused surface acoustic wave field which is excited by an IDT fabricated in the shape of a circular arc and propagating on the metallized surface rather than on the free surface of the TZ-LiNbO₃ is visualized hy means of the Schlieren method. Prom the photographs of the wavefronts taken by using a pulse laser as the light source and of the intensity distribution taken by using He-Ne laser, the total focusing procedure, the wavefronts and the intensity distribution in the focal region, and the rapid change of the phase near the first pair of the minima are demonstrated. These informations are important for the investigators who devote to study the focusing phenomena.
Two parameters, i.e., the focal length and the focus width are measured. It is shown that the foical length formed on the metallized surface is smaller than that formed on. free surface by a factor of 0.7, and the focus width is the same. It means that the focusing performance on the metallized surface is more efficient. In addition, the feature of being a plane wave in the focal region （more than 30 wavelengths） is quite good, and it is important for the SAW elastic convolvers.