ZnO Planar Focusing Transducer on Using Fresnel Array

Abstract

Fresnel Array can produce focusing of acoustic beam. In this paper we will discuss high frequency Fresnel Arrays which have specially designed electrode structure with ZnO piezoelectric film. The sputtered C-axis oriented piezoelectric film and special electrode structure make longitudinally vibrated transducer arrays. These transducer arrays can generate odd order convergent and divergent beams. But the zero order or even order ones are eliminated. In order to make full use of first order convergent beam we can restrain higher odd order convergent beams by increasing the element width. An important characteristic of Fresnel Array is that its focallength can change linearly with frequency. And hence its focus can quickly scan linearly along the axis. Two kinds of structures, straight and circular strip arrays, are developed to produce line and point focuses, respectively.

We have developed these two planar transducers with center frequency of 400 MHz and bandwidth of about 200 MHz. For the fused quartz their foci can scan along the depth in the range of 8 mm. Their focusing properties were optically observed. And some prepared experiments were also done for NDT and acoustic imaging.