Frequency-adjusted Fresnel lens design for a broadband transducer with varying thickness

Fresnel lens design has been used for annular arrays. In this case, the surface area of each concentric ring is the same. By doing so, the phase shift between adjacent transducer elements remains constant when focusing at depth. Recently, a 40 MHz transducer has been proposed with increased bandwidth by varying the thickness of the piezoelectric material from center of the transducer to the edge. Although the equal area design can maintain uniform spectral weighting for such a transducer, this results in non-uniform phase shift between adjacent transducer elements. To maintain uniform phase shift, the surface area of each transducer element needs to be adjusted based on its center frequency. It is the hypothesis of this study that the frequency adjusted Fresnel lens design can provide larger depth of field (DOF) than conventional Fresnel lens design. A finite-element analysis software was used. The model consisted of six transducer elements, electrodes, two matching layers, a backing layer, and the water interface. Properties of the 36deg-rotated, Y-cut LiNbO3 were used in the model. The mean thickness of the piezoelectric material was set at 91.75 mum, corresponding to an operating frequency at 40 MHz. Given that the height difference between adjacent subelements was 10 mum, the thicknesses of the center and outermost subelements were 66.75 mum and 116.75 mum, respectively. It is found that the frequency adjusted Fresnel lens design can providing the best overall performance (bandwidth, DOF and lateral resolution).