High-efficiency Fresnel acoustic lenses

Acoustic Fresnel lenses have emerged in recent years as an alternative to the conventional spherical lenses for focusing sound waves in applications such as acoustic microscopy. Fresnel lenses offer the advantage of near-planar geometry and, therefore, ease of fabrication compared to spherical lenses. The Fresnel acoustic lenses reported so far, however, have the disadvantage of low efficiency; only about 40% of the input signal is directed towards the focus. The design and fabrication of “binary” acoustic Fresnel lenses that offer much higher efficiencies are described. These lenses, while being still nearly planar, have multiple phase levels to achieve phase shifts other than 0 and 180 degrees as used in conventional, two phase Fresnel lenses. Acoustic Fresnel lenses were fabricated at frequencies of about 1 MHz and 170 MHz. Measurements of the focusing efficiency and point spread function have been performed to characterize the operation of these lenses. Focusing efficiencies in excess of 80% have been achieved with these lenses. The measurements compare well to theoretical simulations