Focussing of ultrasonic waves in air using a micromachined Fresnel zone-plate

An ultrasonic Fresnel zone-plate has been micromachined from a thin stainless steel disk for the purpose of focussing high-frequency ultrasonic waves in air. This zone-plate, which was designed with a focal length of 7.5 mm, consisted of five concentric stainless steel rings that were separated from one another by narrow air-channels. The zone plate was fitted to a micromachined air-coupled capacitance transducer source, and the radiation profile measured by a second 1 mm diameter air-coupled capacitance receiver. It is shown that the resultant spot size (for an operating frequency of 580 kHz) was 0.8 mm, with a 3 mm depth-of-held. Comparison to an existing theoretical model shows that the zone-plate is operating as might be expected, but that improvements might be obtained by further research. (C) 1997 Elsevier Science B.V.