FOCUSING OF ULTRASONIC-WAVES IN AIR USING A MICROMACHINED FRESNEL ZONE-PLATE

An ultrasonic Fresnel zone-plate has been micromachined from a thin st ainless steel disk for the purpose of focussing high-frequency ultraso nic waves in air. This zone-plate, which was designed with a focal len gth of 7.5 mm, consisted of five concentric stainless steel rings that were separated from one another by narrow air-channels. The zone plat e was fitted to a micromachined air-coupled capacitance transducer sou rce, 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 z one-plate is operating as might be expected, but that improvements mig ht be obtained by further research. (C) 1997 Elsevier Science B.V.