Chromatic aberration of an air-coupled ultrasonic Fresnelzone-plate

摘要

A micromachined Fresnel zone-plate has been used to focusnultrasonic waves in air over a range of frequencies (450 to 900 kHz).nThe zone-plate was mounted upon a planar micromachined air-coupledncapacitance transducer, which was capable of generating toneburstnultrasonic waves in air over a wide frequency bandwidth (<100 kHz ton2 MHz). A second air-coupled capacitance detector (apertured to 200nΜm) was scanned in the field of the zone-plate source in order tonimage the generated ultrasonic field at various frequencies ofnoperation. It was found that the ~680 Μm spot size of thenexperimental zone-plate did not vary appreciably with changingnfrequency, whereas the focal length increased markedly with increasingnfrequency (from ~5 mm at 450 kHz up to ~15 mm at 900 kHz). Thesenfindings are shown to be in excellent agreement with previously reportedntheoretical predictions by the authors