Chromatic aberration of an air-coupled ultrasonic Fresnel zone-plate

A micromachined Fresnel zone-plate has been used to focus ultrasonic waves in air over a range of frequencies (450 to 900 kHz). The zone-plate was mounted upon a planar micromachined air-coupled capacitance transducer, which was capable of generating toneburst ultrasonic waves in air over a wide frequency bandwidth (<100 kHz to 2 MHz). A second air-coupled capacitance detector (apertured to 200 microm) was scanned in the field of the zone-plate source in order to image the generated ultrasonic field at various frequencies of operation. It was found that the ~680 microm spot size of the experimental zone-plate did not vary appreciably with changing frequency, whereas the focal length increased markedly with increasing frequency (from ~5 mm at 450 kHz up to ~15 mm at 900 kHz). These findings are shown to be in excellent agreement with previously reported theoretical predictions by the authors.