A circularly focused and scanned acoustic scanner

This paper describes a new type of electronically scanned and focused acoustic imaging device called the cylindrical grating acoustic scanner. With this device, a new transducer called the circular-edge-bonded transducer (CEBT) is devised that is bonded on one end of a cylindrical substrate and can generate a surface acoustic wave propagating along the cylindrical surface of the substrate. An array of grooves is fabricated on the cylindrical surface of the substrate to serve as a scattering grating. The periodical grooves scatter a chirped surface-wave pulse coherently into a focused bulk-wave "ring" that scans at the surface-wave velocity. The focal length and resolution can be adjusted by changing the chirp rate and time-bandwidth product of the chirp, respectively. A 2.3-MHz circular scanner with 25-cm field of view and 2.6-mm resolution has been constructed and studied. Acoustic image of an artificial defect in an aluminum pipe is obtained.