Characterization of ultrasonic transducers by means of double-exposure holographic interferometry

Abstract

Double-exposure holographic interferometry is applied to measure the surface displacements of ultrasonic transducers. Interferograms revealing detail surface displacements are produced using iron-doped lithium niobate crystals as the recording medium. Lead zirconate titanate disks poled in the longitudinal thickness mode possessing deposited, shaped electrodes are examined. Transducers with circular, strip, rectangular, and rosette electrode shapes are investigated. The displacement measurements reveal that the electrode “drags” the rest of the transducer surface. Thus the unmetallized portion of the transducer also contributes to the acoustic field. Pressure field calculations using the experimentally measured displacements are performed. It is shown that proper geometrical design of the electrode yields reduction of the sidelobes in the far field and smoothing of the maxima and minima in the near field, and improved focusing via reduced acoustic beam divergence. Most importantly, this work reveals that the unmetallized portion of the transducer contributes significantly to the displacement and to the resulting pressure fields.