Optical characterization of surface-displacement fields of high-frequency transducers

This paper describes a non-contact optical technique to characterize surface-displacement fields of high-frequency transducers. We have developed an ultra-broadband (1 GHz), fine-resolution (0.5 microns), optical interferometer to map the displacement and pressure fields of high-frequency transducers beyond the limits of conventional hydrophones and laser vibrometers. Several transducers, including a 40-MHz annular array and a 100-MHz, zinc oxide (ZnO), focused transducer were characterized. The transducers were excited with an impulse, and their surface vibrations were detected using a Michelson interferometer. The surface of the transducer was raster scanned to map the 2D displacement amplitude and phase distributions. Transducer surface response was measured in air and compared to reference pulse-echo measurement on a glass slide in water. A high numerical aperture (NA = 0.55) optical lens was used to detect the vibrations from a 0.5-micron spot at each scan location in 100-μm raster-scan steps. The optically measured 6-dB bandwidth response and central frequency were in agreement with the pulse-echo measurements. We observed a minimum surface-displacement detection sensitivity of 100 fm/Hz^1/2.