An increasing number of ultrasound diagnostic systems employ harmonic imaging, and center frequencies as high as 15 MHz are now commonly used in clinical practice. However, currently available hydrophone probes are not fully adequate to characterize the acoustic output of such nonlinear systems. This paper reviews the current status of piezoelectric polymer hydrophone probes and discusses their limitations in ultrasound field measurements (such as limited knowledge of the frequency responses beyond 20 MHz and physical dimensions leading to spatial averaging error). Against this background, a down-tapered fiber optic probe is presented as an alternative tool for probing ultrasound fields with ample bandwidth and without finite aperture effects. The results of the preliminary experiments indicate that once fully developed, the optic probe with a tip diameter of about 7 micrometers will provide a desirable and adequate device for ultrasound field measurements up to 100 MHz. The fiber optic probe could also meet the challenges posed by high intensity focused ultrasound (HIFU) and other therapeutic applications.
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