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The high acoustic intensities generated by HIFU systems cause conventional hydrophones to fail before measurements can be reliably made. To address this challenge, we present a new piezoelectric needle hydrophone, which is resistant to cavitation while possessing a flat frequency response (+/-3 dB from 1 to 10 MHz) and a small effective aperture (400 micron effective diameter). This hydrophone has been used in a high intensity field (1.5 MHz tone burst of 30 microseconds and 3% duty cycle, with rarefactional pressures exceeding 4 MPa and positive pressures exceeding 15 MPa) without degradation in the hydrophone's performance, as indicated in before-and after calibration checks of the device.

The goal of this work was to measure directivity of a reflectance-based fiber-optic hydrophone at multiple frequencies and to compare it to four theoretical models: Rigid Baffle (RB), Rigid Piston (RP), Unbaffled (UB), and Soft Baffle (SB). The fiber had a nominal 105 μm diameter core and a 125 μm overall diameter (core + cladding). Directivity measurements were performed at 2.25, 3.5, 5, 7.5, 10, and 15 MHz from ±90° in two orthogonal planes. Effective hydrophone sensitive element radius was estimated by least-squares fitting the four models to directivity measurements using the sensitive element radius as an adjustable parameter. Over the range from 2.25 to 15 MHz, the average magnitudes of differences between the effective and nominal sensitive element radii were 59 ± 49% (RB), 10 ± 5% (RP), 46 ± 38% (UB), and 71 ± 19% (SB). Therefore, directivity of a reflectance-based fiber-optic hydrophone may be best estimated by the RP model.

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