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The improved heterodyne system is presented to measure the frequency response of photodetectors (PDs) with bandwidth about 100 MHz utilized in ultrasonic applications. In order to eliminate the periodic oscillation following the frequency tuning of the acousto-optic frequency shifters (AOFSs) in the measurement results, the AOFSs’ double-passed scheme in Michelson interferometer is used. Compared with the AOFSs’ single-passed configuration in Mach-Zehnder interferometer, the double-passed scheme can avoid additional optical alignment during the process of the frequency tuning and then reduce its induced measurement uncertainty. With two double-passed AOFSs connected in parallel, the experimental setup is designed to demonstrate the feasibility of the improved heterodyne system in the frequency range from 500 kHz to 135 MHz. Experimental results with measurement uncertainty are provided and discussed.

This paper is intended to estimate the spatial averaging effects of the hydrophone in field characterization of focused transducers. The analytical model derived was based on Fresnel approximation and enabled modifications of the results for both on-axis and off-axis measurement arrangements. The axial and lateral field distributions in the focal plane measured by the hydrophone were derived for both non-attenuating and attenuating fluids in the model. The spatial averaging effects result in underestimating the absolute acoustic pressure amplitudes, and overestimating the cross-sectional size of the beam directivity pattern in the focal plane. The validity of the Rayleigh integral and the Fresnel approximation for curved transducers were discussed. The results indicate that, for (where k is the circular wave number, ah is the chord from the center of curved surface to the boundary and b is the effective radius of the hydrophone), the Fresnel approximation remains valid for all locations of the hydrophone in the focused field. Particularly, in the focal plane, the spatial averaging effects of the hydrophone on the directivity pattern of the acoustic beam can be neglected for (where a and lf are the effective radius and focal distance of the focused transducer).

Purpose – This article aims to propose a new measurement method for ultrasonic power based on self-reciprocity theorem which turns the estimation of ultrasonic power to the measurement of first echo current and open-circuit voltage of the driving source. Design/methodology/approach – The formula for ultrasonic power is derived which has corrected the position of pressure reflection coefficient on the interface of water and steel. The diffraction correction for focusing transducers is evaluated using numerical computation of the Rayleigh integral. One way to estimate the reflection coefficient of focusing beams on heterogeneous interface is also depicted. Findings – Comparison experiment with radiation force balance method demonstrates that ultrasonic power measurement using self-reciprocity is sound in theory and feasible in practice. Originality/value – It has a better capability of anti-environmental interference and, thus, can be extended to low-level and high-frequency power measurements.