Measurement of Sound Velocity in Water Using Optical Probe and Acoustical Holography

Abstract

We propose a method for a determination of a three‐dimensional sound velocity using optical computerized tomography (O‐CT) and near‐field acoustical holography (NAH) for measuring a phase object. Ultrasonic waves affect a phase of the light passing through radiated sound fields. The zeroth order diffraction light including sound pressure information is acquired by an avalanche photodiode. Eighteen projections are acquired by rotational scanning, and each projection along the optical axis is obtained by single linear scanning and electronically quadrature‐detected as the complex amplitude. The complex sound fields are reconstructed by O‐CT in a region of 40 × 40 mm2. Then the sound field on another plane is propagated using NAH from the acquired sound fields, and the sound field on the same plane is reconstructed by O‐CT. Comparing the phase of propagated and reconstructed sound fields in wavenumber domain, we can obtain the sound velocity in three dimensional space in a region of 30 × 30 × 10 mm3. The experimental results are in agreement with the reference value measured by another mean.