Determination of Sound Velocity in Three-Dimensional Space by Optical Probe

Abstract

We propose a method of determining a three-dimensional sound velocity using a Michelson interferometer, optical computerized tomography (O-CT) and near-field acoustical holography (NAH). Ultrasonic waves affect the phase of the test light passing through radiated sound fields. The zeroth-order diffraction light including sound pressure information is electrically acquired by an avalanche photodiode (APD). Projection data along the optical axis is obtained by single linear scanning in the range of ±30 mm and electronically quadrature-detected as a complex amplitude. Eighteen projections are acquired in the range of 0≤θ<π rad, and the complex sound fields are reconstructed in a region of 40×40 mm2 by O-CT. Then another plane separated by 1 mm is propagated using NAH from the acquired sound fields, and the same plane is reconstructed. Comparing the phase of the reconstructed and propagated sound fields in wave number domain, we determine the three-dimensional sound velocity in a region of 40×40×1 mm3. The experimental results are in agreement with the reference value.