An optical method for measuring information of sound fields using the Mach-Zehnder interferometer is described. The Mach-Zehnder interferometer is advantageous for obtaining quantitative measurements of phase values in comparison to other visualization methods. In this study, we simultaneously investigate a two-dimensional data acquisition system for the projection image of an optical phase deviation in proportion to sound fields below the frequency of 2.16 MHz. In the experiment, the charge-coupled device (CCD) camera is used as a function of a mean value detector. The experimental results of the optical phase deviation along the ultrasound axis are useful as primary data for estimating the sound pressure. The two-dimensional measurement system using a CCD camera proposed in this paper has the advantages of acquiring the data in a short time when used in an environment where the measurement conditions change.

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In this paper, we propose a method of noncontact measurement of sound fields, radiating from an ultrasonic transducer, using a Mach–Zehnder interferometer. The deviation of refractive index, which depends on sound pressure, was transformed into the intensity of the interference light using the interferometer, in which a light beam passes through the measured object. Projection data used in the computerized tomography (CT) method along the optical axis was obtained by single linear scanning in the range of ±40 mm and was electronically quadrature-detected as the complex amplitude. The 36 projections were acquired by the rotation of the sound fields in the range of 0≤θ≤π (rad) with an interval of π/36 rad. Finally, the complex sound fields were reconstructed in the region of 48×48 mm2 by the CT method. The plane of reconstruction was 5 mm apart from the plane of the transducer. The ultrasonic transducer used in the experiment was driven at a frequency of 1.416 MHz and had a diameter of 8.0 mm. The experimental results were approximately in agreement with the numerical results.

SUMMARYIn this paper we discuss the acquisition of sound field information from optical interference patterns detected by a CCD camera. Although noncontact measurements of wide-beamwidth sound fields based on diffraction techniques such as the Schlieren method yield projected images with high SN ratios, the problem is that sound field information is not projected correctly. In this work, the measurements were made using a Mach-Zehnder interferometer. Because interference measurements measure light that has passed through a real focus generated by the sound fields, these measurements have a clear advantage over methods like the Schlieren method, in that they create no points at which there is a mismatch between the received light and the diffraction spots. By using CCDs as two-dimensional time averaging detectors, wide-range sound fields can be acquired at high speeds. In these experiments, we estimated the optical phase excursion caused by the sound field over an interval along the sound axis from 0 to 25 mm using an ultrasonic wave transducer 8.0 mm in diameter, operating at 1.4 MHz, with a beamwidth of 7 mm. Because there were bands along the sound axis within which our computational algorithm was unable to provide estimated values for the phase deviation, we augmented our primary data set with three other data sets obtained by shifting the phase of the reference light used to generate the interference pattern in the primary measurement by +0.20 π, +0.48 π, +0.94 π (rad), and combining the resulting data sets to interpolate through the gaps in the primary data set, thereby realizing a continuous estimate along the whole sound axis. In this continuous estimate, the data we used to interpolate the primary data set came mostly from the interference pattern whose phase relative to the first measured pattern was in the neighborhood of π/2 rad. By averaging the four estimate results together, we were able to decrease the percentage of no-data sectors over the entire range 2%. The estimated phase deviation was a gradually decreasing function with distance, starting with a value of about 0.7π rad.

Optical probe has the advantage of non-contact sensing and quick response. To combine the optical probe with an optical