Optical detection of evanescent ultrasound waves in water

Abstract

Acoustical perturbation by targets smaller than the wavelength can generate evanescent waves, which decay with the distance of propagation. By putting such targets immediately under the free surface of water, the diffracted acoustical field can excite the surface membrane before evanescence, and it produces a static ripple because of the radiation pressure. A collimated beam of light reflects at the perturbed surface, and it becomes modulated in phase. In this work we show experimental results where we achieve an optical image of the acoustical evanescent waves produced by an array-like target. Arising from the theory, we present a numerical calculus of the optical image produced by the ultrasonic field diffracted from the target in order to verify the experimental results. With this method, we look for a spatial resolution smaller than acoustical wavelength, for normal incidence and plane waves. In our experimental device, we use a sound wavelength in water greater than 1.5 mm, generated by a PZT transducer. We can resolve an array of 1.0 mm of periodicity.

Estimation of skin elasticity by measuring surface wave velocity under impulse stimulus using compact optical sensors

“…In contrast, Raleigh wave that propagates on the superficial layer of the medium has also been studied [4][5][6][7][8][9][10]. It has been proved both theoretically and experimentally that low frequency (under 1000 Hz) surface wave velocity is related to the tissue elasticity and can be measured by noncontact methods [10].…”

“…Surface wave propagation can be measured by piezoelectric elements [4], ultrasound [5] and optical devices [6][7][8][9][10]. Among the optical techniques, [6,7] uses a simple setup based on optical defocusing.…”

“…Among the optical techniques, [6,7] uses a simple setup based on optical defocusing. [8][9][10] detect surface vibration by measuring the phase of the specular reflection interferometrically.…”