Optical phase contrast measurement of ultrasonic fields

Abstract

This report describes an optical phase contrast imaging technique for the measurement of wide bandwidth ultrasound fields in water. In this method, a collimated optical wavefront ( l = 810 nm) impinges on a wide bandwidth ultrasound pulse. The method requires that refractive index perturbations induced by the ultrasound field be sufficiently small. Specifically, on exit from the acoustic field, the phase of the optical wavefront must be proportional to the ray sum of local density taken in the direction of propagation of the incident optical wave. A similar restriction is placed on the dimensions of the ultrasound pulse. Repeated measurement of this phase as the ultrasound field is rotated through 180 about an axis normal to the direction of propagation of the incident optical wave generates the Radon transform of the ultrasonically induced refractive index perturbation. Standard tomographic reconstruction techniques are used to reconstruct the full three-dimensional refractive index perturbation. A simple two-lens imaging system and an optical signal processing element from phase contrast microscopy provide a method of directly measuring an affine function of the desired optical phase for small optical phase shifts. The piezo-and elasto-optic coefficients (the first partial derivatives of refractive index with respect to density and pressure) relate refractive index to density and pressure via a linear model. The optical measurement method described in this paper provides a direct, quantitative measurement of the piezoand elasto-optic coefficients (from the density or pressure fields).

Optical measurement of ultrasonic Poynting and velocity vector fields

“…The expression in (10) gives an intensity that is related to phase delay and, thus, integrated refractive index perturbation by a simple affine transformation. The pitch of the CCD camera and magnification of the lens system used in this report provide a system resolution of approximately 70 µm in all three spatial directions [21], [24]. A more detailed report on the application of this technique to ultrasound field measurement is available in [24].…”

“…For more detailed explanations, the reader is referred to [20], [21], [24]. Both of these methods utilize a precisely timed and repeatable collision between an optical pulse with quasi-planar wavefronts and an ultrasound pulse.…”

“…These are reviewed briefly for the benefit of the reader in Section II. For a more detailed description of these methods, the reader is referred to [20], [24].…”

A Simple Technique for Visualizing Ultrasound Fields Without Schlieren Optics

“…Pitts et al (1994) proposed techniques that can determine an ultrasound field more precisely by detecting a phase shift of light integrated over its path transmitted through an ultrasound field. Two techniques, the phase recovery technique (Pitts et al 1994) and phase contrast technique (Harigane et al 2013;Pitts et al 2001), were used for phase detection. The phase recovery technique estimates the phase shift by an iterative calculation using a magnitude image acquired at a known distance.…”

Imaging local acoustic pressure in microchannels

“…Other investigators derived a value of α ac 1.26 × 10 −10 Pa −1 assuming a linear dependence of electric susceptibility on density [42].…”