Optical Measurement of Wide Bandwidth Ultrasound Fields
Todd A. Pitts, Ph.D. May 1999
Detailed, accurate, three-dimensional, optical measurements of an instantaneous refractive index perturbation in an optically transparent medium may be obtained from measurements of scattered optical intensity alone. The Method of Generalized Projections (MGP) allows incorporation of these measurements into an iterative algorithm for computing the optical phase as the solution of a fixed point equation. The complex optical field amplitude, computed in this manner, is unique up to a constant unit magnitude complex coeffcient. The three-dimensional refractive index distribution may be computed via the Fourier Slice Reconstruction (FSR) algorithm from the optical phase data under the assumption of weak optical scattering. If the refractive index perturbation is induced by an acoustic field, then acoustic pressure, Poynting vector, and particle velocity fields can be computed from these measurements. These field quantities may be obtained at any point throughout the temporal evolution of the sound field, either under mild assumptions on its angular plane wave spectrum or via a minimal number of measurements. Interaction of a wide bandwidth ultrasound pulse with various materials, including biological tissues, may be effectively studied via this measurement method.