Focusing on Plates: Controlling Guided Waves using Negative Refraction.

Elastic waves are guided along finite structures such as cylinders, plates, or rods through reflection, refraction, and mode conversion at the interfaces. Such wave propagation is ubiquitous in the world around us, and studies of elastic waveguides first emerged in the later part of the 19(th) century. Early work on elastic waveguides revealed the presence of backward propagating waves, in which the phase velocity and group velocity are anti-parallel.

Analysis of the time reversal operator for a scatterer undergoing small displacements.

The method of the time reversal operator decomposition is usually employed to detect and characterize static targets using the invariants of the time reversal operator. This paper presents a theoretical and experimental investigation into the impact of small displacements of the target on these invariants. To find these invariants, the time reversal operator is built from the multistatic response matrix and then diagonalized.

Construction of the temporal invariants of the time-reversal operator.

This paper proposes a method to construct the temporal Green's function from a scatterer to an array of transducers in a waveguide using free-space back propagation of the eigenvectors of the time-reversal operator (TRO). The monostatic Green's function is obtained as an eigenvector of the TRO which is known with an arbitrary phase; thus the impulse response cannot be obtained by a simple inverse Fourier transform. Assuming that the monochromatic fields obtained by the back propagation of the eigenvectors are in phase at the focal point, the phase correction is determined.

Iterative high-resolution wavenumber inversion applied to broadband acoustic data.

This paper presents the results of an iterative high-resolution wavenumber inversion algorithm applied to acoustic waveguides in which wave propagation is described by propagating modes. Following work on the first description of the inversion algorithm, a generalization to broadband signals is performed and experimental data are investigated. The algorithm leads to dispersion curves for propagating modes in the case of an ultrasonic waveguide and an elastic plate.

Characterization of an elastic target in a shallow water waveguide by decomposition of the time-reversal operator.

This paper reports the results of an investigation into extracting of the backscattered frequency signature of a target in a waveguide. Retrieving the target signature is difficult because it is blurred by waveguide reflections and modal interference. It is shown that the decomposition of the time-reversal operator method provides a solution to this problem.