Implementation of a time-reversal cavity by confining a time-reversal mirror in a waveguide

Abstract

Time reversal is an original method to focus ultrasonic waves through heterogeneous lossless media. Nevertheless, to conduct a perfect time-reversal experiment, the wave field coming from an acoustic source should be recorded on a closed surface covered with transducers and surrounding the region of interest. This led to the concept of time-reversal cavity. In practice, its implementation is not easily realizable and the time-reversal operation is only performed on a restricted area named a time-reversal mirror. Some aberrating medium may diffract a part of the wavefield coming from the acoustic source outside the limited aperture of the time-reversal mirror. Hence, focusing quality may be strongly degraded. To approach the optimal focusing obtained with a time-reversal cavity, a channel bounded by totally reflecting walls was used to guide the wave field between the source and the time-reversal mirror. The previously lost information was then recovered to focus on the source location. Therefore, the combination of a time-reversal mirror with a waveguide can simulate a time-reversal cavity with only a small number of transducers. Experimental results give good agreement with a wave propagation model using a finite differences scheme. Both show that the focusing is now the same as that in a homogeneous medium.