Theoretical and experimental analysis of focusing techniques through liquid-solid interfaces

Different approaches have been extensively studied in order to obtain optimal focused beams through liquid-solid interfaces. In this paper, we compare two different focusing techniques: the standard Fermat´s surface (FS) method and the time reversal (TR) approach. A FS transducer is designed such that the propagation times between the points on its surface and the desired focal point are equal. This method suffers from limitations, it needs the a priori knowledge of the geometry of the interface as well as the feature of the propagating medium. The TR method is based on the concept of time reversal of ultrasonic fields and takes into account the whole information coming from the sample. This technique is self adaptive and only requires the presence of a target in the solid sample, In this paper, we are interested in focusing an acoustic pressure field through a plane interface separating a liquid and a solid. Numerical and experimental results obtained with a 2D array are presented to compare the performances of the two techniques. These results demonstrate the ability of the TR technique to compensate automatically for the distortions induced by the interface