INFLUENCE OF BOUNDARY-CONDITIONS ON TIME-REVERSAL FOCUSING THROUGH HETEROGENEOUS MEDIA

This letter presents a way to overcome ultrasonic focusing degradation s through strongly diffracting nondissipative layers. A first set of e xperiments shows that, using finite aperture transducer arrays, neithe r the time-reversal technique, nor other focusing techniques are able to achieve proper focusing through this kind of aberrator. These exper imental results show the limits of a finite aperture time-reversal mir ror compared to the theoretical behavior of a time-reversal cavity. To simulate a time-reversal cavity, totally reflecting walls are set bet ween the time-reversal mirror and the aberrator. The experiments and n umerical simulations presented in this letter show that as soon as we introduce these reflecting boundaries, the time-reversal focusing beco mes optimal and the spatial resolution is strongly improved. (C) 1998 American Institute of Physics. [S0003-6951(98)02420-6].