The iterative time reversal mirror: A solution to self-focusing in the pulse echo mode

Accurate focusing in an inhomogeneous medium is difficult to implement. In order to focus on a reflective target we have extended the concept of optical phase-conjugate mirrors, valid for monochromatic signals, to broadband pulses such as those used in ultrasound echography. The transducer’s linear response to the acoustic pressure allows one to replace the phase conjugation by a time reversal operation on the pulse echo signals. The time reversal mirror is an array of transmit–receive transducers. A first incident wave is reflected by the target. The received signals are stored in shift registers, reversed in time, and then reemitted. The main advantage of this process is that waves distorted by the propagation through an aberrating medium are corrected by the mirror operation and the back propagation through the medium. When the medium contains several reflectors, this time reversal process can be iterated in order to focus on the most reflective one. We present theoretical results on this principle, numerical simulations, and experimental results with a 1-D array working at a central frequency of 3 MHz.