TIME-REVERSAL OF ULTRASONIC FIELDS .1. BASIC PRINCIPLES

Time reversal of ultrasonic fields represents a way to focus through an inhomogeneous medium. This may be acomplished by a time-reversal mirror (TRM) made from an array of transmit-receive transducers that respond linearly and allow the incident acoustic pressure to be sampled. The pressure field is then time-reversed and re-emitted. This process can be used to focus through inhomogeneous media on a reflective target that behaves as an acoustic source after being insonified. The time-reversal approach is introduced in a discussion of the classical techniques used for focusing pulsed waves through inhomogeneous media (adaptative time-delay techniques). Pulsed wave time-reversal focusing is shown using reciprocity valid in inhomogeneous medium to be optimal in the sense that it realizes the spatial-temporal matched filter to the inhomogeneous propagation transfer function between the array and the target. Time-reversal focusing technique provides the optimal inputs to the transducer elements by maximizing the pressure at the target location. The research on time-reversed wave fields has also led to the development of new concepts that are described in the paper: time-reversal cavity that extends the concept of the TRM; iterative time-reversal processing for automatic sorting of targets according to their reflectivity and resonating of extended targets. In the last part of the paper the fundamental differences between a TRM and a phase-conjugated mirror are described.