Arnaud Derode

Time-reversed acoustics

The objective of this paper is to show that time reversal invariance can be exploited in acoustics to create a variety of useful instruments as well as elegant experiments in pure physics. Section 1 is devoted to the description of time reversal cavities and mirrors together with a comparison between time reversal and phase conjugation. To illustrate these concepts, several experiments conducted in multiply scattering media, waveguides and chaotic cavities are presented in section 2. Applications of time reversal mirrors (TRMs) in hydrodynamics are then presented in section 3. Section 4 is devoted to the application of TRMs in pulse echo detection. A complete theory of the iterative time reversal mode is presented. It will be explained how this technique allows for focusing on different targets in a multi-target medium. Another application of pulse echo TRMs is presented in this section: how to achieve resonance in an elastic target? Section 5 explores the medical applications of TRMs in ultrasonic imaging, lithotripsy and hyperthermia and section 6 shows the promising applications of TRMs in nondestructive testing of solid samples.

Acoustic time-reversal through high-order multiple scattering

The application of time-reversal mirrors (TRM) to media with very high-order multiple scattering is presented. Random sets of up to 2000 steel rods are considered. When a pulsed wave (typically a 1 /spl mu/s pulse) traverses such a medium, it undergoes many scatterings before reaching,the TRM. The resulting pressure field spreads in time, up to 300 times the initial pulse duration; it is recorded, time-reversed and retransmitted through the same disordered medium. Surprisingly, the time-reversed waves are found to converge to their source and recover their original waveform and duration, unlike one could have expected given the high order of multiple scattering involved and the usual sensitivity to initial conditions of time-reversal processes. In addition to this, the observed resolution of the time-reversed waves was greatly increased, and found to be one sixth of the theoretical limit for the arrays aperture.

The notion of coherence in optics and its application to acoustics

Wave phenomena are mathematically described in similar terms, even though they are of different nature, as is the case for light and sound. Yet, the basic parameters such as velocity and frequency are so different that some of the concepts defined in optics are no longer relevant in acoustics, in particular the notion of spatial and temporal coherence. We briefly present the historical evolution that led to a clear definition of coherence in optics, then we show that it is not adequate for pulse-echo ultrasound and we propose an alternative definition. This leads us to a modified and extended version of an essential theorem in the optical theory of coherence: the Van Cittert-Zernike theorem.

Multiple scattering filter: Application to plane defect detection in a nickel alloy

The ultrasonic inspection of polycrystalline media remains a challenge. The high noise levels due to interaction between the wave and the microstructure limit the efficiency of classical ultrasonic techniques to detect a defect in a coarse grain structure. The aim of this work is to reduce the influence of multiple scattering in order to increase the information obtained from the defect. The technique introduced here is based on array probes for the acquisition of the medium’s response matrix by full matrix capture, after which a filter based on random matrix theory is applied. Here an improvement of this technique is applied on nickel-based alloy mock-ups that present an unfavourable grain structure and well known bulk and plane defects. The results in normal incidence and with an angle array probe of 128 elements and 5u2005MHz of central frequency are compared to classical phased array probe techniques.

Dynamic coherent backscattering of ultrasonic pulsed waves

We report the measurements of dynamic transport properties of an acoustic pulsed wave propagating in a strongly disordered 2D medium using dynamic coherent backscattering. The coherent backscattering peaks are recorded with an array of transducers. Both transport speed and diffusion coefficient are determined accurately.