J. de Rosny

Time reversal of wideband microwaves

In this letter, time reversal is applied to wideband electromagnetic waves in a reverberant room. To that end a multiantenna time reversal mirror (TRM) has been built. A 150MHz bandwidth pulse at a central frequency of 2.45GHz is radiated by a monopolar antenna, spread in time due to reverberation, recorded at the TRM, time reversed, and retransmitted. The time-reversed wave converges back to its source and focus in both time and space. The time compression is studied versus the number of antennas in the TRM and its bandwidth. The focal spot is also measured thanks to an eight-channel receiving array.

Theory of the time reversal cavity for electromagnetic fields

We derive a general expression of the electric dyadic Green function in a time-reversal cavity, based on vector diffraction theory in the frequency domain. Our theory gives a rigorous framework to time-reversal experiments using electromagnetic waves and suggests a methodology to design structures generating subwavelength focusing after time reversal.

Volume coil based on hybridized resonators for magnetic resonance imaging

We present an electromagnetic device based on hybridization of four half-wavelength dipoles which increases the uniformity and the strength of the radio-frequency (RF) field of a Magnetic Resonant Imaging (MRI) apparatus. Numerical results show that this Hybridized Coil (HC) excited with a classical loop coil takes advantage of the magnetic hybrid modes. The distribution of the RF magnetic field is experimentally confirmed on a 7-T MRI with a gelatin phantom. Finally, the HC is validated in vivo by imaging the head of an anesthetized rat. We measure an overall increase of the signal to noise ratio with up to 2.4 fold increase in regions of interest far from the active loop coil.

Application of the DORT method to the detection and characterization of two targets in a shallow water wave-guide

The decomposition of the time-reversal operator (DORT in French) is an active array detection technique. It requires the measurement of the array response matrix K(/spl omega/) and consists in the analysis of the eigenvalues and the eigenvectors of the time-reversal operator K*K which provides information on the presence and localization of scatterers in the medium. It was shown that the DORT method allows to separate and localize pointlike scatterers in a shallow water wave-guide [J. Acoust. Soc. Am. Mordant et al. (1998) and Fotegot et al. (2003)]. Here, we extend the study to the detection and frequency characterization of two spherical targets. Small scale ultrasonic experiments are performed with a 3.9 MHz 24 elements transducer array and two spheres of 2 and 3 mm diameter in a 31 mm deep wave-guide. These scatterers correspond to 15 < ka < 25 leading to a non-isotropic scattered field. We have developed a theoretical model taking into account the wave-guide and the acoustic properties of the spheres and using the partial waves decomposition of the scattered field. We calculate the singular values of the array response matrix. This theoretical approach is in good agreement with the experimental results. This 1/325/sup th/ scale ultrasonic experiment corresponds to a shallow water experiment with a 12 kHz Vertical Linear Array (VLA).

Theory of the Time-Reversal Operator for a Dielectric Cylinder Using Separate Transmit and Receive Arrays

The DORT method applies to scattering analysis with arrays of transceivers. It consists in the study of the time-reversal invariants. In this paper, a large dielectric cylinder is observed by separate transmit and receive arrays with linear polarizations, E or H, parallel to its axis. The decomposition of the scattered field into normal modes and projected harmonics is used to determine the theoretical time-reversal invariants. It is shown that the number of invariants is about 2k 1 a , where a is the cylinder radius and k 1 the wave number in the surrounding medium. Furthermore, this approach provides approximated expressions of the two first invariants for a sub-resolved cylinder, i.e., when the cylinder diameter is smaller than the resolution width of the arrays. The two first invariants are also expressed in the small object limit for k1a. AMS subject classifications. 35B40, 35P25, 45A05, 74J20,78M35.

Design of a time reversal mirror for medium scale experiments

A rigid 24-element acoustic source-receiver array has been jointly developed by ATLANTIDE, LOA and ECA. The array, based on COTS equipment, is specified to be easily deployed from a pier and to allow simple and low cost at sea experiments with water depths up to 20 meters. Dedicated to underwater detection and focusing applications in very shallow water, the instrumentation can be deployed in various acoustic time reversal configurations: detection of drifting objects, acoustic barrier, communication with AUV, etc. The system performance based on the time reversal concept and more particularly on the Time Reversal Operator Decomposition method (D.O.R.T.) is currently being estimated by experiments in a pool and in the bay of Brest.

Far-field imaging with a multi-frequency metalens

A metalens, i.e., a dense array of identical resonators, allows to image an object pattern at subwavelength scale from far field radiation field. Here, we show that the efficiency can be improved when the resonant frequencies of the cell are distributed over a given frequency range. Because in such systems each eigen mode is localized, the subwavelength image is built from a spectral analysis of the radiated field. A simple model based on coupled resonant dipoles is used to find the best frequency distribution. This multifrequency metalens approach is validated using a flat array of split ring resonators. We experimentally demonstrate the subwavelength resolution of such a device at microwave range.

First tests of the DORT method at 12 kHz in a shallow water waveguide

The Decomposition of the Time Reversal Operator (DORT method) has been introduced by Prada et al. (1996) as a generalization of the iterative time-reversal process. The effectiveness of this method for ultrasonic detection and selective focusing on different scatterers in an inhomogeneous media has been shown. The robustness of the method in an ultrasonic wave guide has been demonstrated by Mordant et al. (1999). Whereas the performance of classical detection methods usually decreases due to multiple reflections at the waveguide interfaces, the DORT method takes advantage of the wave guide boundaries and multi-path propagation in order to improve spatial resolution. In this work, we present the first experiment in a realistic shallow water waveguide with a 24 elements source receiver array at 12 kHz central frequency. The DORT method is applied in order to detect two targets at 27 m distance. Numerical and experimental backpropagation of the dominant singular vectors of the array response matrix enable the localization of the individual targets.

Auto-Focalisation, Communication and Sonoluminescence with Acoustic Time Reversal

The basic limitations of Time-Reversal Mirrors and the means to overcome them are first discussed: especially the concept of acoustic sink is introduced. Then, time-reversal experiments in chaotic cavities and multiple scattering media are presented. In the case of time reversal in chaotic cavities, we show an experimental realisation of the acoustic sink. As to time reversal in multiple scattering media, it is interpreted as a robust estimator of both the time and space correlation function of the multiply scattered signals. Using this approach, the difference between broadband time-reversed acoustics and monochromatic phase conjugation is highlighted. Finally, two recent applications of adaptive focusing to communication on the one hand and to boosting sonoluminescence on the other hand are presented.

Time-reversal focusing in a range-dependent ocean

Shallow water time-reversal focusing has been successfully applied in ultrasonic laboratory experiments as well as in the ocean. For simplicity the ocean is usually considered as a range-independent waveguide. As a consequence, an omnidirectional vertical time-reversal array should refocus the wave not only at the source but also on a ring surrounding the array. In a real environment, the ocean bottom may be sloping. In this work, the influence of a slight inclination of the ocean bottom is studied. We show that the symmetry breaking yields an additional focusing in azimuth. We present ultrasonic experiments, which are well supported by a theoretical model based on modal decomposition and a stationary phase approach. The shape of the focal region in depth, range and azimuth is determined. The experiments are carried out in a water tank, at a central frequency of 3 MHz. The height of the waveguide ranges between 10 mm and 13.5 mm, its length is 500 mm. The inclination of the bottom is controlled by a step motor. A piezoelectric array is used to perform the TR operation. A needle hydrophone is used to check the accuracy of focusing. In the theoretical approach, the nature of the bottom can be taken into account, which is shown to significantly change the results. This is confirmed by numerical simulations performed in a realistic ocean environment.