Analogic Time Reversal mirror based on an Atomic Processor – ATRAP

Wave propagation through complex media leads to signal dispersion, wavefront deformation, interference and attenuation, resulting in poor temporal and spatial focusing. More specifically, the multipath propagation distorts and lengthens the signal. In reverberating media, the distortion is so strong that the initial signal may not be recognized, limiting the efficiency or throughput of the communication.

Among the many techniques addressing this issue, time reversal is a particularly powerful and universal technique since it does not require preliminary knowledge of the propagation medium: time reversal actually takes advantage of the multiple scattering that occurs during propagation. It consists in generating a short pulse from one emitter at a spatial position A and recording the detected signal amplitude on a detector at position B. This signal corresponds to the impulse response of the transmission channel between A and B. Then this signal is time-reversed and generated from an emitter placed at B. Due to the invariance of the wave propagation equation by the time-reversal transformation, the wave emitted at B propagates through the medium and refocuses both temporally and spatially at position A. A strong reverberation actually helps the quality of the refocusing. The transmission channel from B to A is now usable by pre-correcting the signal emitted from B.

First proposed for acoustic waves and applied to the medical domain, time-reversal-assisted temporal and spatial focusing was later demonstrated with narrowband electromagnetic waves in a reverberating chamber. Applied to wireless communications and radar in natural or urban environments, time-reversal of electromagnetic waves is very appealing because it would greatly improve their directionality and range. However, such applications lead to four demanding performance criteria (phase fidelity, µs signal duration compatibility, GHz bandwidth and sub-ms latency), which are impossible to satisfy simultaneously with the existing state-of-the-art techniques.

The ATRAP project aims at developing the first time-reversal architecture for electromagnetic signals fulfilling all the requirements for focusing wideband RF waves in non-stationary complex media. This architecture is fully analog so as to avoid the limitations due to analog-to-digital conversion. It is based on an atomic processor where optical coherent transients are implemented in a rare-earth ion-doped crystal. This project will have interesting applications and developments in electronic warfare and in wireless communications.