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Reference : UMR7587-ANNLOU-002
Workplace : PARIS 05
Date of publication : Thursday, April 22, 2021
Scientific Responsible name : Anne Louchet-Chauvet
Type of Contract : PhD Student contract / Thesis offer
Contract Period : 36 months
Start date of the thesis : 1 October 2021
Proportion of work : Full time
Remuneration : 2 135,00 € gross monthly
Description of the thesis topic
Time reversal is a technique based on the invariance of the wave propagation equation in an inhomogeneous medium. It ensures spatial and temporal refocusing of a wave in such a medium, after having recorded the transmission channel signature (ie the response to a short pulse). When the propagation medium is non-stationary, the transmission channel characteristics keep changing. The time-reversed signal must be sent before the medium has changed too much, otherwise the refocusing will not be effective. It is therefore crucial to minimize the latency time between the measurement of the impulse response and the emission of the time-reversed impulse response. In the first demonstrations of time-reversal with RF waves, analog-to-digital converters (ADC) were used, limiting the processing bandwidth. In the aim of reaching the GHz regime, the latency time becomes problematic because of the limited sampling rate of ADCs. We propose to design a fully analog solution to avoid this conversion step.
At Institut Langevin we design original analog architectures for the processing of optically-carried radiofrequency signals. These architectures rely on coherent light-matter interaction in rare-earth ion-doped crystals cooled down to a few kelvin. In these crystals, one can make the atomic medium emit a light pulse with a controlled temporal shape by using the photon echo phenomenon. We will use this photon echo process to generate the time-reversed waveform. We recently proposed a promising time-reversal architecture  whose processing bandwidth already exceeds the previously published designs . While the first results are encouraging, a lot remains to be done to validate its potential for wave refocusing in a non-stationary medium.
In this PhD project, the time-reversal figures of merit such as its bandwidth, latency time, and fidelity will be the figures of merit that we will have to study and optimize. Spectroscopic characterizations of the rare-earth doped crystal will be necessary to identify the fundamental limits to these parameters. The final validation of the architecture will include testing our time-reversal processor on RF signals in a reverberating cavity available at Institut Langevin.
 A. Louchet-Chauvet, Analog time-reversal of optically-carried RF signals with a rare earth ion-doped processor with broadband potential, 2018 International Topical Meeting on Microwave Photonics (2018)
 H. Linget, L. Morvan, J.-L. Le Gouët and A. Louchet-Chauvet, Time-reversal of optically-carried radiofrequency signals in the microsecond range, Optics Letters 38, 643 (2013).
Institut Langevin is a joint research unit between ESPCI Paris and CNRS. It obtained in 2011 the French government label “LABEX” (laboratory of excellence) for the project WIFI (Waves and Imaging: From FuInstitut Langevin was set up in 2009 by Mathias Fink. It was created by merging two laboratories: the "Waves and Acoustics laboratory", founded and directed by Mathias Fink, and the "Optics Physics laboratory" led by Claude Boccara. The aim was to create a laboratory of which the activities would cover all kinds of waves.
Funded by the ANR (Agence Nationale de la Recherche)
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