Adaptive optics for optical wireless communication through fog

Optical wireless communication (OWC) represents an attractive solution to address the problems of radio frequency saturation and growing need for bandwidth. Futhermore unlike the conventional radio frequency systems, optical frequencies can be deployed without regulation rules.

However, atmospheric disturbances such as fog or clouds are an obstacle to the fast deployment of this technology, which remains promising for ground-satellite links or terrestrial links between two buildings. Indeed, when a modulated optical beam passes through a scattering medium, the signal undergoes temporal ans spatial distorsions that increase with the number of scattering events. When the density of the fog droplets gets high, the communication signal becomes unreadable.

Over the past decade, it has been demonstrated that it is possible to focus light through highly scattering materials using a spatial light modulator that, thanks to its large number of actuators, can control the phase of the different waves generated by multiple scattering. The approaches of imaging or focusing through complex media that have emerged in the literature are generally applied to static or weakly turbid media. For such media, the strategy consists in computing a transmission matrix which establishes a relationship between the input and the output complex field. But due to the large number of actuators to address, this calibration procedure is slow and becomes redhibitory for dynamic media such as thick fogs or clouds.

For these very turbid media, the phase conjugation (or time reversal) technique must be used to simultaneously phase the different modes generated by diffusion since the characterization of the channel is not necessary. This iterative method requires the implementation of two wavefront sensors on either side of the transmitter and receiver systems. One of the key parameters in the definition of phase conjugation system is the decorrelation time. Because of granular intererences, after passing through a scattering medium, the field of a coherent optical beam is composed of speckles whose light intensity is spatially randomly distributed. Preliminary works carried out recently by ONERA and the LKB show that the decorrelation time of the speckles that characterizes the medium can be compatible with the bandwidth of actuator mirrors available on the market. This means that it should be possible to implement a time reversal method for OWC through fog.

It is extremely complex to modelize the impact of a very turbid medium on an electromagnetic field. It is therefore necessary to combine modeling and experimental approach to estimate the performance of phase conjugation. The works proposed in this thesis consists in evaluating the gain brought by the adaptive correction in a multi-scattering environment in terms of range and signal to noise ratio of a communication signal. We will focus on the strategies to preserve the temporal chaos signal of a Quantum Cascade laser at 4 μm that allows highly secure communications.

The first step of the work will be to evaluate from models the degradation of the communication signal of a Quantum Cascade laser after propagation through fog (temporal elongation, maximum rate, ...). The ability of the adaptive optics to preserve the integrity of the transmitted communication data will be evaluated taking into account the properties of the scattering medium.

In the second part of the thesis, experimental measurements under low turbidity conditions will be performed using a laboratory fog greenhouse. The feasibility of the time reversal technique will be evaluated. An adaptive optics communication architecture will be proposed and its performances estimated under various conditions.

This thesis will be directed by Frédéric Grillot (Télécom ParisTech) with a co-supervision of Matthieu Boffety (IOGS) and Béatrice Sorrente (ONERA), and a close collaboration with Sylvain Gigan (LKB).


Required background:
Engineering school, Master in optics, physics, telecommunication
The candidate should have a taste for theory and experimentation.

Application deadline: May 31, 2022

Contacts:
frederic.grillot@telecom-paris.fr
matthieu.boffety@institutoptique.fr
beatrice.sorrente@onera.fr

References:
[1] B. Blochet, L. Bourdieu, and S. Gigan, «Focusing light through dynamical samples using fast continuous wavefront optimization », Optics Letters, Vol. 42, Issue 23, 2017.
[2] O. Spitz, A. Herdt, J. Wu, G. Maisons, M. Carras, C.-W. Wong, W. Elsäßer & F. Grillot, « Private communication with quantum cascade laser photonic chaos », Nature Communication,
[doi.org].
[3] C. Sauvage, C. Robert, B. Sorrente, F. Grillot, et D. Erasme, « Study of short and mid-wavelength infrared telecom links performance for different climatic conditions », SPIE Remote Sensing,Strasbourg, Septembre 2019.



Modifié 2 fois. Dernière modification le 25/04/22 15:07 par BSorrente.