Waves control in complex media

In so-called complex media, such as a room in a house for microwaves or optical fibers presenting disorder for light, waves are distorted by a large number of reflections or scattering events during their propagation. This results in mixing, in an apparently random manner, the information carried by the waves. Thus, complex environments can have a deleterious effect on many applications, such as telecommunications or imaging.

The Control of Waves in Complex Environments team studies the techniques of spatial and/or temporal modulation of the wavefront to combat these effects. Some of this work even takes advantage of the properties of disorder to create original devices.

Part of our recent works has focused on telecommunications. Indeed, wave control allows for increased data rates, range, and/or decreased consumption of telecommunications systems. However, other applications have been targeted, such as analog computing, target localization, or the development of MRI antennas.

The theme mainly studies three wave domains:

• the Microwave Domain,
• the Optical Domain,
• the Acoustic Domain.

Microwave Domain

Following the development of reconfigurable metasurfaces in the microwave regime, analogous to spatial light modulators used in optics, several exploitation paths for these have been envisaged. In the microwave regime, electromagnetic waves undergo numerous reflections on the walls and a room typically acts like a reverberating cavity. The typical use of the metasurface is to integrate it onto a wall of these cavities and thus modify the boundary conditions.

Telecommunications

A first direct application of these metasurfaces is related to the start-up from the laboratory, GreenerWave, founded in 2016, concerns the optimization of wireless telecommunications. We have shown that it becomes possible to adapt the environment to optimize the Shannon capacity of a transmission channel in an urban environment to improve the transfer rate [1]. In collaboration with Orange, a metasurface with continuous phase control was exploited to make passive transmissions that recycle the ambient electromagnetic field [2]. Finally, thanks to the pioneering work of the Langevin Institute and its expertise in metasurface modeling, it is widely recognized as promising devices for telecommunications under the name of RIS (Reconfigurable Intelligent Surface) [3].

Analog computing

Microwave setup for analog computing in a reverberating cavity. Image adapted from [4].

In addition to the field of telecommunications, several usage paths for these metasurfaces are being studied. For example, it has been proposed to use these metasurfaces to perform analog computing [5]. By placing reconfigurable metasurfaces in the room, we can adjust the way waves are reflected by the walls. Thus, they perform the desired computing operations – simply by allowing Wi-Fi waves to bounce in an apparently arbitrary but controlled manner.

Target Localization

Also, with the help of these metasurfaces, we have demonstrated that it may be possible to locate intruders who try to remain unnoticed by exploiting multiple configurations [6].

Contacts

• Julien de Rosny
• Fabrice Lemoult

Optical Domain
Wavefront control setup allowing to measure the transmission matrix of a scattering medium or a multimode optical fiber. Image from [7].

Today, most long-distance communications rely on single-mode optical fibers, which are reaching their limit in terms of data rate. Multimode optical fibers are particularly interesting as they offer the possibility to use spatial diversity (the different spatial modes) to increase data rates. However, the dispersion and the coupling between the modes due to fiber defects and its spatial conformation limits their use. The optical transmission matrix, whose measurement was made for the first time at the Langevin Institute in scattering media [8], allows the full characterization of the input/output relationship of the optical fiber and thus to study its properties and to find new applications.

Telecommunications

In order to mitigate the effect of disorder in these fibers, we characterized the effect of local disturbances and showed that it is possible to find a basis of channels, different from the fiber modes, that can be insensitive to disorder [9], which is particularly interesting for telecommunications in a channel whose intensity of disturbances may vary over time.

Example of a channel not sensitive to a local disturbance, the intensity profile at the output remains the same with (bottom) or without (without) disturbance. Image adapted from [10].

Analog computing

We recently showed that this disorder, generally considered purely detrimental, can be exploited for optical computing [11].

All-Fiber Wavefront Control

Moreover, in collaboration with the University of Jerusalem, we showed that modifying the boundary conditions of a fiber, largely responsible for the disorder, can be exploited to spatially shape the beam, similarly to a spatial light modulator, or to focus the output field [12]. By characterizing the system and then modulating the input wavefront, any chosen linear operation can be performed by a disordered multimode optical fiber in a reconfigurable manner.

Contacts

• Sébastien Popoff

Acoustic Domain

Page under construction

Contacts

• Fabrice Lemoult
• Mathis Fink