Multimode optical fibers are back on the map!

For a few years now, multimode optical fibers have been the focus of renewed interest from researchers and industry. Indeed, "single-mode" fibers, in which light can only follow one path, seem to have reached a performance plateau.
To study multimode optical fibers (MMF), researchers from the Langevin Institute (ESPCI Paris - PSL, CNRS) and the Hebrew University of Jerusalem have developed a method to quickly characterize the properties of these fibers, which evolve over time. With this technique, the team has also revealed modes that are insensitive to external perturbations, which were previously one of the main obstacles to the use of MMFs.
Their work is published in the journal Physical Review X.

Optical fibers are everywhere! In telecommunication, in medical imaging... They constitute a dense network, essentially composed of single-mode fibers, i.e. in which only one signal circulates. The problem is that the fibers are almost saturated, in terms of quantity of information transmitted, but also in terms of infrastructure. How to multiply the data rates without multiplying the production costs? The solution may lie in multimode fibers. They were widely used in the 70-80’s. Because they were prone to crosstalk and signal distortion, with a high sensitivity to external disturbances, they were replaced by single-mode fibers for long-haul communications.
To characterize an optical fiber, researchers are interested in determining its transmission matrix: the operator that determines the impact of the fiber between the input and output to reconstruct the original signal. Until now, knowing this matrix was a real technical challenge: complex alignment, no automation, and rapid fluctuations.
However, the team at the Langevin Institute has managed to overcome this obstacle, as explained by Sébastien Popoff, CNRS research fellow and co-author of the study:
Our method allows us to know this transmission matrix in about 1 minute, and it is automatic, without alignment problems.

The researchers measured several matrices for different deformations of the fibers (similar to those caused by the passage of a truck for example).
This study allowed them to identify information transmission channels in these MMFs that are not very sensitive to deformation, a promising avenue for increasing data rates at a lower cost.

The future of optical fibers may well lie in multimode fibers. With space savings, manufacturing cost savings, and performance gains, MMFs may well be the future of the field.

Associated publication

Learning and Avoiding Disorder in Multimode Fibers
Maxime W. Matthès, Yaron Bromberg, Julien de Rosny, and Sébastien M. Popoff
Phys. Rev. X 11, 021060 (2021) – Published 21 June 2021

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