Multimode, few-mode or multicore optical fibers

When presented, optical fibers are generally classified into two distinct families: multimode optical fibers and single-mode optical fibers. The latter are often highlighted for the very high data rates they enable, which form the basis of the information highways that data is exchanged on a daily basis, notably via the submarine cables that line the ocean floor. The term "single-mode fiber" most often refers to a fiber structure consisting of a single waveguide (a single core) allowing a single optical path (a single mode). Throughout this article, we'll look at different types of fiber that allow light to propagate in different modes within a single core (slightly multimode fibers and multimode fibers), or in single modes supported by different cores (multicore fibers). We'll also mention the case of fibers with multiple cores, each supporting several modes. We'll see that, while they share many properties with single-mode fibers, these fibers also have their own specificities in terms of guiding, manufacturing or processing properties. This last point covers, in particular, the need to be able to selectively excite the spatial paths available. Added to this is the need to implement specific optical techniques to be able to characterize them. After some general information on optical fibers, several of these specificities are presented, distinguishing between the two sub-families of multimode fibers (including slightly multimode fibers) and multicore fibers.

The opening up of multiple spatial paths within a single fiber, if mastered and understood, constitutes a richness that can be put to good use in numerous research fields with varying degrees of application orientation. Today, for example, research into optical telecommunications, the development of intense laser sources, bio-medical imaging, non-linear photonics and propagation in complex media can draw on a wide variety of available fiber structures, of which we present a few examples. The multiplicity of spatial paths, however, requires particular attention to the notion of coupling, whether between cores or between modes. We'll see, moreover, that the intensity of this coupling can be chosen and adjusted as a function of the opto-geometric properties of the fiber core(s). This point will be particularly illustrated by recent developments in the telecom context, where these fibers could rapidly find their place alongside "classic" single-mode and multimode fibers.

At the end of the article, readers will find a glossary and a table of acronyms and notations.