In-fibre devices enable a vast array of critical photonic functions ranging from signal conditioning (amplification, dispersion control) to network management (add/drop multiplexers, optical monitoring). These devices have become the mainstays of fibre-optic communication systems because they provide the advantages of low loss, polarisation insensitivity, high reliability, and compatibility with the transmission line. The majority of fibre devices reported to date are obtained by doping, designing or writing gratings in the core of a single mode fibre. Thus, these devices use the fibre only as a platform for propagating light - the device effect itself is due to some extraneously introduced material or structure (dopants for amplification, gratings for phase matching etc.). There exists another, relatively less explored degree of freedom afforded by fibres: the ability to co-propagate more than one mode. Each mode may have a uniquely defined modal dispersion and propagation characteristic. The author describes the variety of fibre devices enabled by few-mode fibres, i.e. fibres that typically support two to four modes with suitably tailored dispersive properties. It is shown that the unique dispersive properties of various modes in conjunction with the ability to couple between them with gratings lead to devices that offer novel solutions for dispersion compensation, spectral shaping and polarisation control, to name a few.
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