A new frontier for optical telecommunications
Optical fibres are used to transmit light energy in a guided manner without electromagnetic interference. This propagation can take place in single-mode or multimode: in single-mode fibres, the light signal propagation takes place in one mode only and compared to multimode fibres, there is less signal attenuation and dispersion.
Research on data transmission in multimode optical fibres dates back some 40 years, when the existence of particular light pulses, called spatiotemporal or multimode solitons, was predicted, capable of propagating indefinitely along the fibres, thanks to a delicate compensation mechanism between dispersive and non-linear effects.
In recent decades, optical solitons, which have become essential building blocks in the construction of ultra-short pulse laser light sources, have also been proposed as ideal vehicles for transmitting data in the fibre-optic information highways that form the backbone of the internet. Theories developed over recent years have attributed a stable and repetitive evolution to solitons in multimode fibres as they propagated along the fibre, yet this periodic pulse evolution has never been observed experimentally.
Today, studies by the research group coordinated by Stefan Wabnitz of the Department of Information Engineering, Electronics and Telecommunications (DIET) at Sapienza University, developed as part of an advanced research project funded by the European Horizon 2020 programme through the European Research Council (ERC), shatter commonly accepted theoretical predictions.In the work, carried out in collaboration with the University of Brescia and the Russian State University of Novosibirsk and published in the journal Communications Physics, it was demonstrated for the first time at an experimental level that multimode solitons do not follow periodic behaviour, but, on the contrary, these pulses spontaneously evolve towards single-mode waveforms, i.e. they propagate in the fundamental mode of the fibre.
An additional unexpected observation was that as they propagate, the solitons acquire a fixed time duration, which depends only on the wavelength of the light radiation injected at the fibre entrance. The typical time duration of these light pulses, at standard telecommunication wavelengths, was found to be extremely small (100-200 femtoseconds) and almost independent of the time duration of the original laser pulse, which is coupled to the fibre input.
This research also provided theoretical and numerical support for the experimental observations, identifying as an essential condition for the formation of such pulses the coincidence of three distinct length scales: the length associated with fibre non-linearity, that associated with temporal broadening due to chromatic dispersion, and that associated with temporal slip or 'walk-off', i.e. the distance by which the modes of a fibre temporally separate due to modal dispersion.
So far, multimode fibre transmission has allowed the exploitation of the Space Division Multiplexing (SDM) technique using each mode of the fibre as an information channel. With this mechanism, several input transmission channels share the same available output transmission capacity, i.e. several signals are combined into one (called multiplexed) output on the same physical link.
"This study - says Mario Zitelli of Sapienza University, who carried out the experimental verifications - "opens up the possibility of realising a solitonic SDM, with channels realised by groups of modes with different velocities, where each channel transmits an elementary amount of information through the propagation of a single space-time soliton, characterised by high luminous power and strong robustness".
"The use of space-time solitons of fixed duration," adds Zitelli, will make it possible to create particularly stable multimode fibre lasers, thanks to the natural predisposition of the light pulse to assume a precise time duration."
"Our work," concludes Stefan Wabnitz, "clarifies the role of space-time solitons in a multimode fibre and contributes to research efforts on the development of new optical transmission techniques and lasers, which will lead to an increase in fibre transmission capacity and the development of new optical sources of ultra-short, high-energy pulses.
These results pave the way for a new type of multimode fibre-optic propagation, applicable in telecommunications and fibre lasers.
Conditions for walk-off soliton generation in a multimode fiber - Mario Zitelli, Fabio Mangini, Mario Ferraro, Oleg Sidelnikov, Stefan Wabnitz - Communications Physics 2021, 4:182. DOI: https://doi.org/10.1038/s42005-021-00687-0
Department of Information Engineering, Electronics and Telecommunications