nonlineAr Multimode and mUlticore optical fiberS for multIple appliCations

Optical fibers are of utmost importance in several areas. Their use has revolutionized the telecommunication landscape, leading to ultrafast data transmission in the Internet network. In addition, they are widely used in sensor technology, medicine, astronomy, spectroscopy and lasers, just to name a few.
A ray of light that propagates into an optical fiber may have many different shapes, called modes. For example, a mode may have circular shape, similar to a beam of light projected onto the wall by a laser pointer. Most of the present optical fiber technology is based on single-mode fibers, where only one mode can propagate and then be exploited, that is the mode having circular shape. Recent years have witnessed a huge interest in multimode (MM) and multicore (MC) fibers, which support the propagation of several modes besides the circular one. The exploitation of many different modes paves the way to new opportunities beyond what is possible in single-mode fibers. However, this comes at the expense of a much more complex dynamics whose understanding is far from being complete, which includes several kind of nonlinear interactions between the different modes.
It is in this framework that the project AMUSIC has arisen, with the expectation to advance the state-of-the-art understanding of nonlinear effects in MM and MC fibers. The project aimed at developing an optimized numerical platform for the analysis of nonlinear effects in MM and MC fibers, and then at investigating new devices where nonlinear effects are exploited in view of important applications in several key-areas, from tunable MM optical amplifiers and oscillators up to novel high-power fiber lasers.

The first part of the project has been dedicated to the development of a numerical platform, which has been implemented in the form of a software toolbox with files written in Matlab and Mathematica languages. This toolbox allows one to simulate the propagation of, and interaction among, many modes in MM- and MC fibers, as well as to design MM/MC fiber-based devices (e.g. amplifiers and lasers) and to predict their performance. This platform has been widely used through the whole project.
The initial numerical investigation led to a new understanding of the nonlinear multimode dynamics and to major results.
First of all, the observation and comprehension of long-haul vector soliton data transmission, which is based on the nonlinear interaction among 2 polarization modes of an optical fiber. These numerical results have been obtained in collaboration with the group of Dr. J.Fatome at the University of Bourgogne, and have found full confirmation in experiments led by Dr.Fatome. These outcomes, which gained the cover of the prestigious journal Nature Photonics (Vol.11,Issue 2,Feb 2017, https://www.nature.com/nphoton/volumes/11/issues/2 ), demonstrate that multimode nonlinear interactions can be exploited to achieve almost undistorted data transmission, which may strongly reduce the power consumption and complexity of the current Internet network.
The project has also allowed developing novel fundamental ideas for the design of broadband MM parametric amplifiers and oscillators, which have led to the first experimental implementation of a fiber-based wavelength converter of several different modes. These results represent an important step towards a new generation of efficient all-optical devices that may be essential key-technology in the Internet network of the future.
Finally, the project has allowed modelling and running some preliminary numerical simulations that clearly show a novel, robust mechanism to achieve coherent beam combination in MC fiber lasers. These simulations show that nonlinearity may induce a strong coupling between the modes in different cores, such that the light beams in each core organize themselves in regular patterns. Under proper conditions, the beams may add in phase in a single core, giving rise to a single “giant” beam characterized by an extremely high power density.

As previously outlined, all the mentioned outcomes represent a step-forward with respect to the state-of-the art understanding and exploitation of nonlinear effects in MC and MM fibers.
From a longer term perspective, the successful outcomes of this project could lead to a breakthrough technology in strategic and emerging areas. The vector soliton data transmission, along with fiber-based wavelength converters and the novel ideas for the design of wideband MM amplifiers, represent key-knowledge in view of the development of a new Internet network being more efficient, faster and less power consuming than the present one.
The study of MM oscillators paves the way to novel fiber-based optical sources accessing frequencies that are not achievable when using single-mode fibers. These may be frequencies in the mid-infrared spectrum, leading to new fingerprinting techniques for personalised, non-invasive detection of disease by breath analysis.
Finally, the initial investigation of coherent combination in MC fibers opens the way to novel compact and powerful lasers providing extremely narrow light beams, to be used for medicine and for precise material processing (e.g. cutting).

Periodic Reporting for period 1 - AMUSIC (nonlineAr Multimode and mUlticore optical fiberS for multIple appliCations)

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