Big fat clean light pipes: Novosibirsk researcher passes a milestone that could lead to delivering high power and high quality light beams over multimode fibers

Since the invention of fiber optics in 1960, a key question has remained so far unanswered: how to transport light with a multimode optical fiber from a powerful and bell-shaped laser beam, without compromising its spatial coherence? Answering to this important question could open the way to using multimode glass fibers for a variety of key photonic technologies, from lensless endoscopy to laser cutting and oil well drilling, and even help solving the internet capacity crunch. A team of researchers from the Novosibirsk State University, the University of Limoges, France, the University of Burgundy Franche-Comté, France, and the University of Brescia, Italy, discovered an elegant and robust way to cancel the natural tendency of multimode optical fibers to scramble the pixels of images that they carry, because each of them travels with its own speed. Quite strikingly, they found that by simply increasing the beam power above a certain level, all image pixels suddenly synchronize and travel in unison, thus keeping intact the initial bell-like shape of the laser beam. The results of this work were published on 13 march 2017 in the journal “Nature Photonics”.

Optical glass fibers provide a natural means of transporting laser beams over long distances without spreading them in space, in virtually loss free manner. More than 99% of all fibers in the world are the so-called single-mode fibers. In them, light is guided by total internal reflection in a micrometer sized thin hair of glass known as the fiber core. These fibers do allow for the propagation of bell-shaped light beams, however because of their extremely small cross-section the optical energy that they can deliver is also very small.

For the delivery of powerful laser beams, which is a must in a variety of industrial applications like metal cutting, welding and drilling, the only solution is to use glass fibers with a very big cross-section. Light propagating in these fat pipes of light is however naturally spread in hundreds of individual modes of oscillation, like the music of a symphonic orchestra resonates in a noise-like myriad of independent instruments before the concert begins. As a result, a light beam with initial smooth bell-shaped profile is quickly transformed into a cacophony of colors (see figure, left-panel).

Stefan Wabnitz, 2016 Megagrant winner at the Physics Department of the Novosibirsk State University and his colleagues have found that, in the presence of a sufficiently strong nonlinear coupling among all of the fiber modes, they get mutually synchronized and pulsate in unison, as when the director signals to the musicians that the concert begins. The spectacular result of this nonlinear synchronization of oscillators, which is fully analogous to the synchronization of coupled pendula discovered by the dutch scientist Christiaan Huyghens in the 17^th century, is that the superposition of hundreds of modes coherently vibrates in the glass pipe to sustain a clean, bell shaped, fat beam of light.

The results of this work have been supported, among others, by the Ministry of Science and Education of the Russian Federation through the Megagrant programme, and the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie initiative.

Reference of the article:

Spatial beam self-cleaning in multimode fibres by K. Krupa, A. Tonello, B. M. Shalaby, M. Fabert, A. Barthélémy, G. Millot, S. Wabnitz, and V. Couderc, Nature Photonics, DOI: 10.1038/nphoton.2017.32