The current bottleneck with current supercontinuum sources, however, is that they are based on optical fibers that support a single transverse intensity profile or mode, which inherently limits their optical power. What's more, conventional optical fibers are made of silica glass with transmission limited to the visible and near-infrared region of the spectrum. Extension of supercontinuum light to other wavelength regimes such the mid-infrared requires optical fibers made of so-called soft glasses, but these possess a lower damage threshold than silica, limiting even more the power of the supercontinuum beam.
This leads to a self-cleaning mechanism that yields supercontinuum light with high power and a clean beam profile. As well as their many applications, they also provide a means of studying fundamental physics effects such as wave turbulence", says Professor Goëry Genty, the leader of the research group at Tampere University.
While these fibers have recently attracted significant attention from the research community, their use has been, up to now, restricted to the visible and near-infrared. In collaboration with the group of Profs. Buczynski and Klimczak at the University of Warsaw (Poland) and the group of Prof. Dudley in the University of Burgundy France-Comté (France), the Tampere team demonstrated for the first time the generation of a two-octave supercontinuum from the visible to mid-infrared in a non-silica graded-index fiber with a self-cleaned beam.
"This problem has now been solved by using a particular design that utilizes two types of lead-bismuth-gallate glass rods with different refractive indices drawn to yield a nanostructured core. The result is a graded-index fiber with an effective parabolic refractive index profile with transmission up to the mid-infrared, and, as cherry on the cake, enhanced nonlinear light-matter interactions," says researcher Zahra Eslami.
Great Potential in Diagnostics and Monitoring
The mid-infrared is of crucial interest as it contains the characteristic vibrational transitions of many important molecules.
"The novel solution will lead to more efficient supercontinuum light sources in the mid-infrared with many potential applications e.g., for pollutant tagging, cancer diagnostics, machine vision, environmental monitoring, quality and food control," explains Genty.
The researchers anticipate that this novel type of fiber will very soon become an important and standard material for the generation of broadband sources and frequency combs.
The research was carried out at Tampere University and within the Academy of Finland Flagship for Photonics Research and Innovation (PREIN).