Researchers at Tampere University collaborated with colleagues at the University of Warsaw and the University of Burgundy France-Comté to create a supercontinuum light source in the mid-infrared, using a nonsilica, graded-index, multimode fiber. The collaborators generated a two-octave supercontinuum in the fiber, which could create a path toward bright, ultrabroadband light sources for applications in the mid-infrared that require high spatial beam quality and high power. The researchers said the optical fiber could be used in environmental sensing, molecular fingerprinting, microscopy, medical diagnostics, gas monitoring, spectroscopy, lidar, and more. The team added that it believes the fiber could become a standard tool for the generation of broadband sources and frequency combs. “As well as their many applications, [the fibers] also provide a means of studying fundamental physics effects such as wave turbulence,” professor Goëry Genty said. Using two types of glass with different refractive indexes and that are stacked in a specific arrangement, researchers developed a multimode fiber with a parabolic refractive index that enabled transmission up to the mid-infrared and high nonlinearity. The spectrum of short pulses of light injected into the fiber massively broadened, to span from the visible to mid-infrared. The light beam remained smooth because of self-cleaning dynamics induced by the parabolic refractive index. Such a light source, with and ultrabroad spectrum, smooth beam, and high power, could be used in environmental sensing or high-resolution imaging for medical diagnostics. Courtesy of Tampere University. To generate a supercontinuum with characteristics similar to supercontinuum spectra in the visible and near-infrared, the researchers constructed an optical fiber using two types of lead-bismuth-gallate glass rods, with different refractive indexes drawn to provide a nanostructured core. The resulting multimode fiber demonstrated an effective parabolic refractive index profile and tenfold increased nonlinearity compared to silica fibers. The parabolic refractive index induced self-cleaning dynamics that kept the light beam smooth. Due to the enhanced nonlinear refractive index, the lead-bismuth-gallate fiber yielded a spectrum with a significantly larger bandwidth than a conventional fiber. When the researchers injected femtosecond pulses into the fiber, a two-octave supercontinuum from the visible to the mid-infrared (700 to 2800 nm) was created. The team conducted a systematic investigation of the supercontinuum-generating mechanism as a function of pump wavelength and pump power. The results showed evidence of nonlinear beam cleaning. The researchers also characterized the relative intensity noise of the fiber in several wavelength bands across the supercontinuum spectrum. Under certain controlled injection conditions, they found clear signatures of self-cleaning dynamics, with a near single-mode spatial intensity distribution at the fiber output. When they compared their experimental results with spatiotemporal numerical simulations, the researchers further confirmed the self-cleaning dynamics. They also performed an experimental comparison with a supercontinuum generated in a graded-index silica fiber. This showed that the spectrum generated in the new, nonsilica optical fiber extended further into the mid-infrared as the result of an enhanced nonlinear refractive index. “The refractive index variation of such graded-index optical fibers leads to periodic focusing and defocusing of the light inside the fiber, which enables coupling between spatial and temporal nonlinear light-matter interaction,” Genty said. “This leads to a self-cleaning mechanism that yields supercontinuum light with high power and a clean beam profile.” Since it was first demonstrated in 2000, supercontinuum laser light has been used in scientific study in areas ranging from metrology to imaging to ultrabroadband remote sensing. The Tampere team said that until now, supercontinuum beam generation in graded-index fibers has been demonstrated using silica fibers only, and supercontinuum beams have been demonstrated in the visible and near-infrared spectral regions only. “This problem has now been solved by using a particular design that utilizes two types of lead-bismuth-gallate glass rods with different refractive indexes drawn to yield a nanostructured core,” researcher Zahra Eslami said. “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.” The research was published in Nature Communications (www.doi.org/10.1038/s41467-022-29776-6).