Researchers at the University of Tampere have successfully developed a novel fiber optic design that enables the generation of rainbow laser light in the electromagnetic region of molecular fingerprints. This new optical fiber with a self-cleaning beam can help develop applications for, for example, tagging pollutants, diagnosing cancer, monitoring the environment and controlling food. The finding was published in the journal Communication Nature.
When an ultrashort, high-power light pulse interacts with a material such as a glass optical fiber, a range of highly nonlinear interactions occur that cause complex changes in the temporal and spectral properties of the injected light. When taken to extremes, such interactions can lead to the generation of a rainbow laser of light commonly referred to as a supercontinuum light source. Since its first demonstration in a special type of optical fiber in 2000, supercontinuum laser light has revolutionized many fields of science, ranging from metrology and imaging at unprecedented resolution to ultra-wideband remote sensing and even sensing. of exoplanets.
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. Additionally, conventional optical fibers are made of silica glass with transmission limited to the visible and near infrared region of the spectrum. The extension of supercontinuum light to other wavelength regimes such as mid-infrared requires optical fibers made of so-called soft glasses, but these have a lower damage threshold than silica, further limiting the power of the supercontinuum beam.
Silica-free optical fiber with a self-cleaning beam
Recently, a different type of optical fiber with a refractive index that varies continuously through the fiber structure has been shown to produce a dramatic increase in supercontinuum power, while preserving a smooth beam intensity profile. “Variation in the refractive index of these graded-index optical fibers leads to periodic focusing and defocusing of light within the fiber that allows coupling between spatial nonlinear light-matter interactions and time. This leads to a self-cleaning mechanism that produces supercontinuum light with high power and a clean beam profile. In addition to their many applications, they also provide a means to study fundamental physical effects such as wave turbulence,” explains Professor Goëry Genty, head of the research group at the University of Tampere.
Although these fibers have recently attracted the attention of the research community, their use has until now been limited to the visible and near infrared. In collaboration with the group of Prs. Buczynski and Klimczak at the University of Warsaw (Poland) and the group of Prof. Dudley at the University of Burgundy France-Comté (France), the Tampere team demonstrated for the first time the generation of a two-octave supercontinuum from visible to mid-infrared in a silica-free gradient index fiber with a self-cleaning beam.
“This problem has now been solved by using a particular design that uses two types of lead-bismuth-gallate glass rods with different refractive indices drawn to give a nanostructured core. The result is a gradient-index fiber with a effective parabolic refractive index profile with transmission down to the mid-infrared and, icing 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 because it contains the characteristic vibrational transitions of many important molecules.
“The new solution will lead to more efficient mid-infrared supercontinuum light sources with many potential applications, e.g. for pollutant labeling, cancer diagnosis, machine vision, environmental monitoring, quality and control. food,” says Genty.
The researchers predict that this new type of fiber will very soon become an important and standard material for the generation of broadband sources and frequency combs.
The research was conducted at the University of Tampere and at the Flagship Academy of Finland for Photonics Research and Innovation (PREIN).