Red does not turn green and infrared light does not suddenly become visible when sent through a light guide. Light does not change its wavelength just like that. Unless, that is, you resort to a trick. An international research team has now been able to use this trick effectively in optical fibers for the first time. They are the first to have succeeded in functionalizing optical fibers in such a way that they transform invisible infrared light into red light. Their special fibers have the potential to be used as miniature light converters in the future. The invention is the result of collaboration between four research groups from the University of Jena's Collaborative Research Center NOA and partners at Fraunhofer IOF, Leibniz IPHT and the universities of Sydney and Adelaide (Australia). The scientists from the research group "Photonics in 2D Materials" led by Dr. Falk Eilenberger, Institute of Applied Physics at Friedrich Schiller University, have now been able to publish their research results in "Nature Photonics". Lead author is doctoral student Quyet Ngo.