Guiding light in solid glass is limited by Rayleigh scattering, tiny fluctuations in the glass density, Hollow Core Fibres don’t suffer from this limitation. Hesham Sakr and team have published results for a collection of hollow core fibres guiding light lower than solid glass fibres at wavelengths 660, 850 and 1060 nm. You can see the paper here, at Nature Communications: https://www.nature.com/articles/s41467-020-19910-7 .
Read more at IEEE Spectrum: https://spectrum.ieee.org/tech-talk/semiconductors/optoelectronics/new-hollow-core-optical-fiber-is-clearer-than-glass