<p>A critical component of optical communications is the availability of a suitable waveguide technology for the transport of electromagnetic waves with low loss over a broad spectral range. In the past four decades, despite extensive research, the attenuation and spectral bandwidth of silica-based optical fibres have remained relatively unchanged, with state-of-the-art fibres offering values of 0.14 dB km⁻¹ and 26 THz below 0.2 dB km⁻¹, respectively. Here we report a microstructured optical waveguide with unprecedented transmission bandwidth and attenuation, with a measured loss of 0.091 dB km⁻¹ at 1,550 nm that remains below 0.2 dB km⁻¹ over a window of 66 THz. Instead of a traditional solid glass core, this innovative optical fibre features a core of air surrounded by a meticulously engineered glass microstructure to guide light. This approach not only reduces attenuation and other signal degradation phenomena, but it also increases transmission speeds by 45%. Furthermore, the approach theoretically supports further loss reductions and operation at wavelengths where broader bandwidth amplifiers exist, potentially heralding a new era in long-distance communications as well as remote delivery of laser beams.</p>