Microsoft-backed hollow-core fiber boffins show speed boost

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.

The quest for long-distance communication has driven human creativity for centuries, from the use of fire beacons at night in the Old and Middle Ages, to the mechanical optical telegraphs of the Napoleonic era, up to the groundbreaking electric telegraphs of the 1850s. The transmission of the first Morse-coded message across the Atlantic via a sub-sea telegraph cable in 1858 was a monumental achievement that shrank geographical divides and revolutionized communication. The realization in the early twentieth century that modulated radio waves could be reflected by the ionosphere further enhanced communication capabilities, thus enabling long-distance communications even in the absence of a direct connection and of a line of sight. However, the inherent noisiness, unreliability and limited bandwidth of radio wave communication prompted the development of higher-quality cables that could transmit multiple voice calls simultaneously. Heaviside’s coaxial cable, with suitably developed conductive and insulating materials, became the technology that underpinned long-distance transmissions for decades. The transition from coaxial cables to optical fibres marked another notable milestone in communication technology. The pioneering work of Kao and Hockham in the 1960s identified the potential of using purified glass for transmitting modulated optical signals (hence information) to kilometre-scale distances, leading to the development of low-loss optical fibres by Corning in the 1970s. This innovation ushered in the era of digital optical communications, which for the last half a century has formed the backbone of global telecommunication networks and enabled the internet revolution. Is a further step ahead possible?

All these breakthroughs were driven by the primary objective to transmit more information, as either more simultaneous messages and voice calls in the analogue electrical era or more bits per second in the digital age. A second, non-negligible goal has always been the reduction of the attenuation (or ‘loss’) of the transmission medium, to increase the distance that a signal could reach before needing regeneration or amplification. Shannon’s mathematical theory of information linked the two goals: lower attenuation required less amplification; the resulting improvement in the signal-to-noise ratio enabled the system to increase its maximum throughput of information.

Upshifting the frequency of the modulated signal carrier from tens of MHz used in the long-distance electrical coaxial cables to hundreds of THz used in optical communications enabled an increase in information throughput of more than a million times. Simultaneously, optical fibres also presented an ultralow level of attenuation of around 0.15 dB km⁻¹, which remained approximately constant over a bandwidth of ~10 THz where optical amplification from erbium-doped fibre amplifiers was available. This was a substantial improvement over coaxial cables, where attenuation was frequency dependent (as √f) and reached much higher values than optical fibres at the top frequencies (for example, ~4.5 dB km⁻¹ at 30 MHz in the transatlantic TAT-6 cable).

Despite unrelented progress in the field of optical communications since 1970, the minimum attenuation of silica glass fibres has remained approximately unchanged for more than four decades: from 0.154 dB km⁻¹ in 1985 to 0.1396 dB km⁻¹ in 2024. The seemingly insurmountable attenuation limit of ~0.14 dB km⁻¹ for information-carrying waveguides has so far hindered further breakthroughs in communication systems. It has also forced technology to converge to this relatively narrow frequency range of only 5% of the carrier frequency (10 THz at around 192 THz).

Having failed in many decades to identify and synthetize a more transparent glass than silica, a potential route to further lower the propagation loss of a long-distance communication waveguide is to avoid the scattering and absorptions introduced by the glass and which cause loss of signal power in telecoms fibres. This can be achieved by transmitting electromagnetic radiation in a hollow region rather than through a solid glass core. Theoretical foundations, early loss estimates and first experiments for cylindrical, metal, hollow waveguides pre-dated the development of ultra-pure glass fibres. Experimental works from Bell Labs in the mid-twentieth century with dielectric-coated metallic hollow pipes (WT4) reached losses as low as 0.5 dB km⁻¹ at frequencies of 70 GHz and impressive capacities of 476,000 voice channels15. The technology was however discarded in the mid-1970s for installation complexities and techno-economic reasons.

New research in the late 1990s and 2000s investigated the potential for achieving ultralow loss at visible/near-infrared frequencies by transmitting light through hair-thin flexible hollow core fibres (HCFs). These glass-based waveguides could transmit light in an air core, thanks to a periodic ‘holey’ cladding around it that created an out-of-plane photonic bandgap. While such research produced an outstanding new tool for scientific investigations, it failed to attain fibres with attenuation below 1 dB km⁻¹ and with adequate modal purity for long-distance communication. It is only with the advent of a second generation of HCFs, guiding light through antiresonances and inhibited coupling effects in sub-wavelength-thick, core-surrounding membranes, and with the introduction of nested tube designs, that the prospect of achieving sub-0.14 dB km⁻¹ losses became viable. Over the last 6 years, through improved designs and engineering, loss in these nested or double nested antiresonant nodeless hollow core fibres (NANFs/DNANFs) has decreased by an order of magnitude, reaching near parity with the fundamental attenuation of silica glass telecoms fibres at 1,550 nm, and lower values at both shorter and longer wavelengths.

In this work, we showcase the latest advancements in hollow core DNANF technology and present the first optical waveguide that surpasses conventional optical fibres in both loss and bandwidth simultaneously. With a measured loss of under 0.1 dB km⁻¹ across an 18 THz bandwidth, this breakthrough result paves the way for a potential revolution in optical communications, enabling unprecedented data transmission capacities, more energy-efficient optical networks and longer unamplified spans.