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.

Throughout history, the development of novel technologies for long-distance communications has had profound influences on societal progress. Landmark scientific discoveries have enabled the transition from short message transmissions via single-wire electrical telegraphs to voice communications through coaxial cables, and ultimately to the optical fibres that powered the internet revolution. Central to these advancements was the invention of novel waveguides to transport electromagnetic waves with lower attenuation over broader spectral ranges. In the past four decades, despite extensive research, the spectral bandwidth and attenuation of silica-based telecommunication fibres have remained relatively unchanged. In this work, we report an optical waveguide with an unprecedented bandwidth and attenuation. Its measured loss reaches 0.091 dB/km at 1550 nm and remains below 0.2 dB/km over 66 THz, substantially better than the 0.14 dB/km and 26 THz achievable with existing technology. Our innovative, microstructured optical fibre replaces the traditional glass core with air, employing a meticulously engineered tubular glass structure to guide light. This approach not only reduces attenuation and other signal degradation phenomena, but it also increases transmission speeds by 50 operation at wavelengths where broader bandwidth amplifiers exist, potentially heralding a new era in long-distance communications.

When a hollow core fiber is drawn, the core and cladding holes within the internal cane geometry are pressurized with an inert gas to enable precise control over the internal microstructure of the fiber and counteract surface tension forces. Primarily by considering the temperature drop as the fiber passes through the furnace and the geometrical transformation of the internal microstructure from preform-to-fiber, we recently established that the gas pressure within the final 'as-drawn' fiber is substantially below atmospheric pressure. We have also established that slight changes in the gas refractive index within the core and surrounding cladding holes induced by changes in gas pressure are sufficient to significantly affect both the modality and loss of the fiber. Here we demonstrate, through both simulations and experimental measurements, that the combination of these effects leads to transient changes in the fiber's attenuation when the fibers are opened to atmosphere post-fabrication. It is important to account for this phenomenon for accurate fiber characterization, particularly when long lengths of fiber are drawn where it could take many weeks for every part of the internal microstructure to reach atmospheric pressure.

We report the fabrication of a hollow-core DNANF with a geometry extensively optimized for minimum loss. Three independent loss measurements average 0.08±0.03 dB/km at 1550 nm, the lowest attenuation ever achieved in an optical fiber.

We report a double-nested antiresonant hollow core fiber designed for ~850nm operation. The measured fiber loss is 0.33dB/km at 850nm across a single span of 10.9km.

Let E be a holomophic vector bundle over a compact Astheno-Kähler manifold ( M, ω ). The authors would prove that E is a numerically flat vector bundle if E is pseudo-effective and the first Chern class c_1^BC ( E ) is zero.

The feasibility of coexisting a quantum channel with carrier-grade classical optical channels over Hollow Core Nested Antiresonant Nodeless Fibre (HC-NANF) is experimentally explored for the first time in terms of achievable quantum bit error rate (QBER), secret key rate (SKR) as well as classical signal bit error rates (BER). A coexistence transmission of 1.6 Tbps is achieved for the classical channels simultaneously with a quantum channel over a 2 km-long HC-NANF with a total coexistence power of 0 dBm. To find the best and worst wavelength position for the classical channels, we simulated different classical channels bands with different spacing between the quantum and classical channels considering the crosstalk generated from both Raman scattering and four-wave-mixing (FWM) on the quantum channel. Following our simulation, we numerically estimate the best (Raman spectrum dip) and worst locations (Raman spectrum peak) of the classical channel with respect to its impact on the performance on the quantum channel in terms of SKR and QBER. We further implemented a testbed to experimentally test both single-mode fibre (SMF) and HC-NANF in the best and worst-case scenarios. In the best-case scenario, the spacing between quantum and classical is 200 GHz (1.6 nm) with 50 GHz (0.4 nm) spacing between each classical channel. The SKR was preserved without any noticeable changes when coexisting the quantum channel with eight classical channels at 0 dBm total coexistence power in HC-NANF compared to a significant drop of 73% when using SMF at $-$24 dBm total coexistence power which is 250 times lower than the power used in HC-NANF. In the worst-case scenario using the same powers, and with 1 THz (8 nm) spacing between quantum and classical channels, the SKR dropped 10% using the HC-NANF, whereas in the SMF the SKR plummeted to zero.

Silica glass optical fibers have revolutionized data transmission, sensing and laser development over the past 50 years. Moreover, dielectric waveguides with a hollow core offer exciting development possibilities beyond traditional technology. Hollow Core Optical Fibers (HCFs) have been fabricated over the past 20 years with various geometries and refinements, yet their attenuation has remained significantly higher than can be routinely achieved in standard silica single mode fibers. Here we present recent developments in Nested Anti-resonant Nodeless Fiber (NANF) design over the last few years and show how this rapidly developing technology has been refined to produce state of the art HCFs at wavelengths between 850 – 1625 nm.

This dataset contains measurement data and processing scripts (Matlab) for the results presented in "Super-broadband on-chip continuous spectral translation unlocking coherent optical communications beyond conventional telecom bands". The paper can be found here: Super-broadband on-chip continuous spectral translation unlocking coherent optical communications beyond conventional telecom bands | Nature Communications Activating Unconventional Wavelength Bands for Coherent Optical Communication by On-chip Continuous Spectral Translation | Research Square

We report the first double-nested antiresonant hollow core fiber. The fiber matches the loss of commercial solid core fibers in the C-band (0.174 dB/km) and fundamentally improves it (0.22 dB/km) in the O-band.

Despite being perceived historically to provide "unlimited" bandwidth, today’s optical communication systems are fast approaching their capacity limits in the conventional telecom bands. To satisfy the ever-increasing capacity demand, opening up new wavelength bands is becoming an appealing solution both in cabled and free-space optical communications in the transparent windows. However, this would ordinarily require the development of whole optical transceivers for any new wavelength band, which is both time-consuming and expensive. Here, we present an on-chip continuous spectral translation based coherent optical communication approach that can exploit existing commercial transceivers to unlock the vast and currently unused potential new wavelength bands rather than dictate the need to develop new coherent transceivers operating directly in those bands. The spectral translators are continuous-wave pumped aluminium gallium arsenide on insulator (AlGaAsOI) nanowaveguides that can provide a continuous conversion bandwidth over an octave. This enables unprecedented coherent transmission in the 2-μm wavelength band using well-developed conventional C-band transmitters and coherent receivers. We demonstrate 318.25-Gbit s -1 Nyquist wavelength-division multiplexed coherent transmission over a 1.15-km hollow-core fibre in the 2-μm wavelength band using this approach. Our demonstration paves the way for coherently transmitting, detecting, and processing signals at wavelength bands beyond the capability of today’s devices.

We measure the changes in transmission properties of two different hollow core fiber types exposed to standard atmosphere over nearly one year. No degradation of transmitted power is observed for the hollow-core NANF studied.

We present a study into the gas pressure and composition within a hollow core optical fiber immediately after fabrication. Results from three different experimental techniques indicate that the initial absolute pressure inside of the hollow core is significantly lower than atmospheric pressure. By measuring the equilibrium height to which water ingresses into the hollow core, we estimate the absolute internal gas pressure to be 20 kPa < ${{\boldsymbol{P}}_{\boldsymbol{i}}}$ < 29 kPa for the fibers reported here that were fabricated using standard techniques. The initial gas composition within the hollow core was studied using Raman and absorption spectroscopy and the evolution of the gas composition provides indirect information about the condition of the silica surfaces inside the fiber. The measurements indicate that these internal surfaces become saturated with atmospheric water vapor as this is drawn into the open-ended fiber, initially due to a pressure gradient post-fabrication. Our findings are an essential foundation for the study of long-term optical and mechanical performance of hollow core fibers and important for accurate characterization of these specialty fibers. The first is becoming increasingly important as commercial applications of these fibers expand.

We demonstrate transient changes in the optical properties, specifically the loss, of antiresonant hollow core fibres (HCFs) due to a combination of the sub-atmospheric gas pressure inside the fibre holes post-fabrication and the subsequent gas induced differential refractive index (GDRI) between the core and cladding elements of the fibre; this is temporarily created while the gas pressures inside the core and cladding elements are evolving after the HCF ends are opened up to surrounding atmospheric pressure. Here we show experimental evidence of this effect in two different HCF designs; for both fibres, the transmitted power initially increases, reaches a maximum, and then reduces to its initial level. We show via gas flow simulations that the timeline of this behaviour is consistent with the gas flow rates into the core and cladding elements of the tubular HCF studied and the subsequent transient differential gas pressure. The experimental results also show (in line with GDRI expectations) that this transmission (loss) change is higher at shorter wavelengths. Our results imply that this transient change in the fibre's optical properties must be considered for accurate fibre characterisation; this is particularly true for long fibre lengths where the equalisation of the fibre's internal gas pressure with atmospheric pressure could take many weeks.

Quantum Key Distribution (QKD) technology has been considered as the ultimate physical layer security due to its dependencies on the physical laws of physics to generate quantum keys. However, for QKD to become functional for practical scenarios, it must be integrated with the classical optical networking infrastructure. Coping with optical nonlinearity from the classical represents a major challenge for QKD systems. In this paper, we take the advantage of the ultra-low nonlinearity of Hollow Core Nested Antiresonant Nodeless Fibre (HC-NANF) to demonstrate the coexistence of discrete-variable quantum key distribution channel with carrier-grade classical optical channels over a 2 km HC-NANF.

We demonstrate the first low-latency 2 -µm-band PAM signal transmission using a hollow-core fiber (HCF). PAM-8 and PAM-16 signals are successfully transmitted over 1.15-km HCF with line rates of 96 Gbit/s and 100 Gbit/s, respectively.

We demonstrate for the first time the coexistence of a quantum-channel and 8×200 Gpbs 16-QAM optical channels with launching powers as high as -9dBm/channel in a 2 km HC-NANF. Comparative analysis with single-mode fibre reveals that the quantum-channel could not be sustained at such power-levels.

The performance of Hollow Core Fibers has improved dramatically over the last 6 years. We report progress of the most successful design, Nested Antiresonant Nodeless Fiber, with losses of 0.28 dB/km.

In this paper, we present results of long-term stability tests of a low-loss (<0.55 dB) hollow core fiber (HCF) to standard optical fiber interconnection prepared by modified gluing-based fiber-array technology. We measured insertion loss of three interconnected HCF samples over a period of 100 days at room temperature, observing a variation in insertion loss of less than 0.02 dB. Subsequently, we placed the HCF samples in a climatic chamber and heated to +85°C in four cycles. Maximum insertion loss variation of 0.10 dB was observed for HCF samples with angled 8° interconnections and only 0.02 dB for a HCF sample with a flat interconnection.