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.

We propose an approach to interconnect a hollow-core fiber (HCF) of arbitrary core size with standard single-mode fiber with perfect mode-field size adaptation and experimentally achieve for the first time insertion loss agreeing with that predicted by simulations. We demonstrate this using three low-loss HCFs, including 1 st window nested antiresonant nodeless fiber (NANF), 2 nd window NANF and the state-of-the-art double NANF (DNANF). The connection with a minimum achieved insertion loss of 0.079 dB was permanently secured via gluing and did not degrade during 4 weeks of continuous measurement. To the best of our knowledge, this is the lowest reported value and is comparable to or lower than the connection between dissimilar single-mode fibers (e.g., standard single-mode fiber and dispersion-compensating fiber). We also show that such connection leads to excellent suppression of higher-order modes coupling, of importance to all applications sensitive to multi-path interference. Importantly, obtaining agreement between simulations and experiments validates for the first time the accuracy of the simulations and opens the door to further optimization via simulations with the ability to subsequently achieve the same result experimentally.

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.

Optical Time Domain Reflectometry (OTDR) is a key technique to characterize fabricated and installed optical fibers. OTDR of emerging Hollow Core Fibers (HCFs) has been demonstrated only very recently, being almost 30 dB weaker than in the glass-core optical fibers. Additionally, it has been challenging to extract useful data from the OTDR traces of HCFs, as the longitudinal variation in the fiber’s geometry, notably the core size or the longitudinal variations of the air pressure within the core result in commensurate changes of the backscattering strength. Here, we demonstrate, for the first time, that the distributed loss and backscattering coefficient in antiresonant HCFs can be separated, obtaining key data about the fiber distributed loss and uniformity. This is enabled by using OTDR traces obtained from both ends of the HCF.

We propose a hollow-core fiber (HCF) end-cap that incorporates a short segment of a single-mode fiber (SMF) that serves as a modal filter. To adapt the end-cap input and output beam to the desired size, the SMF was fusion spliced with short segments of mode-field adapting graded index (GRIN) fibers on both sides. The end-cap is anti-reflective coated to minimize insertion loss and parasitic reflections. The presented proof-of-concept experiments show its ability to suppress coupling into HCFs' higher-order modes. For example, without any end-cap, the extinction ratio between the LP $_{11}$ and the fundamental mode was found to be as low as 9 dB when coupling light from a free-space beam that was misaligned by as little as 1.1 $^{\circ }$ . This was improved to 23 dB when inserting the developed end-cap. Such small angle misalignment often exists when aligning the input beam with 3-axis (x,y,z) stages only (rather than 5-axis that also include pitch and yaw). Finally, we glued the end-cap with the HCF, providing hermetic sealing to the HCF input/output. This is of interest for stable operation in applications that use free-space light launch or require HCF output into free space.

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.

We identify empirical thermal and bending attributes of PMD in short nodeless antiresonant fibers (ARFs). Surprisingly, bend-scaling properties are comparable to solid fibers, albeit with greater magnitude, while temperature dependence is more varied.

We summarize our recent work developing a technique for accurate and nondestructive measurement of the microstructure geometry of nested and double nested antiresonant fibers. We present results showing microstructure variation along a 2.2 km fiber.

We report a PM-16QAM transmission experiment through hollow-core NANF with reduced inter-modal interference. We recirculated 41 C-band channels at 32GBaud up to 1150km with average GMI 7.14 bits/symb. For selected channels we reached beyond 1500km, the current record for long-haul PM-16QAM transmission over NANF.

We demonstrate a 3× thermal phase sensitivity reduction for a hollow-core fiber (HCF) Fabry-Perot interferometer by winding the already low temperature sensitivity HCF on to a spool made from an ultralow thermal expansion material. A record low room temperature fiber coil phase thermal sensitivity of 0.13 ppm/K is demonstrated. The result is of particular interest in reducing the thermal sensitivity of HCF-based Fabry-Perot interferometers (for which existing thermal sensitivity reduction methods are not applicable). Our theoretical analysis predicts that significantly lower (or even zero) thermal sensitivity should be achievable when a spool with a slightly negative coefficient of thermal expansion is used. We also suggest a method to fine-tune the thermal sensitivity and analyze it with simulations.

SummaryQ factor over 90 billion is demonstrated for the first time in hollow core fibre based Fabry-Perot resonators. This is achieved thanks to low-loss of the latest hollow core fibers. Such high Q factor together with other beneficial properties of hollow core fibers such as low thermal sensitivity and high power handling is of interest for a range of applications including laser stabilization in metrology.

We show experimentally that unwanted crosscoupling into higher-order modes of a hollow-core fiber can be reduced to values close to -40 dB when optimizing the input beam size for their suppression rather than optimizing it for the minimum insertion loss.

Unfortunately, we have made a mistake on the name of co-author and missed the DOI of our dataset. It would have no influence on any of our results, and the correct name and DOI are given here.

The first nested anti-resonant hollow-core fiber radiation study is reported. A record near-zero radiation induced attenuation is observed under γ-rays and X-rays. These results open new possibilities for fiber-based applications in radiation environments.

This work proposes a technique to elucidate mode coupling when antiresonant fibers are twisted and bent. From this, we show how the birefringence of a nested antiresonant nodeless fiber changes as function of the deployment.

We demonstrate the simultaneous distribution of stable frequency and data signals over hollow-core fibers (HCF). We show that the use of HCF reduces frequency stability degradation observed in links made of standard single-mode fibers (SMFs) due to nonlinear interaction between the co-propagating signals which is orders of magnitude weaker in HCF.

While the idea of guiding light in an air or vacuum filled core predates the development of ultra-low loss glass in the 1970s, HCFs as we know them today stem from seminal work at the University of Bath in the mid-1990s. Initial fibers were made of silica and air, with a 2D photonic crystal extending indefinitely along the fiber axis to guide light in a core defect (i.e., filled with air or vacuum) by means of a photonic bandgap [1]. The very first reported HCF could only guide light in air through 40 mm of fiber [2], but the breakthrough nonetheless stimulated a first wave of industrial and academic excitement that in the space of only a few years managed to bring the loss of such photonic bandgap guiding HCFs down 1.7 dB/km [3]. Despite predictions that lower than 0.2 dB/km might soon be achievable, loss in such fibers has not been improved substantially since those heroic works. Subsequent research demonstrated that surface scattering fundamentally limits the loss that can be achieved in such HCF type, and large values of intermodal interference (IMI) put an upper bound to its maximum reach.

We propose a new solid-core to hollow-core fiber connection where an SMF is offset-spliced with an angle-cleaved graded index mode field adapter. We show through simulations and an initial experiment that this results in simultaneous low-loss and low backreflection.