Ultra-stable laser sources typically achieve long-term frequency stability by locking to a bulky and costly vacuum-operated Fabry-Perot cavity made of ultra-low expansion material such as ultra-low expansion glass (ULE). In this work, we demonstrate that long-term stability can be achieved with a specially designed fiber interferometer operated at a zero-temperature-sensitivity crossing point, a feature typically achieved only with cavities made from zero-expansion materials. The ultra-low temperature sensitivity is achieved by using a combination of a hollow-core optical fiber that provides the required delay and a short segment of a standard single-mode fiber that provides temperature compensation. Additionally, we placed the interferometer in an airtight aluminum enclosure to mitigate the effect of environmental pressure fluctuations. A laser locked to this interferometer exhibits +/- 550 kHz peak-to-peak frequency variation over 100 h of continuous operation, and a frequency drift below 20 Hz/s. The corresponding Allan deviation reaches 2 x 10(-14)tau (for tau > 100 s), rivaling the performance of miniature ULE cavities. Compared to previously reported fiber and waveguide-based systems operating in ambient conditions, our approach offers superior long-term frequency stability. Owing to its compactness, low cost, and alignment-free design, this system provides a promising solution for deployable frequency references in applications such as geophysics, field spectroscopy, and space-based sensing.

We report the first hollow-core DNANF fiber with ultra-low loss in two separate antiresonance windows. The fiber, featuring a novel hybrid-thickness geometry, measures 0.11dB/km at 1550nm and, simultaneously, a record-low loss of 0.13dB/km at 1015nm.

Power-over-fibre (PoF) can improve future network resilience by simultaneously delivering power and connectivity to remote equipment in situations where a supply based on copper cables is neither feasible nor appropriate. However, in standard silica fibres, the high optical powers have an impact on data signals due to nonlinearities, decreasing maximum data rates. Hollow-core fibre (HCF) as a transmission medium has a marked decrease in nonlinearity, and its applicability to PoF is studied here. Two experiments using 3.1 and 1.2km lengths of HCF are conducted to investigate the benefits of this fibre as a medium to carry high-power optical signals alongside high-throughput data signals. The study of diverse wavelengths highlights the versatility of HCF for power transmission.

Low-frequency acoustic waves exhibit low attenuation and strong penetration, enabling propagation over long distances in water and the solid Earth. This capability underpins applications in natural-hazard monitoring and early warning (earthquakes, tsunamis, volcanic activity), long-range marine sensing and surveillance, and structural and equipment-health monitoring in harsh environments. Traditional piezoelectric hydrophones suffer from high self-noise and electro-magnetic interference, motivating the exploration of fiber optic solutions. However, the operation of fiber solutions at low frequencies suffers from temperature cross-sensitivity due to the inherent thermal sensitivity of conventional optical fibers. Hollow-core fibers (HCFs), which guide light in air/vacuum, provide lower thermal and higher acoustic sensitivity than conventional solid core fibers; however, the extent to which they could address this limitation has never been studied. Here, we present a comprehensive study of the acoustic and thermal sensitivities of HCFs, combining theoretical modeling and experimental validation, aiming for high acoustic sensitivity while keeping thermal sensitivity low, hence reducing unwanted cross-sensitivity. We validate its results experimentally on three HCFs with different structural compositions, quantifying the trade-off between acoustic and temperature sensitivities. Besides, we demonstrate a substantial improvement in temperature-acoustic decoupling by employing an HCF made from Ti-doped ultralow-expansion (ULE) glass, achieving a reduction in thermal sensitivity by more than three orders of magnitude compared with standard single-mode fiber while maintaining comparable acoustic sensitivity. Further, based on the theoretical model we have developed, we show that more optimal coating material could improve the performance of ULE-HCF by another two orders of magnitude. This work thus guides comprehensive design of thermally stable and acoustically sensitive HCF sensors for low-frequency acoustic sensing. Published by Chinese Laser Press under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. https://doi.org/10.1364/PRJ.584448

Hollow-core optical fibres can offer significantly increased radiation hardness relative to solid-core fibres for applications in high-radiation environments such as fusion reactors. Here, we report on gamma and 14 MeV neutron radiation tests of anti-resonant hollow-core fibres guiding in the visible wavelength range, where radiation-induced point defects in silica glass generally cause significantly higher absorption relative to the typically investigated near-infrared range. Characterisation includes spectral loss measurements of radiation-induced attenuation during irradiation followed by post-irradiation Raman scattering gas analysis of the air-filled hollow core. In the gamma-radiation test, hollow-core fibres were exposed to gamma-radiation from Co-60 up to a total dose of 659 kGy(SiO2). There was no evidence of radiation-induced attenuation related to the silica glass microstructure surrounding the hollow core. However, visible-range absorption linked to the radiolytic production of nitrogen dioxide and chlorine in the air-filled core was evident, which was confirmed by the post-irradiation Raman gas analysis. In the neutron test, hollow-core fibres were exposed to 14 MeV neutrons from the deuterium-tritium reaction up to a mean fluence of 4.2×1011 n/cm2. There was no measurable radiation-induced attenuation, but the Raman gas analysis again revealed trace amounts of nitrogen dioxide and elevated amounts of chlorine. These tests provide further evidence of the radiation-hardness inherent to anti-resonant hollow-core fibres and highlight that this radiation-hardness prevails even at visible wavelengths. Future work will explore evacuated cores and inert filling gases, and low-chlorine content silica glass for the fibre fabrication to mitigate absorptions due to radiation-induced gas chemistry.

By optimizing input beam parameters, we demonstrate that cross-coupling into $\mathbf{L} \mathbf{P}_{02}$ mode can be adjusted by changing input beam mode-field diameter via the mode-field adaptation technique.

Battery performance degradation arises from multifactorial and complex electrochemical processes involving all the battery components. Therefore, identifying and mitigating/suppressing the main degradation mechanisms is essential for deploying these technologies. Such a holistic understanding requires advanced and dedicated analytical tools as well as effective strategies to recover battery capacity, extend battery life, and facilitate second-life use and recycling of devices. Here, we develop a set of optical and microfluidic methodologies that enable operando chemical analysis of liquid electrolytes using Raman spectroscopy. We demonstrate that our methods have minimal perturbation on device operation and can be readily embedded in standard manufacturing processes. A bi-directional microfluidic access port allows the extraction and reinjection of carbonate-based electrolytes for analysis over hundreds of electrochemical cycles. Our data show that lithium salt anion (PF6 -) degradation is a major source of electrolyte degradation and capacity loss in industrially relevant Li-ion pouch cells (Ni-rich layered cathodes (NMC811), and graphite anodes). The effects are particularly strong when cycled to an upper-cut voltage of 4.3 V. Finally, we demonstrate that battery capacity loss can be partially reversed by re-infusing pristine, uncycled electrolyte via an integrated microfluidic access port. This strategy not only mitigates degradation and extends battery life but also offers a practical pathway toward second-life applications and adaptable cell chemistries for future battery technologies.

Wavelength widely tunable infrared fiber lasers that simultaneously deliver high pulse energies with narrow linewidths are critical for applications ranging from spectroscopy to nonlinear optics, yet achieving this combination has remained a long-standing challenge. Here, we demonstrate that gas-filled anti-resonant hollow-core fiber Raman laser offers tunability across a broad spectral range from the near-infrared ( 1.4 μm) to the mid-infrared ( 4.6 μm), with near microjoule level high pulse energy and a narrow linewidth of few gigahertz or less. This performance arises from a unique pump laser design together with an optimized selection of gas-filled anti-resonant hollow-core fibers, opening a promising pathway toward compact, high-performance tunable infrared fiber-based sources.

Quantum-dot single-photon sources are promising for quantum communication. Yet, the most advanced devices operate near 900 nm, where standard single-mode fibers experience significant losses. We address this by employing a hollow-core fiber engineered for low-loss transmission at quantum-dot wavelengths, with measured loss of 0.65 dB/km and potentially as low as 0.13 dB/km near 934 nm. The same fiber also supports strong classical signals at 1550 nm without introducing detectable Raman noise. Using this platform, we transmit all four BB84 polarization states from an InAs quantum dot over 340 m with a quantum bit error rate of 0.1% while preserving single-photon purity and indistinguishability even in the presence of a strong classical signal. These results establish an integrated transmission platform that combines a custom-engineered anti-resonant hollow-core fiber with co-existence of classical and quantum channels, enabling practical quantum-dot-based quantum key distribution beyond the conventional telecom bands.

We present a compact all-fiber interconnection component designed for liquid-filled hollow core fiber application. Based on micro-optic collimator technology, this device integrates laser delivery, liquid injection, and output signal collection, offering excellent compactness and robustness.

Frequency conversion of ultrafast lasers is fundamentally constrained by the trade-off between pulse energy and repetition rate, limiting access to regimes critical for fundamental science and industry. Here, we demonstrate a frequency-conversion mechanism in which molecular coherence accumulates across ultrafast pulse trains in gas-filled hollow-core fibers. Unlike conventional nonlinear interactions initiated by individual high-energy pulses, this mechanism relies on the collective buildup of coherent molecular oscillations driven by successive pulses. Using this mechanism, we achieve Raman frequency conversion at repetition rates up to 3 GHz with nanojoule pulse energies. The results establish a regime of nonlinear optical interaction governed by coherence accumulation of gas molecular oscillations with broad implications for ultrafast laser science and frequency conversion technologies.

Hollow-core antiresonant optical fibers (ARFs) have emerged as a contender to succeed standard solid-core single-mode fibers. Their polarization properties are of primary importance for the operation of polarization-sensitive optical systems. Appropriate models are therefore required to design ARFs that can deliver light with precise polarization performance. This work proposes a method based on coupled-mode theory to extract the local polarization properties, in terms of birefringence and dichroism, of bent and twisted ARFs. The method is applied to a 5-tube nested antiresonant nodeless hollow-core fiber (NANF), and the results are discussed, introducing a model for the behavior of birefringence and dichroism as a function of bending radius and twist rate. Once birefringence and dichroism are obtained, their model can be used to study the polarization evolution and the polarization-dependent loss of ARFs, where the equations involved are the same as those of standard fibers. The proposed method is not limited to bend and twist, and can be applied to any perturbed ARFs where the propagation is effectively single-mode.

Antiresonant hollow core fibers are significantly more complex to fabricate than solid core fiber, in part due to the need to maintain the uniformity of the complex microstructure along the entire fiber and the lack of techniques available to profile its geometry non-destructively. In this Letter, we present results showing accurate non-destructive measurements of a hollow core fiber microstructure. By combining two previously reported methods, we overcome the limitations of using either technique alone to yield accurate measurements of crucial geometrical parameters, including the distance between the two fusing points where the capillary microstructure elements meet the surrounding jacket tube and the capillary diameter itself. For the DNANF tested here, this combined method offers a substantial improvement to the middle and outer capillary diameter measurements, reducing the discrepancies from around 6% for each measurement to 0.6% and below.

Today, low-loss hollow-core fibers are slightly multimoded. Although they are usually engineered to have high attenuation of all higher-order modes, unwanted multi-path interference can occur in applications using short fiber lengths. This can be mitigated by coupling into the fundamental mode with minimized unwanted coupling into higher-order modes. Two of these modes are particularly prominent: LP$_{11}$ has the lowest attenuation of all higher-order modes, and LP$_{02}$ shares the same symmetry with the fundamental mode. This shared symmetry was predicted to cause significant unwanted coupling into the LP$_{02}$ when the launch beam mode-field diameter is not carefully optimized. This, however, has not been studied experimentally, to the best of our knowledge. Here, we use an all-fiber launch technique that allows for precise control of the input beam mode-field diameter, and we study the LP$_{02}$ coupling in detail, using two hollow-core fibers with different geometries (i.e., nested antiresonant nodeless fiber and double nested antiresonant nodeless fiber), operating over different antiresonant windows. We confirm that unwanted coupling into the LP$_{02}$ mode is very sensitive to the input mode-field diameter. The results show that the unwanted coupling into both LP$_{11}$ and LP$_{02}$ can be kept below -35 dB in a practical all-fiber launch system. Notably, we show that low fundamental mode loss alone does not guarantee low higher-order mode coupling, underlining the need for precise mode-field diameter matching for higher-order modes control.

We show that hollow-core fibre outperforms single-mode fibre at three-channel 28 GBd DP-16QAM transmission when copropagating with an up to 37.5 dBm feed. There is no sign of power-dependent signal degradation in the hollow-core fibre.

Ultra-stable lasers are fundamental to a growing range of applications, including optical frequency metrology, fundamental physics and quantum sensing. Their outstanding performance is achieved by stabilizing their frequency to Ultra-Low Expansion (ULE) optical cavities. However, the complexity of fabrication and assembly of these systems - even for compact designs - has been limiting their widespread deployment. While micro-resonators and optical fibre delay lines offer alternatives, their performance is significantly limited by thermally-induced frequency drift. Here we demonstrate, for the first time to the best of our knowledge, a laser stabilised to a Hollow Core Fibre (HCF) achieving comparable performance to ULE cavity-stabilised lasers. We achieve a frequency instability of 4.6x10-15 at 1 s and a frequency drift of 88 mHz/s, reducible to 3.7 mHz/s with thermal correction. Furthermore, over 3-year characterization confirms the HCF's predictable long-term behaviour. These results and the simplicity of the HCF-based system pave the way to a high-performance and scalable solution for ultra-stable laser sources.

Pulse compressors are critical components of high-energy ultrafast fiber laser systems. Bulk optical elements like diffraction gratings and chirped mirrors are commonly used to de-chirp and compress output pulses from ultrafast fiber lasers. However, these free-space optics require precise alignment, occupy significant space, and introduce undesired power loss due to their multi-pass configuration. Direct pulse compression through optical fiber provides significant advantages, including all-fiber architecture, compactness, and flexible pulse delivery. However, traditional solid-core fibers face challenges in compressing high-energy pulses due to strong nonlinearity. In contrast, hollow-core fibers (HCFs), which guide light through air rather than glass, significantly reduce nonlinearity, enabling efficient pulse compression and ultrashort pulse delivery. Moreover, HCFs can be designed to provide anomalous dispersions at 1 μm, making them ideal for compressing positively chirped pulses from ytterbium (Yb) fiber lasers.

Hollow Core Fibers require gas pressure during fabrication to achieve a precise geometry. A model and experiment have been developed to understand gas flow into the microstructure that leads to sub-atmospheric pressure in fabricated fibers.