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arxiv: 2605.05642 · v1 · submitted 2026-05-07 · ⚛️ physics.optics

Recognition: unknown

Hollow-Core Fiber for Long-Span Optical Frequency Transfer: Improved Instability and Extended Single-Span Reach

Bo Liu, Dan Wang, Dawei Ge, Huibo Hong, Liuyan Han, Qian Zhou, Rongduo Lu, Ruifang Dong, Ru Yuan, Shougang Zhang, Tao Liu, Xiang Zhang, Yiting Liu, Yucan Zhang, Yu Hua

Authors on Pith no claims yet

Pith reviewed 2026-05-08 07:21 UTC · model grok-4.3

classification ⚛️ physics.optics
keywords hollow-core fiberoptical frequency transferphase-coherent transferfrequency instabilitystimulated Brillouin scatteringthermal sensitivitylong-haul fiber linkoptical metrology
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0 comments X

The pith

Hollow-core fiber enables single-span optical frequency transfer over 152 km with 7.3 x 10^{-21} instability at 10,000 s.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper establishes that hollow-core fiber reduces phase noise, propagation delay, thermal sensitivity, and allows much higher input powers before stimulated Brillouin scattering saturates, compared with standard single-mode fiber. These material advantages support both better short-term and long-term frequency stability and remove the need for amplifiers or repeaters over long distances. In a laboratory 152 km link with 0.18 dB/km attenuation, the authors achieve fractional frequency instability of 7.3 x 10^{-21} at 10,000 s and uncertainty of 1.8 x 10^{-20}. This matters for applications that require phase-coherent links between distant optical clocks, such as relativistic geodesy and distributed metrology, because it simplifies the architecture of continental-scale networks. The results identify hollow-core fiber as a medium that relaxes both stability limits and reach limits at once.

Core claim

Hollow-core fiber exhibits lower fiber-induced phase noise and shorter propagation delay than standard single-mode fiber, yielding improved short-term instability; its much lower thermal sensitivity yields nearly one-order-of-magnitude better long-term instability; and its stimulated Brillouin scattering threshold remains above 34 dBm, allowing high-power injection without saturation. Combined with low attenuation, these properties enable single-span optical frequency transfer over 152 km, demonstrated at 7.3 x 10^{-21} fractional instability at 10,000 s and 1.8 x 10^{-20} uncertainty, opening a route to intercontinental networks without dense repeater infrastructure.

What carries the argument

Hollow-core fiber (HCF) transmission link, which confines light to an air-filled core and thereby reduces material-induced thermal phase fluctuations and Brillouin scattering.

If this is right

  • Short-term instability improves because of lower phase noise and shorter light travel time through the fiber.
  • Long-term instability improves by nearly an order of magnitude because thermal phase fluctuations are suppressed.
  • Single-span reach extends to at least 152 km because the Brillouin threshold allows full-power injection and attenuation is low.
  • The architecture of long-haul frequency networks simplifies by eliminating the need for densely spaced amplifiers and repeater control systems.
  • Intercontinental optical frequency networks become feasible with ultrahigh precision using fewer intermediate stations.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • If the lab advantages hold in installed cables, the number of repeater stations required for a 1000 km link would drop sharply.
  • Optical clock networks for geodesy could be built with lower infrastructure cost and simpler operation.
  • Testing a 300 km or longer HCF span under field conditions would directly test whether the stability scales as expected.
  • The same fiber properties may benefit other phase-sensitive applications such as long-distance quantum key distribution or precision timing.

Load-bearing premise

The measured reductions in thermal sensitivity and the high Brillouin threshold will remain unchanged when the fiber is deployed in the field and subjected to real temperature gradients and mechanical stress over distances beyond 152 km.

What would settle it

A deployed HCF link longer than 152 km that shows either rising long-term phase noise above the lab value or stimulated Brillouin scattering saturation below 34 dBm under outdoor temperature swings.

Figures

Figures reproduced from arXiv: 2605.05642 by Bo Liu, Dan Wang, Dawei Ge, Huibo Hong, Liuyan Han, Qian Zhou, Rongduo Lu, Ruifang Dong, Ru Yuan, Shougang Zhang, Tao Liu, Xiang Zhang, Yiting Liu, Yucan Zhang, Yu Hua.

Figure 1
Figure 1. Figure 1: Experimental setup for optical frequency transfer over HCF and SMF links. The view at source ↗
Figure 2
Figure 2. Figure 2: Phase noise PSDs of the 152 km SMF and HCF links. view at source ↗
Figure 3
Figure 3. Figure 3: Thermal response coefficients of the 6 km SMF and HCF links. view at source ↗
Figure 4
Figure 4. Figure 4: Fractional frequency instability comparison between 6 km SMF and HCF links. view at source ↗
Figure 5
Figure 5. Figure 5: Predicted fractional frequency instability of 1,000 km SMF and HCF optical-frequency view at source ↗
Figure 6
Figure 6. Figure 6: Experimental setup for high-power transmission characterization of HCF and SMF links. view at source ↗
Figure 7
Figure 7. Figure 7: High-power transmission and SBS characteristics of the HCF and SMF links. view at source ↗
Figure 8
Figure 8. Figure 8: Beat note SNR under long distance transmission loss.The measured SNRs of the view at source ↗
Figure 9
Figure 9. Figure 9: a) Calculated SBS threshold as a function of fiber length for HCF links with attenuations of 0.18 dB/km and 0.05 dB/km, and for an SMF link with an attenuation of 0.20 dB/km, shown as orange, dark-red, and blue curves, respectively. b) Estimated maximum transmission length as a function of link attenuation for HCF and SMF links, shown as orange and blue curves, respectively. The left and right axes denote … view at source ↗
Figure 10
Figure 10. Figure 10: Measured fractional frequency instability of the 152 km HCF link at different input view at source ↗
Figure 11
Figure 11. Figure 11: (a) Recorded 43,935 frequency data points from the stabilized 152 km HCF link view at source ↗
read the original abstract

Phase-coherent optical frequency transfer is essential for optical clock networking, relativistic geodesy, and distributed precision metrology. However, realizing coherent optical networks spanning thousands of kilometers in standard single-mode fiber (SMF) generally requires densely distributed amplifiers or repeater stations together with complex operational control, while long-term instability remains limited by thermally driven residual phase fluctuations. Here we show that hollow-core fiber (HCF) can simultaneously improve transfer instability and relax the reach limitation of long-span optical frequency transfer. Compared with SMF, HCF exhibits lower fiber-induced phase noise and shorter propagation delay, supporting improved short-term instability, while its much lower thermal sensitivity supports nearly one-order-of-magnitude better long-term instability. In addition, for long-haul HCF links, no observable stimulated Brillouin scattering induced saturation is found up to the maximum available injected power of 34 dBm, whereas the threshold of an equal-length SMF link remains only a few dBm. Together with the lower attenuation achievable in modern HCF, this enables ultra-long single-span optical frequency transfer. Using a 152 km HCF link with an average attenuation of 0.18 dB/km, we demonstrate single-span optical frequency transfer, achieving a fractional frequency instability of 7.3 x 10^-21 at 10,000 s and a fractional uncertainty of 1.8 x 10^-20. These results establish HCF as a transmission medium that simultaneously improves instability and extends single-span reach, opening a practical route toward future intercontinental optical frequency networks with ultrahigh precision.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 0 minor

Summary. The manuscript reports an experimental demonstration of phase-coherent optical frequency transfer over a 152 km hollow-core fiber (HCF) link with average attenuation 0.18 dB/km. It compares HCF to standard single-mode fiber (SMF), showing lower phase noise, shorter propagation delay, lower thermal sensitivity, and higher stimulated Brillouin scattering threshold (no saturation up to 34 dBm in HCF vs. few dBm in SMF). This enables single-span transfer, yielding a fractional frequency instability of 7.3 × 10^{-21} at 10,000 s and fractional uncertainty of 1.8 × 10^{-20}.

Significance. If the laboratory results hold, this has substantial significance for optical clock networking and precision metrology. The work supplies concrete instability and uncertainty numbers together with direct SMF comparisons, establishing that HCF simultaneously improves long-term stability and extends single-span reach. These strengths support the potential to reduce reliance on dense amplifier chains in large-scale networks.

major comments (1)
  1. Abstract and Results: The central claim that HCF 'opens a practical route toward future intercontinental optical frequency networks' rests on the laboratory-observed reductions in thermal sensitivity and Brillouin threshold persisting in deployed fiber. No field-deployment data, environmental-stress testing, or discussion of performance under uncontrolled temperature gradients, vibrations, or mechanical stress is provided, which is load-bearing for the extrapolation beyond the 152 km lab link.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the positive assessment of the significance of our work and for the constructive feedback. We address the major comment below, agreeing that the extrapolation requires qualification and proposing targeted revisions to the manuscript.

read point-by-point responses

  1. Referee: Abstract and Results: The central claim that HCF 'opens a practical route toward future intercontinental optical frequency networks' rests on the laboratory-observed reductions in thermal sensitivity and Brillouin threshold persisting in deployed fiber. No field-deployment data, environmental-stress testing, or discussion of performance under uncontrolled temperature gradients, vibrations, or mechanical stress is provided, which is load-bearing for the extrapolation beyond the 152 km lab link.

    Authors: We agree that the manuscript does not include field-deployment data or environmental-stress testing, and that this limits the strength of the extrapolation to intercontinental scales. The demonstrated reductions in thermal sensitivity (arising from the anti-resonant guidance and minimal silica overlap) and the elevated SBS threshold (due to the larger effective mode area and hollow-core nonlinearity) are intrinsic waveguide properties independent of the laboratory environment. Prior HCF deployments in telecommunications have indicated robustness to vibrations and temperature variations, though we do not claim equivalence here. To address the concern, we will revise the abstract to replace 'opens a practical route' with 'suggests a promising route based on laboratory characterization' and add a dedicated paragraph in the discussion section. This paragraph will explicitly note the absence of field data, outline why the core advantages are expected to translate (with supporting references to HCF material properties), and identify key environmental factors (temperature gradients, vibrations, mechanical stress) that would require future validation or mitigation strategies analogous to those used in SMF links. revision: yes

Circularity Check

0 steps flagged

No significant circularity: experimental demonstration with no derivation chain

full rationale

The manuscript is an experimental measurement campaign reporting direct observations of phase noise, thermal sensitivity, Brillouin threshold, and frequency transfer instability over a 152 km HCF link. No equations, fitted parameters, or derivations are presented that reduce the central results (7.3e-21 instability at 10 ks, 1.8e-20 uncertainty) to prior inputs by construction. Comparisons to SMF are empirical, and no self-citation load-bearing steps, ansatz smuggling, or uniqueness theorems appear in the text. The work is self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Experimental demonstration relying on established optical physics; no free parameters fitted to data, no ad-hoc axioms, and no new entities postulated.

pith-pipeline@v0.9.0 · 5635 in / 1155 out tokens · 72163 ms · 2026-05-08T07:21:10.613998+00:00 · methodology

discussion (0)

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    2010