Hollow Core Fibres: How Novel Fibres Smashed Loss Records Through Air Guidance8 min read

Silica-glass optical fibres have enabled global connectivity through optical communications networks spanning millions of kilometres [1]. However, as capacity demands accelerate, existing fibre technology is struggling to keep pace due to the fundamental limits of the solid glass medium [2]. Alternative paradigms are necessary to meet future needs. One intensely studied route forward utilizes hollow core fibres containing air rather than solid glass in the core [3]. These unique structures overcome various constraints of conventional fibres and exhibit unusual capabilities.

Hollow core fibres guide light using the principle of total internal reflection (TIR), where light rays propagating along the core undergo near 100% reflection at the core-cladding boundary [4]. To achieve this, the cladding must have an effective refractive index below that of air. Early hollow core fibre designs used metal or dielectric coatings on the interior core surface [5], but suffered from attenuation and reliability issues. In the late 1990s, the emergence of microstructured optical fibres (MOFs) enabled the fabrication of the first hollow-core photonic bandgap fibres (HC-PBGFs) [6], kicking off a new era of research.

Hollow Core Fiber Designs: The Two Routes for Optical Guidance in Air

Photonic Bandgap Fibers: HC-PBGFs contain a microstructured cladding with a periodic array of air holes that creates a photonic bandgap – a wavelength range where light cannot propagate in the cladding [7]. The light inside this bandgap undergoes total internal reflection and thus propagates in the hollow core. The periodic cladding structure provides tight modal confinement and propagation losses below 1 dB/km over a narrow wavelength range [8]. However, fabrication complexity has hindered wider deployment.

Anti-Resonant Fibers: ARF claddings contain multiple glass membranes surrounding an air core [9]. These membranes act as partially reflecting Fabry-Perot resonators to confine light in the core through a combination of anti-resonance and inhibited coupling. Avoiding microstructural features enables simpler fabrication, albeit with higher losses around 1-10 dB/km [10]. ARFs provide extremely broad transmission bandwidths spanning over an octave to meet demands for short and ultrashort high-peak power pulse delivery [11].

Novel Hybrid Hollow Core Fiber: Recently, a novel fibre design combining both PBGF and ARF characteristics was proposed [12]. Termed hollow core nested anti-resonant nodeless fibres (HC-NANFs), this structure contains a series of nested tubular elements in the cladding. Avoiding microstructural nodes provides PBGF-like characteristics while retaining ARF fabrication simplicity. The nested tubes create an additional reflection boundary, enabling strong light confinement through enhanced anti-resonance.

Simulations of the HC-NANF predict remarkably low loss below 0.2 dB/km over a broad 1000 nm bandwidth in the near-infrared [12]. This outperforms any existing hollow core or conventional fibre, with the potential for further improvements. Effective single-mode operation can also be achieved by tuning the nested tube dimensions. The fibre properties hold promise for serving as a versatile platform to replace conventional fibres across diverse application spaces such as telecommunications, ultrafast laser delivery, sensing, and quantum optics.

Hollow Core Fiber Evolution

Photonic bandgap and anti-resonant hollow core fibres have seen intense innovation over the past decade. Competing requirements must be balanced in the designs, including loss minimization, improved confinement, sufficient mechanical stability, and ease of fabrication [13]. Reviewing the progress in hollow core fibre loss reduction helps assess prospects.

Photonic Bandgap Fibers

Early PBGF transmission losses exceeded 1000 dB/km [6]. By 2006, advances in microstructuring enabled mid-infrared guidance under 3 dB/km loss [14]. Losses further dropped below 1 dB/km in the near-infrared by 2015 [8]. However, progress has slowed in recent years as existing designs hit complex fabrication limits. Simplified PBGF architectures removing cladding features are being studied, but have not yet matched earlier performance [15].

Anti-Resonant Fibers

After emerging in 2012 [9], record ARF losses dropped from 30 dB/km to 1 dB/km by 2019 [11]. Unlike PBGFs, ARFs exhibit a smoothly decreasing loss curve as the core diameter grows larger. Recent core scaling experiments revealed ARF losses decaying as λ7/R8, with R the core radius [16]. This dependence suggests the loss could rapidly fall below 0.1 dB/km in a sufficiently large core, competitive with state-of-the-art PBGFs. Larger cores also provide higher damage thresholds and power handling for ultrafast pulse delivery.

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Hollow Core Nested Nodeless Fibers

Leveraging lessons from PBGFs and ARFs, simulations of the proposed HC-NANF predict under 0.2 dB/km transmission loss at 1550 nm [12]. Fabrication simplicity is retained from earlier nodeless ARF designs, while the nested tube geometry enhances confinement similar to a PBGF. Two other important advantages emerge from the simulations:

Rapid Scaling of Loss with Core Size – Due to the nested anti-resonant structure, the loss follows a λ7/R8 dependence on wavelength and core radius. This indicates losses could be rapidly reduced by simply increasing the core size. The team shows losses below 0.1 dB/km may be possible, outperforming any existing hollow-core or solid-core fibre.

Effective Single-Mode Guidance – The dimensions of the nested tubes can be tuned to couple and eliminate higher-order core modes. This resonantly filters out unwanted modes, enabling effectively single-mode behavior which is ideal for many applications like telecommunications.

Remarkable Properties to Unlock New Applications

The proposed HC-NANF combines the strengths of PBGFs and ARFs to push beyond the limits of both existing hollow core fibre platforms. Some of the performance advantages HC-NANFs could provide include:

• Losses below conventional solid-core fibres
• Broad transmission bandwidth exceeding an octave
• Effectively single-mode guidance
• Low latency and non-linearity due to air core
• Wide operating wavelength range
• High laser power handling and damage thresholds

According to the researchers, this exceptional combination of properties could make HC-NANFs key enablers for numerous applications, including:

• High-capacity long haul optical data transmission
• Fiber delivery of ultrafast laser pulses
• Non-linear optics and high peak power handling
• Mid-infrared molecular sensing and spectroscopy
• Low noise interferometry and metrology
• Navigation, gyroscopes, and current sensing

Realizing this application potential demands dedicated efforts to address manufacturing and reliability challenges. Consistent fabrication processes must be developed to transfer simulated designs into high-optical-quality fibres. Improved understanding is also needed of mechanical properties, operating conditions, and potential degradation mechanisms. Finally, system integration work should commence in parallel to facilitate technology transfer upon availability.

With diligent progress, hollow core fibres could unlock unprecedented performance and replace existing solid core fibres for next-generation networks and photonics technologies. Simulations predict immense room for innovation by merging concepts from leading hollow core platforms. Researchers must now rise to the challenge of experimental validation to meet the growing demands of communication systems worldwide.

Future Outlook of Hollow Core Fibres

This review has chronicled the evolution of hollow core fibre technology over 20 years since its inception. Two principal fibre architectures were analyzed: photonic bandgap fibres (PBGFs) and anti-resonant fibres (ARFs). After intense optimization, both platforms now demonstrate losses on par or below conventional solid core fibres. This represents a key milestone towards real-world uptake across a range of photonics applications.

A novel fiber design is also proposed merging the advantages of nested anti-resonant and photonic bandgap concepts. Simulated properties of the hollow core nested nodeless fiber (HC-NANF) predict unprecedented optical performance below 0.2 dB/km loss. Experimental efforts are now critically needed to confirm these simulations and assess integration potential. With diligent progress, hollow core fibres promise immense capacity for technological disruption across communications and sensing. Their unique light-guiding mechanisms continue to offer fresh opportunities to overcome the limits of existing optical systems.

References

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