What is Hollow-Core Fiber (HCF): Principles, Advantages, and Applications?

1. Basic Concept of HCF

Hollow-Core Fiber (HCF) is a revolutionary optical fiber structure that fundamentally differs from traditional solid-core fibers. While conventional fibers guide light through total internal reflection in a solid glass or plastic core, HCF features a hollow central core, where light propagates primarily through air or vacuum, confined by a specially designed microstructured cladding.

Proposed in the 1990s by Prof. Philip Russell’s team, HCF has evolved into multiple mature designs, including:

· Photonic Bandgap HCF (HC-PBGF): Uses periodic microstructures to create wavelength-specific "bandgaps" that trap light in the core.

· Anti-Resonant HCF (HC-ARF): Relies on thin glass membranes to reflect light via destructive interference.

· Kagome Fibers: Employ a lattice-like cladding for broadband transmission.

2. Working Principles

HCF guides light through two key mechanisms:

00001. Photonic Bandgap Effect: Periodic microstructures (e.g., air-hole arrays) create forbidden wavelength ranges, confining light to the hollow core—analogous to electron confinement in semiconductors.

00002. Anti-Resonant Reflection: Thin cladding walls reflect light at specific angles, minimizing leakage into the surrounding glass.

Unlike traditional fibers, >99% of light travels in air, drastically reducing interactions with glass. This enables unique advantages.

3. Five Core Advantages

(1) Ultra-Low Loss & Nonlinearity

· Losses below 0.1 dB/km (approaching theoretical limits).

· Nonlinear effects 10³–10⁵× weaker than solid-core fibers, enabling distortion-free high-power transmission.

(2) Faster Signal Propagation

· Light travels ~30% faster in air (n≈1) vs. glass (n≈1.45), critical for low-latency trading and quantum synchronization.

(3) Superior Laser Handling

· Transmits kW-class continuous lasers and MW-level pulses without thermal damage or nonlinear distortions (e.g., stimulated Raman scattering).

(4) Enhanced Light-Matter Interaction

· The hollow core can be filled with gases, liquids, or nanoparticles for high-sensitivity sensing (e.g., methane detection, biomolecule analysis).

(5) Extreme Environmental Stability

· Radiation-resistant (no radiation-induced darkening).

· Temperature-insensitive (air’s refractive index is more stable than glass).

4. Key Applications

5. Challenges & Future Trends

Current Limitations:

· Complex fabrication (nanoscale precision required).

· Coupling losses with conventional fibers.

· Higher cost than standard fibers.

Future Directions:

· Multi-core HCF for space-division multiplexing.

· Integration with silicon photonics.

· Functionalized coatings (e.g., for dynamic tuning).