Hollow Core Fiber Used As Resonator

Hollow-core fiber (HCF) used as a laser resonator essentially unifies the "cavity" and "waveguide" into one: light oscillates back and forth in air or vacuum, rather than within quartz glass. Below, we will clearly break down its advantages, corresponding performance data, and boundary conditions.

First. Clarify that "using hollow-core fiber as a resonator" has two meanings:

One is that the hollow-core fiber itself forms the cavity: by adding mirrors, fiber Bragg gratings, or ring structures at both ends of the fiber, a Fabry–Pérot (FP) cavity or ring cavity is formed, where the fiber serves simultaneously as the cavity and the gain/light-guiding medium. The thin glass cladding wall itself acts like a natural FP resonant structure, making anti-resonant hollow-core fibers inherently suitable for cavity applications.

The other is using the hollow-core fiber cavity as a reference cavity or feedback element—for example, connecting a section of hollow-core fiber FP cavity to a laser for self-injection locking, frequency stabilization, or mode filtering.

Both cases share a key feature: the overlap between the optical mode field and the glass is extremely low—on the order of 10⁻⁵—effectively bypassing the problematic properties of glass such as nonlinearity, thermal effects, damage, and absorption. This single factor underlies nearly all of the following advantages.

Second. Core Advantages, Itemized

1. Extremely low nonlinearity—the most fundamental advantage

The Kerr nonlinearity coefficient of anti-resonant hollow-core fiber is 3 to 4 orders of magnitude lower than that of solid-core quartz fiber, as the mode field hardly interacts with the glass.

This leads directly to three benefits: high power/energy levels can be sustained inside the cavity without pulse distortion or spectral broadening caused by self-phase modulation, four-wave mixing, or stimulated scattering; narrow linewidths can be maintained even at high power; and ultrashort pulses experience almost no broadening during transmission. (For instance, the Xi'an Institute of Optics and Precision Mechanics has demonstrated high-fidelity transmission of 5–11 μm femtosecond pulses through infrared hollow-core fiber, tolerating peak powers exceeding 16 MW.)

Dispersion is also significantly reduced (approximately 2–5 ps/(nm·km) @ 1550 nm), resulting in cleaner pulses and easier dispersion management in mode-locked cavities.

2. High damage threshold and strong power-handling capability

Quartz exists only in the thin cladding layer, minimizing thermal load and photodamage risk. If the cavity medium is gas, the gas itself has an extremely high damage threshold.

This directly determines how much power the cavity can withstand. For example, researchers at the University of Southampton achieved −15 dB strong self-injection ratio using a hollow-core fiber FP cavity for self-injection locking, relying precisely on the cavity’s high power tolerance and low loss.

3. Extremely low thermal sensitivity—key to frequency stabilization and ultra-stable cavities

This is the most valuable advantage of hollow-core cavities over solid-core ones:

Light propagates in air, which has a low thermo-optic coefficient, enabling FP cavity temperature sensitivity as low as ~0.27 pm/°C.

Measured long-term frequency stability: laser frequency variation within ±600 kHz over 50 hours; 1 s fractional frequency stability of 4×10⁻¹³; Lorentzian linewidth component of 0.2 Hz—outperforming compact commercial low-noise lasers by an order of magnitude.

If ultra-low expansion (ULE) glass is used instead of standard glass in the hollow-core fiber, thermal sensitivity can be further reduced by more than three orders of magnitude.

The significance? Ultra-stable lasers no longer require massive vibration-isolated vacuum chambers and precision temperature control—instead, a compact hollow-core fiber cavity may achieve performance comparable to free-space ultra-stable cavities.

4. Low loss + ultra-wide guidance bandwidth—enabling broad usable wavelength range

The loss of anti-resonant hollow-core fiber at 1550 nm has been reduced to below 0.1 dB/km (0.091 dB/km, 0.08 dB/km; as reported by Yangtze Optical Fiber in 2026, 0.032 dB/km), falling below the theoretical limit of solid-core quartz fiber at 0.14 dB/km—thanks to the low intrinsic cavity loss and high finesse achievable.

The guiding bandwidth enabled by the anti-resonant mechanism can reach approximately 1000 nm (far wider than the tens of nanometers typical for photonic bandgap fibers), allowing transmission windows spanning ultraviolet, visible, near-infrared, and mid-infrared regions, with cavities capable of multi-band reuse.

5. Hollow core enables gas filling or vacuum pumping, offering free choice of gain media

Gases such as C₂H₂, CO, H₂, CH₄, and N₂O can be introduced into the hollow core, enabling direct laser generation via molecular energy-level transitions or Raman gain. Wavelengths are determined by the gas molecules themselves, unbound by the emission spectra of rare-earth ions.

This technology pushes laser operation into spectral bands unreachable by conventional doped fibers: 3.1 μm acetylene

hollow-core fiber lasers (linear cavity, continuous output of 8.23 W) and 4.8 μm CO hollow-core fiber lasers—where bulk quartz absorption reaches up to 13,000 dB/m, rendering conventional fibers completely unusable, while hollow-core fibers remain functional.

Adjustable gas pressure allows tuning of dispersion, gain, and linewidth. Stimulated Brillouin gain in high-pressure gas-filled hollow-core fibers is about six times higher than in solid-core quartz fibers, enabling low-threshold gas Brillouin lasers and highly sensitive sensing applications.

6. Near-vacuum light speed and low latency

Light propagates through hollow cores approximately 45% faster than in solid-core fibers, resulting in a latency reduction of about 30%. This is particularly advantageous for mode-locked cavities requiring precise cavity length, optical frequency combs, and long-cavity delay structures.

7. Mode quality and engineering form

Single-mode or few-mode operation can be achieved with controllable mode field area and excellent beam quality (experimental M² ≈ 1.15–1.18). In ring cavities, natural unidirectional lasing suppresses spatial hole burning and stabilizes modes.

All-fiber configuration offers compactness, electromagnetic interference resistance, and UV radiation tolerance (hollow-core fibers exhibit minimal solarization in the UV range). They easily integrate with fiber couplers, filters, and modulators, providing better alignment and maintenance compared to free-space cavities.

Last. How to make a HCF resonator

The core difficulty of connecting hollow optical fibers as resonant cavities with FBG, ordinary single-mode optical fibers and other devices can be summarized in one sentence: ‌ needs to simultaneously solve three problems ‌ : "severe mode field mismatch", "direct splicing will burn and collapse the internal air holes", and "extremely high Fresnel reflection at the air-glass interface".

Direct splicing - Directly welding hollow-core optical fibers and ordinary solid-core optical fibers/FBGS together

This is currently the most widely used method in the industrial field. The key is not whether it melts or not, but that the ‌ discharge energy parameter must be precisely set to a trade off‌ : it is necessary to ensure that the welding interface is firmly bonded without burning the microstructure/air holes inside the hollow core.

Shinho S-23 hollow core fiber splicer will help you to get a good splicing loss and tension,with a precise alignment techonolgy and high quality of arc controlling.