Hollow-core Fibers

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

A hollow-core fiber is an optical fiber which guides light essentially within a hollow region, so that only a minor portion of the optical power propagates in the solid fiber material (typically a glass). According to the standard physical mechanism for guiding light in a fiber, this should not be possible: normally, the refractive index of the fiber core has to be higher than that of the surrounding cladding material, and there is no way of obtaining a refractive index of glass below that of air or vacuum, at least in the optical spectral region. However, other guiding mechanism can be used:

• One possibility is to exploit a photonic band gap, as can be realized in a photonic crystal fiber with a certain structure. Such fibers are also called photonic bandgap fibers. (Note that not all photonic bandgap fibers have a hollow core.)
• A particularly simple design (also leading to simplified production) is that of the revolver hollow-core fibers [12, 24] containing a pattern of silica rings (with circular or elliptical cross-section) around the hollow core; those are not using a photonic bandgap and cannot be considered as photonic crystal fibers. The fiber preform can be made relatively simply by arranging a number of silica capillaries, and these result in thin glass membranes after drawing into a fiber. A more refined version contains additional smaller rings nested within the larger rings [21, 33], and can provide further reduced propagation losses. A loss reduction can also be achieved already by slightly separating the tubes, avoiding nodes where they would touch each other. The term negative curvature fibers underlines the boundary curvature in a direction opposite that to a ring around the core. Other terms, containing the attribute antiresonant, emphasize the aspect of loss reduction by designing the glass structure for optical anti-resonance, i.e., suitable relative phase changes for reflection at different interfaces.

• Another example for negative curvature fibers is the Kagomé fiber [2, 14], featuring a hypocycloid core-cladding boundary.

Often, such fibers feature a very low overlap of the optical mode field with the solid structure, so that the light propagates mostly in air. The name air-guiding fibers is also used as a general term for hollow-core fibers, but is less precise because it is actually not the air which provides the guidance.

Many hollow-core fibers, particularly those not based on a photonic band gap but rather on simpler anti-resonance structures, have a relatively large hollow core – with a diameter or e.g. 30 times the optical wavelength – and correspondingly large effective mode areas.

Special Properties

Various special properties of hollow-core fibers are relevant for different types of applications:

Wavelength Range with Guiding

The wavelength range in which the photonic bandgap guiding mechanism works is normally quite limited. That can be a limitation for some applications, while it can be exploited for others – for example, for suppressing the propagation of unwanted (e.g. Raman-shifted) light.

That wavelength range with light guidance can be substantially broadened by using a hollow-core fiber with a so-called Kagomé lattice design [2, 14]. That can be useful for supercontinuum generation [10], for example. The operation principle of the Kagome fiber design profoundly differs from that of a photonic bandgap fiber; it does not rely on a photonics bandgap [7, 9, 19]. Some optical properties also differ substantially from those of photonic bandgap fibers. For example, the slope of the chromatic dispersion is lower, which is beneficial for pulse compression [11, 16, 17]. Some designs (particularly those with large mode areas) exhibit a very small overlap of light with the silica structures (order of 0.01%), allowing the guidance of beams with rather high optical peak powers.

Propagation Loss

The propagation losses of hollow-core fibers were initially far higher than for solid-core fibers – in particular when single-mode guidance is required. There are, however, quite effective methods to mitigate that problem [15]. Recently, some hollow-core fibers with much reduced losses – roughly comparable to those of state-of-the-art silica fibers with a solid core in the optimum wavelength region around 1.5 μm, have been achieved and also at shorter wavelengths [35, 36, 52]. Similarly low losses appear to be possible in a wider wavelength region, where silica absorption or scattering is substantially higher. It seems even possible to develop practical telecom fibers with losses below the theoretical limit of solid-core silica fibers [53], which is around 0.15 dB/km at 1550 nm and is essentially set by Rayleigh scattering.

The low overlap of the intensity profile with the glass makes it possible even to guide light at wavelengths where the transparency of the glass material is relatively poor. For example, this has been demonstrated with high-energy pulses from an Er:YAG laser at 2.94 μm [13], and even with kilowatt average powers [45] (notably with single-mode spatial characteristics). Even light from CO₂ lasers at 10.6 μm can be guided with such fibers [3]. Hollow-core fibers are thus interesting for high power beam delivery in a wide range of wavelengths and powers, although limited to spatially single-mode laser sources.

The propagation losses are also much less affected by radiation effects than in solid-core fibers [54]. Hollow-core fibers can thus be regarded as a kind of radiation-resistant fibers.

Low Reflection

In contrast to solid glass fibers, hollow-core fibers exhibit extremely weak end reflections: the usual Fresnel reflections at the fiber ends are essentially absent.

Weak Nonlinearities

The fact that light is primarily guided in air, having only a weak spatial overlap with the glass structure, minimizes nonlinear effects (particularly for ultrashort pulses with high peak power). Note that the Kerr effect in air is about three orders of magnitude weaker than in glass, mostly due to the low density.

This aspect is very relevant for the transmission of light at high powers (see below), e.g. high peak powers of ultrashort pulses, but also for various applications like fiber-optic gyroscopes.

High Damage Threshold

Hollow-core fibers can have a much higher damage threshold in terms of transmitted optical average or peak power than solid-core fibers, again because most of the optical power is in air, and only a tiny fraction in the glass material.

Kilowatt-level continuous powers have been demonstrated [50], which is only possible due to a very low propagation loss. Furthermore, narrow-band light has been transmitted at such power levels without encountering nonlinear limitations.

The peak power in ultrashort pulses transmitted through fibers is usually limited to a few megawatts (for silica fibers, and less for other fiber materials) by the onset of catastrophic self-focusing. For hollow-core fibers, that limit does not apply, and far higher peak powers appear to be possible. The limits appear not to be known yet.

Chromatic Dispersion

Chromatic dispersion of such fibers can be engineered via the fiber design, particularly for photonic bandgap fibers with small mode area. This is also particularly interesting for guiding ultrashort pulses, where substantial amounts of chromatic dispersion and nonlinearity could lead to severe pulse distortions.

Fibers with a large hollow core typically exhibit quite weak chromatic dispersion, with little dependence on the design details. That can also be useful for delivering ultrashort pulses, for example.

High Group Velocity, Low Latency Signal Transmission

The group velocity of guided light in hollow-core fibers is usually close to the vacuum velocity of light, in contrast to conventional silica fibers having group velocities which are roughly 30% lower. This implies substantially lower latency for signal transmission through hollow-core fibers. In some specific application fields of optical fiber communications, such as artificial intelligence and high-speed trading, this is quite relevant. Therefore, such fibers are expected to be applied in data centers, but perhaps also for longer transmission distances.

Raman Interactions in Gases

One may also exploit the high optical intensity in air or in some other gas filled into the fiber – for example, for realizing Raman lasers [5], Brillouin amplifiers [37] or for high harmonic generation [8].

Reduced Coupling to Laser-active Dopants

In some cases, it is useful to have a low overlap of the optical field with the laser-active dopant in a rare-earth doped fiber. For example, this can help to realize a 978-nm Yb-doped fiber laser or fiber amplifier, where it is otherwise more difficult to suppress unwanted emission at longer wavelengths [31].

Main Applications

Being a rather new and not yet mature technology, hollow-core fibers have no well established commercial application areas yet. However, some application areas of hollow-core fibers can be envisaged:

• They would be useful for data transmission, particularly in cases where a very low latency (time delay) is vital – for example, for certain data centers used in high-frequency trading. Also, one may in the future exploit low-loss data transmission in spectral regions where the absorption losses of solid-core silica fibers are too high. By transmitting light with an overall substantially larger optical bandwidth, one may realize higher transmission capacities in terms of data rate.
• Hollow-core fibers can be used for delivering laser radiation in situations where conventional fibers are subject to serious limitations. For example, one can transmit light in wavelength regions where other fibers are very lossy, or for light transmission with high peak power, in particular of ultrashort pulses of light.
• Their naturally low tendency for developing radiation-induced losses make them interesting for special application areas such as space and nuclear plants.
• There are also conceivable applications for specialized markets related to spectroscopy and optical sensing. For example, miniature Raman lasers in special wavelength regions, based on gases, can be realized because the interaction of a light beam guided by a hollow fiber is far more intense than that of a free-space beam in a multipass gas cell. Similarly, a number of other nonlinear functions can be realized with gas-filled hollow fibers [34].
• In high-intensity physics, hollow capillaries with relatively large gas-filled hollow regions are used for nonlinear pulse compression. Although these capillaries are not hollow-core fibers in the usual sense, they have some similar properties.

More to Learn

Suppliers

The RP Photonics Buyer's Guide contains 19 suppliers for hollow-core fibers. Among them:

GLOphotonics

Explore the distinctive hollow-core photonic crystal fiber technology, guiding light within a hollow channel and enveloped by a microstructured cladding. A unique approach that redefines precision and efficiency in the field of photonics.

As a trailblazing industrial leader, GLO takes the forefront in photonics with a diverse array of hollow-core photonic crystal fiber solutions, tailored to meet the unique needs of our valued partners. Our commitment lies in delivering bespoke HCPCF innovations, setting new standards in the industry.

NKT Photonics

Hollow-core photonic bandgap fibers turn conventional fiber technology inside out by guiding the light in a hollow-core. This unique waveguide is ideal for sensing, imaging, and ultrashort pulse applications. Our hollow-core photonic bandgap fibers deliver ultrashort pulses without non-linear effects or material damage and tolerate tight bending without impact on transmission. Hollow-core fibers are radiation insensitive and suitable for harsh radiation environments.

Guiding Photonics

Guiding Photonics fabricates hollow core fibers with a metallic reflective coating on the inner side of a hollow glass or plastic tube. For infrared transmitting fibers, there is also a dielectric layer with a thickness optimized for a specific wavelength range.

Exail

Optical signals in a hollow core photonic bandgap fiber are guided in an air core surrounded by a PBG microstructured region. In addition to the low bend sensitivity, this fiber design exhibits significantly reduced material nonlinearities since more than 95% of optical power is propagating in air. In addition, air/undoped silica provides excellent temperature immunity, which is critical for high performance fiber sensing and metrology applications.

Optical signals in a hollow core anti-resonant fiber propagate in an air core surrounded by a single ring of anti-resonant tube elements. Guidance is based on an anti-resonance from the thin glass membranes constituted by the non-touching tubes surrounding the hollow core. The extremely low overlap of guided power with the surrounding silica, less than 2 · 10⁻⁵, in addition to the large effective mode area, leads to a record-low optical nonlinearity.

few-cycle

The new few-cycle flexible hollow core fiber system allows you to choose various fiber lengths and inner diameters to achieve a desired nonlinear effect. Experimentally measured transmission for multi-mJ femtosecond pulses ranges between 50% and >90%, depending on the application.

The most versatile choice for laser pulse post compression: The few-cycle hollow core fiber supports input energies from 50 μJ to 100 mJ, up to 20 times compression and transmission >90% while keeping the footprint and optical path length at a minimum.

art photonics

art photonics offers hollow glass waveguides based on fused silica capillaries. Different transmission regions in the range 2–18 μm are possible. Inner diameters are between 500 and 1000 μm. A double polymer coating with high mechanical flexibility is used.

UltraFast Innovations

UltraFast Innovations (UFI®) manufactures complete hollow-core fiber compressors to compress high-energy ultrashort pulses by nonlinear spectral broadening in a gas within the hollow-core fiber and subsequent compression with chirped mirrors.

Bibliography

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