Hollow-core Fibers

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 loss. 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 [35]. Similarly low losses appear to be possible in a wider wavelength region, where silica absorption or scattering is substantially higher. It may even be possible to develop practical telecom fibers with losses below the theoretical limit of solid-core silica fibers (around 0.15 dB/km at 1550 nm), which 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 with kilowatt average powers [44]. 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.

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) and makes possible a high damage threshold. Note that the Kerr effect in air is about three orders of magnitude weaker than in glass, mostly due to the low density.

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 is usually close to the vacuum velocity of light. This implies substantially lower latency for signal transmission through hollow-core fibers.

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 [36] 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

The envisaged main application areas of hollow-core fibers are the following:

• They can be used for delivering laser radiation, for example with high average power in wide wavelength regions (but possibly with a narrow linewidth) from the ultraviolet to the mid infrared, or for radiation in the form of ultrashort pulses with high peak power.
• They may be useful for data transmission, particularly in cases where a very low latency (time delay) is vital. Also, one may 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.
• Miniature Raman lasers 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].

Note that although the development of hollow-core fibers started a longer while ago, it took substantial time to develop the technology such that it became suitable for applications. Therefore, it remains to be seen which application areas will be particularly important, and which further applications can be developed.

See also: photonic bandgap fibers, photonic crystal fibers