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, a different guiding mechanism can be used, based on 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. The name air-guiding fibers is less precise, because it is actually not the air which provides the guidance.
The attractions of hollow-core fibers are mainly that the primary guidance in air minimizes nonlinear effects and makes possible a high damage threshold. Also, one can have low chromatic dispersion. Such features are particularly interesting for guiding ultrashort pulses, where substantial amounts of chromatic dispersion and nonlinearity could lead to severe pulse distortions.
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 .
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 .
A general problem of hollow-core fibers is that their propagation losses are substantially higher than for solid-core fibers – in particular when single-mode guidance is required. There are, however, methods to mitigate that trade-off .
Wavelength Range with Guiding
Another issue is the normally quite limited wavelength range in which the photonic bandgap guiding mechanism works. This wavelength range can be substantially broadened by using a different kind of hollow-core fiber with a so-called Kagomé lattice design [2, 12]. 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 [6, 8, 17]. Some optical properties also differ substantially from those of photonic bandgap fibers. Namely, the wavelength range with good transmission can be much broader, which is useful for many applications, including supercontinuum generation . Also, the slope of the chromatic dispersion is lower, which is beneficial for pulse compression [10, 14, 15]. Some designs exhibit very small overlap of light with the silica structures (order of 0.01%), allowing the guidance of beams with rather high optical peak powers.
The RP Photonics Buyer's Guide contains 9 suppliers for hollow-core fibers. Among them:
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|||S. Im et al., “High-power soliton-induced supercontinuum generation and tunable sub-10-fs VUV pulses from kagome-lattice HC-PCFs”, Opt. Express 18 (6), 5367 (2010), doi:10.1364/OE.18.005367|
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|||P. St. J. Russell et al., “Hollow-core photonic crystal fibres for gas-based nonlinear optics”, Nature Photon. 8, 278 (2014), doi:10.1038/nphoton.2013.312|
|||M. Michieletto et al., “Hollow-core fibers for high power pulse delivery”, Opt. Express 24 (7), 7103 (2016), doi:10.1364/OE.24.007103|
|||J. C. Travers et al., “Ultrafast nonlinear optics in gas-filled hollow-core photonic crystal fibers”, J. Opt. Soc. Am. B 28 (12), A11 (2018), doi:10.1364/JOSAB.28.000A11|
|||M. Bache et al., “Poor-man’s model of hollow-core anti-resonant fibers”, J. Opt. Soc. Am. B 36 (1), 69 (2019), doi:10.1364/JOSAB.36.000069|
|||I. A. Bufeto et al., “Catastrophic damage in hollow core optical fibers under high power laser radiation”, Opt. Express 27 (13), 18296 (2019), doi:10.1364/OE.27.018296|
|||W. Li et al., “151 W monolithic diffraction-limited Yb-doped photonic bandgap fiber laser at ∼978 nm”, Opt. Express 27 (18), 24972 (2019), doi:10.1364/OE.27.024972|
|||S. Habib et al., “Single-mode, low loss hollow-core anti-resonant fiber designs”, Opt. Express 27 (4), 3824 (2019), doi:10.1364/OE.27.003824|