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 region. However, a different guiding mechanism can be used, based on a photonic bandgap, 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. Indeed, it has become 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 . Also, one may exploit the high optical intensity in air or in some other gas filled into the fiber – for example, for realizing Raman lasers  or for high harmonic generation .
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 .
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 7 suppliers for hollow-core fibers. Among them:
|||R. F. Cregan et al., “Single-mode photonic band gap guidance of light in air”, Science 285, 1537 (1999) (first hollow-core PCF)|
|||J. Benabid et al., “Stimulated Raman scattering in hydrogen-filled hollow-core photonic crystal fiber”, Science 298, 399 (2002)|
|||P. J. Roberts et al., “Ultimate low loss of hollow-core photonic crystal fibres”, Opt. Express 13 (1), 236 (2005)|
|||F. Couny et al., “Subwatt threshold cw Raman fiber-gas laser based on H2-filled hollow-core photonic crystal fiber”, Phys. Rev. Lett. 99 (14), 143903 (2007)|
|||F. Couny et al., “Generation and photonic guidance of multi-octave optical-frequency combs”, Science 318, 1118 (2007)|
|||G. J. Pearce et al., “Models for guidance in kagome-structured hollow-core photonic crystal fibres”, Opt. Express 15 (20), 12680 (2007)|
|||O. H. Heckl et al., “High harmonic generation in a gas-filled hollow-core photonic crystal fiber”, Appl. Phys. B 97, 369 (2009)|
|||S. Im et al., “Guiding properties and dispersion control of kagome lattice hollow-core photonic crystal fibers”, Opt. Express 17 (15), 13050 (2009)|
|||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)|
|||O. H. Heckl et al., “Temporal pulse compression in a xenon-filled Kagome-type hollow-core photonic crystal fiber at high average power”, Opt. Express 19 (20), 19142 (2011)|
|||A. Urich et al., “Delivery of high energy Er:YAG pulsed laser light at 2.94 μm through a silica hollow core photonic crystal fibre”, Opt. Express 20 (6), 6677 (2012)|
|||P. Ghenuche et al., “Kagome hollow-core photonic crystal fiber probe for Raman spectroscopy”, Opt. Lett. 37 (21), 4371 (2012)|
|||J. M. Fini et al., “Low-loss hollow-core fibers with improved single-modedness”, Opt. Express 21 (5), 6233 (2013)|
|||Ka F. Mak et al., “Tunable vacuum-UV to visible ultrafast pulse source based on gas-filled Kagome-PCF”, Opt. Express 21 (9), 10942 (2013)|
|||K. F. Mak et al., “Two techniques for temporal pulse compression in gas-filled hollow-core kagomé photonic crystal fiber”, Opt. Lett. 38 (18), 3592 (2013)|
|||B. Debord et al., “Hypocycloid-shaped hollow-core photonic crystal fiber, Part I: Arc curvature effect on confinement loss”, Opt. Express 21 (23), 28597 (2013)|
|||P. St. J. Russell et al., “Hollow-core photonic crystal fibres for gas-based nonlinear optics”, Nature Photon. 8, 278 (2014)|
|||M. Michieletto et al., “Hollow-core fibers for high power pulse delivery”, Opt. Express 24 (7), 7103 (2016)|
|||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)|
|||M. Bache et al., “Poor-man’s model of hollow-core anti-resonant fibers”, J. Opt. Soc. Am. B 36 (1), 69 (2019)|
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