Hollow-Core Photonic Crystal Fibers: A Historical Overview

Since their first successful demonstration in the 1960’s [1], optical fibers have revolutionized the field of telecommunications. They are ideal for transmission of signals as they are made of relatively cheap materials (silica glass), which are also non-conductive; thus, contrary to electric wires, they can be used without the risk of external electromagnetic interference. In addition, the attenuation rates of state-of-the-art optical fibers are now extremely low (about 0.2 dB/km), easily achieving long-haul signal transmission, especially when used in combination with intermediate amplifiers.

Despite their excellent performance in the transmission of optical signals, conventional optical fibers suffer from several limitations. First, light in an optical fiber propagates in a bulk medium, glass, and so suffers from both absorption losses (from the constituents of the glass itself and from impurities) and radiative attenuation (Rayleigh scattering from density fluctuations frozen in the fiber). Secondly, the strict geometry and refractive index profile of standard optical fibers impose certain frequency dispersion on the propagating field that cannot be easily engineered for different requirements and applications. Finally, there has always been a desire for greater versatility in the fiber design for better performance in more specialized applications, such as high power delivery, tailored nonlinearity, sensing, high birefringence and the hosting of different materials, to mention a few.

It was these motivations that finally led to the emergence of a new class of fiber, the photonic crystal fiber (PCF). In a PCF the fiber core is surrounded by a microstructured cladding, usually consisting of a periodic array of air holes in silica. Such a cladding is a two-dimensional photonic crystal, hence the term PCF. When the core index of a PCF is higher than that of the cladding, waveguiding still takes place via total internal reflection at the core/cladding interface just as with step-index optical fibers. However the great versatility in the design of the PCF structure gives the fiber a wealth of new properties that have found many interesting applications. Moreover, most PCFs are made of a single material (silica) and this makes the engineering of the fiber properties much easier than for conventional optical fibers.

The idea of a hollow-core photonic crystal fiber (HC-PCF) first emerged in the early 1990’s [2], when it was realized that light can be confined in air by means of a photonic bandgap (PBG). This is a frequency region where light cannot propagate in a photonic crystal. It was a new departure in the field of optics, as it introduced the idea of bandgaps for light propagating out of the periodicity plane of the photonic crystal. Until that time, the existence of frequency stop-bands in periodic dielectric structures was only predicted for in-plane propagation in photonic crystals with a strong refractive index contrast [3, 4, 5]. In HC-PCFs, the air core is surrounded by a two-dimensional periodic cladding that exhibits photonic bandgaps for certain ranges of the propagation constant of the core modes; light is, thus, confined in the air core. Moreover, it was theoretically demonstrated that photonic bandgaps for out-of-plane propagation can exist in structures with such a low refractive index contrast as silica and air [6]. This finally led to the fabrication of the first working HC-PCF in 1999 [7].

HC-PCFs are interesting in many ways. On a fundamental level, there is basic scientific interest in the photonic bandgap that the fiber exhibits. Furthermore, in a HC-PCF light is guided in air; this can potentially mean low levels of nonlinearity, delivery of high-power pulses and the transmission of frequencies that cannot propagate in glass. It also offers diffractionless propagation of light in air in a good quality well-confined mode over distances that can now reach the order of kilometers [8]. This opens up a new ‘highway’ for light/matter interactions, both linear (such as laser-induced guidance of atoms and particles [9, 10]) and nonlinear (for example, gas-based nonlinear optics [11, 12]).

References

[1] K. C. Kao and G. A. Hockham. Dielectric-fiber surface waveguides for optical frequencies. In Proc. IEE, 1966.

[2] P. St. J. Russell. Photonic crystal fibers. Science, 299:358–362, 2003.

[3] E. Yablonovitch. Inhibited spontaneous emission in solid-state physics and electronics. Physical Review Letters, 58(20):2059–2062, 1987.

[4] S. John. Strong localization of photons in certain disordered dielectric superlattices. Physical Review Letters, 58(23):2486–2489, 1987.

[5] P. R. Villeneuve and M. Pich´e. Photonic band gaps in two-dimensional square and hexagonal lattices. Physical Review B, 46(8):4969–4972, 1992.

[6] T. A. Birks, P. J. Roberts, P. St. J. Russell, D. M. Atkin, and T. J. Shepherd. Full 2-D
photonic bandgaps in silica/air stuctures. Electronics Letters, 31(22):1941–1943, 1995.

[7] R. F. Cregan, B. J. Mangan, J. C. Knight, T. A. Birks, P. St. J. Russell, P. J. Roberts, and D. C. Allan. Single-mode photonic band gap guidance of light in air. Science, 285:1537–1539, 1999.

[8] B. J. Mangan, L. Farr, A. Langford, P. J. Roberts, D. P. Williams, F. Couny, M. Lawman, M. Mason, S.Coupland, R. Flea, H. Sabert, T. A. Birks, J. C. Knight, and P. St.J. Russell. Low loss (1.7 dB/km) hollow core photonic bandgap fiber. In Optical Fiber Communication Conference. Paper PDP24, 2004.

[9] F. Benabid, J. C. Knight, and P. St. J. Russell. Particle levitation and guidance in hollow-core photonic crystal fiber. Optics Express, 10(21):1195–1203, 2002.

[10] F. Benabid, G. Antonopoulos, J. C. Knight, and P. St. J. Russell. Particle levitation and guidance in hollow-core photonic crystal fiber. In Photon02 Conference (Structured Optical Materials), page 92, Cardiff, 2002. Paper OP3a.6.4.

[11] F. Benabid, J. C. Knight, G. Antonopoulos, and P. St. J. Russell. Stimulated Raman scattering in hydrogen-filled hollow-core photonic crystal fiber. Science, 298:399–402,
2002.

[12] F. Benabid, G. Bouwmans, F. Couny, J. C. Knight, and P. St. J. Russell. Ultra-high efficiency laser wavelength conversion in a gas-filled hollow-core photonic crystal fiber by pure stimulated rotational Raman scattering in molecular hydrogen. Physical Review Letters, 93:123903–1–4, 2004.

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