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Variable-radius Hollow-core fibers Disclosure Number: IPCOM000030687D
Publication Date: 2004-Aug-23

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We show how core collapse in a hollow fiber, with its resulting thickening effects on regions adjacent to the core, can be used to maintain bandgap confinement at a fixed wavelength for a bandgap based fiber while at the same time achieving large changes in core diameter. In an exemplary structure based upon an actual fabricated Bragg type fiber that confines 10.6 micrometer light in a hollow core, we demonstrated that a factor of two reduction in core diameter can be obtained while preserving strong bandgap confinement via an approximately 30 percent relative core collapse. In this way, one can vary the core diameter of a given hollow bandgap fiber along its length in order to optimize for different circumstances, such as bends of couplers, or for different applications.

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Variable-radius hollow-core fibers

Steven G. Johnson, Aaron Micetich, Burak Temelkuran, Marin Soljacic, Ori Weisberg, Steve Jacobs,

Maksim Skorobogatiy, and Jim Goell

17th August 2004

1 Introduction

Photonic bandgap fibers employ a regular, typically periodic, arrangement of two ma- terials to create a photonic bandgap that prohibits penetration of light in certain fre- quency/wavevector ranges determined by the refractive indices and geometric param- eters. Examples include Bragg and OmniGuide1 fibers [2, 3, 4], which employ con- centric rings of two or more materials (a "1d" bandgap), and "photonic-crystal" fibers which employ a 2d-periodic array of index modulation (typically via 2d lattices of air holes) to create a "2d" bandgap [5]. In both cases, the existence of a photonic bandgap a unique effect compared to traditional index-guided fibers: light can be guided within a hollow core (a region with a lower index of refraction than the bandgap cladding). By confining light in a hollow core, one can surmount intrinsic material losses, nonlin- earities, and imperfections that are associated with solid optical media-for example, one can design a fiber to carry a wavelength such as 10.6µ m for which no low-loss drawable materials are known. In this invention, we present the idea of using a vari- able core radius along a hollow-core fiber axis to optimize the structure for differing conditions and applications. Furthermore, we demonstrate how to use a new degree of fabrication freedom in hollow-core fibers, the core-collapse ratio, in order to alter the core radius of a continuous fiber without sacrificing bandgap guidance at a fixed operating wavelength.

  In such a hollow-core bandgap fiber, much as for a hollow-metallic fiber [6, 7, 8], the core diameter is a critical parameter. Larger cores mean that a smaller fraction of the light penetrates into the cladding, and thus e.g. cladding absorption and imperfec- tion losses are lower. On the other hand, larger cores also support more optical modes and intermodal coupling is worse (e.g. at bends or other fiber nonuniformities)-this may result in e.g. worse total bending losses or worse peak loss rates in a bend (the heating from which may limit optical power capacity). Thus, in general, there are tradeoffs between different loss mechanisms and other figures of merit for larger or smaller cores, and the optimal balance of these tradeoffs will depend upon the fiber

   1OmniGuide fibers use concentric Bragg mirrors whose indices and thicknesses, in a planar geometry, form omnidirectional refLectors for all angles and polarizations (with refLectivities > 95% for all angles up to at least 80[openbullet]) [1].


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bend (smaller core)

Figure 1: Schematic application of a variable-radius hollow-core fiber to a bend. It is likely that the optimal core radius in the bend region (to minimize intermodal coupling) is sma...