Tunable micro-cavities in photonic band-gap yarns and optical fibers

Abstract

The vision behind this work is the fabrication of high performance innovative fiber-based optical components over kilometer length-scales. The optical properties of these fibers derive from their multilayer dielectric photonic band-gap structure that exhibits omnidirectional reflectivity. The theoretical tools needed to design, analyze and optimize such structures are introduced. We show that defect layers in these otherwise periodic structures act as optical micro-cavities that enable precise design of the fibers' spectral response. Fabrication of these composite fibers by thermal drawing of a macroscopic preform in the viscous state requires solving material selection challenges in order to identify pairs of materials with high refractive index contrast and similar thermo-mechanical properties. Operational wavelengths ranging from the UV to the IR are demonstrated and made possible by the wavelength scalability of the photonic band-gap structure and accurate knowledge of the materials' dispersion relation afforded by broadband spectroscopic ellipsometry. The fundamentals of this technique, which is used to characterize a number of dielectrics, semi-conductors and metals, are surveyed. Two fiber structures are then explored: fibers for external reflection and hollow-core transmission fibers.

(cont.) We demonstrate that the resonance wavelength of Fabry-Perot cavities embedded in reflecting fibers can be tuned reversibly under applied elastic strain or external illumination at 514 nm. A simple opto-mechanical model is developed to assess the mechanical tuning efficiency while a review of the photodarkening effect in chalcogenide glasses and accurate measurements of the amplitude and response time associated with its transient component are presented to explain and optimize the optical tuning scheme. Modulation of the fibers' reflectivity near their cavity resonant wavelengths is demonstrated at various frequencies. Based on these results, we show that optical micro-cavities in transmission fibers can induce very high group-velocity dispersion as a result of the interaction between the propagating core modes and the lossy cavity resonant mode(s). Widely tunable dispersion is achieved using a mechanical tuning scheme. Applications for these fibers and future research directions are envisioned.