The vision behind this work is the fabrication of high performance innovative fiber-based optical components over kilometer length-scales. Their optical properties 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 micro-cavities that enable precise design of the fibers'' spectral response. Two structures are explored: fibers for external reflection and hollow- core transmission fibers. 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. Experimental data is compared to computed data based on opto-mechanical and photodarkening models. We then show that optical micro-cavities in transmission fibers can induce significant group-velocity dispersion as a result of modal interactions. Applications for these fibers and future research directions are envisioned.