In one aspect, the invention relates to optical fibers and systems that include such fibers. In another aspect, the invention provides an optical fiber that includes a core, a cladding surrounding the core, a layer surrounding the cladding, and a region between the layer and the cladding. The region can comprise an index of refraction that is different than an index of refraction comprised by the cladding. In one embodiment, the region can include a void containing air or a liquid. The void can be evacuated. The region can include a solid, such as, for example, a polymer. The layer can contact the cladding. The fiber can comprise rare earth ions.