F:End filters: etalon on the beveled facet of a fiber with an outdiffused core

FiEnd (fiber end) etalon filters have been made by multilayer deposition directly on the beveled end facets of fibers. Both standard depressed-cladding single-mode fibers and beam-expanded fibers made by thermal diffusion of the core dopant of standard fibers were used. The fiber end facets were polished at appropriate bevel angles to eliminate back reflection. It was found that, if a standard fiber is used, the bandpass of the FiEnd etalon filter is asymmetric and skewed toward shorter wavelengths. It was also shown that to make reflection free FiEnd etalon filters on beveled fiber and facets with bandpasses smaller than about 5 nm, it is necessary to use fibers with mode field diameters larger than those of standard fibers.<>