A selective filtering device for use in optical communications systems takes the form of a transversely coupled Fabry-Perot interferometer. One described embodiment comprises a first length of monomode optical fiber transversely coupled to a second fiber in a coupling region. One end of each fiber at opposite respective ends of the coupling region is provided with a suitable highly reflective surface, or example, an evaporated gold/aluminum deposit. In operation, a light input may be modified by the resonant cavity behavior of the Fabry-Perot cavity formed between the mirrored ends to provide filtered or enhanced outputs. The outputs may be further modified by alternative or additional light input via the ends of the fibers.