devices to enhance the reliability of optical networks and to reduce the cost of repair are disclosed in this invention . in particular , fiber optic union adapters with built - in protective isolation prevent a damaged , connectorized fiber optic cable from degrading other fiber optic terminations within the network should they be physically connected . the fiber optic union comprises a split sleeve with an interior channel and a fiber stub centrally located within the interior channel . the fiber stub prevents the ferrules of two different cables from making direct physical exchange but allows them to make efficient optical exchange . opposite ends of the fiber stub are optically polished to enable physical contact to the ferrules of fiber optic cables with low insertion loss and low backreflection . devices to achieve low loss isolated interconnection between cables containing dissimilar fiber types by use of a fiber taper are further disclosed .