Devices to enhance the reliability of optical networks and to reduce the cost of repair are disclosed in this invention. In particular, compact and inexpensive fiber optic union adapters with built-in protective isolation prevent the transfer of damage from one connectorized fiber optic cable to another. The fiber optic union includes a split sleeve with an interior channel and a fiber stub centrally located within the interior channel. The fiber stub makes direct optical contact with the cable endfaces to enable efficient optical transmission between interconnected cables while providing a low loss, low back reflection adiabatic transition between the waveguide cores of the two cables.