A device for providing an expanded mode field from a single mode optical waveguide is formed by fusing a length of single mode optical fiber with a length of fiber rod absent a cladding and adiabatically tapering the fused region. The part that remains free serves for receiving respective metal reinforcing strips ) which may be riveted or welded to the sheet metal members. The second end portion is located on a second lateral side of the center portion. The interlocking means are formed such that the thermoplastic material is at least partially hardened in the cavities of the sieve.