A vertical directional coupler fabricated using wafer fusion is demonstrated with a very short coupling length of 62 &mgr;m. The optical propagation loss introduced by the fused layer is investigated. An excess loss of 1.1 dB/cm at 1.55 &mgr;m was measured for waveguides which incorporate a fused junction near the core region. Fused vertical couplers make it possible to realize three-dimensional waveguide structures and compact switching arrays and they solve some of the topology problems of large switch arrays. ©1998 American Institute of Physics.