A unidirectional electro-optic modulator based on an asymmetrical highly multimode waveguide coupler is proposed. To achieve high switching performance in a guided-wave coupler, a grating-based dumping wall is placed on the larger waveguide. This dumping effect makes the asymmetric highly multimode waveguide coupler possess a highly efficient unidirectional coupling process. A high modulation depth of 99&percent; is experimentally achieved at 633 nm wavelength with an index modulation of3.0×10−4.This device can be used for multimode optical interconnection systems such as data communications and fiber sensor networks. ©1998 American Institute of Physics.