We describe the properties of dielectric couplers wherein tunability is achieved by inserting graphene layers in the region which separates the waveguides. We show that, in spite of the atomic thickness of graphene, the strong dependence of its optical parameters on the bias voltage allows coupling between the waveguides, and then the beat length of the coupler, to be dramatically changed. We also demonstrate that simple analytical models based on classic waveguide theory can be efficiently applied for the analysis of complex graphene-based electro-optical structures.