▎ 摘　　要

We investigate the variation of the graphene-waveguide coupling strength G, whose magnitude is the crucial parameter of efficient graphene-based electro-absorptive optical modulators, with the waveguide geometry and its graphene coverage. By sweeping a wide geometrical parameter space, we show that G can be improved up to few times by optimising the waveguide width and height. We find that the high values of G cannot be simply related to the modal confinement factor and assess the impact of a small detachment of graphene from vertical waveguide boundaries. Using perturbation theory, we demonstrate that the modulation depth to the insertion loss ratio of a graphene-based modulator is always independent of the geometry and determined by the residual conductivity of graphene.