Coupling of electromagnetic energy between parallel single‐mode dielectric waveguides is investigated theoretically. Expressions giving the decrease in coupling length due to an antisymmetric change in refractive index are derived. The minimum parallel‐guide path length,l, required for switching or modulation is determined from coupling length calculations. It is shown that, for large waveguide separations,&Dgr;nl=3&lgr;/4, where &lgr; is the free‐space wavelength and &Dgr;nis the magnitude of the antisymmetric index change. Calculations for slab and square‐cross‐section waveguide geometries indicate that antisymmetric index changes are much more effective than symmetric changes for switching or modulation.