Optical switching and modulation in parallel dielectric waveguides

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, Δnl=3λ/4, where λ is the free-space wavelength and Δn is 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.