The three-waveguide nonlinear directional coupler switching characteristic is studied using the eigenmodes of the structure (i.e. supermodes). This model, which is more exact than the one used in previous works, has allowed to confirm the three waveguide configuration as an alternative to the two waveguide nonlinear coupler. Moreover, an analytical expression for the critical power at which the switching between the output branches occurs has been found.
Artigas, D., Olivas, J., Dios, V., Canal, F. Supermode analysis of the three-waveguide nonlinear directional coupler: the critical power. "Optics communications", Octubre 1996, vol. 131, núm. 1-3, p. 53-60.