Nonlinear coupling of modes: a new approach to all-optical guided wave devices

Intensity-dependent nonlinearities in waveguide structures which support more than one guided mode can lead to coupling between these modes. We have analyzed the interaction of the two lowest- order modes in a general symmetric waveguide structure. In this analysis we assumed that the index changes are not large enough to affect the mode shapes, however, ail the wave-mixing terms are included to account for the nonlinear phase shifts and power exchange between the modes. We have found that as the power is increased the beat period between the modes increases, while power is exchanged at the same periodicity. Above a critical power the two modes are phase-locked and a relatively constant field distribution is propagating along the guide. These effects can be the basis of a number of all-optical waveguide devices. For device applications controlled excitation and extraction of the modes are required. We discuss several such configurations and their applicability for all-optical signal processing.