Coherent interaction between two orthogonal travelling-wave modes in a microdonut resonator for filtering and buffering applications.

We theoretically investigate the coherent interaction between two orthogonal travelling-wave modes in a microdonut resonator symmetrically coupled to two bus waveguides. An analytical model has been developed to describe this structure using transfer matrix method. The simulation reveals that the two-mode microdonut can exhibit either a flat-top response or all-pass transmission, governed by the resonance spacing. Then, we implement analytical simulations to characterize the device and analyze the influence of coupling efficiencies and propagation losses of two resonant modes on behavior. Consequently, finite difference time domain simulations have been performed. The numerical results validate our theoretical analysis, and optical buffering effect is demonstrated in a pulse propagation simulation, when the two resonances are aligned. In addition, we show that the device function can be switched between flat-top filtering and all-pass filtering by tuning the local refractive index in a microdonut. Hence, this structure is promising for on-chip optical filtering and buffering applications.