Integrated Optical Mode Converter With Vertical Coupling

We propose the design of an integrated optical mode converter consisting of the single-mode ring resonator, single-mode input access waveguide, and multimode output access waveguide. Access waveguides are designed under the ring forming the stack structure with vertical gap therefore device has vertical mode coupling. The higher-order mode in the multimode waveguide is phase-matched with the fundamental mode in the ring but all other modes are mismatched, the widths of the ring and single-mode waveguide are equal, therefore excitation of the fundamental mode in the single-mode waveguide leads to the inducing the higher-order mode in the multimode waveguide. We used finite difference time domain simulation of the device and found that straight access waveguides give the best performance of the mode conversion at the device output in comparison with the bent “pulley-coupling” scheme. The simulation shows high mode purity at the device output. Because of the resonant character of the spectrum, proposed device is WDM-compatible and can be used for mode division multiplexing systems.