Vertical mode transition in hybrid lithium niobate and silicon nitride-based photonic integrated circuit structures

This Letter presents an optical mode transition structure for use in Si₃N₄/LiNbO₃-based hybrid photonics. A gradual modal transition from a Si₃N₄ waveguide to a hybrid Si₃N₄/LiNbO₃ waveguide is achieved by etching a terrace structure into the sub-micrometer thick LiNbO₃ film. The etched film is then bonded to predefined low pressure chemical vapor deposition Si₃N₄ waveguides. Herein we analyze hybrid optical devices both with and without the aforementioned mode transition terrace structure. Experimental and simulated results indicate that inclusion of the terrace significantly improves mode transition compared to an abrupt transition, i.e., a 1.78 dB lower mode transition loss compared to the abrupt transition. The proposed transition structure is also applied to the design of hybrid Si₃N₄-LiNbO₃ micro-ring resonators. A high-quality factor (Q) resonator is demonstrated with the terrace transition which mitigates undesired resonances.