Ultra-low-loss and broadband mode converters in Si₃N₄ technology

Si₃N₄ grown by low pressure chemical vapor deposition (LPCVD) on thermally oxidized silicon wafers is largely utilized for creating integrated photonic devices due to its ultra-low propagation loss and large transparency window (400 nm to 2350 nm). In this paper, an ultra-low-loss and broadband mode converter for monolithic integration of different materials onto the passive Si₃N₄ photonic technology platform is presented. The mode size converter is constructed with a vertically tapered Si₃N₄ waveguide that is then buried by a polymer or an Al₂O₃ waveguide. The influence of the various design parameters on the converter characteristics are investigated. Optimal designs are proposed, in which the thickness of the Si₃N₄ waveguide is tapered from 200 nm to ∼40 nm. The calculated losses of the mode converters at 976 nm and 1550 nm wavelengths are well below 0.1 dB for the Si₃N₄-polymer coupler and below 0.3 dB for the Si₃N₄-Al₂O₃ coupler. The preliminary experimental results show good agreement with the design values, indicating that the mode converters can be utilized for the low-loss integration of different materials.