Monolithic Integration of Al₂O₃ and Si₃N₄ Toward Double-Layer Active-Passive Platform

Low-cost, high-performance integration technologies are instrumental for active-passive integrated photonics devices. The monolithic integration of Al₂O₃ and Si₃N₄ is studied, enabling to combine the promising optical features of Si₃N₄ with the excellent optical gain characteristics of rare-earth-ion doped Al₂O₃. The Al₂O₃ and Si₃N₄ layers are separated by a thin SiO₂ film and coupled by adiabatically width-tapered Al₂O₃ and thickness-tapered Si₃N₄ waveguides. In this paper, a detailed characterization of the couplers, as well as a study of the influence of the different design parameters and fabrication tolerances on the final device performance is presented. Test structures are characterized under transverse electric (TE) polarization. Measured loss per coupler is as low as 0.26 ± 0.03 dB at the wavelength of 1030 nm, and below 0.24 dB in the spectral window of 1460-1635 nm. Lateral misalignment of ±1 μm results in less than 0.6 dB increase of the coupler loss at 1030 nm, and the tolerance of misalignment goes up to 1.7 μm at the investigated longest wavelength of 1635 nm without introducing extra coupler losses. The reported integration technology paves the way toward a double-layer platform monolithically integrating Si₃N₄ and rare-earth-ion doped Al₂O₃ for active-passive photonic functionalities.