Waveguide amplifier in the hybrid platform of thin film lithium niobate and silicon nitride

Various erbium-doped thin film lithium niobate (TFLN) amplifiers have been developed to compensate for optical loss in photonic integrated circuits (PICs). However, the co-integration of such amplifiers and passive optical devices remains underexplored, despite the fact that integrating amplifiers with key material-based devices (e.g., silicon nitride, Si 3 N 4 ) is vital for the development of large-scale PICs. In this work, we propose the heterogeneous integration of Er 3+ : TFLN and Si 3 N 4 waveguide via a low-temperature bonding technique. By bonding an Si 3 N 4 strip to a planar Er 3+ : TFLN layer, a loaded waveguide is formed to confine the optical field within the Er 3+ : TFLN gain medium, achieving a high optical field overlap ratio of ∼84% that enables efficient optical amplification. For a 1.2-cm waveguide, a signal enhancement of ∼14.4 dB and an internal net gain of ∼7.25 dB/cm are demonstrated. This method eliminates lattice mismatch and thermal expansion coefficient mismatch between different materials, theoretically allowing extension to the hybrid integration of photonic elements fabricated from arbitrary materials. Additionally, the Si 3 N 4 strip is defined by conventional UV photolithography, which is fully compatible with standard CMOS fabrication processes. This work provides a viable scheme for heterogeneous PICs, effectively circumventing the issues of lattice and thermal expansion coefficient mismatches between dissimilar materials.