Low-Loss Highly Tolerant Flip-Chip Couplers for Hybrid Integration of Si3N4 and Polymer Waveguides

In this letter, low-loss and highly fabrication-tolerant flip-chip bonded vertical couplers under single-mode condition are demonstrated for the integration of a polymer waveguide chip onto the Si₃N₄/SiO₂ passive platform. The passively aligned vertical couplers have a lateral misalignment between polymer and Si₃N₄ waveguide cores of ±1.25 μm. Low-loss operation has been experimentally demonstrated over a wide spectral window of 1480-1560 nm, with measured coupler losses below 0.8 dB for Si₃N₄ taper angles below 1.2°, in good agreement with the calculated values. Furthermore, thermal shock test results show less than 0.1 dB degradation, indicating a robust coupling performance.