A low-loss and compact edge coupler for TE-polarized 800-nm-thick Si3N4 waveguides

Efficient fiber–chip coupling remains a key challenge for compact and application-oriented silicon nitride (Si 3 N 4 ) photonic integrated circuits. The 800-nm-thick Si 3 N 4 platform enables compact routing and flexible dispersion engineering, but its tightly confined waveguide mode produces a small facet mode field and, consequently, a substantial mismatch with the focused mode of a lensed fiber. Here, we experimentally demonstrate a compact, directly merged dual-waveguide edge coupler for TE-polarized coupling on an 800-nm-thick Si 3 N 4 platform. Two laterally separated taper arms support an enlarged TE-like even supermode at the chip facet, which is gradually transformed into the fundamental mode of a single high-confinement waveguide over a total length of 120 µm. Among ten characterized devices, the minimum measured TE-polarized fiber–chip–fiber insertion loss is 2.26 dB at 1550 nm. Under the nominal input/output symmetry assumption, this corresponds to an effective average loss of 1.13 dB per facet and an effective per-facet efficiency of 77.1%. For one representative device, the measured effective per-facet loss varies by less than 1 dB relative to its minimum within the tested 1500–1590 nm window. The measured output-side transverse 3-dB alignment half-widths for TE polarization are approximately ± 1.6 μ m and ± 1.3 μ m in the horizontal and vertical directions, respectively. By combining compact footprint, low insertion loss, and experimentally verified tolerance, the proposed dual-waveguide edge coupler provides a practical approach for fiber coupling in high-confinement Si 3 N 4 photonic integrated circuits.