Design of high-performance silicon nitride edge coupler based on subwavelength grating

Fiber-to-chip optical interconnects are critical for enabling efficient signal transmission in photonic integrated circuits, especially in data centers and optical communication systems. However, the large mismatch in mode field diameter between standard single-mode fibers and integrated waveguides leads to significant coupling losses. In this work, a compact, broadband, and polarization-insensitive subwavelength grating (SWG) edge coupler is proposed for silicon nitride (Si₃N₄) waveguides. The coupling performance is evaluated using the finite-difference time-domain (FDTD) method and eigenmode expansion (EME). The designed coupler exhibits a compact length of 55μm with low coupling loss and wide bandwidth. When coupled with a standard single-mode fiber, the minimum coupling losses for the TE0 and TM0 modes are 0.18dB and 0.35dB, respectively. At 1550nm, the corresponding coupling losses are 0.24dB (TE₀) and 0.35dB (TM₀). The 0.4dB bandwidths are 490nm for TE₀ and 350nm for TM₀, demonstrating excellent broadband performance. The alignment tolerance analysis shows that the coupling loss fluctuation of the TE₀/TM₀ mode is 0.5dB/1.4dB in the horizontal deviation of 2μm, and the coupling loss fluctuation of the TE₀/TM₀ mode is 0.8dB/0.7dB in the vertical deviation of 2μm. Compared to conventional single-taper edge couplers, the proposed design achieves higher broadband coupling efficiency and improved tolerance to fiber misalignment, making it highly suitable for practical packaging and integration.