Integrated photonics has undergone significant development in the past two decades, particularly silicon photonics, thanks to the mature complementary metal oxide semiconductor (CMOS) process flow. Despite advancements in silicon photonic chip technology and their widespread applications in various sectors such as lidar, medical detection, and military defense, a significant hurdle remains in the efficient transfer of information between these chips and the external world via optical fibers. Since current manufacturing processes are unable to provide the best properties in a single-material optical system, achieving low-loss transmission between different material systems becomes extremely important. Edge coupling emerges as one of the most promising methods for information exchange in optical chips compared with grating coupling, which exhibits lower coupling efficiency in high-power systems. According to the refractive index of the materials used, edge couplers can be categorized into two types: 1) waveguides with low refractive index and large cross-section size, mainly involving SiO_x and SU-8 glue; 2) waveguides with high refractive index and small cross-section size, with representative materials such as SiON and Si₃N₄. With the development of alignment accuracy in stepper exposure during the optical chip manufacturing process, the manufacturing requirements of multilayer waveguides are gradually being met. Consequently, more research is focusing on waveguides with high refractive index and small cross-section size. However, most proposed edge couplers are based on waveguides of three layers or are based on doping or deep etching of the cladding SiO₂. To simplify the manufacturing process while maintaining high coupling efficiency, we propose and optimize a novel edge coupler based on a cross-type heterogeneous multi-core waveguide. By guiding light with silicon nitride waveguide and silicon waveguide together, this design utilizes one less layer of Si₃N₄ waveguide and eliminates the need for deep etching of SiO₂ and the use of SiON.

The research methodology involves a comprehensive design and simulation process using the MODE Solutions module. The proposed edge coupler is based on a cross-type heterogeneous multi-core waveguide structure (Fig. 1). This structure integrates silicon nitride and half-etched silicon waveguides to achieve high mode field matching between optical fibers and silicon strip waveguides. The structural parameters to be optimized include the width and spacing of the Si₃N₄ waveguides denoted as w and d, as well as the width and spacing of the half-etched Si waveguides denoted as w_Siand d_Si. To simplify the optimization process, we divided it into two steps. First, we optimize the w and d parameters using cross-type Si₃N₄ waveguides, with all five waveguides having the same dimensional parameters. The overlap integral is utilized to measure the mode field matching efficiency, as shown in Fig. 2. In the subsequent optimization step, one Si₃N₄ waveguide at the bottom was replaced by a half-etched Si waveguide, while the w and d parameters are set to the optimized values obtained in the first step. The simulation results of this step are presented in Fig. 3. Inspired by the research work of Maegami, we employ his method of analyzing the coupling sections of the adiabatic taper to design our adiabatic coupler. This involves calculating and comparing the effective refractive index of the single waveguide and the combination of the two waveguides. Based on these calculations, we divide the adiabatic taper into lower coupling and higher coupling sections. In each section, we optimize the length of the taper, respectively.

The simulation analysis of the proposed edge coupler, conducted using advanced simulation tools, reveals exceptional optical coupling performance. At the wavelength of 1550 nm, the device demonstrated a high mode field matching efficiency, which is crucial for efficiently transferring light between the silicon photonic chip and the high numerical aperture fiber (HNAF). The simulation results indicate a coupling efficiency of 97.1% for the TE mode and 97.5% for the TM mode, highlighting the effectiveness of the design in minimizing optical loss during the coupling process (Fig. 6). The bandwidth capability of the coupler was also evaluated, showing that the design supports a bandwidth of 350 nm while maintaining a polarization-dependent loss (PDL) within ±1%. Additionally, the coupler exhibits an acceptable 1 dB alignment tolerance, allowing for approximately ±1.01 μm displacements for both TE and TM modes (Fig. 8). The simulation results further demonstrate that the proposed edge coupler has a high process tolerance for SiO₂ thickness error and horizontal distance error of the silicon nitride waveguides. This resilience to manufacturing imperfections underscores the design’s industrial applicability and potential for reliable, large-scale production. In conclusion, the performance analysis confirms the proposed edge coupler as a promising solution for high-efficiency silicon photonics integration.

With the increasing adoption of Si₃N₄ waveguides alongside SOI waveguides, we introduce a high-coupling-efficiency cross-type heterogeneous multi-core waveguide edge coupler that avoids the need for deep etching or doping of the SiO₂ cladding. The manufacturing process for this structure is relatively straightforward, involving the addition of two layers of Si₃N₄ waveguides to the traditional SOI chip manufacturing process. Apart from the interlayer distance of the two Si₃N₄ waveguides, which may not be suitable for multi-project wafer (MPW) fabrication, the remaining processes are compatible with MPW production. Simulation results reveal that the designed edge coupler achieves a coupling efficiency of 97.1% (TE mode at 1550 nm, coupling loss of 0.13 dB) and 97.5% (TM mode at 1550 nm, coupling loss of 0.11 dB) when coupled with high numerical aperture fibers. These fibers can be linked to standard single-mode fibers through thermally expanded core (TEC) technology, resulting in a coupling loss of less than 0.10 dB. Therefore, the total coupling loss is estimated to be less than 0.23 dB (TE mode) and 0.21 dB (TM mode). Additionally, the designed edge coupler exhibits good manufacturing tolerances for waveguide thickness, width, and SiO₂ cladding thickness, indicating promising prospects for industrial applications.