Simulation study of a highly efficient dual-apodized SiN Grating Coupler using a metal bottom reflector on a Glass Substrate for Co-Packaged Optics

We present a high-efficiency, glass substrate-based dual-apodized amorphous silicon (a-Si)/silicon nitride (SiN) grating coupler (GC) with a bottom reflector, optimized for Co-packaged Optics (CPO) applications. The bottom reflector enhances directionality by reflecting and recoupling light dissipated beneath the substrate, while an a-Si overlayer incorporating subwavelength grating (SWG) metamaterial engineering enhances grating strength and effectively controls the emitted light distribution. Additionally, inverse apodization improves phase matching and reduces diffraction losses. Through Finite-Difference Time-Domain (FDTD) simulation-based optimization, the proposed GC achieves a coupling efficiency (CE) of -0.83 dB and a 1 dB optical bandwidth of 50 nm at 1.31 µm, demonstrating compatibility with deep ultraviolet (DUV) lithography and high fabrication and alignment tolerances. The device maintains CE degradation within ± 0.5 dB under key parameter variations, making it a promising solution for low-loss optical interconnects in CPO systems.