Design and Experimental Demonstration of a High-Performance 2 × 2 Restricted Interference MMI Coupler-Based Optical Power Splitter for C-Band Applications

Silicon waveguide-based optical splitters are fundamental components in the development of advanced functional circuits, including optical switches, routers, modulators, and various optical logic circuits. Among the various approaches, multimode interference (MMI) waveguides are widely employed in optical splitters due to their broad bandwidth, high fabrication tolerance, stability, efficient light confinement, and low transmission loss. In this study, we present the design of an optical splitter based on restricted interference mechanisms, where the precise positioning of input pairs and careful adjustment of the MMI region length are key factors. By applying interference theory, we successfully reduce the length of the RI-MMI coupler. The design undergoes extensive optimization through rigorous 3D-FDTD simulations to ensure optimal performance. Subsequently, we fabricated 11 chip designs using 193-nm deep ultraviolet (DUV) photolithography and plasma-enhanced chemical vapor deposition (PECVD) processes. Performance measurements, conducted through subwavelength grating couplers (SWGC), reveal that our optimized design achieves very low insertion loss (IL) (<5 dB) and minimal reflection ( $\lt -23$ dB) across the entire C-band. Additionally, the devices exhibit low ripple, a nearly input-independent coupling ratio (CR), and balanced splitting ratios within a 10 nm range centered around 1555 nm to 1565 nm. Notably, the core component of the splitter is housed within an ultra-compact footprint of $6~\mu $ m $\times 65~\mu $ m. These exceptional characteristics position the proposed device as a promising candidate for large-scale integrated optical circuits in telecommunications applications.