Broadband Angled Arbitrary Ratio SOI MMI Couplers With Enhanced Fabrication Tolerance

In this article, we design and demonstrate new angled multimode interference (MMI) couplers in silicon-on-insulator that allow arbitrary selection of coupling ratios, while simultaneously achieve low-chromaticity response and improved fabrication variations tolerance. The proposed couplers rely on paired-interference design approach in tandem with a new angled multi-mode junction to mitigate chromatic aberration and geometry-induced waveguide modal shift in conventional angled MMI coupler design. Test devices with footprint of 4.8 μm × 75 μm showed full coupling ratio range addressability with insertion loss below 1.2 dB. Spectral responses across the extended C band (1525 nm - 1575 nm) were also characterized, where coupling ratio varied by less than 2.3% across the spectral region of interest. Furthermore, coupler characteristics variations against line-width and thickness deviations are numerically analyzed, demonstrating robustness within typical fabrication tolerances.