Design and Optimization of Polymer-Based 2×4 MMI Splitters for Photonic Integrated Circuits

Polymer-based photonic integrated circuits are promising for board-level optical interconnects due to their low-cost fabrication, mechanical flexibility, and compatibility with large-area processing; however, efficient multi-input power routing using passive components remains challenging. In this work, we present the design, simulation, and optimization of a step-index polymer 2×4 multimode interference (MMI) splitter operating at 1550 nm, targeting compact dual-input power distribution for board-level photonics. The device is realized on the SUNCONNECT organic–inorganic hybrid polymer platform using NP001L2 (n = 1.573) as the core and NP216 (n = 1.560) as the cladding, and analyzed using the beam propagation method (BPM) in RSoft BeamPROP. Starting from self-imaging theory, the multimode-section width and length, access-waveguide width, input separation, taper geometry, and nonuniform output-port spacing are systematically optimized through staged parameter sweeps to recover stable self-imaging under dual-input excitation. The optimized splitter achieves near-symmetric four-way power distribution with output power splitting ratios of 24.59%, 24.39%, 25.00%, and 26.02%. The corresponding excess loss is 6.13 dB (TE) and 6.18 dB (TM), with a low polarization-dependent loss of 0.04 dB, indicating robust polarization behavior despite increased modal complexity. The results reveal that input parity control, width-driven mode expansion, and asymmetric port placement are critical design levers for dual-input polymer MMIs. This study provides a practical optimization framework for polymer-based multi-source MMI splitters and highlights the key limitations that must be addressed for low-loss scalable board-level optical interconnects.