Ultracompact Multimode Interference Coupler Designed by Parallel Particle Swarm Optimization With Parallel Finite-Difference Time-Domain

An ultra-compact multimode interference (MMI) coupler with a length similar to 10 mu m for dense photonic integration is designed by parallel particle swarm optimization (PSO) with parallel finite-difference time-domain (FDTD). The MMI coupler uses a strongly-guiding deep-etched InP platform and the modal analysis of this strongly-guiding deep-etched InP waveguide gives the cross section of the single-mode nanophotonic waveguide, the geometry of the multimode interference region, and the etched-depth requirement. An efficient global optimizer combining parallel PSO algorithm and parallel FDTD is developed, which speedups the optimization and enables automatic nanophotonic design. Two 2 2 multimode interference couplers with a respective coupling ratio of 50%: 50% and 90%: 10% are designed as numerical examples using the global optimizer developed and the optimization result shows a ultra-compact multimode interference coupler with a length of 10 m can be achieved with a low excess loss, which is numerically verified by a parallel three-dimensional FDTD.