Compact and low-loss inline optical power monitor with an MMI-based Ge/Si hybrid waveguide

As the scale and complexity of silicon photonic chips increase rapidly, driven by demand as well as the maturity of large-scale photonic integration technologies, optical power monitors (OPMs) with a small footprint and insertion loss are demanded. Here, a compact and low-loss inline power monitor is proposed and demonstrated with a direct-through waveguide consisting of a short Ge layer over a silicon multimode interference (MMI) segment where the optical field is relatively weak. Therefore, only a small fraction of light would be absorbed to monitor the incident power, with little influence on the through light. Our analysis shows that with proper design of the dimensions of the MMI segment, a Ge layer large enough for the foundry's design rules can be accommodated. With further optimization of the MMI's shape using a constrained particle swarm algorithm, a fabricated device sample by standard CMOS processes exhibits an insertion loss of ∼ 0.4 dB and a responsivity of 15 mA/W at 1550 nm. Its compact footprint of 14.5 × 10 μm could enable high-density integration in large optical networks-on-chip.