Waveguide couplers with new power splitting ratios made possible by cascading of short multimode interference sections

We show that it is possible to obtain 2×2 waveguide couplers with new power splitting ratios for cross coupling of 7%, 64%, 80% and 93% by cascading two short MMI sections. These couplers have simple geometry and low loss. They offer valuable new possibilities for designing waveguide power taps, high-Q ring resonators, ladder-structure optical filters, and loop-mirror partial reflectors.