Compact High-Performance Multimode Interference Couplers in Silicon-on-Insulator

We demonstrate compact multimode interference (MMI) couplers on silicon-on-insulator that exhibit both high performance and relaxed tolerances. The devices are fabricated with a single shallow etch step on 1.5- $\mu$m-thick silicon, and have a footprint of 12.8 $\mu$m$\,\times\,$456 $\mu$m, including 100-$\mu$m-long tapers at input and output. In a Mach–Zehnder configuration, we achieve an extinction ratio larger than 22 dB for transverse-electric polarization and larger than 20 dB for transverse-magnetic polarization. This extinction ratio is maintained in the full 1520- to 1580-nm band and for MMI width variations in a range of 0.45 $\mu$ m. We also show appreciable tolerance differences between paired and general excitation MMIs.