Parabolic MMI Coupler for 2 × 2 Silicon Optical Switch With Robustly High Extinction Ratio for Four Paths

We propose and experimentally demonstrate a Mach-Zehnder silicon thermo-optic switch based on a parabolic multimode interference coupler. By judiciously designing the parabolic structure, the
$2 \times 2$
MMI can exhibit extremely small imbalance of output power around 0.1 dB over the C band with good fabrication tolerance according to simulation. As such, for all four possible switching paths, high average extinction ratios over 35 dB can be achieved from 1520 nm to 1580 nm in experiment, with maximum values concurrently over 40 dB. The proposed parabolic MMI occupies a small area of merely
$2.4 \mu \mathrm{m} \times 9.5 \mu \mathrm{m}$
.