2×2 SOI Optical Switch with Robust High Extinction Ratio on All Paths Enabled by Parabolic MMI Coupler

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 an extremely small imbalance of output power around 0.1 dB over the C band and have good manufacturing tolerances 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 the experiment, with maximum values concurrently over 40 dB. The proposed parabolic MMI occupies a small area of merely
$2.4 \mu \mathrm{m}\times 9.5\mu \mathrm{m}$
.