Ultra-Compact Subwavelength-Grating-Assisted Polarization-Independent Directional Coupler

We propose and experimentally demonstrate an ultra-compact, low-loss polarization-independent directional coupler on the silicon-on-insulator (SOI) platform. By exploiting subwavelength gratings in the directional coupler, the coupling strength could be made equal for both transverse-electric and transverse-magnetic polarizations. The demonstrated polarization-independent directional coupler has a device length of only 4.5 μm and achieves complete cross-coupling with a low excess loss of <;1 dB over a bandwidth of ~65 nm. The reported devices also demonstrate robust fabrication tolerance.