Optimizing polarization-diversity couplers for Si-photonics: reaching the -1dB coupling efficiency threshold.

Polarization-diversity couplers are low-cost industrially-scalable passive devices that can couple light of unknown polarization from a telecom fiber-mode to a pair of TE-polarized wave-guided modes in the Silicon-on-Insulator platform. These couplers offer significantly more relaxed alignment tolerances than edge-coupling schemes, which is advantageous for commercial fiber-packaging of Si-photonic circuits. However, until now, polarization-diversity couplers have not offered sufficient coupling efficiency to motivate serious commercial consideration. Using 3D finite difference time domain calculations for device optimization, we identify Silicon-on-Insulator polarization-diversity couplers with 1,550 nm coupling efficiencies of -0.95 dB and -1.9 dB, for designs with and without bottom-reflector elements, respectively. These designs offer a significant improvement over state-of-the-art performance, and effectively bridge the "performance gap" between polarization-diversity couplers and 1D-grating couplers. Our best polarization-diversity coupler design goes beyond the -1dB efficiency limit that is typically accepted as the minimum needed for industrial adoption of coupler devices in the telecoms market.