Silicon Perfectly-Vertical Polarization Diversity Grating Coupler With Near-Zero Polarization Dependent Loss

Polarization diversity grating couplers (PDGCs) are fundamental components in photonic integrated circuits for polarization manipulation or multiplexing. However, the polarization dependent loss (PDL) is inherent in off-vertical fiber coupling. Additionally, there is an urgent demand for PDGCs with surface-normal emission to interface with multi-core fiber. Here, we demonstrate perfectly-vertical PDGCs using the inverse design method, featuring high coupling efficiencies and near-zero PDLs. Both 2-port and 4-port configurations are proposed and fabricated. Simulation results show that the 4-port one has an ultra-low PDL of < 0.005 dB across a broad wavelength range of 100 nm. Metal mirrors beneath the gratings are introduced to furtherly enhance the coupling efficiencies. Experimentally, ultra-low PDL of < 0.2 dB over a wavelength range of 100 nm is achieved for both configurations, and the PDL around the central wavelength is near zero, as 0.03 dB / 0.02 dB for the 2-port / 4- port one. The peak coupling efficiency is -5.3 dB / -5.0 dB for the 2-port / 4-port configuration, and -2.9 dB / -2.4 dB in the case of enhancement by bottom metal mirrors.