Subwavelength-grating-assisted broadband polarization-independent directional coupler

Abstract

This Letter presents both numerical and experimental results of a polarization-independent directional coupler based on slot waveguides with a subwavelength grating. The measured coupling efficiency is 97.4% for TE and 96.7% for TM polarization at a wavelength of 1550 nm. Further analysis shows that the proposed subwavelength grating directional coupler has a fabrication tolerance of $\pm 20\mathrm{nm}$ for the grating structure and that the coupling efficiencies for the two polarizations are both higher than $-0.5\mathrm{dB}$ ($\sim 89\%$), exceeding the entire C-band (1525–1570 nm) experimentally.