Design and numerical verification of a polarization-independent grating coupler using a double-layer approach for visible wavelengths applications.

In this paper, a double-layer approach is proposed to design a compact four states polarization-independent grating coupler (GC). The proposed polarization-independent GC is designed to couple a 700 nm polarized light propagated in a 150 nm Gallium Phosphide (GaP) waveguide to a polarization-maintaining fiber. The double-layer approach is based on the deposition of GaP gratings designed to couple the transverse magnetic (TM) light over the GaP gratings designed to couple the transverse electric (TE) light. The two layers are separated by a Hydrogen silsesquioxane (HSQ) with an optimum thickness of 20 nm. The grating periods and the fill factors of the two layers are optimized to achieve maximum possible coupling efficiency (CE). The proposed method resulted in relatively high CEs of 39.2, 31.1, and 23.3% for the TE, TM, and 45°/-45° linearly polarized light, respectively. The polarization-dependent loss (PDL) is 1 dB, 1.26 dB, and 2.26 dB corresponding to TE-TM, TM-45°/-45°, and TE-45°/-45°, respectively. The performance of the double-layer approach is numerically verified by the two-dimensional (2D) finite element algorithm (FEM) using COMSOL software. The proposed method suggests a novel and simple approach to design a compact four states polarization-independent GC that could be used in integrated (on-chip) photonic communication circuits. [ABSTRACT FROM AUTHOR]