High-Performance 2 x 2 Bent Directional Couplers Designed With an Efficient Semi-Inverse Design Method

Directional couplers (DCs) are the basic elements for constructing various silicon photonic devices, including Mach-Zehnder switches (MZSs). Here, we propose a novel 2 x 2 bent directional couplers based on Bezier curves designed with an efficient semi-inverse design method. Two DCs were designed to operate at center wavelengths of 1.55 mu m and 2 mu m, with splitting ratios maintained between 45 to 55% over broad bandwidths of 130 nm and 160 nm respectively, and excess losses <0.05 dB. The present 1.55/2 mu m DCs also have ultra-compact footprints of 16 x 1.8/24 x 2.5 mu m(2), which are much smaller than conventional DCs. The fabricated DCs show good fabrication tolerance of +/- 10 nm waveguide width deviation. The present bent DCs are then used for realizing thermo-optic MZSs for the 1.55/2 mu m wavelength-bands, which show a bandwidth of >90 nm for achieving low excess losses of <0.3 dB and high extinction ratios of similar to 18/20 dB.