Etched Waveguide Grating Variable 1 $\,\times\,$2 Splitter/Combiner and Waveguide Coupler

We propose and test a silicon waveguide grating which serves dual functions: as a 1 times 2 variable integrated beam splitter/combiner and as an out-of plane diffractive element for coupling light between a single-mode optical fiber and a 500-nm-wide silicon-on-insulator waveguide. An integrated Mach-Zehnder interferometer made with this novel functional element had over 20-dB extinction ratio. The splitting ratio can be tuned by changing the launch position of the optical fiber. The grating coupler had over 36% coupling efficiency and a 1-dB spectral bandwidth of 37 nm.