An out-of-plane grating coupler for efficient butt-coupling between compact planar waveguides and single-mode fibers

We have designed and fabricated an out-of-plane coupler for butt-coupling from fiber to compact planar waveguides. The coupler is based on a short second-order grating or photonic crystal, etched in a waveguide with a low-index oxide cladding. The coupler is optimized using mode expansion-based simulations. Simulations using a 2-D model show that up to 74% coupling efficiency between single-mode fiber and a 240-nm-thick GaAs-AlOx waveguide is possible. We have measured 19% coupling efficiency on test structures.