Design of an optimized grating coupler for thick SOI rib waveguides

The use of grating couplers for fiber to chip coupling is a promising technique, allowing wafer scale testing with no need for facet preparation. Albeit grating couplers have been extensively studied for Si-wire waveguides, the implementation on SOI micrometric rib waveguides remains challenging, mainly due to excitation of higher order modes. In this work we show for the first time that monomode excitation of the grating can be achieved by properly designing the access waveguide. With the proposed approach the circuit can be fabricated using standard i-line stepper litography with a single etch step. The optimized design for a 1.5μm thick silicon layer achieves a simulated coupling efficiency of 65.6% at 1.55μm. Preliminary experimental results yield a coupler loss of 4dB.