Compact grating couplers between a single-mode fiber and nanophotonic silicon-on-insulator waveguide

We present a high-efficiency broadband grating coupler for coupling between silicon-on-insulator (SOI) waveguides and optical fibers. The grating is only 14 mu m long and 11 mu m wide, and the size of the grooves is optimized numerically. For TE polarization the coupling loss to single-mode fiber is below 1 dB over a 35-nm wavelength range when using SOI with a two-pair bottom reflector. We used Focused Ion Beam (FIB) processing methods to fabricate the grating structure. The tolerances to fabrication errors are also calculated. The diffractive grating structures are designed to be inherently very directional by adding a silicon overlay before grating definition. 57% coupling efficiency at a wavelength of 1.54 mu m is experimentally demonstrated on devices fabricated used FIB. By optimizing the grating parameters, we theoretically show that 83% grating output coupling efficiency can be obtained for a uniform grating structure.