HIGHLY EFFICIENT BROAD-BAND WAVEGUIDE GRATING COUPLER WITH A SUB-WAVELENGTH GRATING MIRROR

A new diffractive device for light coupling between a planar optical waveguide and free space is proposed. The device utilizes a second order waveguide grating to diffract the fundamental waveguide mode into two free propagating beams and a sub-wavelength grating (SWG) mirror to combine the two free propagating beams into a single beam. The Finite Difference Time Domain (FDTD) simulations show that the SWG mirror improves the coupling efficiency of the waveguide fundamental mode into the single out-coupled beam from about 30 % to 92 %. A high efficiency (>80 %) is predicted for a broad wavelength range of 1520 - 1600 nm. The proposed device is compact (~ 80m in length), it eliminates the need for blazing the waveguide grating, and it is simple to fabricate using standard CMOS processes.