Ultra-compact broadband high efficient grating coupler

We present an ultra-compact metal reflector based grating coupler for coupling light between optical fiber and nano photonic waveguides. The coupler is designed and simulated based on Finite Difference Time Domain (FDTD) method. The lateral dimension of the grating waveguide has been tapered down from 10 μm to 400 nm within 8 μm length to interface with nano photonic waveguides using half overlay triangular waveguides. We have achieved a coupling efficiency of 94% (-0.25dB) using metal reflector and after the cascaded triangular waveguides, the efficiency remains as high as 80% (-0.95dB) with 1-dB bandwidth of 80 nm. The total length of the coupler is as low as 24 μm which provides very small foot print in silicon (Si) photonic ICs.