Compact and spectrally selective asymmetric co-directional coupler using slow light photonic crystal waveguide

The design of an ultra-compact narrow band transmission co-directional coupler suitable for 100GHz wavelength division multiplexing applications is reported. The originality of the considered approach comes from the use of laterally coupled strip and photonic crystal waveguides characterized by strongly different group indices at the phase-matching condition. The performed FDTD modeling demonstrates the improvement of the spectral selectivity by a factor of 70 with respect to the state of the art conventional ADCs. A 1nm half-width transmission band is achieved for a 55μm coupler length designed for fabrication in the silicon photonic technology corresponding to a length bandwidth merit factor of 0.055nm×mm.