Compact and Highly Spectral Selective Asymmetric Co-Directional Coupler Using Slow Light Photonic Crystal Waveguide

This letter addresses the design of an ultra-compact narrowband transmission co-directional coupler suitable for 100-GHz wavelength-division-multiplexing applications. The originality of the considered approach stems from the use of laterally coupled strip and photonic crystal waveguides, the latter one operating near the high group index flat region of the dispersion diagram. The performed finite-difference time-domain modeling demonstrates the improvement of the spectral selectivity by a factor of 70 with respect to the state-of-the-art conventional symmetric co-directional couplers. A 1-nm half-width transmission band is achieved for a 55-μm coupler length. The corresponding length bandwidth merit factor is 0.055 nm×mm. The spectral selectivity improvement is achieved without significant extra-loss penalty. The design performed in view of implementation in silicon photonic technology is compatible with the existing micro-nano-fabrication facilities.