Flat-Top, Sharp-Edge Add-Drop Filters Using Complementary-Misalignment-Modulated Grating-Assisted Contradirectional Couplers

We demonstrate flat-top, sharp-edge, silicon add-drop filters using apodized grating-assisted contradirectional couplers (contra-DCs). Periodic sidewall corrugations are imposed on both sides of two asymmetric strip waveguides for contradirectional coupling between the first two TE supermodes of a two-waveguide system. Gaussian apodization is applied to suppress the sidelobes. By laterally shifting the positions of both waveguide Bragg gratings in a complementary manner, which is called complementary lateral-misalignment modulation, our devices avoid phase errors which impair the spectral responses for conventional lateral-misalignment modulated contra-DCs. We demonstrate a complementary lateral-misalignment modulated (CLMM) contra-DC that has a flat-top, rapid roll-off, drop-port spectral response with a sidelobe suppression ratio (SLSR) of 20 dB. We also demonstrate a series-cascaded CLMM contra-DCs that has a higher SLSR of 39 dB and a sharper filtering edge. ©