Add–drop filter based on apodized surface-corrugated gratings

We report on the fabrication of a grating-based add–drop filter in SiON planar waveguide technology. We achieved apodization of the Bragg grating by concatenating subgratings with various duty cycles. We present the theoretical and experimental dependence of the coupling coefficient on the duty cycle, which leads to a minimum coupling coefficient of 30%. With a breeder genetic algorithm we were able to find optimal apodization profiles within this limited coupling coefficient range. The final device is compatible with a 100-GHz channel spacing and has a bandwidth utilization factor of 36%.