Silicon Add-Drop Filter Based on Multimode Bragg Sidewall Gratings and Adiabatic Couplers

A silicon photonic add-drop filter is demonstrated with sidewall Bragg gratings in a multimode strip waveguide. The operating principle is based on the contradirectional coupling between the TE0 mode and different higher-order modes inside the multimode waveguide, which depends on the symmetry property of the periodic refractive-index perturbations. In order to relax the fabrication tolerance, a pair of adiabatic couplers is used to add/drop the optical signal at the output/input ports of the multimode waveguide. Experimental results show that the functionality of add/drop filtering is successfully achieved with a low insertion loss of 0.6 dB. We also show that by sweeping the multimode waveguide width from 700 to 1400 nm, the bandwidth can be flexibly tuned from 16 to 2 nm.