Polarization-insensitive silicon photonic filter with multimode waveguide gratings

A polarization-insensitive optical filter is proposed and demonstrated with 340-nm-thick silicon photonic waveguides. It consists of a polarization-insensitive mode (de)multiplexer and a polarization-insensitive multimode waveguide grating (MWG). The MWG is designed optimally according to the polarization-insensitive phase-matching condition. Meanwhile, longitudinal apodization is introduced in the MWG to suppress the sidelobes. For the fabricated silicon photonic filter based on a single stage of MWG, the 1-dB bandwidth as large as ∼13 nm, the excess loss is ∼1 dB and the sidelobe suppression ratio is about 10 dB for both polarizations. In particular, the performance is improved greatly by introducing more than one MWGs in cascade, and the fabricated device has a sidelobe suppression ratio as high as >30 dB.