Spectral-Distortionless, Flat-Top, Drop-Filter Based on Complementarily-Misaligned Multimode-Waveguide Bragg Gratings

An integrated-optics, flat-top drop-filter with low excess loss, and high sidelobe suppression ratio (SLSR), is demonstrated. The filter is based on apodized multimode waveguide Bragg grating (AMWBG) developed on a silicon-on-insulator (SOI) platform. Antisymmetric periodic refractive-index perturbations are imposed on both sides of the waveguide to couple light contra-directionally between the fundamental (TE0) and the first-order transverse electric modes (TE1). To achieve a Gaussian apodization profile, the lateral-misalignment between the two sides of the waveguide is modulated by adjusting the positions of both sides of the grating features in a complementary manner. In this way, the undesired phase errors, which exist in conventional misalignment-modulated Bragg gratings and distort the grating responses, can be eliminated. Experimental results show that our AMWBG has a high-quality flat-top response, a high SLSR of >21 dB, a 3-dB bandwidth of 8 nm, a low excess loss of 0.24 dB, and a high roll-off rate at the filter edge. We also demonstrate a dual-stage filter that has a flat-top, drop-port response with a 3-dB bandwidth > 7.5 nm, a SLSR >40 dB, and an excess loss<; 0.5 dB.