Narrow-Band Add-Drop Filter Based on Phase-Modulated Grating-Assisted Contra-Directional Couplers

A narrow-band silicon photonic add-drop filter is demonstrated by using phase-modulated grating-assisted contra-directional couplers (contra-DCs) on the silicon-on-insulator platform. The apodization coupling is achieved by modulating the phase of one grating in a contra-DC, while keeping the other grating uniform. As the local effective index of the gratings remains almost unchanged, the designed apodized contra-DC has a large designing flexibility and a high sidelobe suppression ratio (SLSR). To obtain a narrow bandwidth, weak coupling is designed. Single-stage and dual-stage filtering are both studied. The experimental results show that single-stage filtering device has a 3-dB bandwidth of 0.9 nm and an SLSR of 18 dB. Dual-stage filtering device has a 3-dB bandwidth of 0.5 nm and an SLSR of 23 dB. Our devices are promising for on-chip applications that require narrow bandwidths such as wavelength-division-multiplexed channel filters.