Anti-reflection grating-assisted contra-directional coupler with large corrugation width

The grating-assisted contra-directional coupler (contra-DC) is one of the commonly used add-drop filters serving for coarse wavelength-division-multiplexing (CWDM) systems due to its flat-top and wide-band response. However, the free-spectral-range (FSR) of contra-DC is limited by the back reflection to the input port caused by the intra-waveguide coupling. In this paper we propose an anti-reflection grating-assisted contra-DC by simply increasing the sidewall corrugation width. The intra-waveguide coupling coefficient can be decreased to 0 when the corrugation width is increased to a certain value. The simulation result shows that the transmission at drop port is-0.42 dB, while the back reflection to the input port is only-22.0 dB. The 4-channel cascaded contra-DCs with wavelength range of-60 nm are also simulated, all the channels are not affected by the intra-waveguide coupling. We believe the proposed contra-DC with large corrugation width can be applied to the CWDM system requiring a broad spectrum range.