Forward-Dropping Contradirectional Coupler-Based Add-Drop Filter with Antisymmetric Photonic Crystal

We propose novel forward-dropping, grating-assisted contra-directional coupler (CDC)-based, silicon photonics add-drop filters. The CDC is designed to enable cross-coupling between the TE0 mode in the bus waveguide and the TE1 mode in the drop waveguide. An anti-symmetric photonic crystal (PhC) is placed at the input side of the drop waveguide to act as an ultra-compact, broadband, mode-conversion reflector, which reflects the backward TE1 mode into the forward TE0 mode. This mode conversion is essential to prevent the forward drop signal from coupling back to the bus waveguide. An asymmetric Y-branch serving as a broadband mode demultiplexer is used at the output of the drop waveguide to provide the signal add and drop functionalities of the filter. The proposed add-drop filter with a ~4.9 $\mu {\mathrm{ m}}$ -long PhC is experimentally demonstrated, which shows a center wavelength and a 3 dB bandwidth of ~1521 nm and ~4 nm, respectively. A 4-channel forward-propagating wavelength demultiplexer is also verified by cascading 4 stages of the filters.