Optical Add-drop Filters using Cladding-modulated Sub-wavelength Grating Contra-directional Couplers for Silicon-on-Insulator Platforms

We propose and demonstrate compact, add-drop filters using cladding-modulated, sub-wavelength grating (SWG), contra-directional couplers (CDCs) on a silicon-on-insulator platform. In our device, two asymmetric SWG waveguides, integrated with periodically loaded Bragg segments along the sides of the waveguides, are used to achieve contra-directional coupling between the fundamental and next higher-order transverse electric modes of the two-SWG-waveguide system. We present results for a 286.8-μm-long CDC that has a 3 dB bandwidth of 13.4 nm at the drop-port with a low excess loss of 0.8 dB. This device also has an 11.5-nm region at the through-port over which the extinction ratio is greater than 40 dB.