Silicon photonic contradirectional couplers using subwavelength grating waveguides.

We propose and demonstrate a novel design of contradirectional couplers in which a subwavelength grating (SWG) waveguide replaces one of the asymmetric waveguides of the conventional designs. The fabricated devices in silicon-on-insulator (SOI) platform show over 35 dB suppression of undesired codirectional coupling and larger than 120 nm operating range free from the interference of intrawaveguide reflections thanks to the large optical phase-mismatch between the segmented SWG waveguide and its nearby continuous waveguide. We study the effects of tailoring the period of the SWG waveguide, the gap distance between the two waveguides, and the coupling length on the spectral characteristics of the device where changing the gap distance from 100 nm up to 500 nm allows for bandwidths from 18.2 nm down to 0.9 nm.