Spectral engineering of subwavelength-grating-based contradirectional couplers.

We tailor the spectral characteristics of silicon photonic contradirectional couplers (Contra-DCs), where the design of the coupler is based on placing a subwavelength grating (SWG) waveguide next to a strip waveguide. By tapering the gap distance between the SWG and strip waveguides, we demonstrate a compromise between sidelobe suppression and pass-band/stop-band extinction ratio such that the performance of the device as a potential optical (de)multiplexer is improved. The designs with different pass-band bandwidths of 12 nm, 9 nm, and 6 nm show 10 dB to 20 dB sidelobe suppression ratio and 15 dB to 35 dB extinction ratio. We also obtain a resonant transmission peak in the stop-band of the spectral response of the device by introducing a π phase shift into the gratings of the SWG waveguide. The resonant peak has 1 nm bandwidth and 7 dB extinction ratio, where the use of the SWG waveguide in the structure of such coupler allows the characteristics of the resonant peak to be highly sensitive to the cladding material, which is of strong desire in integrated sensing applications.