Wavelength-Insensitive Mid-Ir Directional Coupler

We demonstrate mid-infrared wavelength-insensitive directional coupler based on two mechanisms, namely rib waveguide dispersion engineering and subwavelength grating (SWG) dispersion engineering. Compared to conventional directional coupler, the rib waveguide dispersion engineered and SWG dispersion engineered directional coupler achieve a 4-fold and more than 4-fold enhancement in the operation bandwidth for 100% coupling ratio respectively.