High-performance grating couplers for mid-infrared applications based on multiparameter optimization of geometry characteristics

This paper presents a novel design of grating couplers in the SiPh platform, as part of the development of a gas sensing system, that measures specific gas concentration through on-chip absorption based spectroscopy in the mid-infrared. The grating is based on 400 nm SOI rib waveguide platform, targeting gaussian emission profile. We investigate both uniform and non-uniform grating designs. We show maximum coupling efficiency optimized by 65% at the center wavelength of 4.3 mu m, and broadband response. A tolerance study investigating the Si height and etch depth variations indicate very good tolerance to fabrication imperfections across the investigated spectral range of 4.25-4.35 mu m. The proposed methodology can be used to engineer grating coupler emission profiles for other applications and platforms.