Silicon subwavelength grating coupler with ultra-high reproducibility, ultra-wide bandwidth, and ultra-low back reflection

Subwavelength grating (SWG) couplers have attracted great attention in silicon photonics due to the flexible capacity of index tailoring for light coupling. With the intrinsic high refractive index (RI), silicon SWG couplers usually suffer from poor tolerance to device fabrication errors, moderate coupling bandwidth, and large back reflection. Although tremendous efforts have been made to solve one of the problems, it is challenging to overcome the main limitations, simultaneously. Here, by deliberately designing the SWG at a low effective RI slope region on an ultra-thin silicon layer, we demonstrate light coupling with ultra-high reproducibility, ultra-wide bandwidth, and ultra-low back reflection in a single silicon SWG coupler. The SWG coupler has an experimental coupling efficiency of -5.5 dB at a wavelength of 2.27 μm with a 1-dB bandwidth of ∼160 nm and a back reflection of -21.2 dB. The high fabrication reproducibility of the SWG couplers is also verified by using high-resolution nanofabrication facilities and a silicon photonics foundry. Moreover, leveraging the low back reflection feature, we further demonstrate the feasibility of achieving perfectly vertical fiber-to-chip light coupling. The study paves an avenue toward cost-effective silicon SWG couplers with high performance and excellent reproducibility.