Large Beam Size Grating Coupler in Silicon-on-Insulator Using Fully Etched Subwavelength Gratings

Abstract

Several emerging applications of silicon photonics, including sensing, ranging, and optical trapping, require fixed, well-collimated beams that enable interaction with targets placed centimeters away from the chip. Generating such beams without using bulk-optic lenses entails radiating lightwaves with diameters of hundreds of microns directly from the chip. Gratings with sufficiently low strength have so far only been shown in the silicon nitride platform using specialized shallow etch steps; in silicon-on-insulator the implementation becomes much more challenging due to the increased index contrast. Here, the first silicon-on-insulator grating capable of radiating such large beams is reported. Using a fully etched, double-period subwavelength structure, with feature sizes compatible with deep-ultraviolet lithography, a beam diameter in excess of $350\mu \mathrm{m}$, with a $54\%$ radiation efficiency, is experimentally demonstrated.