Compact, Broadband, and Wide-Angle Optical Coupling for Silicon Waveguide

A novel grating coupling scheme with simultaneously broadband and wide-angle efficiency for unidirectionally coupling freely propagating light into a silicon waveguide is proposed and theoretically investigated. The coupling structure is composed of two cascaded subgratings with proper spectral and angular intervals. A semi-analytical theory based on coupled-mode model is developed to facilitate the design. Proof-of-principle demonstrations show that the spectral and angular full-width half-maximums (FWHMs) are simultaneously broadened to nearly three times at a specific wavelength or incidence angle, compared with the reference grating of the same number of periodic features. The spectral FWHM reaches 200 nm, covering E + S + C or C + L + U communication bands, and meanwhile, the angular FWHM is up to 8 degrees; if the spectral FWHM is diminished to 40 nm only covering C band, the angular FWHM will be as large as 40 degrees. This paper may be of great interest in related applications such as biochemical sensing and optical interconnect.