Analysis and design of broadband grating couplers

The authors have assessed the ability of thin-film dielectric gratings to convert an incident guided wave into outgoing beams whose widths and intensities are maintained nearly constant over a large wavelength band. For this purpose, they have improved and extended a previously developed analytical approach and used it to investigate the pertinent coupling characteristics of typical planar gratings having symmetric rectangular or blazed triangular profiles. The results reveal that, by judiciously designing the grating configuration, it is possible to achieve broadband coupling operation over a wavelength ratio of 2:1 or larger. It is also found that rectangular gratings can offer wider bandwidths, but they couple energy almost equally into the cover and the substrate beams. By contrast, blazed gratings allow somewhat narrower bandwidths, but they can transfer energy almost entirely into either one of those two beams