ANALYSIS AND DESIGN OF BROAD-BAND GRATING COUPLERS

We have assessed the capability of thin-film dielectric gratings to co nvert an incident guided wave into outgoing beams whose widths and int ensities are maintained nearly constant over a large wavelength band. For this purpose, we have improved and extended a previously developed analytical approach and used it to investigate the pertinent coupling characteristics of typical planar gratings having symmetric rectangul ar or blazed triangular profiles. Our results reveal that, by judiciou sly designing the grating configuration, it is possible to achieve bro adband coupling operation over a wavelength ratio of 2:1 or larger. We also find that rectangular gratings can offer wider bandwidths, but t hey couple energy almost equally into the cover and the substrate beam s. By contrast, blazed gratings allow somewhat narrower bandwidths, bu t they can transfer energy almost entirely into either one of those tw o beams.