Abstract: A grating coupler is disclosed. The grating coupler comprises a grating formed in a waveguide formed in a silicon layer of a silicon on insulator photonic component. The grating has a series of sequentially arranged grating elements i, each comprising an alternating etched trench section having a length Le and an etch depth e, and an unetched tooth section having a length L0. Each grating element has a total length of ?i and a fill factor Fi. The fill factor Fi of each grating element varies across the grating coupler according to an apodization function dependent on the location of the grating element. The total length of each grating element ?i varies across the grating coupler such that the effective refractive index of each grating element causes the Bragg condition to be satisfied for all of the grating elements of the grating.