Using an anti-reflection coating with a grating coupler

Embodiments herein describe a photonic platform where an AR coating is disposed between an optical grating and a semiconductor substrate. In one embodiment, the optical grating is disposed within an insulative layer. A first side of the insulative layer provides an optical interface where an external optical source can transmit an optical signal into, or a receive an optical signal from, the grating. A second, opposite side of the insulative layer contacts the AR coating. When the external optical source transmits light through the first side of the insulative layer, some of the light passes through the grating and reaches the AR coating. The AR coating prevents this light from being reflected back to the grating by the semiconductor layer which can cause interference that varies the coupling efficiency of the grating.