Anti-phase reflection coating maximizing the directionality of grating couplers.

We present numerical demonstrations that anti-phase reflection coatings (APRCs) on the core layers of grating couplers (GCs) return anti-phase field into the core layers and cancel the downward scattering from the gratings by destructive interference to improve the upward directionality of the GCs while the output power per unit length is reduced. Investigating simplified models of GC, we reveal the effect of APRCs semi-analytically and quantitatively. The APRCs can be combined with other enhancement measures, like deep gratings and backside mirrors, to tailor an appropriate output power per unit length while achieving high upward directionalities cooperatively.