This chapter presents the analysis of a plasmonic grating coupler using the FDTD method. For the conventional coupler, the shape of the dielectric grating on the metal substrate is rectangular. In this case, the field coupled to the grating propagates in both directions on the metal surface at normal incidence. When this coupler is used for an optical link, half of the coupled power is wasted. At oblique incidence, the coupled field propagates unidirectionally on the metal surface. However, it is relatively difficult to precisely control the angle of incidence. To overcome this problem, a trapezoidal dielectric grating coupler is proposed for unidirectional propagation at normal incidence. Based on Snell’s law, the trapezoidal grating is designed so that the metal substrate receives oblique incident light through the trapezoidal grating even under normal incidence. It is found that the coupling light in the grating propagates unidirectionally on the metal surface even at normal incidence. It should be noted that the slanted shape of the trapezoidal grating is relatively difficult to fabricate. Therefore, a grating coupler with stepped dielectrics is also investigated. The field propagates unidirectionally on the metal surface at normal incidence. Finally, the possibility of reducing the reflectivity by adding an anti-reflection (AR) coating is considered. It is shown that the reflectivity is significantly suppressed at 1.55  \(\upmu \) m and the field can propagate in one direction.