Experimental Characterization of Particle Swarm Optimized Focusing Non-Uniform Grating Coupler for Multiple SOI Thicknesses

We report the first experimental characterizations of the coupling efficiencies of a series of e-beam fabricated focusing non-uniform grating couplers interfaced with a horizontal fiber coupling-scheme and designed for multiple Silicon-On-Insulator thicknesses, spanning between the 220 and 340 nm standard platforms. The design of nonuniform grating couplers is tackled either from an engineering perspective focused on to the optimization of the structures for this particular coupling-scheme; and a physics perspective related to the investigation of the scattering process of each structure for a complete comprehension of its performance in support to the experimental findings. Measured coupling efficiencies, up to 83% for the 340 nm platform at 1550 nm, are reported showing to be in excellent agreement with the theoretical findings. The physics behind the scattering process of each structure is investigated taking advantage of the Finite Difference Time Domain method to obtain a deeper understating of the coupling efficiencies. Consequently, a new parameter, the integrated leakage factor, together with the variation of the effective refractive index, across the local pitches, are calculated. Our belief is that any coupling structure for grating coupling can benefit from the proposed designed approach, as a widely applicable technique.