Diffractive and Subwavelength Grating Couplers for Microphotonic Waveguides

We review our advances in diffractive and subwavelength grating coupler structures for microphotonic waveguides. We present a subwavelength grating fiber-chip edge coupler with a loss as low as 0.9 dB and with minimal wavelength dependence over a broad wavelength range exceeding 200 nm. We also present fiber-to-chip surface grating couplers based on subwavelength effective medium. The effective medium refractive index is engineered to control the strength of the grating and thereby maximize coupling efficiency, while mitigating back reflections at the same time. We analyze the fabrication tolerances of the coupler, which are of particular relevance for large scale photonic fabrication. Furthermore, we present the first grating coupler for micrometric silicon rib waveguides, which is particularly challenging since the coupler waveguide region is multimode. We experimentally demonstrate grating couplers in $1.5\ \mu {\rm m}$ thick silicon ridge waveguides with a coupling efficiency of $-2.2\ {\rm dB}$ and a 3 dB bandwidth of 40 nm. An inverse taper is used to match the fundamental mode of the interconnection waveguide with the optimum grating coupler excitation field, with negligible higher order mode excitation. The coupler was fabricated using i-line stepper lithography and single etch step.