Silicon nitride grating coupler with suppressed waveguide backreflection and adaptive grate design.

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We present both experimental and simulation results for a fully etched, C-band GC fabricated in an 800 nm silicon nitride platform that significantly reduces backreflections. They are minimized by truncating the initial grates, which deflect reflected light at an oblique angle and excite higher-order modes in the tapered waveguide that is filtered out. Insertion losses resulting from this modification of the grating coupler are mitigated by an adaptive redesign of the grates that corrects incurred errors in the generated phase front. While 2.4 dB of excess insertion losses is first introduced by the grate truncation, reshaping of the grates into ellipses of varying eccentricities reduces the excess losses to less than 1 dB. At the same time, the suppression of the waveguide-to-waveguide backreflection by 10 dB relative to a baseline device without the grate truncation is maintained. This demonstrates improved backreflection with a minimal cost in coupling efficiency and without an increase in the complexity of the fabrication process. Experimentally, insertion losses of -8.8 dB are achieved with a 3 dB passband of 65 nm and less than -25 dB waveguide-to-waveguide backreflections. Simulations show that with the over- and under-cladding thicknesses of a previous fabrication run, this passband and backreflection level can be achieved with -6.4 dB insertion losses.