Most grating couplers for silicon photonics are designed to match the approximately 10 μm mode-field diameter (MFD) of single-mode telecom fibres. In this letter, we analyse grating-coupler designs in the Silicon-on-Insulator (SOI) platform in a wide range of MFDs (4–100 μm) and related footprints, to give a physical understanding of the trends in efficiency and lineshape of the corresponding coupling spectra. We show that large-footprint grating couplers have an intrinsic Lorentzian lineshape that is determined by the quasi-guided photonic modes (or guided resonances) of the corresponding photonic crystal slab, while small-footprint grating couplers have a Gaussian lineshape resulting from the k-space broadening of the incident mode. The crossover between the two regimes is characterized by Voigt lineshapes. Multi-objective particle-swarm optimisation of selected small-footprint apodized grating-couplers is then used to locate the “Pareto fronts;” along which the highest coupling efficiency is achieved for a given bandwidth. This approach identifies several high-efficiency 220 nm SOI grating coupler designs with 1 dB bandwidths exceeding 100 nm. Such grating couplers are ideally suited for broadband photonic applications, such as wavelength-division multiplexing and environmental sensing, and are compatible with commercially available ultra-high numerical aperture fibres.
Grating couplers in silicon-on-insulator: The role of photonic guided resonances on lineshape and bandwidth
M. Passoni, D. Gerace, L. Carroll, L. C. Andreani; Grating couplers in silicon-on-insulator: The role of photonic guided resonances on lineshape and bandwidth. Appl. Phys. Lett. 23 January 2017; 110 (4): 041107. https://doi.org/10.1063/1.4974992
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