Abstract
Ultra-longitudinal-compact S-bends with flexible latitudinal distances (d) are proposed and experimentally demonstrated with ultralow loss and fabrication-friendly structures by three steps based on numerical optimization. During the first step (curve optimization), insertion losses (ILs) of S-bends are significantly reduced by optimizing transition curves based on Bézier curves. During the second step (shape optimization), the ILs are further minimized by varying the widths of S-bends to increase optical confinement. In the third step (curvature optimization), considering ease of fabrication, an optimization of curvature radius is used to ensure that all feature sizes for the S-bends are larger than 200 nm. Simulation results show that for S-bends with footprints of 2.5× d μm2, the ILs are less than (0.19, 0.045, 0.18, 0.27) dB in a wavelength range of 1400–1700 nm when d is set as (3, 6, 9, 12) μm, respectively. Then, the S-bends of 2.5× 3 μm2 and 2.5× 12 μm2 are fabricated on a commercial 220-nm silicon-on-insulator (SOI) platform. Experimental results show that the ILs of both are less than 0.16 dB in a wavelength range of 1420–1630 nm. The lowest ILs are 0.074 dB and 0.070 dB, respectively. Moreover, in addition to the ultralow ILs and ease of fabrication, our design is flexible for designing S-bends with a flexible value of d, which makes our approach practical in large-scale photonic integrated circuits.
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