Ultra-Compact X-Shaped Waveguide Crossings With Flexible Angles Based On Inverse Design

When photonics integrated circuits (PICs) become more massive in scale, the area of chip can't be taken full advantage of with 2x2 waveguide crossings with a 90 degrees intersection angle. Crossings with small angles would be a better idea to further improve the area utilization, but few works have researched 2x2 crossings with different angles. In this paper, in order to have an ultra-compact footprint and a flexible intersection angle while keeping a high performance, we report a series of compact X-shaped waveguide crossings in silicon-on-insulator (SOI) waveguides for fundamental transverse electric (TE0) mode, designed by using finite-difference frequency-domain (FDFD) numerical analysis method and a global optimization method. Thanks to inverse design, a compact footprint as small as 4.5 mu m(2) and various angles between two input/output waveguides of 30 degrees, 45 degrees, 60 degrees, 80 degrees and 90 degrees are achieved. Simulation results show that all crossings have good performance of insertion losses (ILs) within 0.1 similar to 0.3 dB and crosstalks (CTs) within -20 similar to-50 dB in the wavelength range of 1525 similar to 1582 nm. Moreover, the designed crossings were fabricated on a commercially available 220-nm SOI platform. The measured results show that the ILs of all crossings are around 0.2 similar to 0.4 dB and the CTs are around -20 dB similar to-32 dB; especially for the 30 degrees intersection angle, the crossing has IL around 0.2 dB and CT around -31 dB in C band. Besides, we theoretically propose an approach of a primary structure processing technique to enhance the device performance with a more compact tbotprint. This technique is to remove the redundant structures in conjunction with the electric field distribution during the optimization procedure of inverse design. For the new 90 degrees crossing structure produced by it, simulation results show that ILs of 0.29 +/- 0.03 dB and CB of -37 +/- 2.5 dB in the wavelength range of 1500 similar to 1600 nm are achieved and the footprint is shrunk by 25.5%. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement