Inverse-design of an Ultra-compact and Broadband Mode Size Converter

Mode size converters are fundamental building blocks of photonic integrated circuits, connecting waveguides with different mode spot sizes. In this paper, we propose a new design approach based on gradient-based pixel merging method. The method combines a gradient-assisted discrete search with an additional merging process. As a proof of concept, we inversely design and experimentally demonstrate a compact mode size converter on a silicon-on-insulator platform. The converter, with a compact size of 5 μm × 10 μm offers low insertion loss (IL), wide bandwidth and 20-times mode size squeezing for fundamental transverse electric mode. The minimum feature size of 100 nm × 100 nm also enhances the feasibility of fabrication. The experimental results show that the mode size converter, optimized through the merging process, can achieve an IL around 2.2 dB and an optical bandwidth of around 100 nm.