Ultra-Compact 1 × 4 Optical Power Splitter Based on Variable-Length Segment Optimized Inverse Design

Fixed-length segment (FLS) optimization method offers a way to realize the high-efficiency analog inverse design of nanophotonic devices. However, due to the limitation of the variable dimensions and restricted search space, this method can hard to simultaneously achieve large bandwidth, compact size, and efficient performance when dealing with high-dimension design. Here, we propose a highly efficient variable-length segment (VLS) based inverse design method, aiming to solve complex analog inverse design and fully demonstrate the targeted performance. It divides the optimized region into several tapered segments of unequal length and inserts a subwavelength transition waveguide between each tapered segment, which can expand the search space of the algorithm, thus making it easier to obtain a better locally optimal solution. As typical complex proof-of-concept examples, a 1 × 4 power splitter on a silicon-on-insulator (SOI) platform is chosen to demonstrate the validity of our design paradigm. The simulation results show that, compared with the conventional FLS, VLS has about 4–5 times higher efficiency and obtains better optimization performance. In our experiment, the fabricated device has a compact footprint of 9.8 μm × 4.9 μm and is complementary metal oxide semiconductor (CMOS) compatible. The measured insertion loss and the uniformity are less than 0.58 dB and 0.8 dB, respectively. In addition, the tolerances to fabrication errors are also investigated. Our work may find important applications in the advanced design of future nanoscale high-quality optical devices.