Ultra-low-loss broadband multiport optical splitters

A major challenge in inverse design of optical splitters is to efficiently reach platform nonspecific designs constrained to multiple functional requirements: arbitrary splitting ratio, low insertion loss, broad bandwidth and small footprint. While the traditional designs fail to fulfill all these requirements, the more successful nanophotonic inverse designs require substantial time and energy resources per device. Here, we present an efficient inverse design algorithm that provides universal designs of splitters compliant with all above constraints. To demonstrate the capabilities of our method, we design splitters with various splitting ratios and fabricate 1 x N power splitters in a borosilicate platform by direct laser writing. The splitters show zero loss within the experimental error, competitive imbalance of <0.5 dB and broad bandwidth in the range 20 - 60 nm around 640 nm. Remarkably, the splitters can be tuned to achieve different splitting ratios. We further demonstrate scaling of the splitter footprint and apply the universal design to silicon nitride and silicon-on-insulator platforms to achieve 1 x 5 splitters with the footprints as small as 3.3 pm x 8 pm and 2.5 pm x 10.3 pm, respectively. Owing to the universality and speed of the design algorithm (several minutes on a standard PC) our approach renders 100 greater throughput than nanophotonic inverse design.