Triangular cross-section multimode interference couplers in polymer enabled by 3D printing

Multimode interference (MMI) couplers have been long considered as key enabling components for splitting optical signals. Building on recent advancements in two-photon polymerization-based direct laser writing (2PP-DLW) and the capability to operate with full 3D freedom, we report on MMI couplers with an unconventional triangular cross-section. We developed first in their kind MMI couplers operating around the 1550 nm telecom wavelength that receive a 2- micrometer diameter waveguide at the input and generate a 3 or 4-node interference pattern in a triangular lattice at the output. As a proof-of-concept application, we developed an interconnection component that splits light from an input single-core fiber into four cores of a triangular core layout multi-core optical fiber. We fabricated the splitter with 2PPDLW technology to experimentally demonstrate a novel type of single-core to multi-core fiber splitter, measuring losses at -3 dB per channel and achieving a length as short as 180 micrometers.