Mode-mixing element 3D-printed directly on a fiber tip for space-division multiplexing

Space-division multiplexing (SDM) offers a promising route to scaling data throughput in fiber-optic networks, but it also introduces challenges such as mode-dependent loss (MDL) and intermodal crosstalk, which increase the computational load on digital signal processing (DSP). Periodic mode mixing has been shown to mitigate these effects by redistributing loss and gain across modes and shortening the effective temporal impulse response over which crosstalk accumulates. In this work, we present a novel and compact mode-scrambling device, 3D-printed directly onto the facet of a few-mode fiber. Our scalable design precisely controls a printed microstructure that strongly couples six spatial modes across a broad spectral range, equalizing modal gains and losses in the SDM link. The device exhibits low insertion loss and small footprint, making it suitable for periodic deployment along the fiber without incurring excessive loss. To the best of our knowledge, this is the first demonstration of a six-mode on-fiber mixer fabricated by 3D printing that meets practical performance requirements, offering a viable path toward scalable, high-capacity SDM transmission systems.