Low-Insertion-Loss and Ultracompact Multicore Fiber Fan-In/Out Device Fabricated by In Situ 3D Nanoprinting

Abstract Space-division multiplexing (SDM) using multicore fibers (MCFs) addresses the capacity bottleneck in single-mode fiber communications. Fan-in/fan-out (FIFO) devices interfacing MCFs with single-mode fiber arrays are essential for SDM systems, yet conventional fabrication suffers from precise alignment, complex packaging, and large footprints. We demonstrate an ultracompact, low-insertion-loss, 4-channel MCF FIFO device fabricated by in situ 3D nanoprinting via two-photon polymerization. The device monolithically integrates parabolic total internal reflection (TIR) bridge structures onto the end faces of a 4-core MCF and a single-mode fiber array, eliminating active core-to-core alignment and postfabrication assembly. The 3D structure has a height of only 60 μm and functions as a reflective 4f imaging system, ensuring diffraction-limited mode-field transfer. The FIFO exhibits a low insertion loss of ∼0.46 dB, intercore crosstalk below −50 dB at 1550 nm, and broadband operation across the S, C, and L bands. Its viability is validated through an offline 24 GBaud Dual-Polarization Quadrature Phase Shift Keying (DP-QPSK) recirculating transmission experiment over 1058.4 km (14 loops × 75.6 km) covering 186 channels, with all bit-error rates below the FEC threshold. With simple fabrication, low loss, alignment-free operation, and ultracompact footprint, this technique presents a promising and scalable solution for efficient photonic interfacing in SDM systems.