Diffractive Fan-In Couplers for Multicore Optical Fibers Fabricated on the Fiber Tip via Two-Photon Polymerization

This study presents novel fan-in fiber-optic couplers based on diffractive optical elements (DOEs), designed for precise and uniform excitation of all cores in multicore fibers (MCFs). The proposed couplers utilize custom-designed DOE phase profiles to split an incident Gaussian beam radiation into multiple focused spots that match the spatial layout of the target MCF cores. Four structures were designed using a modified Gerchberg–Saxton algorithm for operation at 1550 nm and 655 nm, addressing four-core and seven-core MCFs. All couplers are based on a single-DOE architecture, which constitutes a clear advantage of the presented solution. Their performance was first validated through numerical simulations, which confirmed the intended optical behavior. The structures were then fabricated using two-photon polymerization (TPP) with sub-micrometer resolution. Experimental results confirmed proper functionality consistent with the simulation data. One of the couplers was printed directly on the tip of a four-core fiber, enabling precise axial positioning and angular alignment relative to the fiber input facet. These results highlight the feasibility of TPP-fabricated on-fiber fan-in couplers as compact and efficient solutions for next-generation fiber-optic applications in sensing and telecommunications.