Optics 2026

Multi-ring thulium-doped silica fibres represent a promising architecture for active fibre devices operating in the 1.7–2.1 µm spectral range. Unlike conventional fibres with uniformly doped cores, this design employs a controlled radial distribution of active ions, offering opportunities to tailor the refractive-index profile and modal field distribution. However, preserving the intended dopant architecture during preform fabrication and subsequent fibre drawing remains a significant technological challenge. Broadening of doped-region boundaries, and changes in the local glass structure may alter the coordination environment of the active ions and affect the optical properties of the resulting fibre, This study investigates the radial distribution of Tm³⁺ ions and their local structural environment in a multi-ring silica fibre preform fabricated using Modified Chemical Vapor Deposition with Chelate Doping Technology (MCVD-CDT). Micro-area X-ray fluorescence (XRF) mapping and electron probe microanalysis (EPMA) are used to determine the radial dopant concentration profile and assess the preservation of the designed core architecture. Extended X-ray absorption fine structure (EXAFS) spectroscopy at the Tm L3-edge provides complementary information on the local coordination environment, including the Tm–O bond distances and coordination number. The measurements are performed at the PolyX beamline of the SOLARIS synchrotron in Kraków.By combining spatially resolved elemental analysis with local-structure spectroscopy, this work establishes an experimental basis for evaluating dopant-profile preservation and structural changes during preform fabrication. The findings are intended to guide process optimisation and the development of multi-ring active fibres for emission near 2 µm.

Władysław Owsiński is a PhD student in Chemical Sciences at AGH University of Krakow in Poland. He graduated in 2024 from the Faculty of Physics and Applied Computer Science at AGH University of Krakow. His research focuses on thulium-doped silica fibres, their local structural characterisation, and the optimisation of fabrication processes for novel active fibre architectures. He is co-author of DOIs: 10.1051/epjconf/202638906001, 10.1364/OME.588270, 10.1117/12.3103407 and participated in STL 2026 and IQCLSW 2026.Władysław Owsiński is a PhD student in Chemical Sciences at AGH University of Krakow in Poland. He graduated in 2024 from the Faculty of Physics and Applied Computer Science at AGH University of Krakow. His research focuses on thulium-doped silica fibres, their local structural characterisation, and the optimisation of fabrication processes for novel active fibre architectures. He is co-author of DOIs: 10.1051/epjconf/202638906001, 10.1364/OME.588270, 10.1117/12.3103407 and participated in STL 2026 and IQCLSW 2026.