Research progress on characterization techniques for doping uniformity in rare-earth-doped optical fiber materials

Rare-earth-doped optical fibers are key gain media for high-power fiber lasers and optical fiber amplifiers. The distribution uniformity of rare-earth ions and co-dopant elements directly affects pump absorption, gain distribution, refractive-index profiles, mode stability, photodarkening behavior, and long-term reliability. This paper reviews recent progress in techniques for characterizing doping uniformity in rare-earth-doped silica fibers and their preforms from three complementary perspectives: elemental composition and local structure, refractive-index distribution, and optical response. Elemental-composition and chemical-state analyses provide information on the spatial distribution, local enrichment, diffusion, and chemical states of dopants, while local-structure characterization further reveals the coordination environments of rare-earth ions, glass-network structures, and defect centers. Refractive-index distribution measurements can be used to evaluate core geometry, central depressions, profile distortions, and non-axisymmetric structures. Optical-response characterization reflects effective absorption, active-ion emission distributions, energy-transfer processes, quenching behavior, and excited-state dynamics. The applicable objects, spatial scales, sample-preparation requirements, quantitative reliability, advantages, and limitations of different methods are comparatively discussed. Since no single technique can simultaneously satisfy the requirements of quantitative compositional analysis, large-area spatial coverage, nondestructive testing, and functional-response evaluation, multiparameter joint characterization and the establishment of quantitative correlations among composition, local structure, fabrication processes, and optical properties will be important directions for future research on doping-uniformity characterization of rare-earth-doped optical fibers.