Tailoring Al‐F‐P‐Si Glass Network for Yb ³⁺ ‐Doped Fibers With Enhanced Photodarkening Stability

Yb 3+ ‐doped fiber lasers are widely used for high‐power applications but suffer from photodarkening (PD), limiting long‐term performance. We report a novel fabrication approach for alumino‐fluorophosphosilicate (AFPS)‐based Yb 3+ ‐doped fibers using the modified chemical vapor deposition–vapor phase chelate delivery (MCVD–VPCD) process. Strategic fluorine co‐doping during core deposition suppresses P 2 O 5 evaporation, minimizes central dip formation in the refractive index profile, reduces the numerical aperture, and mitigates Yb–Yb pair formation. Optimized preforms with high Yb 3+ concentration (∼4000 ppm) with nearly 1:1 Al:P ratios resulted in the formation of a stable glass network containing AlPO 4 units, in a repeatable manner without structural distortions. A comparative PD‐induced loss analysis revealed that Yb‐doped AFPS fibers exhibit ∼5.4‐fold and ∼3.5‐fold improvement toward long‐term stability than conventional Al–Si–Yb fibers and P 2 O 5 ‐rich Al–P–Si–Yb fibers, respectively. The overall results highlight AFPS as a robust host glass for high‐power laser applications with improved PD resistance.