Mixed-cation effect for enhanced L-band spectral broadening in Er3+/Yb3+ co-doped fluoro-sulfo-phosphate glasses

The rapid expansion of data-intensive technologies such as 5G, AI, and cloud computing is pushing optical communication systems to their limits, necessitating the development of advanced materials to extend bandwidth into the L- and U-bands. Fluoro-sulfo-phosphate (FSP) glasses, owing to their multi-anionic compositions and tunable network structures, offer a promising platform for next-generation broadband fiber amplifiers. In this work, a mixed-cation approach employing K + and Ba 2+ is employed to manipulate the local structural environment and enhance glass-forming ability, lifetime (8.7–9.22 ms for Er-doped, ∼10 ms for Er 3 + /Yb 3+ co-doped), emission cross-section (0.59-0.70 × 10 − 20  cm 2 ), energy transfer efficiency (∼85 %), and thermal quenching resistance. Despite their similar sizes and electronegativities, K + and Ba 2+ exhibit contrasting field strengths and polarizabilities, enabling a controlled balance between network connectivity and local disorder. Thermodynamic modeling and experimental results confirm that the introduction of mixed cations significantly broadens the glass-forming region. The addition of AlF 3 further strengthens the network, allowing for the preparation of stable K/Ba-FSP glasses with improved mechanical, thermal, and chemical properties. Spectroscopic analysis reveals a substantial enhancement in L-band emission efficiency and gain characteristics, with performance improvements ranging from 30 % to 150 % over conventional glass hosts. These findings position K/Ba-FSP glasses as a robust and high-performance candidate material system for broadband rare-earth-doped fiber amplifiers and lasers.