Large-mode-area highly Yb-doped photodarkening-free Al2O3-P2O5-SiO2-Based fiber

In this letter, photodarkening (PD) problem in lasers were solved by utilizing Al₂O₃-P₂O₅-SiO₂ (PAS) ternary glass matrix. It was shown earlier that almost all available Al³⁺ and P⁵⁺ ions in this matrix become four-fold coordinated, and are paired with formation of AlPO₄ joins having a refractive index below that of the pure silica glass. The structure of the AlPO₄-SiO₂ glass network is very similar to that of pure SiO₂ glass. Therefore the properties of active fibers (including PD) are defined mainly by the dopant (aluminum or phosphorous) being in excess, which keeps its usual coordination. This study has shown that presence of the AlPO₄ join significantly increases the solubility of the rare-earth ions as compared to pure silica and phosphorosilicate glasses. These properties make the PAS matrix with a small excess of P₂O₅ a promising candidate for large mode area highly Yb-doped PD-free fibers.