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

Large-mode area (LMA) Yb-doped fibers are routinely used in high-power CW and pulsed fiber lasers and amplifiers. Solubility of Yb³⁺ ions in pure silica glass is rather small; therefore co-doping with Al2O3 is required to suppress clustering. Single-mode operation in LMA fibers (core diameter ∼ 20 µm) is possible, when the refractive index difference between the core and the cladding (Δn) is about 0.002 or less. Both Al2O3 and Yb2O3 have a relatively high molar refractivity (∼0.0025/mol.% and 0.001/wt.%, respectively), and therefore only a small Al2O3 concentration can be incorporated into a highly Yb-doped LMA fiber. Clustering of Yb³⁺ ions results in photodarkening (PD) - degradation of the laser properties with time due to a grey loss increase. Utilization of the phosphorosilicate glass matrix allows one to suppress PD almost completely (the grey loss due to PD is 10–100 times smaller in comparison with that in the aluminosilicate fiber) [1], but a typical Δn in such fibers is about 0.005–0.015.