Abstract

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 (∆n) 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 ∆n in such fibers is about 0.005-0.015.

© 2011 Optical Society of America