Structural and spectroscopic properties of Yb:Al-P co-doped silica fiber preform materials

We demonstrate a pathway for producing large-core fiber preforms with optimized characteristics for high-power fiber laser applications. Reactive powder sintering (REPUSIL) was used for producing large volumes of doped silica fiber preforms with predictable, homogeneous, and precise compositional profiles, focusing on formulations near the equimolar dopant ratio of P:Al = 1 to suppress Yb-related photodarkening (PD). Spectroscopic and structural properties are related to PD performance for both preform and optical fiber materials. All preform samples exhibit a radial dip-free, step-like refractive index profile with relative average index fluctuations of less than 2%. Reduced excess PD loss of 10 dB/m after 21 h exposure time and lower PD rates are obtained when the P content is adapted at a slight excess over Al, even when the overall Al content is high.