Ytterbium-doped silica fiber (YDF) has important applications in material processing and scientific research. The photodarkening (PD) effect, which originates from the formation of color centers, can decrease the laser output power over 1,000 h by about 10% and will seriously restrict the power stability of the fiber laser. However, the nature of the PD color centers has not been adequately elucidated until now.
In a recent study, a team from the Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences has made a systematical study in PD color centers of YDF. Related work was published on Journal of Non-Crystalline Solids.
In their experiment, a series of YDF core glasses were irradiated with a 193 nm ArF excimer laser for different durations. The species of the UV laser-induced color centers were identified by radiation-induced absorption (RIA), in-situ photoluminescence (PL), and continuous-wave electron paramagnetic resonance (CW-EPR) spectroscopic techniques.
They found that the oxygen hole center and the Yb2+ ion pairs were primarily responsible for the UV laser radiation-induced darkening, and their formation was highly dependent on the charge balance between the Yb3+ ion and its ligand.
The study also showed that, for the Yb3+/Al3+ co-doped silica glass, the Yb3+ ions were surrounded by electronegative [AlO4/2] and electroneutral [SiO4/2]0 groups, and the electronegative [AlO4/2] group favored the formation of the Yb2+ and Al-OHC pairs.
This work helps to understand the underlying mechanism of PD effects in YDF and it was supported by the National Natural Science Foundation of China.