We have analyzed the cooperative luminescence of an ytterbium-doped silica fiber in which the ytterbium concentration is quite large, i.e., ∼ 10 21 cm −3 . The presence of a fast decaying component in a decay profile of 2 F 5/2 → 2 F 7/2 transition indicates the formation of ytterbium pairs in the fiber. The ytterbium cooperative luminescence spectrum is characterized by two peaks at 488 nm and at ∼512 nm, respectively. Overall intensity of the visible cooperative upconversion luminescence is proportional to excitation power at 980 nm. However, as fiber length increases, intensity of the shorter-wavelength side of the cooperative luminescence spectrum becomes smaller than the other portion, which results in a narrower spectral bandwidth.
In this study, detailed investigation of photodarkening in Yb - doped aluminosilicate fibers is reported. The cooperative ::: luminescence loss influenced by photodarkening is measured simultaneously with 633 nm probe loss. The quadratic ::: dependence of photodarkening and cooperative luminescence versus dopant concentration was observed. The results ::: indicated lower cooperative luminescence loss at 513 nm than expected and provided new pieces of information to the ::: photodarkening mechanism description.
We prove that groups acting geometrically on delta-quasiconvex spaces contain no essential Baumslag-Solitar quotients as subgroups. This implies that they are translation discrete, meaning that the translation numbers of their nontorsion elements are bounded away from zero.