Photodarkening in Yb-doped silica fibers: influence of the atmosphere during preform collapsing

摘要

We investigated the influence of the atmosphere during preform collapsing (Cl_2/O_2, He, reducing gases as CO and H_2) on the photodarkening process in ytterbium doped silica fibers. The measurements were performed for a probe wavelength of 633nm in-situ during cladding pumping at 915 nm in dependence on the Yb inversion. The equilibrium values of the core excess loss were found to be remarkably lower in the fibers from preforms collapsed with the treatment of He or reducing gases, whereas the photodarkening rate constants are rather similar. Subsequent measurements of the fluorescence properties (pump wavelength 915nm or 976 nm) were carried out at the photodarkened fibers and compared to earlier results obtained at fibers as drawn.Laser experiments with the different fibers at low Yb inversion (no photodarkening) show a decrease of the laser slope efficiency in parallel to the degree of reduction of the doped core glasses. For the He treatment, an optimum of lowered photodarkening loss and reasonable laser efficiency can be obtained.