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The sol-gel based granulated silica method has several advantages for the production of active optical fibers. It offers a high degree of freedom regarding the usable dopants and co-dopants, the maximum possible dopant concentration and the homogeneity of the dopants. The freedom in controlling the co-dopant concentration enables the full control to tailor the refractive index of the core. Several ytterbium (Yb) doped, aluminum (Al) and phosphorus (P) co-doped double cladding silica fibers with varying Al and P co-dopants concentrations have been produced by the sol-gel based granulated silica method, in order to study the influence of the different co-dopants concentrations on the fibers performance. To do so, we fixed the Yb concentration to 0.3 at% in all the fibers, as well as the cores and claddings diameters to 10 and 125 micrometer respectively. The variation of the core-cladding refractive index steps due to the different Al and P co-dopants concentration have been confirmed by measuring the one dimensional and two dimensional refractive index profile of every fiber by two different measurement apparatus, resulting in core-cladding refractive index steps that correspond well with the fibers compositions. In addition, the effect of the different co-dopants concentration on the fibers performance have been investigated by measuring the upper state lifetimes and the lasing performance (slope efficiency) of the fibers. We observed different fluorescence lifetimes among the differently co-doped fibers, and different slope efficiencies that are well correlated with the corresponding lifetime of each fiber. One of the fibers featured 60% slope efficiency at 1030nm by pumping the fiber at 976nm by a fiber-pigtailed laser diode (LD).
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