Core-cladding interface instability as one of the source of additional losses in high doped optical fibers

For the first time "viscous fingers" along the core-cladding boundary were detected in the fiber preforms with high concentration of Ge and Al in the core. The "viscous fingers" deform the waveguide structure and can cause the additional loss. The proposed theoretical model and observation of viscous fingers allow one to explain the origin of additional optical losses in fibers based on glasses with different viscosities. Also we can make several proposals to decrease these additional losses. The most obvious recommendation consists in matching of the viscosities of core and cladding.