Refractive index change of multicomponent glass films for bismuthate erbium-doped waveguide amplifiers prepared by radio-frequency magnetron sputtering under different magnetic fields of cathode

Bismuthate erbium (Er)-doped waveguide amplifiers consist of an Er-doped core surrounded by an Er-free cladding film with lower refractive index. The propagation loss of a waveguide critically depends on both the thickness and the refractive index of its core and cladding films; hence, these two properties of such films must be controlled. We studied the influence of magnetic fields on a batch-to-batch variation of the refractive index of Er-free multicomponent cladding glass films deposited by radio-frequency magnetron sputtering. We successfully controlled the variation in the refractive indices of the films to within 0.001 throughout the lifetime of the target by applying the design techniques of a weak magnetic field and flat magnetic field lines to a magnetic array for sputtering. We also found that the self-bias voltage maintained a high value irrespective of target consumption. This phenomenon is thought to be related to stabilization of the deposition rate and the refractive index of the films under the experimental sputtering conditions.