Instrumentation for Simultaneous Non-Destructive Profiling of Refractive Index and Rare-Earth-Ion Distributions in Optical Fiber Preforms

We present a non-destructive technique for a combined evaluation of refractive index and active-dopant distribution in the same position along a rare-earth-doped optical fiber preform. The method relies on luminescence measurements, analyzed through an optical tomography technique, to define the active dopant distribution and ray-deflection measurements to calculate the refractive index profile. The concurrent evaluation of both the preform refractive index and the active dopant profiles allows for an accurate establishment of the dopant distribution within the optical core region. This combined information is important for the optimization and development of a range of advanced fibers, used, for example, in a high-power fiber lasers and modern spatial-division-multiplexing optical communication systems. In addition, the non-destructive nature allows the technique to be used to identify the most appropriate preform segment, thus increasing fiber yield and reducing development cycles. We demonstrate the technique on an Yb3+-doped aluminosilicate fiber preform and compare it with independent refractive index and active-dopant measurements. This technique will be useful for quality evaluation and optimization of optical fiber preforms and lends itself to advanced instrumentation.